Fact Sheet - Slurry Injection of Drilling Wastes
Underground Injection of Drilling Wastes
Several different approaches are used for injecting drilling wastes into underground formations for permanent disposal. Salt caverns are described in a separate fact sheet. This fact sheet focuses on slurry injection technology, which involves grinding or processing solids into small particles, mixing them with water or some other liquid to make a slurry, and injecting the slurry into an underground formation at pressures high enough to fracture the rock. The process referred to here as slurry injection has been given other designations by different authors, including slurry fracture injection (this descriptive term is copyrighted by a company that provides slurry injection services), fracture slurry injection, drilled cuttings injection, cuttings reinjection, and grind and inject.
Types of Slurry Injection
The two common forms of slurry injection are annular injection and injection into a disposal well. Annular injection introduces the waste slurry through the space between two casing strings (known as the annulus). At the lower end of the outermost casing string, the slurry enters the formation. The disposal well alternative involves injection to either a section of the drilled hole that is below all casing strings, or to a section of the casing that has been perforated with a series of holes at the depth of an injection formation.
Many annular injection jobs are designed to receive wastes from just one well. On multi-well platforms or onshore well pads, the first well drilled may receive wastes from the second well. For each successive well, the drilling wastes are injected into previously drilled wells. In this mode, no single injection well is used for more than a few weeks or months. Other injection programs, particularly those with a dedicated disposal well, may inject into the same well for months or years.
A related process involves injection into formations at pressures lower than the formation's fracture pressure (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. Wastes are ground, slurried, and injected, but the injection pressures are considerably lower than in the case of slurry injection. The most notable example of this process occurs in 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 even under a vacuum. A commercial waste disposal company has established a series of subfracture injection wells at several locations in east Texas. These wells have served as the disposal points for a large percentage of the drilling waste that is hauled back from offshore platforms in the Gulf of Mexico for onshore disposal.
How Is Slurry Injection Conducted?
As a first step, the solid or semi-solid drilling waste material is made into a slurry that can be injected. The waste material is collected and screened to remove large particles that might cause plugging of pumps or well perforations. Liquid is added to the solids, and the slurry (or the oversize 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 or other physical properties.
When the slurry is ready for injection, the underground formation is prepared to receive the slurry. First, clear water is rapidly injected to pressurize the system and initiate fracturing of the formation. When the water is flowing freely at the fracture pressure, the slurry is introduced into the well. Slurry injection continues until an entire batch of slurried material has been injected. At the end of this batch, additional water is injected to flush solids from the well bore, and then pumping is discontinued. The pressure in the formation will gradually decline as the liquid portion of the slurry bleeds off over the next few hours, 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 occurs continually and storage space is inadequate to operate in a daily batch manner, injection must occur continuously as new wells are drilled. Most other injection jobs are designed to inject intermittently. They inject for several hours each day, allow the formation to rest overnight, and then repeat the cycle on the following day or a few days later.
Slurry injection activities at onshore locations are generally subject to the requirements of the Underground Injection Control (UIC) program. The UIC program is administered by the U.S. Environmental Protection Agency but can also be delegated to state agencies (see the Regulatory section of this website for the specific EPA and state agency requirements for the UIC program). At offshore locations, the UIC program does not apply because underground sources of drinking water are not present. The Bureau of Ocean Energy Management, Regulation and Enforcement issues guidelines for injection and approves slurry injection on a case-by-case basis.
Geologic Conditions That Favor Slurry Injection
Different types of rocks have different permeability characteristics. Although rocks appear solid, they are made up of many grains or particles that are bound together by chemical and physical forces. Under the high pressure found at depths of several thousand feet, water and other fluids are able to move through the pores between particles. Some types of rock, such as clays and shale, consist of very small grains, and the pore spaces between the grains are so tiny that fluids do not move through them very readily. In contrast, sandstone is made up of cemented sand grains, and the relatively large pore spaces allow fluids to move through them much more easily.
Slurry injection relies on fracturing, and the permeability of the formation receiving the injected slurry is a key parameter in determining how readily the rock fractures, as well as the size and configuration of the fracture. When the slurry is no longer able to move through the pore spaces, and the injection pressure continues to be applied, the rocks will crack or fracture. Continuous injection typically creates a large fracture consisting of a vertical plane that moves outward and upward from the point of injection. Intermittent injection generates a series of smaller vertical planes that form a zone of fractures around the injection point. Fractures that extend too far vertically or horizontally from the point of injection can intersect other well bores, natural fractures or faults, or drinking water aquifers. This condition is undesirable and should be avoided by careful design, monitoring, and surveillance.
Most annular injection jobs inject into shale or other low-permeability formations, and most dedicated injection wells inject into high-permeability sand layers. Regardless of the type of rock selected for the injection formation, preferred sites will be overlain by formations having the opposite permeability characteristics (high vs. low). When available, locations with alternating sequences of sand and shale are good candidates to contain fracture growth. Injection occurs into one of the lower layers, and the overlying low-permeability layers serve as fracture containment barriers, while the high-permeability layers serve as zones where liquids can rapidly leak off.
Database of Slurry Injection Jobs
Argonne National Laboratory developed a database with information on 334 injection jobs from around the world (part of Veil and Dusseault 2003). The three leading areas representing slurry injection in the database are Alaska (129 records), Gulf of Mexico (66 records), and the North Sea (35 records). Most injection jobs included in the database feature annular injection (296, or more than 88%), while the remainder (36 or 11%) used dedicated injection wells. These figures reflect the large number of annular injection jobs reported for Alaska.
Most injection jobs were conducted at depths shallower than 5,000 feet; many occurred in the interval between 2,501 and 5,000 feet. The shallowest injection depth reported was 1,246 to 1,276 feet in Indonesia, and the deepest was 15,300 feet at an onshore well in Louisiana.
The reported injection rates range from 0.3 bbl/minute to 44 bbl/minute. The reported injection pressures range from 50 pounds per square inch (psi) to 5,431 psi.
Most wells in the database were used to inject drill cuttings. Many were also used to inject other types of oil field wastes, including produced sands, tank bottoms, oily wastewater, pit contents, and scale and sludge that contain naturally occurring radioactive material (NORM). The table notes the number of records that reported injectate volumes within specified ranges. The data show that more than 83% of the injection jobs in the database involved less than 50,000 bbl of slurry. The largest job reported in the database involved more than 43 million bbl of slurry injected in several wells associated with a dedicated grind-and-inject project at Prudhoe Bay on the North Slope of Alaska.
Problems were reported in fewer than 10% of the database records. The most common problem was operations-related (e.g., plugging of the casing or piping because solids had settled out during or following injection; excessive erosion of casing, tubing, and other system components caused by pumping solids-laden slurry at high pressure). Environmental problems associated with slurry injection are rare but are of much greater concern. Few documented cases of environmental damage caused by slurry injection exist. Unanticipated leakage to the environment not only creates a liability to the operator, but also generally results in a short-term to permanent stoppage of injection at that site. Several large injection jobs have resulted in leakage to either the ground surface or the sea floor in the case of offshore wells. The most likely cause of these leakage events is that the fracture moved upward and laterally from the injection point and intersected a different well that had not been properly cemented or a natural geologic fault or fracture. Under the high downhole pressure, the injected fluids seek out the pathway of least resistance. If cracks in a well's cement job or geological faults are present, the fluids may preferentially migrate upward and reach the land surface or the sea floor. In situations involving closely-spaced wells, the potential for communication of fluids between wells should be carefully evaluated.
Various examples taken from the literature provide a range of cost comparisons for using oil-based muds and injecting the cuttings, using synthetic-based muds and discharging the cuttings, and hauling drilling wastes to shore for disposal. Although many of the references show that slurry injection is the most cost-effective option at the studied site, no single management option is consistently identified as the least or the most costly. This confirms the importance of conducting a site-specific cost-benefit analysis. In addition to the economic considerations of the initial disposal well design, it is useful to conduct a thorough evaluation of the fracturing and injection well design to insure that long-term mechanical isolation can be achieved in the proposed injection zone.
Three factors are critical when determining the cost-effectiveness of slurry injection:
(1) The volume of material to be disposed of – the larger the volume, the more attractive injection becomes in many cases. The ability to inject onsite avoids the need to transport materials to an offsite disposal location. Transportation cost becomes a significant factor when large volumes of material are involved. In addition, transporting large volumes of waste introduces safety and environmental risks associated with handling, transferring, and shipping. Transportation also consumes more fuel and generates additional air emissions.
(2) The regulatory climate – the stricter the discharge requirements, the greater the likelihood that slurry injection will be cost-effective. If cuttings can be discharged at a reasonable treatment cost, then discharging is often the most attractive method. Regulatory requirements that prohibit or encourage slurry injection play an important role in the selection of disposal options.
(3) The availability of low-cost onshore disposal infrastructure – several disposal companies have established extensive networks of barge terminals along the Louisiana and Texas coasts to collect large volumes of wastes brought to shore from offshore Gulf of Mexico platforms. They subsequently dispose of them through either subfracture injection or placement into salt caverns at onshore locations. Through the economy of scale, the onshore disposal costs are not high, and much of the offshore waste that cannot be discharged is brought to shore and disposed of at these facilities. Most other parts of the world do not have an effective, low-cost onshore infrastructure. Thus, in those locations, onshore disposal is often a greater environmental and human health risk than the onsite disposal options.
Argonne, 2003, "An Introduction to Slurry Injection Technology for Disposal of Drilling Wastes," brochure prepared by Argonne National Laboratory for the U.S. Department of Energy, Office of Fossil Energy, National Petroleum Technology Office, September, 20 pp.
Puder, M.G., B. Bryson, and J.A. Veil, 2003, "Compendium of Regulatory Requirements Governing Underground Injection of Drilling Wastes," prepared by Argonne National Laboratory for the U.S. Department of Energy, Office of Fossil Energy, National Petroleum Technology Office, February.
Veil, J.A., and M.B. Dusseault, 2003, "Evaluation of Slurry Injection Technology for Management of Drilling Wastes," prepared by Argonne National Laboratory for the U.S. Department of Energy, Office of Fossil Energy, National Petroleum Technology Office, September.