A waste management plan documenting the waste strategy, storage (including facilities and locations) and handling procedures should be developed and should include a clear waste tracking mechanism to track waste consignments from the originating location to the final waste treatment and disposal location.
Significant additional waste stream specific to onshore oil and gas development activities may include:
- Drilling fluids and drilled cuttings
- Produced sand
- Completion and well work-over fluids
- Naturally occurring radioactive materials (NORM)
Drilling fluids and drilled cuttings
The major source of pollution in the drilling system is the drilling mud and cuttings from the bit.
Drill mud: Drill mud (also called cutting mud) is a complex colloidal mixture of water, bentonitic clays, chemical additives, and trace amounts of oil from cuttings of the hydrocarbon-bearing zones. This mud serves several purposes in oil drilling as it is circulated down the inside of the rotating drill from the surface and backup the annular space between the drill pipe and the drilled hole. At the drill bit/rock interface, it cools and lubricates the cutting action. As it flows up the annular space, it lifts rock chips which can then be screened out at the surface. Most important, the column of mud creates hydrostatic pressure which keeps pressurized oil or gas from being expelled uncontrollably (a “blowout”).
Much of this drill mud is recycled, but after repeated use it picks up fine rock particles and water soluble subsurface minerals until it is no longer economically practical to recondition it. The colloidal mass can then be separated from the water either by centrifugal processes or by simply allowing it to settle in a pit. The remaining fluid is then disposed off by deep injection. Much progress has been made in the last decade in the employment of low toxicity mud additives.
Drill cuttings: Drill cuttings are the pieces of rock and soil removed from the ground as a drill bit cuts a hole for a well. The volume of rock cuttings produced from drilling is primarily a function of the depth of the well and the diameter of the wellbore. It has been estimated that between 0.2 barrels and 2.0 barrels (8.4 and 84.0 gallons) of total drilling waste is produced for each vertical foot drilled.
Drilling operations also produce air emissions, such as exhaust from diesel engines and turbines that power the drilling equipment. The air pollutants from these devices will be traditionally associated with combustion sources, including nitrogen oxides, particulates, ozone, and CO. Additionally, hydrogen sulfide may be released during the drilling process.
The process of well completion may produce waste of which the most prominent is stimulation. Unused hydrochloric acid must be neutralized if acid stimulation is being used, and paraffins and any other dissolved materials brought to the surface from the formation must also be disposed. In addition, solid wastes such as waste cement and metal casing may remain from the casing process.
The primary functions of drilling fluids used in oil and gas field drilling operations include removal of drilled cuttings (rock chippings) from the wellbore and control of formation pressures. Other important functions include sealing permeable formations, maintaining wellbore stability, cooling and lubricating the drill bit, and transmitting hydraulic energy to the drilling tools and bit. Drilled cuttings removed from the wellbore and spent drilling fluids are typically the largest waste streams generated during oil and gas drilling activities.
Numerous drilling fluid systems are available, but they can generally be categorized into one of two fluid systems:
- Water-Based Drilling Fluids (WBDF): The continuous phase and suspending medium for solids (or liquid) is water or a water miscible fluid. There are many WBDF variations, including gel, salt-polymer, salt-glycol, and salt-silicate fluids;
- Non-Aqueous Drilling Fluids (NADF): The continuous phase and suspending medium for solids (or liquid) is a water immiscible fluid that is oil-based, enhanced mineral oil-based, or synthetic-based.
- Diesel-based fluids are also available, but the use of systems that contain diesel as the principal component of the liquid phase is not considered current good practice.
Typically, in most drilling fluids the solid medium used is barite (barium sulfate) for weight, with bentonite clays as a thickener. Drilling fluids also contain a number of chemicals that are added depending on the downhole formation conditions.
Drilling fluids are circulated downhole and routed to solids control system at the surface facilities where fluids can be separated from the cuttings so that they may be recirculated downhole leaving the cuttings behind for disposal. These cuttings contain a proportion of
residual drilling fluid. The volume of cuttings produced will depend on the depth of the well and the diameter of the hole sections drilled.
The drilling fluid is replaced when its rheological properties or density of the fluid can no longer be maintained or at the end of the drilling program. These spent fluids are then contained for reuse or disposal (NADFs are typically reused).
Feasible alternatives for the treatment and disposal of drilling fluids and drilled cuttings should be evaluated and included in the planning for the drilling program.
Alternative options may include one, or a combination of the following:
- Injection of the fluid and a mixture of cuttings into a dedicated disposal well
- Injection into the annular space of a well
- Storage in dedicated storage tanks or lined pits prior to treatment, recycling, and / or final treatment and disposal
- On-site or off-site biological or physical treatment to render the fluid and cuttings non-hazardous prior to final disposal using established methods such as thermal desorption in an internal thermal desorption unit to remove NADF for reuse, bioremediation, land farming, or solidification with cement and/or concrete. Final disposal routes for the non-hazardous cuttings solid material should be established, and may include use in road construction material, construction fill, or disposal through landfill including landfill cover and capping material where appropriate. In case of land farming it should be demonstrated that sub-soil chemical, biological, and physical properties are preserved and water resources are protected;
- Recycling of spent fluids back to the vendors for treatment and re-use.
- Consider minimizing volumes of drilling fluids and drilled cuttings requiring disposal by:
- Change out and minimizing the amount of residual fluid on drilled cuttings;
- Use of slim-hole multilateral wells and coiled tubing drilling techniques, when feasible, to reduce the amount of fluids and cuttings generated.
- Use of high-efficiency solids control equipment to reduce the need for fluid
- Pollution prevention and control measures for spent drilling fluids and drilled cuttings should include:
- Minimizing environmental hazards related to residual chemicals additives on discharged cuttings by careful selection of the fluid system.
- Careful selection of fluid additives taking into account the technical requirements, chemical additive concentration, toxicity, bioavailability and bioaccumulation potential;
- Monitoring and minimizing the concentration of heavy metal impurities (mainly mercury and cadmium) in barite stock used in the fluid formulation.
- The construction and management measures for surface storage or disposal pits should be applied to cuttings and drilling fluid pits. For drilling pits, pit closure should be completed as soon as practical, but no longer than 12 months, after the end of operations. If the drilling waste is to be buried in the pit following operations (the Mix-Bury-Cover disposal method), the following minimum conditions should be met:
- As far as possible, the pit contents should be dried out
- If necessary, the waste should be mixed with an appropriate quantity of subsoil (typically three parts of subsoil to one part of waste by volume)
- A minimum of one meter of clean subsoil should be placed over the mix
- Top soil should not be used but it should be placed over the sub-soil to fully reinstate the area.
- The pit waste should be analyzed and the maximum lifetime loads should be calculated. A risk-based assessment may be necessary to demonstrate that internationally recognized thresholds for chemical exposure are not exceeded
Produced sand originating from the reservoir is separated from the formation fluids during hydrocarbon processing. The produced sand can be contaminated with hydrocarbons, but the oil content can vary substantially depending on location, depth, and reservoir characteristics. Well completion should aim to reduce the production of sand at source using effective downhole sand control measures.
Produced sand should be treated as an oily waste, and may be treated and disposed off along with other oil contaminated solid materials (e.g., with cuttings generated when NADFs are used or with tank bottom sludge).
If water is used to remove oil from produced sand, it should be recovered and routed to an appropriate treatment and disposal system (e.g., the produced water treatment system when available).
Completion of well work-over fluids
Completion and well work-over fluids (including intervention and service fluids) can typically include weighed brines, acids, methanol and glycols, and other chemical systems. These fluids are used to clean the wellbore and stimulate the flow of hydrocarbons, or simply used to maintain downhole pressure. Once used these fluids may contain contaminants including solid material, oil, and chemical additives. Chemical systems should be selected with consideration of their volume, toxicity, bioavailability, and bioaccumulation potential. Feasible disposal options should be evaluated for these fluids. Alternative disposal options may include one, or a combination of, the following:
- Collection of fluids if handled in closed systems and shipping to the original vendors for recycling
- Injection to a dedicated disposal well, where available
- Inclusion as part of the produced water waste stream for treatment and disposal. Spent acids should be neutralized before treatment and disposal
- On-site or off-site biological or physical treatment at an approved facility in accordance with the waste management plan.
Naturally occurring radioactive materials (NORM)
Depending on the field reservoir characteristics, NORMs may precipitate as scale or sludge in process piping and production vessels. Where NORM is present, a NORM management program should also be developed so that appropriate handling procedures are followed. If removal of NORM is required for occupational health reasons, disposal options may include:
- canister disposal during well abandonment;
- deep well or salt cavern injection;
- injection into the annular space of a well or disposal to landfill in sealed containers.
Sludge, scale, or NORM-impacted equipment should be treated, processed, or isolated so that potential future human exposures to the treated waste would be within internationally accepted risk-based limits. Recognized industrial practices should be used for disposal. If waste is sent to an external facility for disposal, the facility must be licensed to receive such waste.
The following principles should be followed for chemicals used in the onshore oil and gas sector:
- Use chemical hazard assessment and risk management techniques to evaluate chemicals and their effects. Selected chemicals should have been tested for environmental hazards;
- Select chemicals with least hazard and lowest potential environmental and / or health impact, whenever possible;
- Use of Ozone Depleting Substances should be avoided.