Geothermal well technology and potential applications of subterrene devices, 1974

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Geothermal well technology and potential applications of subterrene devices, 1974

  1. 1. @-93s- LA-5689-MS1y. Informal Report w Reporting Date: July1974 W Issued: August 1974 fr; *e 1 i 4 Geothermal Well Technology and Potential Applications o Subterrene f Devices - A Status Review by John H Altseimer . I D alamos scientific laboratory of the University of California LOS ALAMOS, NEW MEXICO 87544 UNITED STATES ATOMIC CNERGY COMMISSION . CONTRACT W-74 OS-E N G 36 OlSTRlBU v N OF THIS DOCUMENT I UNLIMITED 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 report was prepared as an account of work sponsored by the UnitedStates Government. Neither the United States nor the United States AtomicEnergy Commission, nor any of their employees, nor any o their contrac- ftors, subcontractors, or their employees, makes any warranty, express or im-plied, o assumes any legal liability o responsibility for the accuracy, com- r rpleteness or usefulness of any information, apparatus, product o process dis- rclosed, or represents that its use would not infringe privately owned rights. In the interest of prompt distribution, this LAMS re- port was not edited by the Technical Informationstaff. Work partially supported by a grant from the National ScienceFoundation, Research Applied to National Needs (RANN). Printed i the United States of America. Available from n National Technical Information Service b U.S.Department of Commerce 5285 Port Royal Road Springfield, Virginia 22151 price: Printed Copy $4.00 Microfiche $1.45
  5. 5. NOTICE This report was prepared as an account of work sponsored by the United States Government. Neither the United States nor the United States Atomic Energy Commission, nor any of their employees, nor any OF their contractors, subcontractors, of their employees, . makes any warranty, express or Implied, or assumes any legal UabUity or responsibility For the ICCUI~CY, com- pleteness or usefulness of any information, apparatus, product or process disclosed. or represents that its usebd would not infringe privately owned rights. CONTENTS I, INTRODUCTION 1 A. Objectives 1 B. History of Geothermal.Energy (GTE) Exploitation 1 C. General GTE Industry Status 2 1. Economic Incentives 2 2. Geothermal Resource Data 3 3. Environmental Pollution Awareness 3 4 Federal Support . 3 5. State Support 4 11. GEOTHERMAL WELLS: CURRENT TECHNOLOGICAL AND COST STATUS 4 A. Exploration 4 1. Extent an4 Nature of the Resource 4 2 Surface Prospecting Techniques . 5 3. Exploration and Discovery Wells 6 4 costs . 7 B. Problems in Completing Geothermal Wells 8 1. Dependence on Oil-and Gas-Drilling Equipment and 8 Methods 2 . Geological Factors 8 3 Surface Sites and Equipment . 9 4 Drilling . 9 5. Casing and Cements 12 6. Drilling Fluids and Muds 12 7. Downhole Measurements and Samples 13 8 . costs 13 9 Summary of Current Drilling Problems . 13 C. Production Operational Problems 14 1. Production Loss With Time; Reinjection; and 14 Augmentation 2. Large Surface Piping 14 3. Corrosion and Scaling 15 4 Desirability of Higher Electric Power Generating . 15 Efficiency 5 Summary of Current Operational Problems . 6 1 111. CONCEPTUAL APPLICATIONS OF SUBTERRENE DEVICES TO 16 GEOTHERMAL WELLS A. Subterrene Program History 16 B Basic Subterrene Technology . 19 1. Basic Types of Penetrators- 19 2. Other Coring Designs 21 3 Rock Glass Hole Linings . 22 C. Current Problems That Subterrenes Might Help Solve 23 or Eliminate iii DISTRIBUTION OF THIS DOCUMENT IS UNLl m
  6. 6. D Potential Subterrene Applications in Geothermal Wells . 24 0 E Subterrene Program Accomplishrpents Still Required . 25 IV. CONCLUSIONS 26 REFERENCES 26 Appendix: Contacts Made to Discuss Geothermal Well Drilling 29 Problems 1I - ’ iv
  7. 7. . GEOTHERMAL WELL TECHNOLOGY AND POTENTIAL APPLICATIONS - OF SUBTERRENE DEVICES A STATUS REVIEW by John H. Altseimer ABSTRACT The past, present, and some future aspects of the geothermal energy (GTE) industry have been reviewed with special attention given to geothermal well- drilling problems. Geothermal wells can be produced with present equipment and methods, mostly derived from the oil and gas industry, but costs are relatively high. Short-term improvements are needed in drilling rigs and auxiliary surface equipment, drill bits, bit-bearing lubrication systems, tubular goods, high-temperature muds and cements, logging and downhole sampling equipment, directional control equipment applicable to geothermal conditions,and in the use of a data bank for GTE wells to help optimize drilling programs. Two types of wells are needed: ( ) small-diameter wells 1 for exploration, reinjection, and disposal purposes, and (2) larger-diameter wells for production. To develop and greatly expand the use of GTE in the future, new methods and equipment are needed to penetrate hard abrasive rocks and to provide hole stabilization and support at the very high temperatures and other extreme conditions which can be encountered in GTE wells. New Los Alamos Scientific Laboratory concepts for penetrating rocks by use of rock- melting processes (called Subterrene concepts) offer potential solutions to sbme difficult GTE well-production problems. I. INTRODUCTION recalls Plato writing about ancient Atlantis in his A. Obj ectives Critias as follows: "Poseidon, as he was a god, A primary objective of this study is to present found no difficulty in making special arrangements a summary status review of geothermal well technol- for the center of this island, bringing two streams ogy, which includes drilling and operational prob- of water from under the earth, Which were caused to lems. A secondary objective is to begin the analy- astend as springs, one of hot water and the other of sis and evaluation of new concepts of making a hole cold". by means of melting processes (called Subterrene The first experimental attempts to produce concepts at the Los Altlmos Scientific Laboratory)to electric power from GTE began in 1904 at Larderello, improve the methods of producing geothermal wells. Italy, under the direction of Prince Piero Ginori Besides the use of published literature and data, Conti, general manager of the Larderello works. many personal discussions were held with people in These works had been developed by a Frenchman,c various fields of the drilling and GTE industries Francesco de Larderel, in 1818, primarily as a 1 in an effort to arrive at correct and objective con- source of boric acid. In a paper delivered ih 1924 clusions. A list of the contacts made is presented Prince Conti reported that in 1905 a 20-hp engine in an appendix. was run by the natural steam supply to generate B. History of Geothermal (GTk) Ernloitation plant lighting power. By 1912, the development of Man has made various uses of naturally occurr- a 250-kW steam turboalternator was initiated with 1 ing hot water for thousands of years. Indeed, Lieb subsequent success. Ir 1914, the erection of the t 1
  8. 8. r e l l o Power S t a t i o n was begun, which housed GTE because o i l - d r i l l i n g technologies o f f e r a con- urboalternators of 2500 k each, two opera- a1 and one spare. W From these e a r l y beginnings venient way of entering i n t o these new e n t e r p r i s e s . Union O i l became a c t i v e a t The Geysers i n t h e 1960s c, r e l l o Power S t a t i o n has increased i t s out- while others l i k e Signal, Standard O i l of California, 4 MW i n 1969. With current technology, Sun, P h i l l i p s , Occidental, Gulf, and Getty a r e a l l u t may be near t h e maximum f o r t h a t loca- now a c t i v e i n t h e western United S t a t e s along with a growing group of small s p e c i a l t y companies. For r e l l o i s a vapor-dominated reservoir. A example, i n t h e KGRA bidding i n California, r e f e r r e d eservoir e x i s t s at The Geysers i n northern t o previously, 17 companies submitted as many a s 56 i a , I n 1960 The Geysers f i r s t became opera- bids. Various problems i n t h e p a s t had slowed t h e t 12.5 MW.3 Since the-several 110-MW com- growth of t h e GTE industry. Some problems a r e d i s - onsisting of two 5 - W subcornplexes, 5M appearing because of developments external t o t h e loped. A t t h e end of 1973, P a c i f i c industry and others a r e being taken c a r e of by accel- i c Co. reported a t o t a l capacity of erated e f f o r t s of t h e involved organizations o r r The geyser^,^ which became t h e l a r g e s t agencies. A discussion of t h e important f a c t o r s G E f i e l d i n t h e world. T More wells a r e a f f e c t i n g t h e industrys growth follows. i l l e d including one completed at 2898 m 1. Economic Incentives. The promise of genera- eepest producing steam well i n the t i n g low-cost e l e c t r i c power has probably kept t h eworld. Current estimates of t h e t o t a l power produc- GTE industry a l i v e , although t h e large-scale develop- a b i l i t y of The Geysers range from 1000 t o ment of f o s s i l - f u e l i n d u s t r i e s with easy and pre- e). An indication of t h e p r i v a t e s e c t o r s d i c t a b l e p r o f i t s has g r e a t l y overshadowed GTE. Also, a t i o n of t h a t areas p o t e n t i a l is given by t h e hydroelectric power p l a n t s were well understood and, 2 r e s u l t s of t h e January 22, 1974, bids f o r 202 km where water was a v a i l a b l e , such p l a n t s were b u i l t .(50,000 acres) of &own geothermal Resource Areas To f u r t h e r lessen t h e incentive t o build GTE power 2(KGRAsj in California. One 9.5-km (2340 acres) p l a n t s , large nuclear p l a n t s were advertised a s t h eKGRA brought a b i d of $3,200,000 or 337,000 $/km2 solution t o a l l power problems. I t i s , however, be-(1367 $/acre), and t h e average f o r 1 2 KGRAs was coming evident t h a t f o s s i l and nuclear power p l a n t s 4156,000 $/km2 (631 $/acre). have problems of c o s t , environmental pollution, and Encouraged primarily by Larderello successes, limited f u e l resources, which make other comple-GTE i n t e r e s t s sprang up a l l over t h e world. This menting energy sources, e.g., geothermal and s o l a r ,was t o be expected because GTE manifestations occur look increasingly a t t r a c t i v e . Finally, t h e poweri n many locations, being usually associated with industry, because of heavy f i n a n c i a l commitment re-volcanic a c t i v i t i e s and evidences of c r u s t a l p l a t e quirements, is an extremely conservative industryphenomena. Thus, G E a c t i v i t i e s occur from t h e T which measures changes i n decades r a t h e r than merelyAleutian Islands and southern Alaska down t h e west years.coasts of the North and South American continents, A b r i e f look a t t h e economics of GTE is worth-westward under t h e P a c i f i c through N w Zealand and e while. McMillan, who helped promote The GeysersOceania and up thrqugh Japan plus another volcan- development i n t h e 1950s presented comparative costi c a l l y a c t i v e zone extending through Hawaii. Mani- data i n a paper a t Pisa, Italy. While now severalf e s t a t i o n s a l s o appear through t h e mid-Atlantic years old t h e paper is valuable because it comparesOcean, including Iceland, i n t o the southern Atlan- c o s t s f o r d i f f e r e n t types of p l a n t s i n t h e same geo-t i c , up eastern Africa, and i n t o the Red Sea and graphic area. In 1968, i n California, The Geysers cMediterranean areas with sizeable extensions east- fuel and operating c o s t s were 2.75 mills/kWh, com-ward across Asia. A t l e a s t 19 ongoing GTE power pared with 5.78 m i l l s f o r t h e Humbolt Bay nucleardevelopment p r o j e c t s a r e presently s c a t t e r e d around plant and with 3.28 t o 10.32 m i l l s f o r a l l othert h e world i n t h e areas approximately outlined above. power p l a n t s i n t h e s t a t e , excluding hydroelectric. 7C. General GTE Industry S t a t u s Using s l i g h t l y d i f f e r e n t assumptions, Goldsmith Many o i l companies a r e becoming i n t e r e s t e d i n estimates a GTE e l e c t r i c i t y cost of 5.33 mills/kWh, bi2
  9. 9. which compares favorably with McMillans data for for many years. One example is a detailed (3733 re-other power plants (average, 7 mills/kWh, excluding ferences) compilation by Waring et al.13 on the ther-hydroelectric). Included in a recent survey report mal springs of the United States and the world. How-by the State of Oregon Public Utility Commissioners ever, national geothermal resource estimates in theOffice, R. Bowen, State Economic Geologist, shows literature differ by orders of magnitude, becausethat GTE power costs are 46, 61, and 82% of nuclear, different assumptions are made as to what is or iscoal, and hydroelectric costs, respectively. For not economically feasible. The same phenomenondry-rock geothermal reservoirs being studied at the occurs in fossil-fuel resource estimates. Although 9Los Alamos Scientific Laboratory, Brown et al. show total resources remain the same, proven reserve es-in a preliminary study a power cost of 4.7 mills/ timates change when, for example, oil prices risekwh in 570 K rock in the New York area, compared and make formerly uneconomic and therefore unavail-with 11.8 and 13.3 mills for nuclear and coal, re- able resources suddenly economic and qualified asspectively. At Cerro Prieto in Mexico where a 7s-W proven reserves.plant with 15 supporting wells is operating by In GTE, confusion about resources and reservesflashing superheated water, the Mexican Department will continue until much more exploration and pro-of Geothermal Resources recently released cost data duction technology is established. The most ad-showing that the cost of the production installation vanced GTE power systems today are probably theamounted to 264 $/kW.l0 A previous release showed vapor-dominated types such as The Geysers. Yet, thean electricity cost of 8 mills/kWh. In Russia, exact nature of the thermal reservoir at The Geyserswhere enormous reservoirs of hot water are known, is not clear. The large energy resource in theKremnjov et a1.12 also conclude that, in many cases, Imperial Valley hot brine is still not being eco-GTE is competitive with other types of energy sys- nomically exploited because of technical diffi-tems. culties. Methods for using the geopressurized GTE Note that costs such as those quoted above are resources in the Gulf of Mexico are still quitefor small GTE installatiods and for very large coal speculative. Finally, the dry-rock concepts at theor nuclear power plants. Paver from these latter Los Alamos Scientific Laboratory which may make manytypes would be even more costly if generated in of the high GTE reserve predictions come true aresmall plants. Indeed, one advantage of GTE power still in an early stage of research.generation is the possibility of distributing many 3. Environmental Pollution Awareness. Hope-smaller plants over the country in lieu of fewer fully, the GTE inhustry will be able to maintain itsvery large installations with their attendant huge claim of being environmentally acceptable. Environ-cross-country power-transmission lines and right-of- mental awareness is a new phenomenon that could ben-way land-area consumption. Another economic attrac- efit the industry. However, H2S pollution insteadtion is the short construction time for a GTE power of SO pollution is not much of an improvement. 2 7 Also, thermal pollution becabse of the lower thermalplant. Goldsmith quotes Pacific Gas and ElectricCo. as estimating only 28 months needed to build a efficiency in GTE conversion to electric power (vis-110-MV system at The Geysers, from approval and 2-vis fossil-fueled thermal systems) is a disad-equipment purchase confirmation to commercial ser- vantage. Conversely, direct use of thermal energyvice. At Cerro Pyieto. Mexico, drilling was re- other than for electric-power generation could re-cently started for a new 7s-MW complex," which is duce the overall thermal pollution problem whilescheduled for power production in July 1976, or , making a significant cQntribution to efficient en-1ess)thanthree years from initiation. ergy utilization. 2. Geothermal Resource Data. A major factor in 4. Federal Support. Until recently, federalrestricting the growth of the industry has been the interest in GTE developmetit has not been great. Thelack of geothermal resource data, including detail- Geothermal Steam Act of 1 9 d 4 was passed to facili-ed understandings of how various types of reservoirs tate the issuance of leases on public lands for thebehave and can best be exploited. The United States purposes of GTE developments. However, three yearsGeological Survey has been gathering resource data elapsed before the final regulations were issued on 3
  10. 10. cember 21, 1973.l’ While not a technical problem, Another preliminary i n d i c a t o r of GTE i s t h elack of d e f i n i t i v e f e d e r a l guidelines put t h e p r i - measurement of near-surface h e a t flow i n Heat Flowvate s e c t o r i n a p o s i t i o n where funding f o r G E pro- T 2 Units [HFU; one HFU being equal t o 1 pcal/cm as -j e c t s was i n h i b i t e d . Federal funding of technology 2 (0.0418 W/m ) ] . Knowing t h e average or normal valuedevelopment has also been low and fragmented among of h e a t flow f o r a region, one can then d e t e c t t h eten d i f f e r e n t government agencies. l6 Many problems abnormal a r e a s which may be s u i t a b l e f o r e x t r a c t i n gare assuming lesser importance as r e g u l a t i o n s aredefined, l e a s i n g competitions are held, and f e d e r a lgeothermal RED funds are increased and coordinated. heat. world i s - Very approximately, t h e average HFU f o r t h e value is -1.3, 1.5.18 I n t h e eastern United S t a t e s t h e whereas i n t h e western states it is 5. . S t a t e Support. A l l t h e western states are -2.0. Thus, HFU considerations would lead t o t h ee i t h e r accepting GTE as a v i a b l e energy source or conclusion t h a t t h e western United S t a t e s is a goodseem t o b e r a p i d l y becoming aware of its p o t e n t i a l s . place t o search f o r GTE. Again, heat flow is onlyActive p r o j e c t s or s t u d i e s are i n progress i n , a t one i n d i c a t o r i n a complex system. t , California, Oregon, Washington, Montana, Two United S t a t e s Geological Survey (USGS) sci-Idaho, Utah, Nevada, Arizona, New Mexico,and Colo- e n t i s t s , L. J. P. Muffler and D. E . White, have pub-rado. On t h e west c o a s t , where l a r g e power-load l i s h e d estimates of GTE resources. I n a 1965 paper,c e n t e r s e x i s t , i n t e r e s t i s high. Oregon and Wash-ington have both r e c e n t l y issued state-sponsoredstudies 8,17 o f energy resources, and both states de- White” t h e o u t e r 10 k o f t h e e a r t h t o be m - c a l c u l a t e d t h e amount of h e a t a v a i l a b l e i n c a l ) or several thousand times t h e heat re- 4 x lo2’ Jvoted a s i g n i f i c a n t p o r t i o n of t h e i r e f f o r t t o geo- presented by t o t a l world coal resources. In a 1972thermal energy. paper,20 Muffler and White estimated t h e t o t a l s t o r e d heat i n a l l geothermal r e s e r v o i r s t o a depth of 1011. GEOTHERMAL WELLS: CURRENT TECHNOLOGICAL AND km, including reservoirs of molten rock, abnormally * COST STATUS hot rocks of low permeability, and deep sedimentaryA. Exploration basins of near-normal conductive heat flow such as 1. ExtentandNature of t h e Resource. A s noted t h e U.S. Gulf Coast and Kazahkstan i n t h e U.S.S.R.e a r l i e r , GTE resource areas of t h e vapor-dominated They a r r i v e d a t a value o f a t least 4 x Jor hot-water types e x i s t throughout t h e world. c a l ) or about equal t o t h a t o f t h e world’s coal re- es or hot-water springs a r e i n d i c a t o r s source. B assuming an e f f i c i e n c y value f o r con- y and occur from t h e Rocky Mountain v e r t i n g t h e above GTE resource i n t o e l e c t r i c i t y ,S t a t e s westward t o t h e coast; hot s p o t s are known Muffler and White conclude t h a t G E is u n l i k e l y t o Tt o extend even out i n t o t h e Pacific Ocean basin. supply more than perhaps 10%o f domestic or worldAlaska has many hot-spring i n d i c a t i o n s of GTE. e l e c t r i c a l power. However, i n favorable areas suchH a w a i i , because of its inherent volcanic nature, power may be of major importance. They a l s o con-o f f e r s promise o f having l a r g e G E resources. T Yet, clude t h a t knowledge and extent of our geothermalWaring” points out t h a t Hawaii does not have many resources is s t i l l inadequate and new, and more ef-hot springs because t h e volcanic material is frag- f i c i e n t ways t o use t h i s resource are needed.mented and porous, leading t o a water t a b l e only a One o f t h e new approaches t o G E is t h a t of Tf e w feet above sea l e v e l . In t h e eastern United hydraulically f r a c t u r i n g hot d r y rock and then ex-S t a t e s , Waring lists numerous hot springs i n V i r - t r a c t i n g thermal energy with c i r c u l a t i n g water. i n i a and West Virginia, followed by Arkansas and This technique is i n t h e research s t a g e a t t h e LosGeorgia. The above d a t a are only rough i n d i c a t i o n s Alamos S c i e n t i f i c Laboratory (LASL) . Potter21 a to f G E resources because a s i g n i f i c a n t GTE reser- T C LASL estimated t h e t o t a l a v a i l a b l e United S t a t e s a Ev o i r i s a complex system depending on l o c a t i o n and resource using t h e hot-rock method t o b e 2 x J.n a t u r e of t h e h e a t source, on water supply and re- (For comparison, t h i s amount of h e a t is about 3000charging c h a r a c t e r i s t i c s , on i n t e r r e l a t i o n of per- times t h e 7 x J of t o t a l energy used i n t h e U.S.meable and nonpermeable s t r a t a , and on t o t a l volume i n 1970.)22 He f u r t h e r estimated t h a t 13 x J -7of t h e system. could be used d i r e c t l y as thermal energy and t h e W4
  11. 11. remainder t o generate e l e c t r i c power equivalent t o TEMPERATURE FC) 488,000 GW-years. Using a gross averaging method,ti t h e l a t t e r i s equivalent t o a power output of 4,880,000 MW f o r an e f f e c t i v e G E lifetime of 100 T years. The average value of 4,880,000 MW compares favorably with t h e estimated t o t a l U.S. e l e c t r i c - power requirement o f 2,000,000 MW i n t h e year 2000. 23 Noteworthy is t h e fact t h a t t h e dry-rock method . could b e applied i n every state. In t h e vapor-dominated and hot-water r e s e r v o i r s now known, t h e maximum well depths are 3 km, and i n a good r e s e r v o i r t h e temperatures might be between 470 and 570 K. The highest temperatures, 660 K, have been measured i n t h e hot-water systems a t Cerro P r i e t o , Mexico. The Geysers, t h e downhole pressure is - I n t h e vapor-dominated system a t 3500 kPa (500 p s i ) and a t Cerro P r i e t o t h e pressures can be TEMPERATURE IT) as high as -12,000 kPa (1700 p s i ) . The temperature c h a r a c t e r i s t i c s i n a conventional water-based re- Fig. 1. Temperature p r o f i l e s controlled by conduc- 24 t i v e gradients within t h e "normal" range s e r v o i r have been explained by D. E. White. (A,B.C) and above "normal" (D), and by con- Figure 1 is reproduced from t h i s work and shows tem- v e c t i v e t r a n s f e r o f heat i n hot-water sys- tems of d i f f e r e n t subtypes, compared with perature-versus-depth changes f o r various conductive reference boiling-point curve F (see t e x t ) . and convective h e a t - t r a n s f e r conditions. Lines A, (Taken from White.)24 C, and B show d a t a f o r temperature gradients i n a purely conductive s i t u a t i o n of 8, 25, and 50 K/km, Yellowstone Park. These systems tend t o b e self- Areas r e s p e c t i v e l y , and might b e considered normal. s e a l i n g due t o mineral deposits forming near-surface are known with abnormal gradients such as 75 K/km r e s t r i c t i o n s t o t h e flow and can b u i l d up pressures ( l i n e D) which would r e s u l t i n 500 K a t 3 k depth. m and temperatures exceeding those o f Curve F. Curve The Reference Boiling-Point Curve F is a p l o t o f J i l l u s t r a t e s a steam-producing s i t u a t i o n where steam water b o i l i n g temperature a t t h e h y d r o s t a t i c pres- i s being generated i n a deep brine-water body which s u r e corresponding t o t h e depths shown and includes e x i s t s below 1400 m. Higher steam temperatures and a temperature c o r r e c t i o n f o r water density. Curve enthalpies may b e obtainable as t h e b r i n e becomes E i l l u s t r a t e s r a p i d upflow of water s t a r t i n g a t a more concentrated and b o i l i n g occurs a t g r e a t e r moderate temperature a t depth and b o i l i n g as it ap- i n i t i a l temperatures. proaches t h e o u t l e t and discharging a t 373 K. Curve 2. Surface Prospecting Techniques. To s t a r t a G starts a t a high temperature similar t o E but with GTE exploration one must determine an objective, f a low upflow rate. Thus, t h e heat has time t o con- e.g., is a low-grade o r a high-grade heat source re- ductively d i s s i p a t e and t h e water a r r i v e s a t a low quired? Some examples: (1) heating o f buildings, s u r f a c e temperature of 320 K. Curve H i l l u s t r a t e s greenhouses, mines, etc., can be done with 350 K. s a l i n e water i n t h e Salton Sea system. A t 550 K, (2) i n d u s t r i a l processing such as water d e s a l i n a t i o n Curve H crosses t h e reference b o i l i n g Curve F and could be done with 450 K, (3) e l e c t r i c power genera- then exceeds t h e reference temperature because of t i o n requires 450 K and preferably higher, and (4) s a l i n i t y effects on t h e b o i l i n g point. Because of a multitude o f new a p p l i c a t i o n s would open up i f t h e low-permeability sediments i n t h e Salton Sea temperatures > 670 Kwere t o becofie a v a i l a b l e by system t h e r e i s a s t r o n g conductive-gradient e f f e c t means of new heat-extraction techniques. The next on Curve H a t shallow depths. Curve I i l l u s t r a t e s s t e p s would usually be l i t e r a t u r e surveys and f i e l d a high-temperature system at depths similar to t h e surveys. Noteworthy, f o r G E l i t e r a t u r e , i s an ex- TW systems observed a t Wairakei, New Zealand, and a t t e n s i v e bibliography of worldwide G E r e p o r t s , T 5
  12. 12. sponsored by the National Science Foundation and c Geophysical Methods. One of the most use- .published by the New Mexico Institute of Mining and ful geophysical tools is the measurement of deep-Technology, Socorro, New Mexico.in 1971. 25 Anothercurrent project, using all available well data, is to sounding electrical resistivity, which is primarily dependent on the effective porosity, temperature, 30 cmap subterrenean temperature conditions across the and salinity of the interstitial water at depth.North American continent. This is being done by This method is fast and convenient, and can be usedthe Amercian Association of Petroleum Geologists, to probe down to at least several kilometers, It hasTulsa, Oklahoma. Methods used for field tests are been demonstrated to be very effective in New Zealand.listed below. Meidav” reviewed the method at the U.N. 1970 meet- a Geochemical Analyses. The geochemical . ing in Pisa, Italy. Recent developments in electro-analyses of hydrothermal waters produce data on magnetic methods may become especially useful inwater condition at depth (steam or liquid), tempera- geothermal areas of low resistivity. At leastture at depth, source of the water, chemical compo- three different types of electromagnetic apparatussition at depth, and character of the water to be are being developed. Aeromagnetics have also beenhandled at the surface. D E White26 described . . used in attempts to locate hot plutons where watergeochemical methods at the U N Meeting in Pisa, . . can cause changes leading to reduced magneticItaly. Geochemical indicators of subsurface tem- proper tie^.^^ Gravimetric measurements are alsoperatures are sometimes called geothermometers, the normally made across an anomalous area.use of which are described by Fournier, White, and d Seismic. For the exploration of hot water .Tr~eSdell.~~ Fournier and Truesdell have also or vapor-dominated reservoirs, active seismic methodsstudied the problem of mixing hot and cold waters. 28 are only indirectly useful, active seismic data b Thermal Measurements. Aerial infrared sur- . being used primarily to define subsurface geologicveys of large areas have been used by the United structure and basement-rock depths and configura-Nations in Ethiopia and Kenya to delineate above- t i o n ~ . However, if future drilling capabilities ~~normal surface temperature^.^ The USGS has also allow exploitation of very hot intrusions or magmasdetected some warm areas by this method. This tech- then the active seismic methods could become muchnique has severe limitations, the signal-to-noise more interesting and applicable. Some success alongratio making it difficult to accurately detect heat- these lines is indicated by Furumoto.35flux anomalies less than about 100 to 150 times the Passive seismic methods are directly useful forn0-1;~’ however, the contrast needed may be much defining water or vapor reservoirs, one use being toless, depending on the terrain being surveyed. detect micro-earthquakes which seem to be prevalent Using boreholes, thermal gradients with depth in geothermal reservoirs. Alsathere is usually anare measured and heat flows calculated. Very shallow abundance of background noises caused by water or gas(-1-m-deep) holes are usually not worthwhile because movements. Exceptions to this statement are possi-of near-surface effects. Holes to 150 m have bccn ble, however--the most notable current example beingm r e successful although the data must always be eval- the high-heat-flow area being ktudied at Marysville,uated for effects of ground-water movement. Typical Montana, which is one of the most quiet spots known 18boreholes are drilled 100 mm ( i . in diameter 4 n) in the U S ..with a 20- or 25-mm (0.7-to 1-in.) pipe cemented 3 Exploration and Discovery Wells. With all .into the hole. The pipe is filled with water and the exploratory methods available, the presence of a viable GTE reservoir must still be proven bycapped, and after equilibrium is established atemperature traverse is made from the top down. 31 drilling. These exploratory holes should be used,CombsJo writes about gradient and flux-calculation according to Combs and Muffler,34 for measurements -results of 20 HFU average in a New Zealand field, of temperature and pressure profiles, permeability, porosity, lithology, stratigraphy, fluid composi-120 times normal near Steamboat Springs, Nevada,and of at least 67 HFU at Yellowstone’s Firehole tions, other geophysical logs, and production tests.Basin. They further state that these data, which can only be obtained from a borehole, are essential in: i6
  13. 13. "1. Estimating the ability of the geothermal reser- TABLE IW voir to produce sufficient energy over B suffi- ciently long time to be economically attractive. AVERAGE DEPTH AND COST PER DEPTH TOTAL UNITED STATES IN 1972* 2. Distinguishing between different models of the Type of Wells E Average Cost** geothermal system, with the aim of accurately pre- per Meter, $/m dicting production characteristics under varying Average Total exploitation conditions. 3. Calibrating and Depth, Weighted refining geophysical and geochemical methods for ---km Oil Gas Dry Average 1.3 44.10 45.10 25.10 35.90 recognizing and delineating geothermal systems." (4300) (13.40) (13.80) (7.70) (11.00) The above described uses of exploration holes 1.9 52.95 53.80 34.80 45.20 are consistent with todays capabilities for drill- (6200) (16.10) (16.40) (10.60) (13.80) ing, downhole measurements, and lbgging. An addi- 2.6 6.0 80 82.90 54.40 65.00 tional desirable capability, not yet available, is (8500) (20.70) (25.30) ( 6 6 ) (19.80) 1.0 that of continuing drilling economically past the *Numbers in parentheses are ft and $/foot. anticipated production zone towards the source of **Includes drilling and casing. heat that drives the reservoir. This could mean penetrating into hard igneous and metamorphic rocks at very high temperatures. The objectives and to prepare for production in these 1.5-km wells, would be to gain a better understanding of the basic costs 33 to 50 $/m (10 to 15 $/ft). Thus, costs of system and to determine whether reinjection fluid, geothermal wells range from 100 to 250 $/m (30 to or even additional fresh water, might be added at 75 $/ft) or approximately two to five times higher the lower hotter depths to percolate upward into than the average costs of oil or gas wells given in the production zone. The latter could greatly aug- Table I. Greider presented other cost data: Sur- ment and artificially stimulate both the production face exploration costs run from $75,000 to $90,000 and useful life of the reservoir. At the Geothermal per typical area of interest. Only one out of four Resources Research Conference in Seattle, in Septem- of these areas would probably justify an exploratory ber 1972,36 two recommendations were that high pri- hole, resulting in $300,000 to $380,000 per drill- ority be given to immediate improvement of explora- able prospect. Only one of four of these prospect tion methods and to the development of cheaper wells would be worth running pipe and completing for drilling methods in high-temperature formations. These improvements would result in improved -- extensive testing. The three unsuccessful wells would cost $100,000 to $200,000 each and the com- pleted wells $150,000 to $250,000. The net aver- reservoir and economic models which, the conference attendees also concluded, were sorely needed. age cost for each prospect worth completing and 4. Costs. Let us start ,thisdiscussion on testing extensively is then $650,000. Perhaps one costs with a look at average costs for oil and gas of four of the completed prospect wells would re- wells as presented in the 1972 Joint Association sult in the discovery well of a reservoir large Survey Report on drilling costs37 and presented in enough to be commercially attractive. The ratio of Table I . total wells drilled to each discovery well is 16:l. For a number of reasons the costs of geothermal Creider feels that this is a realistic ratio as the wells are greater than those of averake oil and gas industry matures after the large easily located wells. Greider" of Chevron Oil compiled cost data reservoirs are drilled. AxtellS9 of Phillips Petro-? on geothermal exploration wells as follows: Wells leum Corp. agreed that 16:l is reasonable and cited to depths of 1.5 km (SO00 ft) in most geothermal historical data showing five discovery wells out of provinces in sedimentary basins in the U. S . average 75 exploration geothermal wells for a 15:l ratio. 65 to 100 $/m (20 to 30 $/ft). In remote areas or Greiders definition of a good discovery well is one in those with interbedded volcanic rocks, costs run defining a reservoir capable of 275 M ( ) power out- We from 100 to 200 $/m (30 to 60 $/ft). To run casing put. A summary of his cost model is shown in Table ItIs 7
  14. 14. TABLE I1 given in Section I11 where rock-glass-lined holes I COST TO PRODUCE A DISCOVERY WELL are discussed. B. Problems in Completing Geothermal Wells FOR A 275-MW(e) RESERVOIR 1. Dependence on Oil-and Gas-Drilling Cost,$ % of Total Equipment and Methods. The.methods used in makingLand acquisition (nontech- nical leasing, bonus, geothermal wells are essentially the same as those rentals, etc.) 3,580,000* 45 for oil or gas wells. Indeed, this very fact some-Drilling (12 unsuccessful + what impedes GTE development because geothermal well 4 completed holes) 2,600,000 32 drillers are forced to use materials and equipmentSurface exploration (geol- ogy, geochemistry, geo- that are not necessarily best for geothermal wells physics) 1,840,000 23 with their higher temperatures and corrosive condi- tions. Problem examples are: (1) only oil-well Total 8,020,000 100 tubular goods and bits are available, (2) muds and*Considering the recent high bids made at the Jan. cements are not checked out for high-temperature 22, 1974 KGRA competitions in California. these costs are probably low. use because suitable high-temperature laboratoryIn Table 11, drilling amounts to 32% of the costs equipment is not available, and ( ) bit-bearing 3and is significant. Tabie I1 can also be used to lubrication systems are not designed to.withstandindicate the possible overall exploration drilling GTE temperatures. This dependence is designated ascosts to, e.g., the year 2000. Recalling Muffler an important consideration in the NSF-sponsored 40and Whites estimate that GTE might supply 10% of study of impediments to geothermal developmentthe nations electrical power (see Section 11, A 1 ) being done by Bechtel Corp. for The Futures Group,and the AEC predictionz3 of a total U.S. power ca- Inc. 2 . Geological Factors. The geological char-pacity of 2,000,000 MW(e) by the year 2000, onearrives at 200,000 M ( ) supplied by GTE in 2000. We acter of The Geysers, a vapor-dominated reservoir,This is low compared to the estimate36 of 395,000 is described by Cr~mling.~ The area is underlainMW(e) by 2000 made at the 1972 Seattle by the Jurassic-Cretaceous Franciscan formation --Geothermal Research Conference. However, using an eugeosynclinal sequence of graywacke, shale, ba-the 200,000-MW(e) estimate, 727 of Greiders salt, and serpentine. These Mesozoic rocks were275-MW(e) plants would be needed. Neglecting any uplifted and complexly faulted into a series ofeffects of capital costs and inflation, the total horsts and grabens and during Pleistocene time,costs for exploration drilling only for the 727 volcanic rocks were erupted onto the then erodedplants would be $2,600,000 x 727 = $1.89 x 109 . surface. The Geysers is at one end of a graben withThis simple calculation of exploratory drilling Franciscan graywacke overlain by basalt,which iscosts indicates that the costs are high enough to then overlain by serpentine. In some places theeasily justify the cost of research leading to low- basalt and serpentine have been downfaulted intoer exploration drilling costs. The much higher the underlying graywacke, the geometry of the faultscosts associated with the production and reinjection indicating that the thermal areas are located on thewells will be discussed later. fissures closest to the serpentine body. The Qua- A factor worth noting is that the hole size of ternary volcanic rocks indicate the presence ofan exploratory well is almost always the same as if young hot magma at depth. The rocks are hard andit were a production well, the reason being that a badly jointed. Not all geothermal areas have the charactersuccessful discovery well can then be easily con-verted to a production well. With current cemente& described above. For example, large sedimentary basins can provide hot water. In Hungary, Boldiz- .casing methods it is not feasible to drill a small-er, more economical well and later enlarge the well sar42 describes the Hungarian Basin of the Danubeto production size. A possible method for enlarg- River system as being filled with several kilometersing small exploratory holes to production size is of sand and clay sediments spreading over an area 2 of 92,000 km . No volcanic activity occurred in ,- ,8
  15. 15. this area during the Cenozoic Era, but the heat flow equipped for rapid changes in drilling procedures is higher than normal-- between 0.084 and 0.142 as hole conditions change.45LJ W/m2 12.0 and 3.4 HFU (pcal/cm2 -s)] -- and thermal Another type of surface capability lacking in gradients are 50 to 70 K/km. Water free-flows at GTE work is the so-called Optimized Drilling "con- 357 K from wells of 1.8 km average depth and is cept". This concept has been developed in the oil used for heating buildings and greenhouses. The industry and is described by Lummis.46-48 The con- Imperial Valley, California, is a sedimentary cept envisions the establishment of a standardized basin with static water temperatures approaching data-acquisition system ultimately resulting in a 670 K. well-drilling data bank used to analyze and optimize The large sedimentary basins are usually the drilling of new wells. The data would cover the characterized by above-hydrostatic pressures.34 mud program, drilling hydraulics, bit types, bit Although these strata may be easier t o penetrate rotation, depths, formations, etc. Comparable data with rotary drills than The Geysers type rocks, are relayed to the data-bank location for analysis operations can be complicated by hot corrosive from an active drilling rig. Computer programs water at high pressures. would continuously analyze the data and optimize the Wunderlich4 described how present orogenic modes of operation to minimize total overall cost. activity may be an important factor in influencing Kennedy4 reports cost savings of 30 to 40% on a the heat-flow distribution of the earth and the 14-well program in Wyoming, using the Optimized location of GTE anomalies, in addition to volcanism, Drilling concept. Another example is a 25% savings highly permeable sedimentary basins, and rocks of on two North Sea wells linked by satellite with an high heat conductivity such as rock salt. He Dp-Drilling" headquarters in Tulsa, Oklahoma. states that "Some of the interesting zones of 4. Drilling. In current geothermal wells, abnormally high heat flow show a characteristic drilling is easy in some sedimentary basins and is position relative to young orogenic belts. Usually, very difficult in hard, fractured rocks found, e.g., orogenic fronts are combined with low heat flow at The Geysers. The latter results in high bit wear values in the foreland and high heat flow behind and often in failures of bit bearings due to a com- the front." Further development of Wunderlichs bination of temperature, stress, corrosion, and thesis may furnish guidance for locating new "hot fatigue effects. Cromling41 describes typical ac- spots" associated with young mountainous regions. tual penetration rates of 3 to 6 m/h, using mud, 3. Surface Sites and Equipment. Because geo- and of 3 to 11 m/h using air as the circulating flu- thermal anomalies are often located in volcanic and id for 310-mm (12-1/4 in.)-diam holes. In the final mountainous terrain, the drilling sites Can cause 222-mm (8-3/4 in.) hole into the production zone problems and abnormal expenses. Cr~mling~ldescribes (on air). rates of 7 to 23 m/h are attainable. Of his drilling-site problems at The Geysers very well. course, the overall effective penetration rate can He cites rugged, mountainous terrain; very small be considerably less than the actual drilling rate, drill sites; blasting required often; little top soil as illustrated by the detailed drilling data given and that little sloughing readily; steep roads re- by M a t s u ~ tand shown in Table 111. He states that ~~ quiring tractor towage; pits on various levels, etc. in Japanese steam fields, rock hardness, lost cir- His site costs run from $10,000 to a s much as culation, and other conditions unique to geothermal $40,000. The latter cost is corroborated by Hutchin- wells cause drilling of steam wells to be slower son of Standard Oil of Calif~rnia.~~ Cromling also than drilling for oil or gas at depths ranging from reports that at The Geysers the drill rig should be 500 to 1300 m and working at downhole temperatures overdesigned for the job so as to withstand the of about 370 to 510 K. excessive torque and shock loadings. To broaden our understanding of factors Of significant economic importance to the GTE affecting geothermal well drilling we have studied exploratory drilling industry is the lack of a the activity logss1 for 125 geothermal wells drill- modern, easily moved drilling rig capable of ed in California. The majority of the data were penetrating to depths of up to 4 km and adequately 9
  16. 16. TABLE I11 AVERAGE DRILLING RATES FOR SIX GEOTHERMAL W L S I N JAPAN EL 50 (Data from Matsuo ) Average Penetration Average Total Rat e Overall Rate W11 e Twe Depth, When D r i l l i n g t o Total Depth Designation We1 1 - - m m/d - m/h m/d m/h Shikabe HGSR-1 Production 500 13.9 0.58 10.0 0.42 Matsukawa R-3 Production 210 17.3 0.72 11.4 0.48 Ohnuma R - 1 Production 850 25.0 1.04 12.8 0.53 Onikobe GO-10 Production 1350 18.2 0.76 10.9 0.45 Takinokami Survey 403 16.1 0.67 11.5 0.48 Takenoyu Survey 309 15.2 0.63 8.9 0.37e i t h e r from t h e general Geysers area or from Imperi- range. A s i m i l a r p l o t i s shown i n Fig. 3, buta l Valley, California, and a r e here designated Steam, for o v e r a l l average penetration rates where t h eHard Rock (SHR); and Hot Water, Sedimentary (HWS); average includes t o t a l time f r o m spudding-in t orespectively. The data include 92 S R and 33 HWS H t o t a l depth. In t h e Imperial Valley t h e penetra-type wells. Their depths a r e indicated i n a f r e - t i o n r a t e s peak a t about 1.5 t o 2.5 m/h (4.9 t oquency of occurrence-versus-depth p l o t i n Fig. 2. 8.2 f t / h ) for 42% of t h e wells analyzed. In TheSelected depth i n t e r v a l s a r e 300 m . About 40% of t h e Geysers, 52% of t h e r a t e s peaked a t 1 t o 2 m/h (3.3Imperial Valley wells ranged i n depth from 1300 t o t o 6.6 f t / h ) , somewhat lower than t h e peak r a t e s i n1900 m (4300 t o 6200 f t ) . In t h e Geysers 50% of t h e t h e Imperial Valley. Iwells a r e i n the 1900- t o 2500-m (6200- t o 8200-ft) The same data a r e shown i n Fig. 4 and 5 as 40 c The Geysers vicinity hard rock stem-dominated (92 wells) i - cn s3 0 40 The Geysers vicinity steam-dominated - 6 e c 0 0 020 e E IO 0 3 6 9 1216 - 0 0 3 1 6 2 3 9 1 2 4 0 0 u 0 4 8 1 1 2 2 (m/h) 3 4 2-0 0 Overall Average Renetmth Rate 1 (mlh) 2 (f trh) 3 4Fig. 2. Depths of geothermal wells d r i l l e d i n two Fig. 3. Overall average penetration r a t e s i n geothermal regions .So t y p i c a l geothermal wells.50 10
  17. 17. penetration rates-versus-depths p l o t s . For t h eW I Imperial Valley, t h r e e widely s c a t t e r e d p o i n t s were discarded, and t h e remaining 91% o f t h e d a t a l i e i n -IO an area as shown. S i m i l a r l y , 99% o f t h e d a t a are enclosed i n Fig. 5 f o r t h e Geysers. The d a t a t r e n d s i n d i c a t e a decrease i n penetration rates with depth - -8 i n t h e Imperial Valley and a lesser decreasing t r e n d . r i n t h e Geysers. In t h e Imperial Valley t h e wells % ,-c* are d r i l l e d t o t o t a l depths with muds and a t higher temperatures and pressures than a t The Geysers where -4 t h e d r i l l e r s use air (which is simpler t o handle than hot muds) f o r much of t h e well i n more compe- -2 t e n t rock and a t lower temperatures. The d i f f e r e n c e i n d r i l l i n g techniques may account for t h e fact t h a t , I I I I I 1 I I 0 1 2 - 4 even though t h e strata a t t h e two l o c a t i o n s are (km) i ; v a s t l y d i f f e r e n t , t h e penetration performance is 1 0 I 2 4 I 6 8 0 I 12 h not. The Imperial Valley d a t a average 1570 m Total Well Depth(kft1 (5160 f t ) depth and 1.82 m/h (5.97 f t / h ) p e n e t r a t i o n Fig. 4 . Overall average penetration r a t e vs depth rate. Corresponding averages f o r The Geysers are f o r sedimentary h o t water wells.50 1890 m (6230 f t ) and 1.98 m/h (6.49 f t / h ) . However, 38 - well c o s t s at The Geysers, according t o Greider and C r ~ m l i n g ,are 1.5 t o 2.0 times higher, pro- ~~ bably because of higher c o s t s due t o d i f f i c u l t sit- ing conditions; erosion by high-velocity a i r and steam flow; and high b i t f a i l u r e rates i n h o t , hard, j o i n t e d rocks. I 12 1 The l o s t - c i r c u l a t i o n problem mentioned earlier is t y p i c a l o f The Geysers and o t h e r locations where low ground-fluid pressures e x i s t i n badly f r a c t u r e d o r highly permeable formations. Cromling41 and BuddS2 both v e r i f y t h i s problem a t The Geysers. A s i m i l a r problem occurred i n t h e Jemez Mountains at t h e Los Alamos S c i e n t i f i c Laboratorys second hot- dry-rock experimental well, GT-2, where excessive lost c i r c u l a t i o n i n t h e sedimentary s e c t i o n was en- countered i n t h e first 600 m. This problem is o f t e n solved by changing t h e d r i l l i n g f l u i d from mud t o air. The use of a i r i n a steam w e l l , however, re- s u l t s i n excessive erosion o f t h e d r i l l stem a n d o f a l l o t h e r metal p a r t s i n contact with t h e mixture of a i r , steam, and c u t t i n g s being blown out of t h e well. The flow r a t e o f t h e a i r is f i x e d by t h e bit-cooling and debris-removal requirements, b u t steam entering I I I I I 1 0 2 4 6 a 0 t h e well anywhere-above t h e b i t adds t o t h e t o t a l Total We1 I Depth (kft) volumetric flow rate t o create t h e excessive-veloc- i t y flow condition. The lack of adequate d i r e c t i o n a l c o n t r o l equip-W Fig. 5. Overall average penetration r a t e vs de t h f o r steam-dominated, hard-rock wells. 58 ment i n hard rocks i s a problem. states Cr~mling~~ 11
  18. 18. that directional drilling at The Geysers is expen- need for cement squeezing, i.e., the special appli- cation of high pressure to force the cement back ofsive and adds as much as $100,000 to the cost of awell in difficult situations. MatsuoSo and Suter53 the casing, to recement channeled areas, or to iboth confirm the desire for good directional control correct cement-deficient zones. Cromling reportsand point out the difficulties of attaining it in requiring squeezing in about 50% of the wells athot hard-rock formations. For production wells, The Geysers. Dench listed four possible failure 54especially where the surface sites are expensive modes, which may result if the cementing operationand difficult to set up, directional control would leaves a length of casing not bonded to the hole:allow three or four wells to be drilled from one ( ) water in the annulus expands when the well 1central surface area. Currently available direc- goes into production causing high temperature/pres-tional downhole motors are inadequate in geothermal sure buildup and casing failure, (2) compressivewells because of high temperatures and also because failure of a joint due to heating, ( ) tensile 3water or mud is often used to drive the motors, failure of a joint due to cooling, and ( ) failure 4whereas the drilling fluid may usually be air. of the well-head connection. An example of the 5. Casing and Cements. The presence of hydro- first type of failure is the Matsukawa Well R-4 in 50gen sulfide is not new to the drilling industry and Japan where the casing collapsed at 35 m depth.is relatively common in GTE projects. Dench54 Matsuo reports that such repairs are very difficultreviewed the problems of casing design in the and expensive. Matsukawa Well R-3 also had a casinggeothermal fields of New Zealand and described the failure for some undefined reason when casing frag-problem of cold, damp hydrogen sulfide in contact ments began to be blown out of the well after sevenwith casing at high tensile stresses causing frac- months of production followed ten months later byturing in all grades of casing steels tested. In blowout of a large quantity of casing fragments.one case, failure occurred at only 25% of the One possible reason for the latter type failurenormal ultimate strength. Due to thermal expansion andcasing disintegration is the fact that, typically,and contraction effects the casing joints can be a GTE steam well uses the stepped casing as the pro-loaded either in tension or in compression. Crom- duction-flow passage, which is constantly directlyling recommends buttress joints, which cause in- exposed to the high-velocity steam flow. In oilcreased casing costs, for wells at The Geysers. or gas wells, on the other hand, an inserted pro-Another temperature-caused problem is brought up duction tube of constant diameter is normally usedby Suter who states that casing wear occurs in The which is easily removed for maintenance, if neces-Geysers wells because rubber drill-pipe protectors, sary. A well-drilling system that would produce aused to centrally locate the drill pipe in the well, hole of constant diameter to total depth would fa-fail due to temperature and allow the drill pipe to cilitate the use of a production tube. Also, itrub against the casing. A problem of high operating would avoid sudden turbulent expansion conditionspressure requiring either new high-strength materials across a step that can cause precipitation of saltsor very heavy casing walls was noted by Bealaurier40 from the expanding fluid and their deposition onfor geopressurized wells under both static and the casing walls.dynamic flow conditions. 6 . Drilling Fluids and Muds. The use of air as In geothermal wells, one function of the casing the drilling fluid and the resultant erosion problemcement is to prevent the detrimental intrusion of have already been noted. Water is another readilycold water into the well. It was found early that available and easily handled drilling fluid, whichcommon cement at high temperatures suffered strong can be used if the downhole fluid pressure is notretrogression effects and became highly permeable, above hydrostatic and if lost circulation, should itas reported by Nakajima.” Special cement mixtures exist, can be compensated for by either having suffi-capable of setting up properly at higher temperatures cient makeup-water flow capacity or by pumping allhave been developed and will need further improve- the water and debris directly into the formation withment as temperatures in future GTE developments zero return to the surface. However, if the groundbecome higher. Nakajima Mentioned the frequent12
  19. 19. fluid pressures are above hydrostatic then special of 25 reinjection wells would amount to $3,000.000LJ drilling muds must be composed, which are compatible with high temperatures and meet the following re- to $3,500,000. Thus,the total cost for production and reinjection wells would be -$10,750,000 per quirements: The mud must have the density needed complex. to hydrostatically balance the downhole pressures These costs, which average 94 $/m (29 $/ft), and to otherwise stabilize the hole; its surface are reasonable considering that only about one in tension must be adequate to transport the bit cut- twenty geothermal fields24 would be in a vapor-domi- tings out of the hole; it must serve as a good bit nant field like The Geysers, where drilling is most coolant; it must not dry out and bridge flow pas- expensive. Then, continuing the example started in sages during cementing (leading to casing failures); Section II,A,4, the total costs of production and and it Must not dry out or change chemically during injection wells to develop a complex with 200,000 time-consuming operations such as changing bits. MW(e) power output would be 727 x $10,750,000 = There are many references in the literature to the $7.28 x lo9. This, added to the previously derived high-temperature mud problems in geothermal wells. exploratory drilling cost of $1.88 x lo9, gives us Recently, GreiderS6 of Chevron Oil wholeheartedly the total drilling expenditures, $9.7 x lo9, just endorsed the development of high-temperature muds for finding and setting up the wells. Clearly, GTE and cements. Also, the Bechtel Corp. study4’ lists drilling with current techniques could be very 588 K as the current limit for muds. costly (e.g., running into tens of billions of 7 Downhole Measurements and Samples. The . dollars) making it quite worthwhile and cost-effect- early exploration and modeling phase of a GTE ive to develop new, cheaper techniques and equipment. reservoir requires taking many: downhole measure- Note, again, that the initial assumption of 200,000 ments and samples of the rock and general fluids. MW(e) may be low. Wither~poon,’~ is involved with studies of the who The above discussions of cost centered primar- national geothermal resources, feels that equipment ily on wells in conventional GTE areas where maximum and methods for obtaining the required downhole depths may not exceed 3 km. For hot dry rock and data leave much to be desired. Cores are used to perhaps geopressurized developments the depths and obtain data on the type of strata being encountered, drilling costs could be considerably higher. Costs on fracture orientation, and on other phenomena. increase very rapidly with depth, as illustrated in However, cores are taken in insufficient intervals Fig. 6. The average oil and gas-well costs are and the taking of many more core samples would be shown shaded; typical The Geysers and Imperial highly desirable. The alternative and perhaps ulti- Valley geothermal costs are shown as 160 $/m mate approach to this problem is to develop new (50 $/ft) and 80 $/m (25 $/ft), respectively; for downhole diagnostic techniques. Ground-water sam- depths of 15 km, costs could be - $ 2 0 , 0 0 0 , 0 0 0 to ples are also very important, if they can remain $26,000,000 per well. chemically undisturbed between the time of in situ 9. Summary of Current Drilling Problems. It sampling and the time of chemical analysis. In a has been amply demonstrated that naturally occur- water-based GTE reservoir assessment program the ring hot-water or vapor-dominated geothermal reser- data should, at a minimum, be sufficient to deter- voirs can be penetrated by rotary drilling methods mine downhole temperatpres and pressures, porosity, thdt have been developed for oil and gas wells. permeability, fluid chemistry, and GTE producibili- However, there are factors in geothermal fields tY. such as high temperature, corrosive fluids and 8. Costs. Cost data for a typical exploratory gases, unfavorable siting conditions, and, in many geothermal well have been presented in Section 11, cases, hard abrasive rocks, which combine to make A,4. According to Greider’s data for large-scale the average rotary-drilled geothermal wells more development of 275-MW(e) GTE complexes, the cost expensive than the average oil or gas wells of eom- of 50 production wells per complex would, with parable depth. High well costs could significantly careful planning, average $7,500,000; and the cost impede the expansion of geothermal energy sources 13

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