Recovering Drilling Muds and Drill Cuttings for Beneficial Reuse
Drilling mud—also called drilling fluid—is an essential component of the drilling process. Drilling mud aids in the process of drilling a borehole into the earth. Such holes are drilled for oil and gas extraction, core sampling and a variety of other purposes. The fluid is used to lubricate the drill bit and transport the drill cuttings to the surface . Drill cuttings are broken bits of solid material that are produced as the drill bit breaks the rock. As it circulates up from the drill bit, the drilling mud carries drill cuttings up to the surface, where the mud and the cuttings are separated.
Drill cuttings vary in size and texture, from a very fine silt to gravel. After the drilling process is completed, these small rock cuttings can be taken onshore for treatment or re-injected into the wells with the proper permits. The complete removal of drill cuttings requires a variety of processes, including cutting, pumping, dredging or the use of specially designed underwater vehicles to recover the waste cuttings. Today, there are recovery systems capable of achieving more than 90 percent recovery of drill cuttings.
There are three main types of drilling mud: water-based, oil-based and synthetic-based. The synthetic-based muds are more frequently used because they have less environmental impact and are quicker to biodegrade than water- and oil-based fluids. Drilling fluids are made up of a base fluid, such as mineral oil or synthetic oil-based compounds; weighting agents, such as barite; clay and stabilizing organic material, such as lignite. The main component of drilling mud is bentonite clay, similar to the material that is used in cat litter and gypsum fillers. The clay is mixed with the water, oil or synthetic base, and several compounds are added to the mixture, such as cellulose polymers and barium sulfate to increase viscosity.
Water-based mud (WBM) usually consists of bentonite clay, or gel, mixed with additives such as barite, calcium carbonate or hematite. Various thickeners, such as guar gum, glycol or starch, are added to WBMs to influence the viscosity of the fluid. Other common additives include lubricants, shale inhibitors and fluid loss additives, which are used to control the loss of drilling fluids into permeable formations.
Oil-based mud (OBM) has a petroleum product, such as diesel fuel, as its base fluid and diesel, mineral oil or some other form of oil as its continuous phase with water as its dispersed phase. OBMs may also include barite, clays, emulsifiers, lignite and lime. These types of muds are capable of withstanding greater heat without breaking down and can be used to improve lubricity and reduce friction.
Synthetic-based mud (SBM) uses a synthetic oil as its base fluid. A synthetic liquid forms the continuous phase while brine serves as the dispersed phase. SBMs are most often used on offshore rigs because they have the same properties as OBMs, but the toxicity of the fluid fumes is much lower.
Throughout the drilling process, drilling mud is recirculated, which helps decrease waste by reusing as much mud as possible. When the drilling process is finished, the drilling waste must be disposed of in some way. Drilling mud and drill cuttings, however, both contain traces of hydrocarbons—flammable, organic chemical compounds, which occur naturally in fossil fuels, such as oil and natural gas. Hydrocarbons have varying densities and viscosities, which influence how easily they can be removed from a material.
Most water-based muds are simply disposed of after the drilling job is completed, but many oil- and synthetic-based muds can be recycled. The main obstacle preventing the reuse of drill cuttings is the chemical characteristics of the cuttings, especially hydrocarbons. Through the process of thermal desorption, however, drill cuttings can be treated and beneficially reused.
The average well in the Bakken, North Dakota region of the U.S. generates about 26 semi-truck loads of cuttings that require disposal. In 2012, more than one million tons of drill cuttings were disposal of in special waste landfills in the state of North Dakota alone. The volume of waste from oil and gas exploration gas companies looking for a new disposal solution.
Fortunately, waste streams that are high in hydrocarbons, such as OBMs, are excellent candidates for thermal treatment technologies, such as thermal desorption. In the thermal desorption process, heat is applied either directly or indirectly to drilling wastes to vaporize the volatile and semi-volatile components without incinerating or damaging the soil. One of the best ways to thermally treat drilling wastes is with an indirect heated rotary kiln. A rotary kiln uses hot exhaust gases from fuel combustion to heat the wastes and remove the hydrocarbons, allowing the drilling muds and drill cuttings to be put to beneficial reuse.
In an indirect heated rotary thermal desorption system, the drilling wastes are fed to the feed hopper, which is mounted on a pugmill and equipped with a weight-controlled feed valve. From the pugmill, the waste materials are transferred via a transfer auger to the feed auger, which conveys the waste into the primary thermal desorption unit and maintains a seal or airlock for the system.
The indirect fired rotary kiln is capable of reaching operating temperatures of up to 1,000 degrees Fahrenheit. The kiln operates in an oxygen-deficient environment and under a slightly negative atmosphere. Vapors from the contaminated material are pulled out of the system in a countercurrent direction to the material flow. The clean, dried material is charged to a water-cooled discharge auger that rehydrates and cools the soil. The steam is extracted through a quench stack that removes any particulate from the H2O vapor stream before discharging it into the atmosphere.
Recovered drill cuttings have numerous uses, such as stabilizing surfaces that are more vulnerable to erosion, like roads and drilling pads. Cuttings can also be used as aggregate or filler in concrete, brick or block manufacturing. The U.S. Department of Energy has even researched the possibility of using drill cuttings as a substrate for restoring coastal wetlands, and some trials have been conducted in the United Kingdom using cuttings as power plant fuel .
Recovering drill cuttings and drilling muds is often practical and cost-effective and is an environmentally sustainable process. Recycling and reusing drill cuttings can help companies save money on disposal costs, reduce truck traffic and save money on building well pads and roads.
Recycling drill cuttings and drilling muds can help operators meet disposal regulations, and the proper disposal of such waste prevents the contamination of water supplies and the soil. Through this process, muds and cuttings may be reused, saving money and contributing to the health of the environment.