Solids Control and Waste Management is the process of managing, treating and disposing of waste generated during the drilling a well in particular cuttings and drilling fluid (mud). Depending on the mud system used waste generated can be highly toxic. Appropriate Solids Control and Waste Management practices can significantly lower the amount of waste and its toxicity, as well as the negative effects waste, can have on the well and down-hole equipment.
In general, the waste management strategy is a stage ladder process, whereby reducing the waste at its source is the first option, followed by recycling, and finally treatment and disposal. The subcategory consists of two major areas: 1) Solids collection and control and 2) Waste Treatment and Disposal.
SOLIDS COLLECTION AND CONTROL - is a process of separating solid rock particles in the mud generated by drilling, cleaning the mud, and recovering the mud before returning it back to the system. Depending on the application and mud system used, solids collection and control equipment is comprised of:
- Mud Tanks - mud storage and mixing
- Shale Shaker - separates big solids, as a first stage separation
- Desander - separates medium-sized solids
- Desilter - separates small-sized solids
- Mud Cleaner - a combination of desander and desilter
- Vacuum degasser - separates the air from the mud
- Conveyance System (auger) - transports cuttings on the rig, normally offshore
- Decanter Centrifuge - high-speed & variable-speed centrifuges are required when drilling fluids requires further processing to provide a higher mud recovery rate and comply with stricter environmental standards. Decanter centrifuge can also recover up to 90% of the barite that can be re-used.
- Mud Cooler - to cool the drilling fluid, mostly used in oil-based mud and high-temperature applications
- Cutting dryer - ideally vertical, this device reduces liquid content in cuttings to below the 5% mark. Shakers, mud cleaners and centrifuges would leave cuttings with liquid content on between 10% to 15%.
- Dewatering Unit - allows removal of the fine solids (less than 5 microns) from the water-based drilling fluids, using chemical processes.
- Cuttings collection & transportation system - is a closed-loop system providing a vacuum or pneumatic powered conveyance to transport cuttings for a relatively long distance, e.g. from an offshore drilling rig to a supply vessel.
- Drill Cutting Boxes are small open containers/skips that are used to transport cuttings from drilling rig offshore to the shore base for further treatment and disposal. Depending on the drilling speed, solid equipment on the rig and distance to shore, the number of skips ranges from 150 to 350 per rig.
WASTE TREATMENT AND DISPOSAL is the process of management and disposal of waste generated by drilling fluid and drill cuttings, after the solids collection and control phase. Depending on the mud system used and initial treatment, the water content in cuttings and its toxicity may still be high and further treatment could be required, prior to the final disposal. This stage is not always present and there are instances when non-toxic waste is disposed of immediately. Oil-based mud and synthetic-based mud are considered to be toxic and must go through a treatment and disposal process. There are various options to treat and dispose of the waste and the selection criteria are driven by 1) regulations; 2) costs, 3) type of waste, and 4) what equipment is available. The options available are:
- Cuttings Re-injection (CRI) is used for re-injecting drill cuttings back to the reservoir through separate disposal wells, old non-producing wells or existing production wells. This process returns drill cuttings to its native environment and does not produce any pollution. This is the only on-site, permanent disposal method that is in full compliance with environmental standards. In certain applications, cuttings re-injection is a more cost-effective solution, when compared to skip-and-ship and onshore treatment and disposal.
- Thermal desorption is a process of indirectly heating the waste to increase the volatility of contaminants that frees them from the solids matrix and evaporates them from the cuttings. This, in turn, removes oil and other toxic materials from the processed cuttings and provides less than 0.1% of oil-on-cuttings content, to allow further disposal (landfill and land farming) or recycling (road spreading, construction materials or restoring coastal wetlands). Thermal treatment is the most effective technology to process the waste generated by oil-based mud systems. Current days, technology allows in-situ processing using TCC RotoMill technology with full disposal offshore, given the space and power capacity on the drilling rig allows the installation of the equipment. This completely eliminates the requirements of transporting waste to shore for further processing. Two major types of technology are used: 1) Thermomechanical, whereby friction is the source of heat and 2) Thermal Phase Separation, where propane, natural gas or diesel, combusts in the burners to heat up the cuttings.
- Fixation / Stabilization / Encapsulation used mainly with water-based mud. The technology converts liquid and slurry waste into dry and solid materials with high structural integrity. Materials and chemicals such as calcium oxide, cement, fly ash, mica, and lime are used to treat drill cuttings and encapsulate them. Stabilized drilling waste has been used as a material for road foundations and backfills for earthworks. Although there are some limitations, this method has been approved and successfully used in various locations. Because of the equipment footprint, it has never been used in offshore locations.
Dewatering is a chemical and mechanical process that is used to recover water from the mud and return it back to the drilling fluid.
The table below shows the various attributes and characteristics of available cuttings management strategies.
The cost of Solids Control and Waste Management is largely dependent on the type of mud system used and the life span of the facilities. For example, the costs of a dedicated thermal plant that can reach as much as US$20M can be spread over many wells.