Introduction to Hydrofracturing


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Introduction to Hydrofracturing

  1. 1. College of Agricultural Sciences • Cooperative Extension Water Resources Extension Water Facts #31 Introduction to HydrofracturingThis publication describes the process of hydraulically Wells with barely profitable output were common infracturing, or hydrofracturing a well. It discusses the gas and oil well development. Slow production couldhistory, some common techniques, and introduces have been caused by very small pore spaces in thesome of the equipment and chemicals commonly rock, closed passages at the wellbore, or by waxesused in the operation. The term Hydrofracturing is and algae in the production fluids clogging theoften shortened to “fracing,” or “fracking,” and we will formation. To the person drilling the well, fracing wasuse the short form, “fracing,” here for convenience. In only half as expensive as drilling a new well, and wasthis publication, we remove the surrounding mystery successful in boosting production at least 50% of theand give a basic description of the fracing process. time. Added to these odds, a new well drilled in the same formation had a 50% chance of having theExperts say that every geologic formation and every same production problems. Fracing developed into afracture within each formation is unique. We still sensible way to save money and increasecannot see what goes on thousands of feet productivity.underground. However, microseismic technology hasadvanced through analyzing sound and radio signals, Conventional Verses Unconventionalenabling us to understand much more than we did In unconventional gas reservoirs, the gas is tightlyeven five years ago. The Environmental Protection stored in the rock itself. In conventional gasAgency (EPA) is currently conducting a study on reservoirs, the gas is stored in pore spaces betweenfracing, suggesting we will learn even more about the individual grains. In some limestones, organic matterprocess in the future. decomposes leaving tiny spaces in the rock like a loaf of bread. They are too small to see, but are largeHistory enough to hold fluids, including water, gas, and oil. InFracing is a term used to describe the starting and other limestones, grain packing and chemicalextending of tiny rock cracks, using water, sand, and processes provide open spaces for holding fluids.chemicals at high pressures. These cracks, called Sandstone sand grains pack together imperfectly,“fractures” provide passages allowing trapped leaving tiny pores in the stone, even when composedhydrocarbons to escape. The oil and gas industry first partly of finer silt and clay. As the pores connect, theused fracing in 1947 as a method of increasing fluid can flow through the rock formation, and we relyhydrocarbon yields from rock formations. on this flow to supply wells drilled into the “reservoir rock.” Natural fractures formed in the rock massesIn early well development in the late 1800’s, “shooting over millions of years as the earth’s plates moved anda well” entailed dynamiting a void at the bottom of the collided. These also allow fluids initially trapped withinvertically drilled gas and oil well wellbore, so that the rock to flow. In limestone and sandstoneproduction fluids, usually oil, flowed into what formations, some fracture sets develop horizontally,amounted to a cistern. The oil was then dipped out by parallel to original sediments that settled in beds. Ina bailing bucket. The first well shot was done by conventional wells, vertical holes are drilled into theColonel E.A.L. Roberts on Ladies Well near Titusville, porous rock formation with the hope of interceptingPA, in 1865. Gas pressure in the newly drilled shallow areas where gas has collected. As the verticalwells pushed the oil to the surface freely, resulting in wellbore intercepts fractures oriented perpendicularlythe exciting gushers shown in early oil field to it, drillers are able to collect gas over much largerdevelopment pictures. volumes of the reservoir rock than the porous rock holding fluids next to the well. Fractures in this wayPorous formations may have been plentiful in some permit a well to drain a larger area.areas, but wells eventually slowed in production. 1
  2. 2. Vertical wells placed in conventional reservoirs are the larger casing. The wellbore is continued to belowunable to collect fluids for a long period in very dense the deepest freshwater aquifer. A second casing,formations of shale or coal. The particles forming called the “surface casing” or “freshwater string”, isthese rocks are a mixture of decomposed microbes, suspended inside and cemented in place completelyplants, and animals along with very fine clay minerals. to the surface. The surface casing separates thePore spaces between the components of this fine freshwater aquifers from the oil and gas bearingsediment are so small that as organic materials formations below. It often uses a valve and seal,decomposed through microbial activity, gas and oil called a “seat” on its base.molecules formed and chemically bonded to thesolidified “mud rock” matrix. These fluid molecules A third casing, known as an “intermediate “ string iscannot move until more space is created. placed in a similar manner inside the surface casing and cemented to the surface. This casing isolates gasThe orientation of natural fractures often do make of -bearing rock from the surface casing seat andfluid extraction easy. A vertical wellbore in the provides a foundation for well control equipmentreservoir rock may intercept relatively few of these designed to control fluids from escaping underfractures so the well can only gather fluids from the pressure.formation next to it. To intersect more verticalfractures, the oil and gas industry developed a way to The drilling process continues with each stepinitially drill a well vertically to a certain depth and requiring successively smaller casing sizes thatthen steer the drilling over a gradual arc, so that after extend deeper into the subsurface. The final casinga quarter mile the leading drill pipe and bit can be string is referred to as “production” casing. Severalboring horizontally in the desired direction. Horizontal casing strings cemented in place ultimately separatewellbores may extend over 5,000 feet, enabling the the targeted gas reservoir from freshwater aquifers.driller to hit many more natural fractures in relatively Although variations in well construction are common,thin coal and shale formations. A horizontal well can a typical well is configured like the cross sectionthen collect much more natural gas from the reservoir shown in Figure 1 on the following page.rock. However, in very dense unconventional gasreservoirs, such as shale formations, the gas is so After all casing runs are placed, a well servicetightly stored within the rock itself that fracing has company perforates the production casing so that thebecome a standard part of well stimulation. well can collect fluids from the targeted formation. To accomplish this, the larger drill rig replaced by aPreparing for the Hydrofracturing smaller completion rig or a large crane. The rig orIn drilling a horizontal well, the first portion of the hole crane lowers a tool called a perforating gun into theis bored deeply enough to build a stable well production casing on a cable or by coiled tubing. Thefoundation and prevent surface fluids from entering perforating gun is a long pipe with holes positionedthe well. Sections of steel pipe known as “conductor along its length. The holes are loaded with explosivescasing”, or referred to as “drive pipe”, are placed in connected to a signal line running through the middlethe wellbore. The pipe may be driven into place by of the pipe. The pipe is lowered and pushed in thethe rig, or it may be hung in open hole and cemented casing to a selected formation zone where the well isin position. to collect oil or gas. A technician sends an electrical charge to the tool causing the explosive in each toolIf it is to be cemented, a two-inch space, called “the hole to detonate and punch through the steel casingannulus” is usually left between the casing and the and cement next to it. The next step is to pumpborehole to create a strong bond. A calculated diluted hydrochloric acid down the hole to clean outcement volume is pushed down the inside of the pipe the well, dissolve the cement at each perforatedby placing a system of wiper plug ahead of and zone, and expose the rock formation. The spent acidsealing plug behind it, and pumping water behind it. solution is removed as each zone is washed, andThe cement has nowhere to go at the end of the can be recycled. The gas well is now ready to becasing, and is forced back to the surface of the hole “fraced.”in the annulus outside the pipe. The Fracing OperationCement is given time to harden in a chemical process Figure 2 shows the top of the well before the fracingknown as “curing”. Next, a smaller hole is drilled begins. The pipe extending up from the ground is thedeeper through the inside of the conductor casing, gas wellhead, with a shut off valve above it. The stackboring through any plugs and valve systems placed in of equipment includes the valve, and the six iron pipe 2
  3. 3. Figure 1. Cross section of typical horizontal well drilled into the Marcellus formation (courtesy ofPenn State Marcellus Center for Outreach and Research).lines attached to it which will each carry a specificformula of water, sand and chemicals to frac the well.This arrangement allows service companies toconnect their equipment to the well to do theirspecialized work, and shut off, or “shut in,” the well touncouple and change equipment. Communicationcables running from the top of this assembly allowgeologists and operators to receive well informationas the job proceeds. They can then immediatelychange fluids, pumping rates, or pressure asneeded.Any work in the well must be able to withstand veryhigh pressures and temperatures, approaching 370degrees Fahrenheit at depths sometimes exceeding9,000 feet. The rubber and steel used must havespecial properties to avoid rapid wear or Figure 2. Frac manifold at McKean County site.disintegration. Photo Courtesy of James Clark, 2008. 3
  4. 4. The iron pipes at the well are connected to pump An auger or conveyor runs from the sand trailers ortrucks supplying the pressure for the fracing. The sand hogs to the blender, delivering sand to this unit.pump truck is positioned next to the well, and its Powerful diesel engines drive the conveyor becausepowerful (1,200 to 2,500 horsepower) pumps move the sand is very heavy. There may be several sandthe materials designed specifically for the well into the containers on the site. These giant sand hogs are setformation. These trucks can pump at extremely high in position empty and loaded by an assembly line ofrates (sometimes 500 gallons per minute or more) sand trucks before the fracing can begin. The trailerand high pressures (as much as 14,000 lbs. per shown in Figure 4 can hold about 3,000 cubic feet, orsquare inch) as they need to move enough sand or 111 cubic yards of sand, weighing about 200,000 lbs.ceramic beads, termed “proppant” into the formation This trailer has four compartments so the blender canonce fractures are started. Small wells may use one pump sand, glass, or ceramic beads of very specificpump truck. As Figure 3 shows, many pump trucks sizes into different areas of each formation zone asmay be used in a system on very large wells. The needed. They begin the frac with very fine sand andhigh pressure lines lead directly from the pump truck use coarser sand as the job the valve at the well site, called the “frac manifold.” Figure 4. Sand Hog being positioned at the wellFigure 3. Hydraulic fracturing operation on East site, Courtesy of Universal Well Services, Inc.Resources well, eastern Tioga County. Photo 2009.Courtesy of Robert Hansen, 2010. Since the practice began, fracture operations pumpedBehind the pump trucks is the blender truck, which relatively large quantities of water to carry smallpulls together the material needed to frac the well. quantities of sand into formations to prop them open.The blender is a long mixing machine that takes During the 1980’s, plastic polymer gel use increasedsand, or ceramic bead propping agent, water, and fluid thickness so that much more sand could bechemicals to prepare a gel that carries the propping pumped in stream without dropping it in the pipe.agent into the formation as deeply as possible. The Now, gelled suspensions containing from 20,000 lbs.blender driver adds antiseptic detergent chemicals to (about 10 cubic yards) to 150,000 lbs. (about 75 cubicthe gel to keep the formation rock molecules from yards) of proppant are pumped into each stage of aswelling and clogging the well. The chemical supply frac operation at rates up to 15 barrels (630 gallons)truck will often be a stake-body truck with signage per minute and at pressures reaching 14,000 psi tocalled placards showing if and what types of penetrate the formation effectively. As shown inhazardous chemicals are on board. Most chemical Figure 5 and Table 1, a frac operator may usehazards will be classified by the Department of 150,000 lbs. of sand suspended in a 229,000-gallonTransportation as being corrosive, flammable, or non- gel volume in each perforation stage.flammable. The graph on page 5 (Figure 5) and tableon page 6 (Table 1) give examples of what these The final material delivered to the blender is water.chemical additions may be. If the blender works on One to nine million gallons of water are typically usedmore than one formation zone, the frac operator may to frac an unconventional gas well. This varies,prepare a specific recipe for each. The blender driver depending on how many “zones”, or stages arecontrols the proportions of each ingredient according planned for the well. Water is managed in severalto tests run by geologists during the drilling stage. ways depending on the site. It can be stored in 4
  5. 5. freshwater ponds, usually 10 feet deep, on site or the pump driver or controlling the pumps remotely innearby, and then trucked or piped to several different order to regulate pressure applied in an initial acidsites. Exploration companies may limit pond and wash. As tiny imperfections in the formation rocksingle site excavations to less than five acres to crack, creating new fractures, the process registersreduce permit requirements. Since 2009; however, it right away as a recognizable pressure drop. Next,is not uncommon for 20 or more gas wells to be the operator tells the pump driver to begin pumpingplaced on one larger drilling site or pad. This makes sand suspended in gel to prop the new extendedwater use much more efficient. Frac tanks lined up crack open. All these activities, including pressures,on the site are also used. Water is stored on some timing, and fluids used, are carefully recorded andsites in 500-barrel frac tanks. To supply four million relayed by satellite communications to the owners ofgallons to a well, 200 tanks must be stored on site, the well. With this technology, analysts can knowwith supply lines to fill them and empty them as how far a fracture penetrates into the formation,needed. Whether water is held in frac tanks or in before and after treatment.ponds, on-site space is required on to manage water. Frac operators have traced fractures with microseismic equipment that reach 500 feet or more from the wellbore, particularly if fractures extend to parallel cracks along the formation. Fractures usually deflect along formation boundaries rather than penetrating adjoining formations. After the frac crew pumps the sand, gel and chemical suspension into the well, it pumps water or other fluids behind it to push, or displace, all the sand into the formation and out of the casing. The further it travels, the more likely the fractured formation will stay open, extending the well’s productive life. The entire process to frac one zone in a well will take about three hours. The fracing will begin at the farthest end of the wellbore and move toward the surface. In horizontal well sections, the farthest endFigure 5. Example of a hydraulic fracturing fluid is called the “toe”, and the section closest to the vertical turn is called the “heel”. Each zone that iscombination specified for a shale well, based fraced will be isolated from the rest of the well usingupon composite of Range Resources and tools called packers and rubber frac balls. EachChesapeake Energy information released in July perforation zone in shale wells often produces as2010. Every fracturing zone within a well may be much gas as one conventional vertical well.prepared individually depending on results ofgeologic tests. When crews finish fracing a well, most fluids pumped into the formation are eventually forced back to the surface immediately or over the life of the well. WellsThe entire frac operation is controlled at the Frac in the Pennsylvania watershed region monitored byOperator Van. The frac operator, like a job foreman, the Susquehanna River Basin Commission haveis in charge of the company performing well fracture returned from 8% to 10% of stimulation fluids in theactivities. The frac operator’s van, which looks like a first 30 days. Another large percentage will comecamper, has computers, electronic monitoring and back over the next several months with the producedcommunications equipment from which to direct all gas. This must be separated using a filter before thefracturing operations. The frac operator tells the gas can be sent to the gathering lines, blended toequipment drivers what fluids to pump into the well, correct qualities, and eventually pumped to awaitingwhen, how fast, and how much. markets. It is still unclear what percentage remains unaccounted for, as it cannot be distinguished fromThe gas industry has developed the ability to analyze the waxes, brines, and metals that return with thewell formation porosity by recording pressure drops fluids created in the formation. These are alland electrical signals. The operator watches separated from the gas and must be recycled orpressures and signal continuity in the well; instructing properly disposed of over the well’s producing life. A 5
  6. 6. gas well may be re-fraced several times in the future. least one perforation zone. Every well is treatedShale wells in Pennsylvania have not been re- individually, and every formation treatment is unique,fractured to date. depending on the company and the current research. These examples are shared as a reference only.In July 2010, Range Resources and Chesapeake Individual treatments have progressed to include aEnergy each publicized the chemistry and volume of dozen or more perforation zones in a single well.materials typically used in their well completions and Volumes of materials used will increase considerablystimulations. Figure 5 and Table 1 are composites of for longer horizontal laterals and will be shown in thethe two companies’ information for wells having at well records. Table 1. Example of a fracturing chemical combination in oil and gas wells. 6
  7. 7. Summary More InformationHydraulic fracturing has become a standard practice For more information on water resource issues related toin the gas industry. Fracing is a process that involves Marcellus gas drilling in PA visit: water, sand, and chemical additives to initiate,extend, and prop open formation fractures for the For more information about natural gas exploration andhighest gas production. development in PA visit : oil and gas industry changes continually. Futurestudies and research will help us learn more about Prepared by Scott A. Sjolander (Extension Associate), Jamescontinually evolving fracing and other gas production Clark (Senior Extension Educator), Dana Rizzo (Extension Educator), and John Turack (Extension Educator).technologies and their impacts. Penn State College of Agricultural Sciences research, extension,Acknowledgements and resident education programs are funded in part bySincere thanks to the following reviewers of this edition for Pennsylvania counties, the Commonwealth of Pennsylvania, andhelpful comments and suggestions: the U.S. Department of Agriculture. David Bicko and Patrick Jarrell of Universal Well Service, This publication is available in alternative media on request. Inc. Douglas Mehan, PG, Dallas-Morris Drilling Co. The Pennsylvania State University is committed to the policy Timothy Bruno; Rich Neville; Anthony Oprendek; Eugene W. that all persons shall have equal access to programs, facilities, admission, and employment without regard to personal Pine, PG, Seth Pelepko PG of PA Department of characteristics not related to ability, performance, or Environmental Protection. qualifications as determined by University policy or by state or Thomas Murphy; David Yoxtheimer, PG; and Dr. Michael A federal authorities. It is the policy of the University to maintain an Arthur, PG of Penn State Marcellus Center for Outreach & academic and work environment free of discrimination, including Research. harassment. The Pennsylvania State University prohibits discrimination and harassment against any person because of age, ancestry, color, disability or handicap, national origin, race,References religious creed, sex, sexual orientation, gender identity, orCarlson, Carl. Range Resources Company Presentation, Range veteran status. Discrimination or harassment against faculty,Resources Range Resources Hydraulic Fracturing, July, 2010, staff, or students will not be tolerated at The Pennsylvania Statepage 26 at University. Direct all inquiries regarding the nondiscrimination policy to the Affirmative Action Director, The Pennsylvania StateRange Resources website University, 328 Boucke Building, University Park, PA 16802-getdoc/50e3bc03-3bf6-4517-a29b-e2b8f0afe4f/Well-Completion- 5901; Tel 814-865-4700/V, 814-863-1150/TTY.Reports.aspx. ©The Pennsylvania State University 2011Chesapeake Energy, Inc. Hydraulic Fracturing and Completion.Website: March 17, 2011Drilling/Pages/Hydraulic-Fracturing-and-Completion.aspxClark, James, and Bryan Swistock. Gas Well Drilling & PrivateWater Supplies. Portable Classroom CD. 2008.Marcellus Center for Outreach and Research. Cross Section of aTypical Horizontal Marcellus Well. Website DCNR Department of Environmental Protection. HydraulicFracturing Overview. Website: Inc. Fas Drill® Array Bridge Plug Website Limited Oilfield Glossary, Website: EPA Hydraulic Fracturing Research Study 7