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Solids Disposal and Monitoring

The process of drilling oil and gas wells produces large volumes of drill cuttings. These drill cuttings are broken bits of solid material removed from the wellbore in the process of drilling a borehole. Drill cuttings vary in composition depending on the geological composition of the subsurface, the type of drilling fluid (oil based, synthetic based or water based) and additives used. When drilling a well, drill cuttings return to the surface with drilling fluid (mud) circulated through the wellbore. To minimize environmental impacts, drill cuttings are removed from the drilling fluid with the solids control system, separated from the mud, ground to fine particles, and mixed into a cuttings slurry.

Mudloggers collecting samples out of the shaker

Regulatory requirements vary greatly for the disposal and recycling of drill cuttings and solid wastes generated from oil and gas operations. At the federal level, solid wastes from oil and gas development fall under the Resource Recovery and Conservation Act of 1976 Subtitle D for nonhazardous solid waste.

Due to the variation of this subclass of waste, the U.S. Environmental Protection Agency (EPA) has not published specific regulations for the management of or disposal of these wastes. However, different state, local, and other federal agencies have developed specific and, in many cases, very stringent regulations regarding the disposal of wastes generated during oil and gas drilling operations.

Once at the surface, drill cuttings present both short-term and long-term storage and disposal challenges. The mobile (leachable) fractions of the residues, if not controlled, may eventually enter the food chain of animals or humans. Also, drilling wastes and other solid wastes may generate emissions of volatile organic compounds resulting in violation of air pollution control regulations.

During onshore operations, drill cuttings may be stored temporarily onsite in earthen pits near the drilling rig. Many state and local regulatory agencies require pit liners to prevent the potential contamination of groundwater sources. With a closed loop fluid management system pits are replaced by a series of tanks that hold drilling cuttings that have passed through several shaker screens prior to being processed through a hydro clone and one or more centrifuges.

When the contents of the drill cuttings do not present an environmental risk, regulatory agencies may allow cuttings to remain in the onsite pit, or to be placed in a cuttings trench (for drill cuttings from a closed loop system with reduced moisture content) and then covered with native soils for long-term disposal. Volume and toxicity are two environmental risk criteria for evaluating the content of drill cuttings.

In the past, oilfield pits were typically used for both the temporary storage and final disposal. Such practices often resulted in surface damage due to excessive concentrations of buried hydrocarbons or permanent disposal of produced brines in pits. Modern technology of pit closure involves partial removal of waste from the pit, separation of liquids from solids and treatment of these two phases prior to their final disposal. These treatment methods employ dilution, chemical alteration, and biodegradation mechanisms to reduce the concentrations of pollutants to acceptable levels consistent with intended land use.1 The technique combines the treatment with final disposal of salts, petroleum hydrocarbons and metals. The use of a closed loop fluid management system integrates many of these steps into a flowthrough process. Laboratory analysis of waste composition must be made for each pit or trench in order to evaluate levels of contamination.

Some considerations for onsite burial include the pit or trench depth, soil type, depth to groundwater and proximity to surface water. Burial is generally not recommended in areas with shallow groundwater. Soils with low permeability such as clay may help contain contamination and pit contents within the disposal zone. In addition to low permeable soils, a liner can help to isolate drill cuttings from subsurface soils and groundwater. Operators should select long term storage and disposal sites and conduct grading to ensure that the final grade limits pooling of water above the location of the pit or trench as pooling may increase the risk of leaching contaminants into groundwater.2

An alternative to burial onsite is disposal at an offsite commercial landfill or private centralized waste management facility permitted by a state or federal agency. In some geographic locations, such as in West Texas, New Mexico and Pennsylvania, offsite disposal landfills dedicated to drill cuttings have emerged and provide another offsite disposal option. These landfills generally act as landfarms and consist of a series of liners and geologic barriers, such as clay, that separate the cuttings from any groundwater.2

Land treatment is another disposal option for the management of drill cuttings and pit solids. Land treatment can be performed using techniques such as landfarming and land application or land spreading. Landfarming is the controlled and repeated application of waste to the soil surface, where the naturally occurring micro-organism biodegrade hydrocarbon constituents and other processes, dilute, and attenuate metals, and transform other waste constituents. Landfarming can be a relatively low-cost approach to drilling waste management, however, it does require significant sampling and analysis and ongoing monitoring and may require a site specific permit and surface owner approval.  

Land application, which is also sometimes referred to as land spreading, is the one-time application of waste to the soil surface. Land application must balance the addition of waste against the soil’s capacity to assimilate the waste constituents without destroying soil integrity or causing adverse environmental impacts. Land application as with landfarming requires significant sampling and analysis and monitoring and may also require a specific permit plus surface owner approval.

Other strategies for the management of drill cuttings include recycling or reuse. Road spreading is a technique in which operators reuse drill cuttings to stabilize roads and drilling pads. This technique is not permitted in all areas and generally requires a permit or other regulatory approval. The re-use of drill cuttings for construction of lease roads and well pads may significantly impede interim and final reclamation. When employing this disposal method the operator will need to conduct significant testing and analysis of the drill cuttings and collect samples to identify background soil agronomic and fertility levels. Treating drill cuttings at recycling facilities constructed specifically for this purpose may be one way to increase the possibilities for reusing drill cuttings. Treated drill cuttings also appear as aggregate in concrete, brick and block manufacturing.2

Injection of cuttings into a permitted well or cavern, also known as reinjection, is another method for cuttings disposal, especially in remote areas. 2 In this process, drill cuttings are removed from drilling fluid with the solids control system, separated from mud, ground to fine particles and mixed into a cuttings slurry that is re-injected into a well or cavern.

Incineration is also a method used to dispose of drill cuttings. In this process, thermal systems are used to burn the drill cuttings. However, due to the resulting air emissions, safety concerns of the heat source, and high efficiency required, this disposal method is not widely used and is typically only applied in offshore operations.

  1. Orszulik, S. T. (Ed.). (2008). Environmental technology in the oil industry. Springer.
  2. Drill Cuttings Disposal – An Overview of the 5 Most Common Methods

Images: “Landfill” by Amy Dalrymple, Forum News Service; “Mudloggers collecting samples out of the shaker” by Joshua Doubek via Wikimedia Commons licensed under CC BY-SA 3.0