Drilling mud—also called drilling fluid—aids in drilling a borehole for oil and gas extraction, core sampling and other purposes. The mud is used to lubricate the drill bit and transport the drill cuttings to the surface.1 Drill cuttings are solids particles that are produced as the drill bit breaks the rock. As it circulates up from the drill bit, the drilling mud carries these drill cuttings to the surface, where the mud and the cuttings are separated. Drill cuttings vary in size and texture, ranging from fine silt to gravel. After drilling is completed, these small rock cuttings can be treated or re-injected into wells with the proper permits. For offshore operations, these cuttings must be taken onshore for treatment or disposal. The complete treatment and removal of drill cuttings requires several processes, including cutting, pumping, dredging or the use of specially designed underwater vehicles (offshore) to recover the waste cuttings. Today, some systems can recover more than 90 percent of drill cuttings.
Drilling Mud Bases
The first step in recycling drilling fluid and cuttings removal is understanding the characteristics of each mud type. Drilling mud can be divided into three main categories: water-based, oil-based and synthetic-based. Synthetic-based muds are used more frequently because they have less detrimental environmental impact and biodegrade more quickly than water- and oil-based fluids. Drilling fluids are made up of the following:
- 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, which is similar to the material used in cat litter and gypsum fillers. The clay is mixed with the water, oil or a synthetic base. Several compounds are added to the mixture to increase its viscosity, such as cellulose polymers and barium sulfate.
Water-based mud (WBM) usually consists of bentonite clay or gel that is mixed with additives such as barite, calcium carbonate or hematite. Thickeners—such as guar gum, glycol or starch—are added to WBMs to influence the fluid’s viscosity. Other common additions are 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 central fluid. It includes diesel, mineral oil or another type of oil as its continuous phase and water as its dispersed phase. OBMs may also include barite, clays, emulsifiers, lignite and lime. These mud types withstand higher temperatures without breaking down and can improve the lubricity, reducing friction.
Synthetic-based mud (SBM) uses 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 during drilling operations 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. This reuse of mud helps decrease waste. When the drilling process at a location is completed, operators must dispose of the drilling waste. Drilling mud and drill cuttings, however, contain traces of hydrocarbons. Hydrocarbons have varying densities and viscosities, which influence how easily they can be removed from materials. An average well in the Bakken region in North Dakota generates about 26 semi-truck loads of cuttings that require disposal. In 2012, more than one million tons of drill cuttings were disposed of in special waste landfills in North Dakota alone. The volume of waste from oil and gas exploration has companies looking for new disposal solutions.
Most WBMs are disposed of after the drilling job is completed, but many OBMs and SBMs can be recycled. The main obstacle to the reuse of drill cuttings is the chemical characteristics of the cuttings, especially the presence of hydrocarbons. Through the process of thermal desorption, drill cuttings can be treated and beneficially reused. Fortunately, OBMs and other waste streams that have high hydrocarbon content 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 waste is with an indirect-fired rotary kiln. A rotary kiln uses hot exhaust gases from fuel combustion to heat the wastes and remove the hydrocarbons, allowing the drilling mud and drill cuttings to be safely reused. In an indirect-fired rotary thermal desorption system (see Figure 1), the drilling wastes are supplied to a feed hopper, which is mounted on a pugmill and equipped with a weight-controlled feed valve. From the pugmill, the materials are moved through the 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 reaches operating temperatures of 1,000 F. The kiln runs 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 moved to a water-cooled discharge auger that rehydrates and cools the soil. The contaminated steam is extracted through a quench stack that removes any particulates from the water vapor stream before discharging it to the atmosphere.
The treated drill cuttings have numerous uses, such as stabilizing roads, drilling pads and other surfaces that are vulnerable to erosion. The 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 possible use of drill cuttings as a substrate for restoring coastal wetlands. In addition, some trials using cuttings as power plant fuel have been conducted in the U.K.2 Recovering drill cuttings and drilling muds is often practical and cost-effective and is an environmentally sustainable process. Treating, recycling and reusing drill cuttings can help companies save money on disposal costs, reduce truck traffic, and save money on well pad and road construction. Recycling these materials can help operators meet disposal regulations, and the proper disposal of such waste prevents the contamination of water supplies and the soil.