Once the drilling mud has served its purpose, safe disposal becomes an issue. Savannah Cooper, Worldwide Recycling Equipment Sales, LLC, USA, explains why many in the oil and gas industry are turning to thermal desorption to treat and recycle drill cuttings.
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.
Types of drilling mud
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 stabilising 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.
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 US generates approximately 26 semi-truck loads of cuttings that require disposal. In 2012, more than one million tonnes of drill cuttings were disposed of in special waste landfills in the state of North Dakota alone. The volume of waste from oil and gas exploration companies is taking its toll on these special waste landfills, so drilling contractors have been 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. Thermal treatment uses high temperatures to reclaim or destroy hydrocarbon-contaminated materials. This process is an efficient treatment for destroying organics and reduces the volume and mobility of inorganics, such as metals and salts.
In the thermal desorption process, heat is applied either directly or indirectly to drilling wastes to vaporise 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.
Before the cuttings can be reused, it is necessary to ensure that the hydrocarbon content, moisture content, salinity and clay content are suitable for the intended use of the material. Drill cuttings are considered as contaminated soil until they are treated. If left untreated, the cuttings must be disposed of in a licensed solid waste landfill, but once treated with thermal desorption, the cuttings can be recovered and beneficially reused.
Recovered drill cuttings have numerous uses, such as stabilising 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. Once thermally treated, drill cuttings can be used as construction material in road pavements, bitumen or asphalt in cement manufacturing. Treated cuttings have previously been used as fill material and daily cover material at landfills. The US 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 UK using cuttings as power plant fuel.1
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.
Worldwide Recycling Equipment Sales, LLC in Moberly, Missouri, supplies a full range of equipment for thermal desorption processes. Its systems can be custom-designed specifically to suit individual project needs, and the company has previously designed thermal desorption systems capable of processing up to 10 tph of material.
Thermal desorption unit
The Vulcan® IDR 8440 is a new 40 ft long x 7 ft diameter COR-TEN indirect heated rotary thermal desorption unit. This system is designed to remediate soils and drilling muds contaminated with petroleum hydrocarbons. The Vulcan IDR 8440 comes equipped with a vapour recovery unit (VRU) to capture vapours from the primary thermal desorption unit (PTU). The system is capable of recovering drill cuttings or up to 100 MMBtu/hr of energy for reuse as fuel, reducing the total operating cost and improving total return from projects.
In the thermal desorption unit, the material is fed to the feed hopper, which is mounted on a pugmill and equipped with a weight-controlled feed valve. From the pugmill, the material is transferred via a transfer auger to the feed auger, which conveys the material into the PTU and maintains a seal or airlock for the system.
The indirect fired rotary kiln has four heat zones, and the operating temperature of the drum can reach 1000°F. The kiln operates in an oxygen-deficient environment and under a slightly negative atmosphere. Vapours from the contaminated material are pulled out of the system in a counter-current 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 vapour stream before discharging it into the atmosphere.
The vapour from the primary treatment unit is pulled into a high-efficiency quench scrubber, which operates at 250°F and uses oil as the quench liquid. The quench removes dust particulate and condenses the heavier hydrocarbons in the vapour stream. The condensed oil and sludge are pumped through a set of filters to an oil/water separator and then to the client’s storage tank.
The vapour not condensed in the quench scrubber is pulled through a shell and tube heat exchanger, reducing the temperature to 70°F. The vapour is then pulled into a knockout pot, which removes particulate with a demister pad. The resulting fluid – condensed water and oil – is pumped to an oil/water separator, then to the client’s holding tank.
A high-efficiency positive displacement blower pulls the non-condensable gases from the knockout pot. The gases are then circulated to a thermal oxidiser, where all remaining hydrocarbons are cracked and destroyed at temperatures of up to 2000°F with a residence time of more than two seconds. The heat from the oxidiser can be circulated back to the combustion chamber on the PTU to be used as a fuel source to supplement the burners and reduce overall operating costs.
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.
1. Drilling Waste Management Information System, web.ead.anl.gov/dwm/.
Adapted by David Bizley
Read the article online at: https://www.oilfieldtechnology.com/special-reports/21082014/treat-recycle-and-reuse/