AnIntroduction to

Slurry InjectionTechnologyfor Disposal ofDrilling Wastes

Because wastes areinjected deep into theearth below drinkingwater zones, properslurry injection opera-tions should pose lowerenvironmental andhealth risks than moreconventional surfacedisposal methods.

Table of ContentsWhat Are Drilling Wastes? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2

How Are Drilling Wastes Managed? . . . . . . . . . . . . . . . . . . . . . .3

Underground Injection of Drilling Wastes . . . . . . . . . . . . . . . . .4

Disposal in Salt Caverns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5

Subfracture Injection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5

Types of Slurry Injection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6

How Is Slurry Injection Conducted? . . . . . . . . . . . . . . . . . . . . . .7

Regulatory Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8

Geologic Conditions That Favor Slurry Injection . . . . . . . . . . .9

Fracture Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10

Database of Slurry Injection Jobs . . . . . . . . . . . . . . . . . . . . . . .11

Type and Volume of Material Injected . . . . . . . . . . . . . . . . . . .12

What Types of Problems Have Occurred? . . . . . . . . . . . . . . . .13

Economic Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14

Final Thoughts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15

How to Learn More About Slurry Injection . . . . . . . . . . . . . . .16

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17

1

What Are Drilling Wastes?

Oil and gas wells aredrilled to depths ofseveral thousand tomore than 20,000feet. Drilling oil andgas wells generatestwo primary types of wastes – drillingfluids and drill cuttings.Drilling fluids (alsocalled drilling muds)are used to aid thedrilling process. Mostdrilling muds contain

bentonite clay, water, barium sulfate (barite),and specialized additives. Some types of muds also contain hydrocarbons. Inaddition, just as a homeowner’s electricdrill bores through wood and generateswood particles or shavings, oil and gasindustry drilling systems generate ground-up rock particles known as drill cuttings.

Large volumes of drilling muds are storedin aboveground tanks or pits. Muds arepumped to the bottom of the well throughthe hollow drill pipe and out throughholes in the drill bit. The muds help tolubricate and cool the drill bit, and aid incarrying the drill cuttings to the surface,where the muds and drill cuttings areseparated by mechanical means, usually a vibrating screen. The liquid muds passthrough the screen and are recycled intothe mud system, which is continuously

treated either mechanically or with variousadditives to maintain the desired propertiesfor effective drilling. The solid cuttings,which are coated with mud, are stockpiledfor further processing and final disposition.

The volume of drilling wastes generatedfrom each well varies depending on thedepth and diameter of the well bore;typically, several thousand barrels ofdrilling waste are generated per well. Abarrel (bbl) is the standard unit of volumein the oil fields of the United States andmany other parts of the world. An oil fieldbarrel has a volume of 42 U.S. gallons orabout 0.16 cubic meters. The AmericanPetroleum Institute (API) estimates thatabout 150 million bbl of drilling waste were generated at U.S. onshore wells in1995. About 40 million bbl per year of this waste was solid drill cuttings.

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Mud

How Are Drilling Wastes Managed?

The cuttings at most onshore wells in the United States are placed into a pitnear the well. When the drilling iscompleted, any liquids in the pit areremoved and disposed of, and theremaining solids are buried in place or are spread out on the land surroundingthe well. This process is not approved forsome types of drilling muds and cuttings,or in certain locations with particularlysensitive environmental conditions (suchas wetlands, areas with a seasonally highwater table near the surface, or frozentundra). In these situations, the drillingcompanies must use other methods todispose of or manage the drilling wastes.

Some examples of approved drillingwaste management methods include:

• thermal treatment• biological treatment• discharge to the ocean from

offshore platforms• reuse of solids for fill dirt,

road cover, or other uses• offsite landfilling• subsurface injection

3

CasingAquifer

Confining Layer

Cap Rock

Gas

Oil Sand

Derrick

Schematic

Not to Scale

Cement

Casing

Drill Pipe

Drill Bit

Mud

MudReturn

Pump

Casing

ShakerCuttings

Pit

Underground Injection of Drilling Wastes

Several different approaches are used forinjecting drilling wastes into undergroundformations for permanent disposal. Thisbrochure focuses on slurry injection technol-ogy, which involves grinding or processingsolids into small particles, mixing themwith water or some other liquid to make a slurry, and injecting the slurry into anunderground formation at pressures highenough to fracture the rock. The processreferred to here as slurry injection hasbeen given other designations by differentauthors, including slurry fracture injection(this descriptive term is copyrighted by a company that provides slurry injectionservices, and is therefore not used in thisbrochure), fracture slurry injection, drillcuttings injection (or reinjection), cuttingsreinjection, and grind and inject. Two otherinjection approaches — waste injectioninto salt caverns at relatively low pressure,and injection into formations at pressureslower than the formation’s fracturepressure (subfracture injection) — aredescribed in the following sections.

4

Slurry Processing and Injection EquipmentPhoto Courtesy of Terralog Technologies

Data Acquisition& Control Unit

Slurry Mix Unit Shaker Unit

3-Stage Auger

Pumps

Feed Hopper

SolidInjectate

Hydraulic Skid

Disposal in Salt Caverns

In 1999, the U.S. Department of Energy(DOE) prepared a brochure describingthe use of salt caverns for disposal of oilfield wastes. Salt caverns are created bydissolving underground salt formationsin a controlled manner to create largeunderground “containers” that are filledwith brine (salty water). In the UnitedStates, disposal of drilling waste into saltcaverns is currently permitted only inTexas, although Louisiana is in theprocess of developing cavern disposalregulations. Through August 2002, Texashad permitted 11 caverns at 7 locations.All these caverns may receive oil fieldwastes, including drilling wastes.

5

Incoming Waste

Brine

Above Ground

Below Ground

Salt Cavern

"Container"

Salt Formation

Waste Pile

Subfracture Injection

In certain geological situations, formations may be able to accept waste slurries at an injection pressure below the pressure required to fracture the formation. Wastesare ground, slurried, and injected, but the injection pressures are considerably lowerthan in the case of slurry injection. The most notable example of this process occursin East Texas, where the rock overlying a salt dome has become naturally fractured,allowing waste slurries to be injected at very low surface injection pressures or evenunder a vacuum. A commercial waste disposal company has established a series ofsubfracture injection wells at several locations in East Texas. These wells have servedas the disposal points for a large percentage of the drilling waste that is hauled backfrom offshore platforms in the Gulf of Mexico for onshore disposal.

Types of Slurry Injection

Oil and gas wells are constructed withmultiple layers ofpipe known as casing.A well is not drilledfrom top to bottom at the same diameterbut rather in a seriesof segments. The topsegment is drilledstarting at the surfaceand has the largestdiameter hole. After a suitable depth hasbeen drilled, the holeis lined with casingthat is slightly smallerthan the diameter ofthe hole, and cementis pumped into thespace between thewall of the drilledhole and the outsideof the casing. Next,

a smaller diameter hole is drilled to a lowerdepth, and another casing string is installedto that depth and cemented. This processmay be repeated several more times. Thefinal number of casing strings depends on the total depth of the well and thesensitivity of the formations throughwhich the well passes.

The two common forms of slurry injectionare annular injection and injection into adisposal well. Annular injection introducesthe waste slurry through the space betweentwo casing strings (known as the annulus). Atthe lower end of the outermost casing string,the slurry enters the formation. The disposalwell alternative involves injection to either a

section of the drilled hole that is below allcasing strings, or to a section of the casingthat has been perforated with a series ofholes at the depth of an injection formation.

Many annular injection jobs are designedto receive wastes from just one well. Onmultiwell platforms or onshore well pads,the first well drilled may receive wastesfrom the second well. For each successivewell, the drilling wastes are injected intopreviously drilled wells. In this mode, nosingle injection well is used for more thana few weeks or months. Other injectionprograms, particularly those with a dedi-cated disposal well, may inject into thesame well for months or years.

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Cement

Drill Bit

Casing

Casing

Casing

Drill Pipe

Cement

Deep Casing

Shallow Casing

AnnulusInjection into

Cement

Casing

Tubing

Deep Casing

Packer

PerforatedCasing

Disposal Well

Annular Injection

How Is Slurry Injection Conducted?

Slurry injection involves straightforward mechanical processes such as grinding, mixing,and pumping. The technology uses conventional oil field equipment. As a first step, thesolid or semi-solid drilling waste material is made into a slurry that can be injected. Thewaste material is collected and screened to remove large particles that might cause plug-ging of pumps or well perforations. Liquid is added to the solids, and the slurry (or theoversize material) may be ground or otherwise processed to reduce particle size. Prior to injection, various additives may be blended into the slurry to improve the viscosity orother physical properties. The slurry is injected through a well into the target formation.

When the slurry is ready for injection, the underground formation is prepared to receivethe slurry. First, clear water is rapidly injected to pressurize the system and initiate fractur-ing of the formation. When the water is flowing freely at the fracture pressure, the slurryis introduced into the well. Slurry injection continues until an entire batch of slurriedmaterial has been injected. At the end of this batch, additional water is injected to flushsolids from the well bore, and then pumping is discontinued. The pressure in the forma-tion will gradually decline as the liquid portion of the slurry bleeds off over the next fewhours, and the solids are trapped in place in the formation.

Slurry injection can be conducted as a single continuous process or as a series of smaller-volume intermittent cycles. On some offshore platforms, where drilling occurscontinually and storage space is inadequate to operate in a daily batch manner, injectionmust occur continuously as new wells are drilled. Most other injection jobs are designedto inject intermittently. They inject for several hours each day, allow the formation to restovernight, and then repeat the cycle on the following day or a few days later.

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Pre

ssu

re

Time

24-Hour Cycle

Repose Period

5 67

8

9

101

24

3

45 6

7

1

2

3

8

Pressure vs. Time Plot for Two Daily Cycles of Slurry Injection

Injection Period Injection Period Numbers on the plotare identified below.

1.2.3.4.5.6.7.8.9.

10.

Set upInitiate injectionWater flushStart solids injectionSlurry injectionStop slurry injectionWater flushFracture closurePressure bleed offReturn to background pressure

Change Injection Mix

Three-Day ∆p MeasurementGood Leak-OffToo Slow Leak-Off

Restart

σv

Step-Rate Test

Pre

ssu

re

Time (days)

Long-Term Solids Injection Approach

Regulatory Requirements

Under the Safe Drinking Water Act, theU.S. Environmental Protection Agency(EPA) administers the Underground Injec-tion Control (UIC) program to regulateinjection activities. States can apply toEPA to administer the UIC program, andmany oil- and gas-producing states havebeen delegated UIC program authority.The UIC regulations for oil and gasinjection wells do not specifically saymuch about slurry injection. However,those regulations do specify that theinitiation of new fractures and thepropagation of existing fractures mustoccur within the injection formation. Thefracturing must not extend through anoverlying confining zone or cause migrationof fluids into an underground source ofdrinking water (USDW). (A USDW is anaquifer or its portion which meets thefollowing criteria: supplies any publicwater system or contains a sufficientquantity of groundwater to supply a publicwater system; currently supplies drinkingwater for human consumption or containswater with fewer than ten thousandmilligrams per liter total dissolved solids;and is not an exempted aquifer.) State-runUIC programs must be consistent withfederal regulations and be approved byEPA. EPA may approve differences basedon the uniqueness of the state.

Slurry injection is currently permitted on a regular basis in Alaska, Texas, andCalifornia, and at offshore locations in the Gulf of Mexico and elsewhere in theworld. Oil and gas regulatory officialsfrom the mid-continent states do not usethis procedure widely, primarily becausemost companies there prefer to dispose of drilling waste through burial in pits.

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Some states have formal slurry injectionregulations while others approve slurryinjection under general administrativeauthority. Because the procedures used to approve slurry injection vary greatlyamong states, readers are advised tocontact their state oil and gas agencies for more information.

Several states, most notably Alaska,consider annular disposal incidental to the drilling process, and therefore do not require a UIC permit for that activity.Annular disposal in Alaska is carefullyregulated by the Alaska Oil and GasConservation Commission with criteriasimilar to the UIC program. Injectioncannot contaminate fresh water, causedrilling waste to come to the surface,impair the integrity of the well, or damagean actual or potential producing zone.

Injection activities at offshore locationsare not covered under the UIC programbecause there are no USDWs at thoselocations. The U.S. Minerals ManagementService issues guidelines for injection andapproves slurry injection jobs on a case-by-case basis.

Different types of rocks have differentpermeability characteristics. Althoughrocks appear solid, they are made up ofmany grains or particles that are boundtogether by chemical and physical forces.Under the large pressure found at depthsof several thousand feet, water and otherfluids are able to move through the poresbetween particles. Some types of rock,such as clays and shales, consist of verysmall grains, and the pore spaces betweenthe grains are so tiny that fluids do notmove through them very readily. In con-trast, sandstone is made up of cementedsand grains, and the relatively large porespaces allow fluids to move through them much more easily.

Slurry injection relies on fracturing, andthe permeability of the formation receivingthe injected slurry is a key parameter indetermining how readily the rock fractures,as well as the size and configuration of the

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fracture. When the slurry is no longer ableto move through the pore spaces, and theinjection pressure continues to be applied,the rocks will crack or fracture. Continuousinjection typically creates a large fractureconsisting of a vertical plane that movesoutward and upward from the point ofinjection. Intermittent injection generates a series of smaller vertical planes that forma zone of fractures around the injectionpoint. Fractures that extend too far verticallyor horizontally from the point of injectioncan intersect other well bores, naturalfractures or faults, or drinking wateraquifers. This condition is undesirable and should be avoided by careful design,monitoring, and surveillance.

Most annular injection jobs inject intoshales or other low-permeability forma-tions, and most dedicated injection wellsinject into high-permeability sand layers.Regardless of the type of rock selected forthe injection formation, preferred sites will be overlain by formations having theopposite permeability characteristics (high vs. low). When available, locationswith alternating sequences of sand andshales are good candidates to containfracture growth. Injection occurs into one of the lower layers, and the overlyinglow-permeability layers serve as fracturecontainment barriers, while the high-permeability layers serve as zones whereliquids can rapidly leak off.

Geologic Conditions That Favor Slurry Injection

Aquifer

LowPermeabilityLayer

High Permeability Layer

Fracture Deflected byLow Permeability Layer

Fracture Monitoring

The size, shape, and orientation offractures can be predicted throughcomputer modeling, but it is important to ascertain the dynamics occurringwithin the formation and verify thatfractures are not extending into inap-propriate locations. Several types ofmonitoring devices can provide usefulfeedback to operators about what ishappening underground. Conventionaloil field monitoring methods that rely on lowering logging instruments into a well — including radioactive tracers,temperature logs, and imaging logs —provide some indication of fractureposition. However, such methods arelimited in their capacity to give usefulinformation about fracture geometry andextent because they can only measureconditions within the first few feet from the well.

Two types of external devices that remotelymeasure changes in the rock provide muchbetter information. These can be located atthe surface or at some depth inside of mon-itoring wells. Tiltmeters can detect very smallchanges in the angle of a rock surface beforeand after injection, and show how the rockdeformed after fracturing. Geophones areacoustical devices that can detect micro-seismic events related to fracturing. Bothtechnologies can be located at the surfaceor at some depth inside monitoring wells.These tools are expensive and are typicallyemployed only at dedicated injection wells that are intended for long-terminjection programs.

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Database of Slurry Injection Jobs

As part of an evaluation of the feasibilityof slurry injection technology, ArgonneNational Laboratory developed a data-base with information on 334 injectionjobs from around the world. The threeleading areas representing slurry injectionin the database are Alaska (129 records),Gulf of Mexico (66 records), and theNorth Sea (35 records). Most injectionjobs included in the database featureannular injection (296 or more than 88%),while the remainder (36 or 11%) useddedicated injection wells. These figuresreflect the large number of annularinjection jobs reported for Alaska (121 ormore than one-third of all reported jobs).Injection is being carried out primarily onwells owned by many large multinationalcompanies, but some injection also isbeing undertaken by medium or largeindependent companies. Several servicecompanies and consultants are used by the producers when designing andconducting the slurry injection projects.

Most injection jobs were conducted atdepths shallower than 5,000 feet; manyoccured in the interval between 2,501and 5,000 feet. The shallowest injectiondepth reported was 1,246 to 1,276 feet inIndonesia, and the deepest was 15,300feet at an onshore well in Louisiana.

The reported injection rates range from0.3 bbl/minute to 44 bbl/minute. The reported injection pressures range from50 pounds per square inch (psi) to 5,431 psi.

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Type and Volume of Material Injected

Most wells in the database were used to inject drill cuttings. Many were alsoused to inject other types of oil fieldwastes, including produced sands, tank bottoms, oily wastewater, pitcontents, and scale and sludge thatcontain naturally occurring radioactivematerial (NORM).

Table 1 shows the number of records thatreported volumes within specified ranges.The data show that more than 83% of theinjection jobs in the database involvedless than 50,000 bbl of slurry. The largestjob reported in the database involves morethan 43 million bbl of slurry injected inseveral wells associated with a dedicatedgrind and inject project at Prudhoe Bay on the North Slope of Alaska.

12

Table 1 - Distribution of Total Slurry Volume in Database

<10,000

10,000–50,000

50,001–100,000

100,001–500,000

500,001–1,000,000

>1,000,000

Total

Total Reported Slurry Volume (bbl) Number of Records in Database

87

206

9

13

5

12

332

What Types of Problems Have Occurred?

Problems were reported in fewer than10% of the records. The most commonproblem was operations-related: pluggingof the casing or piping because solids hadsettled out during or following injection.Another significant operational probleminvolved excessive erosion of casing,tubing, and other system componentscaused by pumping solids-laden slurry at high pressure. In some cases, theinjection was unable to keep up with the drilling rate, and cuttings had to be stockpiled. This situation is merelyinconvenient at onshore locations, but can cause drilling to stop at offshorelocations with insufficient storagecapacity. Operational problems areinconvenient and costly to operators who have to stop their normal activities,but such problems do not normallyrepresent a risk to the environment.

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Environmental problems associated withslurry injection are rare but are of muchgreater concern. Few documented cases of environmental damage caused by slurryinjection exist. Unanticipated leakage to theenvironment not only creates a liability tothe operator, but also generally results in a short-term to permanent stoppage ofinjection at that site. Several large injectionjobs have resulted in leakage to either theground surface or the sea floor in the caseof offshore wells. The most likely cause ofthese leakage events is that the fracturemoved far from the injection point andintersected a different well that had notbeen properly cemented. Under the highdownhole pressure, the injected fluids seek out the pathway of least resistance. Ifcracks in a well’s cement job or geologicalfaults are present, the fluids may preferen-tially migrate upward and reach the landsurface or the sea floor.

Economic Considerations

Various examples taken from the literatureprovide a range of cost comparisons — usingoil-based muds and injecting the cuttings,using synthetic-based muds and discharg-ing the cuttings, and hauling drilling wastesto shore for disposal. Although many of thepapers reviewed show that slurry injectionis the most cost-effective option at thestudied site, no single management methodis consistently identified as the least — or,conversely, the most — costly. This confirmsthe importance of conducting a site-specificcost-benefit analysis.

Three factors are critical when determiningthe cost-effectiveness of slurry injection:

(1) The volume of material to be disposedof — the larger the volume, the more attrac-tive injection becomes in many cases. Theability to inject onsite avoids the need totransport materials to an offsite disposallocation. Transportation cost becomes asignificant factor when large volumes ofmaterial are involved. In addition, trans-porting large volumes of waste introducessafety and environmental risks associatedwith handling, transferring, and shipping.Transportation also consumes more fueland generates additional air emissions.

14

(2) The regulatory climate — the stricter the discharge requirements, the greater the likelihood that slurry injection will be cost-effective. If cuttings can be dischargedat a reasonable treatment cost, thendischarging is often the most attractivemethod. Regulatory requirements thatprohibit or encourage slurry injection play an important role in the selection of disposal options.

(3) The availability of low-cost onshoredisposal infrastructure — several disposalcompanies have established extensivenetworks of barge terminals along theLouisiana and Texas coasts to collect largevolumes of wastes brought to shore fromoffshore Gulf of Mexico platforms. Theysubsequently dispose of them througheither subfracture injection or placementinto salt caverns at onshore locations.Through the economy of scale, the onshoredisposal costs are not high, and much of the offshore waste that cannot bedischarged is brought to shore anddisposed at these facilities. Most otherparts of the world do not have an effective,low-cost onshore infrastructure. Thus, inthose locations, onshore disposal is often a more costly alternative.

Final Thoughts

Slurry injection has been usedsuccessfully in many locations around the world for disposing of drilling wastes.Although a few injection jobs have notworked well, the reasons for theseproblems are understood. Similar problemscan be avoided by proper siting, design,and operation. When slurry injection isconducted at locations with suitablegeological conditions and the injectionprocess is properly managed andmonitored, it is generally a safe andenvironmentally preferable disposalmethod. Because wastes are injected deepinto the earth below drinking water zones,proper slurry injection operations shouldpose lower environmental and health risksthan more conventional surface disposalmethods. The costs for slurry injection can be competitive with or even moreattractive than those for other disposalmethods. Drilling waste managementmethods should be assessed for each site individually. Slurry injection will notbe the favored management option fordrilling wastes in all situations; however,in many locations, it compares favorablywith other, more conventionalmanagement options.

15

How to Learn More About Slurry Injection

This brochure is part of a project fundedby DOE to evaluate the feasibility of slurryinjection technology. Argonne NationalLaboratory conducted the evaluation and prepared a technical report and acompendium of relevant state and federalregulations. These two reports are listedbelow and can be downloaded fromArgonne’s website at:http://www.ead.anl.gov/project/dsp_topi

cdetail.cfm?topicid=18

Veil, J.A., and M.B. Dusseault, 2003,Evaluation of Slurry Injection Technologyfor Management of Drilling Wastes,W-31-109-ENG-38, 110 pp, May.

Puder, M.G., B. Bryson, and J.A. Veil, 2003,Compendium of Regulatory RequirementsGoverning Underground Injection ofDrilling Wastes, W-31-109-ENG-39-11, 206 pp, February.

16

DOE also funded an earlier study thatprovides additional information on slurryinjection technology. That study includes a database that provides very detailedinformation on a few slurry injection jobs.

Terralog Technologies USA, Inc., 2002,Development of Improved Oil Field WasteInjection Disposal Techniques,DE-AC26-99BC15222, Arcadia, CA, March 31.

For additional information contact:

John Ford, DOE National EnergyTechnology Laboratory, 918-699-2061,john.ford@npto.doe.gov

John Veil, Argonne National Laboratory,202-488-2450, jveil@anl.gov

17

Acknowledgements

The funding to support this project was provided by the U.S. Department of Energy,Office of Fossil Energy, National Energy Technology Laboratory,

under Contract W-31-109-Eng-38.

Office of ScienceU.S. Department of Energy

September 2003 Argonne National Laboratory is operated byThe University of Chicago for the

U.S. Department of Energy, Office of Science