Oil and Gas Waste Disposal Options
When soils become contaminated with spilled chemicals, hydrocarbons and other waste materials there are several methods of disposing or treating these wastes. The main disposal method is through burial, either on or off site. Treatment occurs via bioremediation (using micoorganisms to convert toxic compounds into less toxic forms); or thermal technologies (using high temperatures to reclaim or destroy hydrocarbon-contaminated material).
The following types of waste disposal and treatment sites are utilized for oil and gas wastes:
- Land Spreading
- Thermal Technologies
- Undergound (Slurry) Injection
- Salt Caverns
The types of oil- and gas-field wastes dumped into pits include: drilling wastes; produced water; and production fluids and wastes. Many of these fluids are poisonous to living organisms.
Lined pits for disposal and storage are sometimes used, but mud, drill cuttings and other materials are frequently discharged into unlined pits,allowing potentially toxic substances to seep into the ground. If improperly fenced, these pits can be a hazard for livestock and wildlife. While it is common for oil and gas companies to drain off fluids from drilling mud pits, it is very common for companies to simply bury the remaining solids in place or spread them on the lease site.
When discharged into unlined pits the toxic substances in pits can leach directly into the soil and may contaminate groundwater. Lined pits can also lead to pollution via ruptures in liners or by overflowing the pit area. These events can result in soil and water contamination, which can have a negative effect on both human and ecosystem health. Read about groundwater contamination from pits in New Mexico.
Landfills are engineered earthen impoundments used for the permanent disposal of various types of wastes (e.g., municipal, hazardous, and industrial). Drilling wastes and other oil field wastes are disposed of in landfills that are permitted to accept oilfield wastes. An operator that has a large number of wellsites in an are may open and operate its own private landfill to dispose of its wastes. In other cases, commercial operations receive wastes from multiple operators. In other cases, oil companies with a large amount of drilling activity in an area may construct and operate private landfills.
In order to conmay be lined with clay or synthetic liners, and systems established to capture fluids that leach from the wastes. Airborne wastes (e.g., particulate matter and dust from the contaminates soils) may be controled using dust suppressants, or by covering wastes with a layer of clean soil or other inert material.
Landfarms are not permanent waste disposal sites. Rather, they are sites designed to reduce the concentration of The oil and gas industry uses land farming to treat oily (hydrocarbon-rich) wastes. Land farming is the controlled and repeated application of wastes to the soil surface. Naturally occurring or introduced microorganisms in the soil break down the hydrocarbons.
Metals and salts are not broken down by the microorganisms, but some metals may be assimilated into the tissues of microorganisms, which can reduce the mobility of metals through the soil. If the concentrations of metals or salts are too high in the wastes, microbial activity may be inhibited, thus reducing the breakdown of hydrocarbons.
Bioremediation of hydrocarbons may be accomplished using a variety of tillage and composting techniques. In all cases, the breakdown of hydrocarbons is maximized by providing the prime conditions for microbial activity, which requires a proper balance of moisture, and nutrients, as well as soil oxygen (Read the Argonne National Laboratory's Fact Sheet on Bioremediation). Thus, it is often necessary to add water, nutrients and additional soil, and aerate the soil to enhance biodegradation of hydrocarbons. During periods of extended dry conditions, moisture control may also be needed to minimize dust. Application rates of water should be monitored and controlled to minimize the potential for runoff or leaching of contaminants.
Land spreading is typically a one-time application of wastes to an area of land. Wastes are spread on the land and incorporated into the upper soil zone (typically upper 6-8 inches of soil) to enhance hydrocarbon volatization and biodegradation. It is important that other constituents, such as metals, salts, acids not be present at levels that will sterilize or permanently impair the soil system.
Thermal technologies utilize high temperatures to destroy or remove hydrocarbons from waste materials. Depending on the final fate of the wastes, additional treatment may be needed to remove metals and salts. Thermal treatment technology generally occurs at a permanent or fixed facility, but some efforts are under way to develop mobile thermal treatment units.
There are two main types of thermal technologies:
- Incineration (e.g., rotary or cement kilns) destroys hydrocarbons by heating them to very high temperatures in the presence of air.
- Thermal desorption (e.g., indirect rotary kilns, thermal phase separation, thermal distillation) involves the application of heat to the wastes, to vaporize volatile and semivolatile hydrocarbons. These gases may be combusted to reduce the emission of toxic components, or condensed and separated to recover heavier hydrocarbons.
In order to inject oil and gas field wastes underground, solids are typically ground into small particles and mixed with water or another liquid to make a slurry. It is this slurry that is injected into underground formations (sometimes at pressures high enough to fracture the rock so that wastes can enter the formation; sometimes at lower pressures, e.g., when an underground formation has ample natural fractures).
There are two methods of injection:
- Annular Injection, where wastes are pumped into the space between two casing strings on an oil or gas well (known as the annulus). At the lower end of the outermost casing string, the slurry enters the formation. Typically, wells utilizing annular injection only inject wastes from a single well, and injection lasts only a few weeks or months.
- Disposal well injection, where wastes are injected down the disposal well to 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. These wells may inject wastes underground for months or years.
According to the Argonne National Laboratory, few documented cases of environmental damage caused by slurry injection exist, although several large injection jobs have resulted in leakage to either the ground surface or the sea floor. Argonne cautions that in situations involving closely-spaced wells, the potential for communication of fluids between wells should be carefully evaluated, as nearby wells can act as a conduit for injected wastes to flow to the surface. Under the high downhole pressure, injected fluids seek out the pathway of least resistance. If cracks in a well's cement job or geological faults are present, fluids may preferentially migrate upward and contaminate freshwater aquifers or daylight at the surface.
Underground salt deposits are found in many parts of the United States and worldwide, typically at depths of 500 to more than 6,000 feet below the surface. Salt caverns are created by dissolving the salt and removing the brine using a process called solution mining. The result is an underground cavern where the salt used to be.
Salt caverns have been used for several decades to store various hydrocarbon products, such as natural gas. More recently, their use as oilfield waste disposal sites has received increased attention. According to Argonne National Laboratory's information on salt caverns, as of the end of 2003 Texas was the only U.S. state to have issued permits for disposal of oil field wastes in salt caverns. Louisiana adopted cavern disposal regulations in May 2003 but has not yet permitted any disposal caverns. Several disposal caverns are also operated in Canada, and, in early 2004, Mexico announced that it was developing regulations for disposal of oil-based muds and cuttings in salt caverns.