Lasl subterrence program rapid excavation by rock melting, 1976, 89p

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Lasl subterrence program rapid excavation by rock melting, 1976, 89p

  1. 1. . .... : . * , . * .,.. .3& .*. . , : ..I I 4-5979-SRdO p UC-3 and UC-66c 775 Issued: February 1977 Rapid Excavation by Rock Melting == LASL Subterrene Program -= September 1973- June 1976 Compiled by R. J. Hanold Contributors J. H. Altseimer P. E. Armstrong H. N. Fisher M. C. Krupka scientific laboratory of the University of California LOS A L A M O S , NEW MEXICO 8 7 5 4 5 ffirmative Action/Equal Opportunity Employer a$ U N I T E D STATES E N E R G Y RESEARCH A N D D E V E L O P M E N T A D M I N I S T R A T I O N C O N T R A C T W-7405-ENG. 3 6 dSTRIBUTION OF THIS DOCUMENT I UNLlMlTED S
  2. 2. DISCLAIMERThis report was prepared as an account of work sponsored by anagency of the United States Government. Neither the United StatesGovernment nor any agency Thereof, nor any of their employees,makes any warranty, express or implied, or assumes any legalliability or responsibility for the accuracy, completeness, orusefulness of any information, apparatus, product, or processdisclosed, or represents that its use would not infringe privatelyowned rights. Reference herein to any specific commercial product,process, or service by trade name, trademark, manufacturer, orotherwise does not necessarily constitute or imply its endorsement,recommendation, or favoring by the United States Government or anyagency thereof. The views and opinions of authors expressed hereindo not necessarily state or reflect those of the United StatesGovernment or any agency thereof.
  3. 3. DISCLAIMERPortions of this document may be illegible inelectronic image products. Images are producedfrom the best available original document.
  4. 4. This work was supported by a grant from the National Science Foundation, Research Applied to National Needs (NSF-RA") and by the US EnergyI Research and Development Administration, Division of Geothermal Energy.I Printed in the United States of America. Available from National Technical Information Service U.S. Department of Commerce 5285 Port Royal Road Springfeld, VA 22161 Price: Printed Copy $5.00 Microfiche $3.00I
  5. 5. CONTENTS ABST~CT 1 I. INTRODUCTION AND SUMMARY 2 A. Objectives 2 B. Technical Approach 2= C. Sumnary 2 11. PROTOTYPE DESIGN AND TEST 6 A. Consolidation Penetrator Development 6 1. Introduction 6 2. S i g n i f i c a n t Technical Achievements 6 B. Extruding Penetrator Development 7 1. Introduction 7 2. 84-mn-diam Extended Surface Penetrator 8 3. High Advance Rate Extruder (HARE) 9 4. M e l t Flow Augmented Extruder 10 5. S i g n i f i c a n t Achievements 11 6. Penetration Rate Paradox 12 C. Coring Penetrator Development 13 D. Thermal Stress Rock F r a c t u r i n g Tool 14 E. G1ass-Formi ng Techno1ogy 15 F. D i r e c t M e l t Heating 17 1. Introduction 17 2. Me1t-Heating Experiments 17 G. S a l t Borehole Plugging by M e l t i n g 19 H. Plasma Arc Experiments 20 I. Rock Laboratory Test F a c i l i t y 111. POWER SOURCE DESIGN AND DIRECTED RESEARCH A. E l e c t r i c Power Sources 1. Intyoduction 2. Heater Development f o r Penetrator Research 3. A l t e r n a t i v e s t o Graphite Resistance Heating B. M a t e r i a l s Sc.ience and Technology 1. I n t r o d u c t i o n 2. Refractory A l l o y - Rock Melt I n t e r a c t i o n s 3.. Power Source M a t e r i a l s. 4. S t r u c t u r a l Glass L i n e r Results C. Refractory A l l o y F a b r i c a t i o n 1. Introduction-6eneral F a b r i c a t i o n Problems 2. High-Temperature Braze Development 3. Si1 i c o n Carbide Conversion Coatings D. Geosciences 1. Introduction 2. Me1t i n g Range Experiments 3. Molten Rock Property Studies . 4. Directed Research I .i i DISTRIBUTION OF THIS DOCUMENT ISUNLIMITED .$"r
  6. 6. IV. FIELD TEST AND DEMONSTRATIONS 40 A. Field-Demonstration Units 40 1. Introduction 40 2. Tunnel L i n i n g Experiment 40 B. Public Demonstrations 41 1. Washington, DC 41 2. Denver, CO 42 3. Tacma, blA 43 C. Mobile Experimental Field U n i t 43 1. Introduction 43 2. Stem Design and Performance 44 3. Proof-of-Concept Field Experiment 45 V. SYSTEMS ANALYSIS AND APPLICATIONS 47 A. Geothermal Well Technology 47 1. Introduction 47 2. Current Technological and Cost Status 47 3. Conceptual Applications of Subterrene Devices t o 50 Geothermal W11s e ‘4. Contacts Made t o Discuss Geothermal Well Drilling 52 Problems B. Geothermal Well Systems and Cost Analysis 53 1. Introduction 53 2. System Model 53 3. Study Results 56 4. Cost Analyses 57 C. Mathematical Modeling and Analysis 59 1. Introduction 59 2. The Thrust-Velocity Relationship for Extruding 61 Penetra tors 3 . Liquid Basalt Thermal Conductivity Investigation 66 4. Stem Cooling w i t h Particle Transport 69 5. M1 t-Heating Analysis e 70’ 6. Power Transmission Analysis of Subterrene Stem 77 D. Applications and Technology Transfer 78 V I . TECHNICAL REPORTS AND PRESENT~TIONS 80 A. Completed L S Technical Reports AL 80 B. Technical Presentations and Journal Articles 81 C. Reports Related t o Subterrene Technology Published by 83 Other Organizationsiv
  7. 7. RAPID EXCAVATION B ROCK MELTING Y -- LASL SUBTERRENE PROGRAM -- September 1973 - June 1976 Compi 1ed by R. J. Hanold Contributors J. H. Altseimer P. E. Armstrong H. N. Fisher M. C. Krupka ABSTRACT Research has been d i r e c t e d a t establishing the technical and economic feasi b i 1i y o f excavation systems based upon the rock-me1 t i n g (Subterrene) t concept. A series o f e l e c t r i c a l l y powered, small-diameter prototype me1t- i n g penetrators has been developed and tested. Research a c t i v i t i e s i n - clude optimizing penetrator configurations, designing high-performance heater systems, and improving r e f r a c t o r y metals technology. The properties o f the glass l i n i n g s t h a t are a u t o m a t i c a l l y formed on the melted holes have been investigated f o r a v a r i e t y o f rocks and s o i l s . Thermal and f l u i d - mechanics analyses o f the m e l t flows have been conducted w i t h the o b j e c t i v e o f optimizing penetrator designs. F i e l d t e s t s and demonstrations o f the prototype devices continue t o be performed i n a wide range o f rock and s o i l types. Primary emphasis has been placed on the development o f a penetrator designed f o r more economical e x t r a c t i o n o f geothermal energy and o f small- diameter penetrators which can be u t i l i z e d i n support o f geothermal energy exploration programs. The conceptual design o f a geothermal energy rock- m e l t i n g system w i l l d e f i n e the surface equipment, the stem, hole-forming. assembly, and debris removal subsystems.. Optimization o f we1 1 design, the t r a d e - o f f o f advance r a t e w i t h operating l i f e , the advantages o f using the melt-glass hole casing f o r well-bore seal-off, r i g automation, and the b e n e f i t s which r e s u l t from the i n s e n s i t i v i t y o f rock m e l t i n g t o formation temperatures and geologic v a r i a t i o n s have also been studied. Subsystem hardware development has been d i r e c t e d toward r e s o l u t i o n o f c r i t i c a l technical questions r e l a t e d t o penetrators f o r dense rock, debris handling, e l e c t r i c a l heater configuration, and e s t a b l i s h i n g pene- t r a t o r l i f e . Laboratory experiments and f i e l d t e s t s provide data f o r f i n a l system design o p t i m i i a t i o n s and i n d i c a t e proof o f a p p l i c a b i l i t y of the concept t o a geothermal w e l l hole-forming system. A f i e l d t e s t u n i t t o fonn r e l a t i v e l y shallow v e r t i c a l holes f o r heat f l o w surveys i n support o f geothermal exploration studies has been designed, fabricated, and f i e l d tested. Experience w i t h t h i s u n i t i s intended t o provide a basis f o r tech-.. nology t r a n s f e r t o the d r i l l i n g industry. P r a c t i c a l applications t o deep geothermal d r i l l i n g w i l l r e q u i r e an ex- tensive development program i n c o l l a b o r a t i o n w i t h the d r i l l i n g industry. progressive steps v i a smaller systems can p o t e n t i a l l y speed the transfer process. 1
  8. 8. I. INTRODUCTION AND SUMMARYA. Objectives Power Source Design and Directed Research The technical e f f o r t s and resources o f the LASL F i e l d Test and Demonstrationsrock-me1 t i n g (Subterrene) program have been d i s t r i b - Systems Analysis and Applications.uted t o y i e l d a balance o f prototype hardware o f i n - The s i g n i f i c a n t r e s u l t s and achievements i n t h ecreasing complexity and size, l a b o r a t o r y experi- research and development program are summarized f o rments, p r a c t i c a l f i e l d - t e s t experience, design and each o f these f o u r technical a c t i v i t i e s i n f o u reconomic analyses, e l e c t r i c heater development, ma- major sections o f t h i s r e p o r t f o r the period Septem-t e r i a l s development and applications, and t h e o r e t i - ber 1973 t o June 1976.c a l studies. The r e s u l t s o f these technical a c t i v - C. Summaryi t i e s were planned t o y i e l d : During the research p e r i o d covered by t h i s sta- The demonstration o f the basic f e a s i b i l i t y t u s report, a r e d i r e c t i o n o f technical e f f o r t was i n i - of rock m e l t i n g as a new excavation t o o l t i a t e d a t the request o f the programlsponsoring or- f o r a p p l i c a t i o n s up t o 400 mn (16 i n . ) i n ganizations. E f f o r t s d i r e c t e d toward the appl ica- diameter. t i o n o f rock m e l t i n g t o large-diameter tunneling Operational and f i e l d - t e s t data f r o m proto- systems were suspended. A vigorous research and de- type devices o f a range o f sizes and con- velopment program leading t o d e e p - d r i l l i n g capabil- i t y f o r geothermal energy systems was i n i t i a t e d i n - figurations, and the v e r i f i c a t i o n o f pre- l i m i n a r y t h e o r e t i c a l modeling needed t o c l u d i n g intermediate development o f equipment f o r scale t o l a r g e r diameters, p r e d i c t p e r f o r - shallow exploratory holes, development of h o l e sta- mance, make cost estimates, and optimize b i l i z a t i o n t o o l s , development and t e s t o f h o t rock advance r a t e and r e l i a b i l i t y . penetrators f o r magma taps, and development o f l a r g e r Refractory m a t e r i a l s technology s u f f i c i e n t l y diameter shallow-hole devices f o r a v a r i e t y o f po- established t o permit p r e d i c t i o n s o f compo- t e n t i a l applications. nent l i f e and t o generate m a t e r i a l s selec- Extruding penetrator systems have demonstrated t i o n c r i t e r i a f o r prototype development the basic features o f rock-melt f l o w handling w i t h needs and p r o j e c t i o n s o f service l i f e f o r debris produced i n the form o f chiTled glass pellets, systems i n p r a c t i c a l applications. glass rods,.or rock wool and c a r r i e d out of t h e stem F i e l d - t e s t experience and operational demon- by the coolant gas flow. Two l a r g e r diameter (84- s t r a t i o n s s u f f i c i e n t t o e x h i b i t the poten- and 86-mm), higher advance rate,molybdenum extruding t i a l u t i l i t y o f smaller diameter prototype penetrator systems were designed, fabricated, and devices and t o p r o j e c t commercial use i n t h e extensively tested i n both laboratory and f i e l d t e s t s . The f i r s t o f these designs incorporates mul- important p r a c t i c a l a p p l i c a t i o n s o f geo- t i p l e heater stacks, m u l t i p l e m e l t f l o w passages, and thermal energy e x p l o r a t i o n programs, econom- a penetrator configuration u t i l i z i n g extended sur- i c a l e x t r a c t i o n o f geothermal energy, and faces f o r enhanced heat t r a n s f e r surface area. This shallow h o r i z o n t a l hole emplacements i n penetrator was used w i t h t h e new Experimental F i e l d loose o r unconsolidated materials. U n i t (EFU) t o produce a 30m-deep hole i n a b a s a l t 0 Theoretical models and a n a l y t i c a l techniques 1edge near Los Alamos as a proof-of-concept experi- needed t o describe t h e heat- t r a n s f e r and ment. The second design incorporates a more conven- f l u i d mechanics o f the rock-melting and t i o n a l me1t i n g body capable o f producing t h i c k e r penetration processes f o r the purposes o f glass l i n i n g s i n dense rock, a s i n g l e annular r e s i s - o p t i m i z i n g configurations, and p r e d i c t i n g tance heater which d e l i v e r s a higher leading edge, performance. heat flux, and m e l t removal e n t i r e l y through t h e cen-B. Technical Approach t r a l e x t r u s i o n p o r t . S i g n i f i c a n t improvements have The technical e f f o r t i s organized i n t o f o u r been achieved i n extruding penetrators, i n c l u d i n g i m -technical a c t i v i t y areas whose functions are: Prototype Design and Test proved c o a x i a l - j e t debris removal systems, higher2
  9. 9. strength extractors, improved high-temperature opera- was d i r e c t e d toward the r e s u l t i n g physical properties i o n and s t a b i l i t y , and "designed i n " instrumentation of the plugs prepared i n t h i s manner. Another tech-qapability . The r e s u l t s from the successful t e s t i n g n i c a l approach investigated for Subterrene penetrators o f these extruding penetrator systems designed t o i n - involves the a p p l i c a t i o n o f plasma arc heating t o i n - vestigate s p e c i f i c aspects o f heater design, debris crease penetration r a t e s and handle the highest melt- removal, and viscous rock-melt f l o w t h a t w i l l lead t o ing-point rocks. While the observation has been higher penetration r a t e s were analyzed t o provide de- noted t h a t a plasma t o r c h r e a d i l y melts a rock sample, sign considerations f o r future penetrator systems. l i t t l e a t t e n t i o n has been given t o the task o f pro- Laboratory t e s t s i n Dresser b a s a l t heated t o 650 K v i d i n g a debris removal system for such a penetra- confirmed the a n a l y t i c a l l y predicted enhanced pene- t i o n device. t r a t i o n rates i n the hot basalt. Density consolida- R e s i s t i v e l y heated, pyrolytic-graphi t e heater t i o n penetrator designs have been developed t o the elements, which r a d i a t e energy t o the r e f r a c t o r y stage where compressed-air-cooled, oxidation-resis- metal penetrator body, have proven t o be most s a t i s - tant, e a s i l y replaceable penetrators a r e i n s a t i s - f a c t o r y f o r the development e f f o r t s . With heater f a c t o r y use f o r both laboratory experiments and f i e l d c a v i t i e s f i l l e d w i t h helium t o enhance the r a d i a l heat transfer, heat f l u x e s of up t o 2 MW/m 2 have been demonstrations i n a wide v a r i e t y o f porous materials i n c l u d i n g t u f f , alluvium, unconsolidated and layered obtained from p y r o l y t i c - g r a p h i t e radiant-heater ele- sediments, saturated ground, and b a s a l t i c gravel. A ments. Larger diameter annular-shaped p y r o l y t i c - high-temperature thermal stress rock f r a c t u r i n g probe graphite heaters have been used t o achieve higher based on rock-me1 t i n g Subterrene technology has been leading edge heat fluxes i n extruding penetrators, developed and made a v a i l a b l e f o r rock mechanics and and heater stacks whose energy generation per u n i t f r a c t u r i n g studies. A 114-mm-diam consolidating l e n g t h have been c a r e f u l l y matched t o the penetrator corer intended f o r use i n porous a l l u v i a l s o i l s has requirements through the use of a n a l y t i c a l modeling been designed, constructed, and laboratory tested. calculations are now routine. A new single-piece The core diameter i s 64 mn,and t h e melting body, carbon c l o t h composite heater has been developed which i s vacuum-arc-cgst molybdenum, i s f a b r i c a t e d which allows greater f l e x i b i l i t y i n heater configura- as a s i n g l e s t r u c t u r a l component. t i o n and has been successfully tested i n the thermal The p o s s i b i l i t y o f arranging the e l e c t r i c c i r - stress rock f r a c t u r i n g probe. Investigations o f new c u i t o f a rock-melting penetrator so t h a t c u r r e n t designs have been i n i t i a t e d , including high-tempera- passes through a molten region and deposits most o f t u r e l i q u i d metal heat pipes, the d i r e c t melt-heating the melting power d i r e c t l y i n t h e m e l t l a y e r j u s t concept, and t h e use o f higher resistance conduction adjacent t o the melting i n t e r f a c e has been recognized heaters which would lead t o s o l i d penetrators o f very f o r a long time. This mode o f rock melting has been high strength f o r use i n deep wells. A t the r e l a - investigated i n recent laboratory experiments and t i v e l y high temperatures encountered by rock-me1 t i n g The success o f a n a l y t i c a l modeling calculations. penetrator systems, most materials react with one an- these preliminary experiments has i n i t i a t e d f u r t h e r other t o some extent, and thermodynamic and k i n e t i c experimental and a n a l y t i c a l research t o d e f i n e the 1i f e t i m e 1i m i t a t i o n s have therefore been investigated. d e t a i l s of the heating mechanisms involved and the S t a t i c c o m p a t i b i l i t y laboratory t e s t s have been per- influence o f the t y p i c a l m e l t r e s i s t i v i t y vs tempera- formed t o determine the corrosion o r d i s s o l u t i o n r e - t u r e behavior on the process. An experimental study actions o f molybdenum and tungsten w i t h t u f f , Jemez of the f e a s i b i l i t y o f forming borehole plugs i n basalt, and g r a n i t i c rocks a t temperatures from 1700 underground s a l t deposits by m e l t i n g and r e s o l i d i - t o 2100 K. These experiments are designed t o inves- f y i n g s a l t s i m i l a r t o t h a t found i n the formation t i g a t e q u a n t i t a t i v e l y the e f f e c t s o f time and tempera- was conducted on the basis t h a t a plug w i t h physical t u r e on the reactions between r e f r a c t o r y metals and and chemical properties close t o those o f the forma- rock-glass me1ts. Techniques for rock-glass property t i o n could be formed by t h i s technique. Since melt- evaluation and optimization are under developmentwing and r e s o l i d i f i c a t i o n o f rock s a l t causes p r a c t i - w i t h the goal o f establishing rock-melt glass as an c a l l y no change i n the chemical properties, emphasis --t u i n si s t r u c t u r a l element t o serve as the hole 3
  10. 10. support d u r i n g penetration. Laboratory experiments field using a field-demonstration unit. A prototypehave confirmed a significant increase i n crush tunnel opening, 2 m h i g h , 2 m deep, and 1 m wide,wastrength and decrease i n permeability for rock-glass Li formed i n a loose alluvial d i r t f i l l . The roof andliners when compared to the original porous materials side walls were formed by melting a series of paral-from which they were formed i n situ. Petrological lel small-diameter horizontal holes i n the looseexaminations of parent rock and derived rock-glass soil f i l l using density consolidation penetrators.samples have been performed. Correlation of petro- The holes were placed sufficiently close togethergraphic information with physical properties and i t s that the glass linings fused and t h u s produced aextension to Subterrene design and performance is the double-walled lining reinforced by webs between thedesired goal. The melting ranges of approximately individual holes.15 different rock types were measured using hot- Efforts have been directed toward the develop-s t a t e microscopy. Penetrator fabrication techno1ogy ment of new analytical and numerical techniques forhas been vastly improved, particularly i n the areas analyzing the combined fluid dynamic and heat trans-of refractory metal forming, the development of mod- f e r performance of melting penetrators and the appli-erate-and high-temperature brazing techniques for dis- cation of these techniques to specific penetratorsimilar metals, and electron beam welding. designs and concepts. Numerical solutions of the The field-test program was expanded w i t h the coupled energy equation and the Navier-Stokes equa-design, construction, and utilization of two portable, tions, including the strong temperature dependencemodularized field-demonstration units (FDUs). These of the rock-melt viscosity, have been obtained. U t i -easily transportable units provided self-contained lizing this viscous rock flow computer program, de-systems for demonstrating smal 1-diameter rock-me1 t i n g tailed calculations have been performed on a varietypenetration system capabilities a t locations away of consolidating and extruding penetrator systems.from the Los Alamos area and were mobilized for pub- The validity of this powerful analytical tool hasl i c demonstrations of the rock-melting process before been established by comparisons with laboratory data,large audiences in Washington, DC; Denver, CO; and and design and scaling to larger diameters and dif-Tacoma, WA. Such field evaluations of penetrator ferent operating conditions can now be accomplished.systems have served to acquaint excavation technolo- Analytical techniques have also been extended togists with the potential of rock melting and have pro- study the fluidized debris removal process in ex-vided valuable data and experience on reliability and truding penetrators. Calculations performed on ex-service l i f e . Numerous penetrations into various un- truding penetrator designs have also confirmed theconsolidated soil samples, including layered samples significant penetration rate increases associatedformed from different loose materials, have been con- with increasing in situ rock temperatures and theducted t o examine the resulting glass liners. The general trend toward higher penetrator efficienciesglass liners have been of good quality,and the smooth with increasing penetration rate, b o t h effects havingtransition across the layered samples was particu- been observed in the laboratory. Results from a two-larly encouraging. The LASL-designed mobile experi- dimensional heat conduction program have been instru-mental field unit (EFU) has been delivered, evaluated, mental in improving the thermal design of penetratorand operated in the field on basalt penetration tests. systems, particularly i n the areas of desired heaterT h i s unit, which can be used with penetrators from 50 performance, thermal control of the coaxial - j e t de-to 127 mn i n diameter, consists of a trailer-mounted bris removal zone, thermal control of the glass form-stem tower, hydraulic power supply, control functions, ing and conditioning zone, and i n evaluating theand a thruster that will support pipe stems 300 m thermal stress distributions and cooling requirementslong. The f i r s t field use of the EFU was i n the pro- in critical penetrator regions. The development ofduction of a planned 30-m-deep hole i n a basalt ledge analytical models has contributed t o the basic under-a t Ancho Canyon u s i n g 84-mn-diameter extruding pene- standing of specific relationships such as the lead-trators. The basic concept of Subterrene systems for i n g edge flux limitations and the thrust-velocity detunneling and excavating loosely compacted formations pendence. O major importance has been the theoret- fwas demonstrated by an experiment conducted i n the ical calculation of material properties when the 4
  11. 11. II experimental values were not appropriate or avail- hot rock penetrations for geothermal energy explora- U e l . The application of these models and techniques tion and production t o emplacements i n arctic perma- o specific designs and the interpretation of t e s t frost. The technology dissemination efforts expended results have received the largest portion of the anal- by members of the Subterrene program have been ex- ysis program effort. Analytical calculations have tensive in both scope and depth. O particular in- f also provided s u p p o r t for the laboratory experiments terest to the current program direction is a report investigating new penetration techniques , including entitled, "Geothermal Well Technology and Potential . direct melt heating and plasma arc heating. .. Applications of Subterrene Devices - A Status Review," The number of novel and conventional potential which has been completed and distributed. This re- applications of Subterrene technology that have been port reviews the past, present, and some future as- investigated continue t o increase and range from deep pects o f the geothermal energy industry w i t h special attention given to geothermal well drilling problems. 5
  12. 12. I 11. PROTOTYPE DESIGN AND TEST A. Consol idation Penetrator Development resulting hole liners were removed and examined. AI 1. Introduction. For porous rock or soft typical result from such a test i s shown i n Fig. ground, the density consol idation Subterrene can be 11-3. used to simplify the excavation process. In these materials, the glass lining formed when the rock- melt cools, i s significantly more dense and there- fore occupies a smaller volume than did the original porous rock. B melting out to a diameter larger y than that of the penetrator, the molten debris from the hole can be entirely consolidated i n the dense glass lining, completely eliminating the necessity for removing debris. Density consol idation pene- trator designs have been developed t o the stage where compressed-air-cooled, oxidation-resistant, easily replaceable penetrators are used satisfacto- r i l y for both laboratory experiments and field de- monstrations i n a wide variety of porous materials, including tuff, alluvium, unconsolidated and layered Fig. 11-1. Consolidating penetrator designs used f o r melting prototype tunnel opening sediments, saturated ground,and basaltic gravel. i n loose alluvial f i l l . . 2. Si gni f i cant Tec h n i ca 1 A h i evemen ts c A prototype tunnel opening (described i n detail i n Sec. IV. A ) was formed in.loose alluvial f i l l using two different 50-mm-diam consolidating penetrator designs. These penetrator designs, which employ replaceable graphite glass formers, are illus- trated i n Fig. 11-1. The solidified rock-melt layer which adheres t o the molybdenum penetrator body and provides oxidation resistance i s clearly visible. A considerable amount of operational field experi- ence was obtained i n this t e s t as over 100 m of sta- bilized hole were formed i n a loose f i l l including both vertical and horizontal penetrations. The a b i l i t y of consolidating penetrators t o produce glass-lined stabilized holes through vari- able, broken, and d i f f i c u l t rock samples was further verified by extensive testing i n a wide variety of samples. Figure 11-2 i l l u s t r a t e s a hole melted i n a conglomerate of Hanford alluvium. containing basal- t i c gravel and large cobbles. Penetration of the largest cobbles without debris removal is achieved by thermal stress fracturing and extrusion of por- tions of the rock melt into the resulting cracks. The a b i l i t y t o provide a continuous glass lining across nonhomogeneous samples was demonstrated i n tests i n which samples comprised of layers of allu- F i g . 11-2. Hole melted i n a conglomerate of Hanfor alluvium containing basaltic gravel and L large cobbles w i t h a consolidating pene- J viums, shales, and tuff were penetrated and the trator. 6
  13. 13. soil density and mechanical compaction. Results i n - dicate an extremely sharp drop i n penetration rate w i t h increasing soil density i n the range of densi- t i e s between 1.7 and 1.8 Mg/m 3 . Penetration of s o i l s w i t h b u l k densities greater than 1.8 by consolidating penetrators is very slow,and there i s l i t t l e possi- b i l i t y of significant mechanical compaction. Successful laboratory tests were conducted w i t h a 50-nnn-diam parabolic penetrator and a 60-mn- diam penetrator with a parabolic leading edge pro- f i l e joined t o a short cylindrical afterbody. Sig- nificant performance data were obtained from theseFig. 11-3. Continuous glass liner produced through tests,and penetration rates almost double the pre- a layered sample of unconsolidated allu- viums. vious rates for the double cone configuration were achieved . A design study was conducted indicating the During the research period covered by t h i s sta-feasibility of an integrated 76-mm-diam extruder- tus report, technical effort was redirected a t theconsol idator hole-forming system. T h i s proposed request of the program-sponsoring organization.system would take advantage of the simplicity of con- Efforts directed toward the development of densitysolidating penetrators, which eliminate the debris consol idation penetrators were terminated, and theremoval operation, for use in porous or low-density program emphasis was placed on the development ofrocks and soils. If hard rock were encountered, the hard rock extruding penetrator systems that could beuniversal stem would permit changing to an extruding applied t o the exploration and production of geother-penetrator for hole advancement u n t i l soft ground mal energy.conditions again prevailed. The emplacement of near- B. Extruding Penetrator Developmentsurface small-diameter u t i l i t y lines represents the 1. Introduction. Extrusion penetrators are re-main application for such an integrated system. quired i n dense materials and are designed to contin- Detailed viscous flow calculations have led uously remove the debris from the borehole. As i n -t o the development of a penetrator geometry incorpo- dicated in Fig. 11-4, the melt flow, confined by therating a parabolic leading edge segment joined t o a unmelted rock and the hot melting face of the pene-cy1 indrical afterbody. The parabolic leading edge trator, i s continuously extruded through a nozzle in the melting face. T h i s material is chilled and freezes shortly a f t e r the circulating cooling fluid impinges upon the extrudate exiting from the melt orifice. The flowing coolant then carries these small fragments up the stem t o the exhaust section. E x t r u d i n g penetrator systems have demonstrated the basic features of rock-melt flow handling w i t h debrissuited for the use of high-te produced in the form of chilled glass pellets, glass rods, or rock wool. These penetrators have been op-leading edge for en erated in the laboratory and in the field, i n verti-conceptual desi cal and horizontal orientations, and i n both hardrating these ad igneous rocks and porous tuffs and alluviums. Three larger diameter (84-to 89-mm) , higher advance rate,oerformance has been extended t o include the effect molybdenum extruding penetrator systems were design- mechanical compaction of the soil surrounding the ed, fabricated, and tested. Significant improvementsmelt zone. Calculations employing these methods show have been achieved i n current extruding penetrators,the dependence of consolidator penetration rates on including improved coaxial-jet debris removal systems, 7
  14. 14. rDEBRlS REMOVAL ZONE MOLYBDENUM Fig. 11-5. Cross section of the 84-m-diam extended surface penetrator me1 t i n g body. DENSE ROCKFig. 11-4. Extruding penetrator concept i l l u s t r a t - from the gas nozzle. The chilled debris is removed ing debris removal technique. through the debris carry-off tube and transported to the surface by the gas stream. A portion of theimproved high-temperature operation and stability, cooling gas flow enters the debris carry-off tubeand "designed-in" instrumentation. A maximum pene- through the gas orifices which provide a boundarytration rate of 0.28 m / s (3.31 f t / h ) in denseba- layer of cool gas along the tube surface reducing i t ss a l t has been demonstrated in the laboratory. temperature and minimizing any tendency for the cool- 2. 84-mn-diam Extended Surface Penetrator. ing rock melt to adhere t o i t s surface. Actual 84-This design introduced the concepts of mu1 t i p l e mm-diam hardware i s illustrated i n Fig. 11-6. Theheater stacks, multiple melt flow passages, and a upper penetrator i s unused,whereas the lower u n i tpenetrator configuration using extended surfaces for was tested in basalt and retains its protectiveenhanced heat transfer surface area. Based on theenhanced surface area, greater heater power, reducedoperating melt layer thickness, and high thrust ca-pability, this penetrator has melted hard rock a t asignificantly faster (- 3X) rate than the earlier66-mn design i t replaces. Figure 11-5 illustratesthe major components of this penetrator design. Theheater consists of three separate stacks of pyro-graphite resistance heater pellets and includeshigher power density pellets near the leading edgeto provide a higher heat flux in this critical re-gion. Electrical power i s supplied to the heatersby three tungsten electrodes. Energy distributionthroughout the penetrator is accomplished by radia-tion from the surface of the heater stacks t o thegraphite receptors and then by conduction throughthe h i g h thermal conductivity molybdenum meltingbody. Molten rock enters the rock nozzle from the Fig. 11-6. Extended surface penetrator hardwareaxial and radial rock flow passages and is rapidly comparing an as-fabricated melting body with an assembly that was tested inchilled by the high-veloc ty cooling gas exiting basalt.8
  15. 15. l a y e r o f s o l i d i f i e d b a s a l t melt. The f l u t e d o r ex- f i g u r a t i o n o f t h i s penetrator corresponds t o t h e de-u e n d e d surface c o n f i g u r a t i o n and t h e mu1t i p l e m e l t s i g n i l l u s t r a t e d i n Fig. 11-4, d e p i c t i n g t h e concept removal passages are c l e a r l y v i s i b l e i n t h i s photo- of extruding penetrator operation. This design was graph. 1 + based on t h e use o f a l a r g e d i a m e t e r annular pyro- This 84-mm-diam extended surface penetrator was graphite heating element which would provide t h e used t o produce a 30-m-deep f i e l d hole i n b a s a l t maximum surface area f o r r a d i a t i o n heat t r a n s f e r (see Sec. I V . C. 3). Design changes based on f i e l d near the c r i t i c a l leading edge region. The annular t e s t experience have eliminated e a r l y d i f f i c u l t i e s design eliminates the azimuthal temperature v a r i a - encountered i n the operation t o produce t h i s 30-m- t i o n s present w i t h t h e use o f three separate heater deep hole. Rock i n t r u s i o n has been eliminated by stacks (a!; i n the extended surface penetrator) and improved c o o l i n g of t h e glass-former region, and plug- f u r t h e r enhances the leading edge heating since both ging o f t h e debris carry-off tube has a l s o been elim- the i n n e r and outer surface of the heater can r a d i - i n a t e d by improved c o o l i n g o f t h e w a l l near t h e de- a t e w i t h equal f l u x density. I n addition, t h e mo- b r i s forming nozzle. The tube temperature i s held lybdenum penetrator body thickness was h e l d t o 5 m below 650 K t o prevent s t i c k i n g of h o t m e l t p a r t i - i n t h i s region t o minimize the temperature drop r e - cles. This has been accomplished by a combination s u l t i n g from heat conduction t o t h e molten rock l a y - of increased gas f l o w and improved coolant passage er. Approximately h a l f o f the t o t a l heater power i s design. Tests of t h i s penetrator system incorpo- developed i n t h e t h i n constant area section embedded r a t i n g a m e l t channel impedance, o r surge suppressor, i n the leading edge. Heater power i s s t e a d i l y r e - t h a t increased the pressure i n t h e molten l a y e r have duced away from the leading edge where t h e m e l t i n g been successful i n suppressing gas bubble production power requirements o f the penetrator are much less. i n t h e me1t and have produced remarkably improved This t a i l o r i n g o f the heater output i s based on de- smooth glass borehole l i n i n g s i n a d d i t i o n t o m i n i - t a i l e d coniputer c a l c u l a t i o n s which are discussed i n mizing any tendency o f t h e d e b r i s removal system t o the analysis section of t h i s report. Polycrystal- surge during t r a n s i e n t operations. Surging occurs 1ine-graphite r a d i a t i o n receptors are used t o i m - when a l a r g e m e l t pool i s formed and the penetrator prove t h e surface absorption c h a r a c t e r i s t i c s o f the i s suddenly pushed i n t o it. This causes a l a r g e heater c a v i t y which i s a l s o f i l l e d w i t h helium gas amount o f l i q u i d rock t o extrude r a p i d l y i n t o t h e t o provide a d d i t i o n a l conduction t r a n s f e r across the c a r r y - o f f system. While the gas system can c a r r y a narrow gap. Debris removal techniques i n t h i s pene- considerable amount o f rock i n excess of t h e steady- t r a t o r system are completely analogous t o those em- s t a t e design, a severe surge can overload it. Suc- ployed i n t h e extended surface penetrator. cessful operation o f t h i s debris removal system has With tin 89-nnn-diam m e l t i n g body, a HARE pene- been demonstrated i n both v e r t i c a l and h o r i z o n t a l t r a t o r was operated i n t h e l a b o r a t o r y a t power l e v e l s orientations. I n a d d i t i o n t o operations i n hard of up t o 24 k and corresponding penetration r a t e s W rock, t h i s penetrator system has been tested i n po- i n b a s a l t rock up t o 0.28 mn/s (3.31 f t / h ) w i t h de- rous Bandelier t u f f w i t h encouraging r e s u l t s . The bris-handling and glass-forming systems f u n c t i o n i n g extrudate was e j e c t e d i n s h o r t rods which were e a s i l y c u l a t e d and experimentally measur- expelled from t h e system by t h e t r a n s p o r t gas flow, power r e l a t i o n s h i p s a r e i n excel- and there was evidence t h a t t h e penetration mode n a d d i t i o n t o hard rock, HARE has (i.e. , extrusiontor d e n s i t y consolidation) could be n compacted Hanford c o n t r o l l e d by varying t h e gas flow r a t e t o the ex- density o f 2.1 Mg/m3) and produc t r u s i o n nozzle and t h e heater power l e v e l . w a l l approximately h i c k using a longer tem- 3. High Advance Rate Extruder (HARE), The HARE pera ture-c:ontroll e former. E a r l i e r broblems design has a more conventional m e l t i n g body capable of i n producing a smooth glass borehole l i n i n g were woducing t h i c k e r glass l i n i n g s i n dense rock, a s i n - l a r g e l y a t t r i b u t e d t o t h e gases released from t h eu e annular resistance heater t h a t d e l i v e r s a higher rock during melting. A modification consisting o f leading edge heat f l u x , and m e l t removal e n t i r e l y a long extrudate tube-nozzle system has been employ- through t h e c e n t r a l e x t r u s i o n p o r t . The basic con- ed t o a l l e v i a t e t h i s problem by c o n t r o l l i n g t h e back- 9
  16. 16. pressure a t the glass-forming section thereby i n - Debris creasing the pressure i n the melt layer and sup- pressing gas bubble production. Figure 11-7 shows Remya HARE penetrator a f t e r testing i n basalt. 4. Melt Flow Augmented Extruder. An attempthas been made t o arrange the geometry of a pene-trator in such a manner that h o t molten rock i sforced t o flow past the leading edge and therebyprevent this region from cooling a t higher penetra-tion rates. A conceptual design is shown i n Fig.11-8 to i l l u s t r a t e this technique. A melt reservoiri s formed on the outside of the conical section andmaintained under pressure by the applied thrustload. The pressure differential resulting from thedebris removal causes the melt to flow forward pastthe leading edge. The hottest (and least viscous)rock flows most easily and carries a significant L R o c k Flow -Iamount of energy forward to prevent cooling of the Fig. 11-8. Conceptual design of a melt flow aug-leading edge and vanishing of the melt layer thick- mented penetrator.ness. A new penetrator design that augments leadingedge conduction heat transfer w i t h forced melt flowhas been developed and i n i t i a l l y tested i n an 86-m-diam version. Heater configuration and debris re-moval system operation are based on the HARE designwhich has demonstrated successful performance i n ex-tensive laboratory testing. The penetrator assemblyfor this melt flow augmented extruder i s illustratedi n Fig. 11-9 prior t o testing. The melt flow pass-age i s on the axis so that material melted near theperimeter of the hole must flow along the cone andover the leading edge i n order to reach the exit Fig. 11-9. Melt flow augmented extruding penetrator w i t h segmented molybdenum glass former. passage. A segmented molybdenum glass former is em- ployed. Tests showed t h a t t h i s penetrator can ac- cept higher power levels a t higher thrust loads w i t hFig. 11-7. HARE extruder a f t e r several tests i n an increase i n penetration rate without an increase basalt showing the molybdenum me1 ting i n melting body temperature. In the initial testing, body w i t h residual rock melt and the rates as high as 0.232 mn/s (2.74 f t / h ) were attained. graphite glass fomer.10
  17. 17. The maximum v e l o c i t y o f the rock m e l t as i t crosses Data from a l a r g e number o f laboratory andU h e leading edge i s approximately 200 times the pen- f i e l d t e s t s have confirmed the operating performance etration r a t e velocity. Despite t h i s very appre- maps and r e p e a t a b i l i t y o f the newer extruding pene- c i a b l e v e l o c i t y enhancement, absolute v e l o c i t i e s are t r a t o r systems. Typical of these data i s the r a t e vs s t i l l q u i t e low,and higher t h r u s t s are required t o power r e l a t i o n implied by laboratory t e s t data f o r provide t h i s hydrodynamic pumping. Preliminary lab- the extended surface penetrator melting i n b a s a l t as oratory t e s t data i n d i c a t e t h a t s i g n i f i c a n t l y higher indicated i n Fig. 11-10. me1t v e l o c i t i e s are required before any appreciable 0 Extruding penetrators have been used success- penetration r a t e increase can be r e a l i z e d as a r e - f u l l y t o nielt samples o f basalt, granite, porous vol- s u l t o f m e l t f l o w augmentation. Further t e s t s t o canic t u f f , and compacted alluvium. Typical labora- b e t t e r define the r o l e o f higher v e l o c i t y rock m e l t t o r y samples o f b a s a l t t h a t have been penetrated i n f l o w i n the leading edge region are planned. hardware development experiments are i l l u s t r a t e d i n 5. S i g n i f i c a n t Achievements. Fig. 11-11, which shows the r e s u l t i n g smooth-lined A1 1 three new extruding penetrator systems holes. A material p a r t i c u l a r l y d i f f i c u l t t o d r i l l have achieved penetration rates i n excess o f 0.2 mm/s. through by conventional techniques i s high quartz- The HARE design was operated i n the laboratory a t pow content g r a n i t e gneiss. Samples o f such a gneiss e r inputs o f up t o 24 k w i t h correspondina advance W were supp1,ied t o the program from a quarry s i t e i n r a t e s i n b a s a l t rock up t o 0.28 mn/s (3.3 f t / h ) . V i r g i n i a f o r melting experiments. Figure 11-12 shows Maximum penetration r a t e s a t t a i n e d exceed those o f a hole me1 ted i n t h i s g r a n i t e gneiss by an extruding the e a r l i e r 66-nnn-diam design by a f a c t o r o f 3 t o 4. penetrator and some o f the associated debris ejected E f f o r t s t o develop advanced penetration sys- i n the form o f short glass rods. Differences between tems capable o f s i g n i f i c a n t l y higher advance rates t y p i c a l b a s a l t debris and the gneiss extrudate are have reached the laboratory experiment stage. Ex- a t t r i b u t e d t o the l a r g e d i f f e r e n c e i n v i s c o s i t y be- periments r e l a t i n g t o d i r e c t m e l t heating and plasma tween the two glass melts and t o the s i g n i f i c a n t v o l - arc heating are i n progress (see Sec. 11. F. and ume f r a c t i o n o f quartz c r y s t a l s i n the gneiss. 11. H). Preliminary design o f a l a r g e r diameter (150- Production o f a 30-m-deep f i e l d hole i n ba- mm) penetrator based upon the HARE c o n f i g u r a t i o n i s s a l t has been completed w i t h the 84-rn-diam extended i n progress. The i n i t i a l m e l t i n g bodies w i l l be made surface penetrator operated from the experimental 0.251 I I I I I I f i e l d u n i t (see Sec. I V . C. 3). As a proof-of-concept experiment f o r a geo- I thermal energy prototype b i t , an extruding penetrator system was used t o m e l t a hole i n a block of Dresser b a s a l t preheated t o 650 K. The r e s u l t s o f t h i s t e s t were compared t o another s e t o f t e s t s conducted a t the same operating conditions w i t h the exception o f the b a s a l t block temperature, which was a t a normal ambient value o f 290 K. The l a r g e specimen o f Dress- e r b a s a l t was surrounded with m u l t i p l e e l e c t r i c a l resistance heaters and brought t o temperature over a 4-112-day heating period. Experimental data con- firmed t h a t the a n a l y t i c a l l y estimated 25% increase i n penetration r a t e i n t h e hot b a s a l t could be eas- I I I 1 I I2 14 1 6 18 20 i l y achieved. Results from t h i s "hot rock" t e s t Power (kW) have established t h a t operatton i n very hot rock i su f e a s i b l e , and t h a t enhanced penetration r a t e i s an Fig. 11-10. Rate vs power r e l a t i o n s h i p f o r 84-mm- diam extended surface penetrator from additional benefit. laboratory t e s t s i n basalt. 11
  18. 18. b u t i t i s not expected t o be a practical material for production penetrators. Information from laboratory and f i e l d t e s t s of extruding penetrator systems is being used to provide the basis for designs of larger diameter pen- etrator systems for high-pressure, deep-drilling geothermal energy applications. 6. Penetration Rate Paradox. Analytical cal- culations based on penetrator i n p u t power, calculat- ed thermal fluxes, and available published transport properties of viscous rock melts have indicated the potential for appreciably higher penetration rates than could be attained i n laboratory tests. While the basic agreement between the analytical calcula- tions and the laboratory established penetrator per- formance data has been very good, two particular areas where this agreement breaks down have been noted. The f i r s t area of concern was that calculat- ed penetrator thrust loads were considerably lowerFig. 11-11. E x t r u d i n g penetrator melted holes i n than those observed i n actual tests. The second typical laboratory samples of basalt. area o f concern was the inability of penetrator sys- tems t o consistently achieve rates i n excess of 0.25 mm/s with allowable body temperatures despite the fact t h a t the analyses indicate higher rates should be possible with acceptable temperatures. Resolu- t i o n of these discrepancies led t o the investiga- tion of molten rock thermal conductivities presented in detail i n the analysis section of this report. The conclusion arrived a t was that the thermal con- ductivity of molten rocks i s appreciably lower than some data in the literature indicates. The source of the problem stems from correctly removing the thermal radiation contribution from the measured effective conductivity to arrive a t the true thermal conductivity based only upon molecular conduction. Using a value of 0.25 W/m-K for the thermal con- ductivity of molten basalt i n the computer simula- t i o n program ( t h i s value i s appreciably lower than the ones used i n previous calculations), i t becomesFig. 11-12. Hole melted in h i g h quartz-content evident t h a t the penetration rate of an extruding granite gneiss with extruding pene- penetrator i s limited by the heat flux that can be trator, including associated debris i n the form of short glass rods. provided a t the leading edge or flow stagnation point. Based on allowable temperatures in the molyb-from Sic-coated graphite which can be fabricated a t denum body, the new calculations indicate that thea fraction of the cost of a refractory metal body. leading edge heat f l u x will r e s t r i c t penetrationLaboratory tests indicate t h a t this material will rates in basalt to approximately 0.25 mmjs. T h i shave a long enough lifetime for testing purposes, lower value of thermal conductivity also results i n a thinner melt layer a t the leading edge for the12
  19. 19. i same heat f l u x . A t h i n n e r m e l t l a y e r necessitates C. Coring Penetrator Development higher t h r u s t loads f o r penetration, and hence t h i s For a p p l i c a t i o n s such as geophysical prospect-Bjalso explains why t h e c a l c u l a t i o n s w i t h a higher ing, i t i s desirable t o e x t r a c t a r e l a t i v e l y undis- thermal c o n d u c t i v i t y always underestimated the ap- turbed core sample t o i d e n t i f y the rock l a y e r s and . 4 - p l i e d t h r u s t load. Away from t h e leading edge, how- f a u l t structures a t various depths. The Subterrene ever, the c o n i c a l shape o f t h e me1t i n g body provides concept o f rock penetration by progressive m e l t i n g a s i g n i f i c a n t geometrical enhancement and much has been expanded t o include a technique f o r obtain- higher penetration r a t e s are possible. This concept i n g g e o l o g i c a l l y i n t e r e s t i n g core samples from t h e was demonstrated i n a l a b o r a t o r y t e s t using a coni- material being penetrated. The c o r i n g concept u t i - c a l shaped penetrator m e l t i n g i n t o a b a s a l t sample l i z e s an annular m e l t i n g penetrator which leaves a t h a t was p r e d r i l l e d t o remove the rock t h a t would rock-glass encased core i n t h e i n t e r i o r . Although normally be melted by t h e leading edge region o f t h e the concept i s a p p l i c a b l e t o e i t h e r the e x t r u s i o n penetrator. With the penetrator body temperature o r d e n s i t y consolidation mode o f melt-handling, i n i - below the operating maximum, sustained r a t e s o f j u s t t i a l emphasis was placed on a consolidating-coring under 1 mm/s were attained. This represents a fac- penetratar. A 114-m-diam consolidating corer i n - t o r o f 4 t o 5 times t h e r a t e t h a t would have been tended f o r use i n porous a l l u v i a l s o i l s has been de- expected i f the leading edge heat f l u x were c o n t r o l - signed, constructed, and laboratory tzsted. The core l i n g the r a t e . This conical penetrator and a sec- diameter i s 64 mn and t h e m e l t i n g body, which i s t i o n o f t h e g l a s s - l i n e d hole i n b a s a l t t h a t i t pro- vacuum-arc-cast molybdenum, i s f a b r i c a t e d as a s i n g l e duced are i l l u s t r a t e d i n Fig. 11-13. Experimental s t r u c t u r a l component. The penetrator has a water confirmation of t h i s a n a l y t i c a l l y predicted r e s u l t c o o l i n g system which represents a departure from t h e l e d t o an i n t e n s i v e research program t o introduce conventional gas systems. Design power l e v e l i s 13 techniques f o r increasing the a v a i l a b l e leading edge kW, and i n i t i a l t e s t i n g was accomplished a t 9 k i n W heat f l u x . t u f f and alluvium w i t h low t h r u s t loads and penetr- t i o n r a t e s i n t h e range fo 0.05 t o 0.15 mn/s. rm During an i n i t i a l t e s t t h e penetrator assembly was allowed t o cool i n place a f t e r making a pene- t r a t i o n i n t o a volcanic t u f f sample i n t h e labora- tory A f t e r breaking away the rock sample and glass hole l i n e r , t h e m e l t i n g assembly i s shown i n place i n F g. 11-14. The end o f the core sample i s v i s i - b l e eaving the core removal tube and a p o r t i o n o f the t r u c t u r a l - g l a s s h o l e 1i n i n g can be seen behind t h e m e l t l n g assembly. A t y p i c a l segment o f t h i s ex- t e r n a l glass l i n i n g and a glass-encased core sample can be seen i n more d e t a i l i n Fig. 11-15. The ap- proximately 18-mm-thick glass hole 1i n i n g r e s u l t s from debris disposal by t h e combined mechanisms o f core removal and density consol i d a t i o n . Although t h e periphery o f the core sample w i l l be melted and an a d d i t i o n a l region w i l l be thermally a l t e r e d , t h e i n t e r i o r region can be preserved when the penetrator i s operating a t the c o r r e c t design conditions. This i s i l l u s t r a t e d i n Fig. 11-16,which i s a cross section o f a Subterrene-produced core i n Green River shale. The dark melted peripheral r e - Fig. 11-13. Conical penetrator and h o l e melted a t gion can be c l e a r l y distinguished from the s o f t high r a t e i n p r e d r i l l e d b a s a l t sample. shale remaining i n the i n t e r i o r . This unique a b i l i t y 13
  20. 20. Fig. 11-16. Cross section o f a Subterrene-produced core i n Green River shale showing glass l i n i n g and s o f t i n t e r i o r region. and d i r e c t i o n s f o r s i g n i f i c a n t performance improve- ments have been indicated. During the research pe-Fig. 11-14. M e l t i n g assembly o f c o r i n g penetrator r i o d covered by t h i s report, technical e f f o r t s were a t completion o f t e s t showing forma- t i o n of glass hole l i n i n g and emerging r e d i r e c t e d a t t h e request o f t h e program-sponsoring core sample. organization and f u r t h e r development o f t h e c o r i n g concept has n o t been pursued. D. Thermal Stress Rock F r a c t u r i n g Tool F r a c t u r i n g by developing thermal gradients i n a rock mass by r a p i d heating o r c o o l i n g o f i t s surface i s a well-known technique t h a t has been employed i n p r i m i t i v e forms o f mining and tunneling since ancient times. The modern excavation i n d u s t r y does n o t t y p i - c a l l y employ thermal s t r e s s f r a c t u r i n g , b u t research being conducted a t t h e U n i v e r s i t y o f Missouri i n d i - cates t h a t t h e technique can be e f f e c t i v e l y u t i l i z e d i n tunneling o f hard rock. Their experiments i n v o l v e d r i l l i n g an a r r a y o f holes i n rock face t o a prede- termined depth and i n s e r t i n g h i g h - i n t e n s i t y heat sources a t t h e hole bottoms. The r e s u l t i n g thermal stresses produce cracks normal t o the rock face and a l s o p a r a l l e l t o t h e rock face i n a plane containing t h e heat sources. Thus t h e heading i s advanced byFig. 11-15. External glass l i n i n g and glass-encased breaking an incremental depth o f the rock i n t o f a i r l y core sample produced by Subterrene c o r i n g penetrator i n volcanic t u f f . r e g u l a r blocks which are subsequently removed by me- chanical means. Requirements f o r the commercial ap-t o "package" a core sample i n a r i g i d glass casing p l i c a t i o n of t h i s technique demand the a v a i l a b i l i t yleads t o t h e p o t e n t i a l f o r obtaining oriented geo- of rugged, reusable, economical heat sources. De-l o g i c a l l y i n t e r e s t i n g samples i n f a u l t e d o r brokenground masses. Detailed a n a l y t i c a l analyses of t h i s vices developed f o r rock m e l t i n g as a p a r t o f t h e Subterrene program have these desired c h a r a c t e r i s t i c s . Li n i t i a l c o r i n g penetrator design have been developed They are capable o f sustained operation a t high14
  21. 21. blA.AloR wSEALASSEMW COMPOSITE HEATER MOCYBDENUM BODY ELECTRODE Fig. 11-17. Cross section of thermal stress rock fracturing tool. temperature in contact with molten rock; they are rugged and reusable, and i n production could be man- ufactured economically. The development of a spe- cial tool for rock fracturing based on rock-melting technology was therefore undertaken. The rock fracturing tool, shown in Fig. 11-17, i s 48 mm diam and 2 m long. The molybdenum heater - body operates a t 1700 K and i s hermetically sealed with helium for protection of the electrically pow- ered graphite composite heating element which oper- - ates a t 2100 K. Peripheral equipment required for operation of the fracturing tool consists of a power supply (40 V , 250 A) and an a i r compressor for cool- Fig. 11-18. Thermal stress fragmentation of a gran- i t e block produced by electrically ing gas (35 SCFM). A,typical power level of 5 kW heated molybdenum fracturing tool. i s required to produce fracturing. Rock fracturing tools have been successfully a gas-cooled graphite glass-forming section directly operated i n basalt, granite, limestone, and sand- behind the me1 ting penetrator, smooth-walled 1inings stone w i t h 3 t o 5 major fractures per hole occurr- were formed i n a wide variety of soft ground and po- ing w i t h i n 475 to 725 s a f t e r reaching operating rous rock mediums which permitted extraction of the temperature. The rock fragments typically range in penetrator assembly w i t h relative ease. Graphite size from 25 t o 140 kg. Figure 11-18 shows the i n i - was selected as the forming material because i t mini- t i a l fractures in a granite block produced by a sin- mized the tendency for the cooling glass to stick or gle rock fracturing tool and the fragmentation of adhere to i t s surface d u r i n g the solidification pro- this block after fracturing from both predrilled cess. The thick glass liners necessitated by density holes. Spacing of the predrilled holes i n the rock consol Ida tion penetrators readily accommodated the face has typically been 0.3 m. I t i s necessary t o presence of even large volumes of unmelted quartz develop fractures t o one or more free, unrestrained crystals without seriously affecting the integrity of surfaces to f a c i l i t a t e breaking out the rock frag- the liner. When experiments were performed with high- ments and to provide a surface t o which cracks can e r thrust loads, the glass walls of the resulting propagate. holes were of much better visual quality than noted E. Glass-Forming Technology previous1.y. I t appears logical t h a t the higher Early experiments w i t h density consol idation thrust loads and associated higher pressures i n the penetrators melting in Bandelier tuff and alluvium rock melt minimize gas-bubble evolution which canw d e m o n s t r a t e d the basic features of forming the mol- cause voids in the glass walls. ten rock into a competent glass hole l i n i n g . Using The use of extruding penetrators in hard compe- tent rock presents a different s e t of conditions for 15
  22. 22. liner optimization. The melting penetrator i s now tube t o significantly increase the pressure drop ofcontinuously extruding molten debris for removal and the entering rock melt. Operating w i t h this flowtransport t o the surface,and the glass liner require- restriction, melt pressure levels a t the glass-form-ments for a stabilized hole i n competent rock are i n g section are increased sufficiently t o suppressvery minimal. Penetrator development effort has gas bubble production while simultaneously minimizingbeen directed toward improving the ability of the the effects of surging a t the debris-forming nozzle.system to control conditions i n the molten rock to Tests with an 84-nim-diam extruder incorporating a re-influence the character and properties of the bore stricted extrudate passage, or surge suppressor, thatlining and the extruded debris. After completing increased the pressure i n the melt have produced re-the 30-m-deep field hole i n basalt, photographs of markably improved smooth glass borehole linings asthe hole wall were taken t o determine i t s surface illustrated i n F i g . 11-19.characteristics. The photographs indicated that two The problem of graphite glass-former wear wasdistinctly different wall conditions were evident. addressed by designing more rugged, abrasion-resis-These conditons were characterized by either (1 1 a t a n t assemblies employing refractory metal segmentsrelatively smooth glass liner of fused basalt w i t h i n place of the all-graphite segments. An i n i t i a la thickness estimated t o be of the order of 1 mn or design used a molybdenum forming r i n g followed by a( 2 ) a surface largely stripped of fused material graphite release r i n g to minimize rock glass stick-leaving behind only slightly glazed virgin basalt. i n g d u r i n g the critical cooling temperature range.Wall surfaces alternate between these two types In the cold condition, the graphite diameter isthroughout the depth of the hole due t o changes i n smaller than the molybdenum so that the abrasionpenetrator glass-former design and operating charac- from the hole wall i s born by the metal parts. Int e r i s t i c s d u r i n g penetration. Both o f these condi- laboratory t e s t s the new glass former produced ations had previously been observed i n laboratory cleanly stripped hole in basalt; i.e., the glass i stests. wiped from the wall by the p i s t o n effect of the form- Initial experiments w i t h extruding penetrators i n g section and the aspiration effect of the extrud-employed a segmented graphite glass-forming section ate nozzle. The seal between the forming section andbased on consolidating penetrator experience. Whilecapable of producing dense glass linings in hardrock, the early systems possessed two major short-comings. The relatively soft graphite was easilyscored by irregularities i n the liner d u r i n g extrac-tion, leading to frequent replacement of the formingsections. This limitation was considered t o be par-ticularly important i n view of the present programdirection toward the development of deep hole sys-tems for geothermal energy production. The secondproblem area concerns the dynamics of the moltenrock layer when melting through basalts which con-tain both carbonate inclusions and bound water, re-sulting i n appreciable gas generation d u r i n g melting.Rapid gas evolution in the melt can result i n voidsi n the glass lining and also surges of molten rockbeing forced i n t o the debris removal nozzle of thepenetrator. Extreme surging can overload the debrisremoval system resulting i n blockage of the debriscarry-off tube w i t h chilled rock debris. Fig. 11-19. Horizontal hole i n basalt. Note smooth This l a t t e r problem area resulted i n an extrud- glass l i n i n g produced by the penetratoring system modification employing a longer extrudate with extrudate tube restrictions.16

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