In zero discharge and other highly environmentally regulated drilling operations, operators are continually seeking new and optimized drilling waste management technologies that offer enhanced environmental stewardship and enable them to safely maximize drilling efficiency and reduce operating cost.
Cuttings re-injection (CRI), thermal desorption and ship-to-shore transfer technologies are among common solutions adopted by operators to manage cuttings waste and comply with regulatory requirements. A number of surface containment and transfer systems are developed and deployed for collection and transportation of drill cuttings from rigs to shore for processing and disposal. These systems provide an ideal solution for zero-discharge operations, however, the increased need for crane use and rig space as well as limitations on the amount, distance and duration of cuttings transport are among the drawbacks of such systems.
Impact of Environmental Regulations on Non-Aqueous Drilling Fluids Cuttings Disposal
Although operating in offshore areas in the search of oil and gas presents great engineering challenges and creates risk of environmental events, the abundant supply of oil and gas in offshore fields is an opportunity for oil and gas operators that cannot be ignored. In 2015 offshore oil production accounted for approximately 30% of global production. More than 50 countries contributed to the 27 million barrel production, with major sources being Saudi Arabia, Brazil, Mexico, Norway and the United States.
Initially, there were few regulations governing the disposal of drilling fluid and drill cuttings in offshore operations. An historical account of regulation of marine pollution in general, including contributions from the oil and gas industry, was provided by Kashubsky (Kashubksy, 2006). Before the onset of environmental regulations, cuttings piles accumulated on offshore sea beds in the most active areas. The Gulf of Mexico and the North Sea accumulated massive oil-based and synthetic-based cuttings piles. Studies on the ecosystem impact of the cutting piles revealed major effects of the cuttings and drilling fluids that were correlated with hydrocarbon content. Mud chemicals and heavy metals from barite contributed to ecotoxicity. Another effect on the ocean environment was oxygen depletion due to the high organic loading on the ecosystem. For the North Sea, oil-based cuttings and associated fluids were the major source of hydrocarbons introduced to the marine environment. This source of hydrocarbon contamination was gradually reduced by environmental regulations for the North Sea from 1996 to 2000. For the United Kingdom Continental Shelf (UKCS), oil-based cuttings must be processed to reduce oil content to less than one percent by weight. Today, cuttings disposal in the North Sea is highly regulated and oil-on-cuttings must be below 1% oil by weight.
A similar pattern of new environmental guide lines was developed for the Gulf of Mexico (GOM). In 1993, the EPA created guidelines for offshore drilling and associated wastes including drilling fluids and cuttings. The current regulations, outlined in the latest National Pollution Discharge Elimination System Permit (NPDES), prohibit the discharge of mud and cuttings from diesel oil and mineral oil mud systems. Synthetic-based mud cuttings may be discharged if the synthetic content is reduced below 6.9 percent by weight. Synthetic-based muds must be made from a small list of pre-approved base oils. Approved synthetic hydrocarbons consist of a few versions of isomerized olefins.
Regulations that severely impacted offshore cuttings disposal pushed the industry to develop alternative disposal methods. These methods include injection, on-site thermal treatment and ship-to-shore strategies. In 2014, 68,000 tons of oil-based cuttings were shipped to land from the UK continental shelf.
Oil-Based and Synthetic Fluid Composition for Drilling Offshore
During drilling of oil and gas wells, drilling fluid waste and drill cuttings are the two major types of waste. The arrival of offshore environmental regulations impacted non-aqueous fluids more than aqueous fluids. The greatest impact was on the selection of base oil for the use of nonaqueous fluids. Before environmental regulations, diesel oil was used almost exclusively as the major liquid component of oilbased fluids, followed by first generation mineral oils. These first-generation mineral oils had significantly higher aromatic and polycyclic hydrocarbon content than the current generation of highly refined, lowaromatic base oils in drilling fluids. The current generation of low-aromatic mineral oils contains extremely low levels of aromatic hydrocarbons, sometimes less than 0.1 percent by weight. Most areas of major oil and gas drilling activity, including the North Sea, have selected highly refined mineral oils with the extremely low levels of aromatic hydrocarbons. The Gulf of Mexico is an exception where synthetic hydrocarbons are tailored to meet specific environmental requirements.
Oil-based and synthetic-based drilling fluids are brine-in-oil fluids with a non-aqueous continuous or external phase and an aqueous internal or dispersed phase. Environmental regulations have had a major impact on the composition of these fluids, especially those that have limited disposal allowance. Emulsifiers stabilize the emulsion. Other components include rheological additives to define the rheological profile and wetting agents to improve dispersability and oil wetting of weight material and incorporated solids. Fluid loss additives control fluid loss to the formation. Lost circulation products are incorporated to control low and medium to whole fluid loss to the formation. In many offshore areas environmental regulations have not banned used of diesel oil, but they do not allow any type of discharge of diesel oil-based fluids and associated cuttings. For most offshore regions low-aromatic mineral oils are used as the oil phase. For the Gulf of Mexico, extensive studies have resulted in selection of isomerized olefins as the approved synthetic base. This selection is based on biodegradation, toxicity and PHA content. Each base oil must be pre-approved based on these three criteria.
|Table 1—Common components of synthetic and mineral mud systems|
|Category||General Chemical Description||Approximate Weight Per Cent||Ecotoxicity Risk|
|Base Oil||Refined mineral oil or synthetic oil||35||hydrocarbons|
|Viscosifiers||Organophilic Clay||1.6||Quaternary Component|
|Emulsifiers||Fatty acid, amidoamine, imidoazolines||2.2||Surface Active|
|Internal Phase||Calcium Chloride Brine||15||electrolyte balance|
|Fluid Loss Additives||synthetic polymer||0.2||Residual synthetic polymers|
|Rheological Modifiers||Fatty acid derivative, polymers||0.5||Surface active|
|Wetting Agents||Sulfonated, synthetic polymers||0.5||Surface active|
|Weight Material||Barite, Hematite, Titanates, Manganese Oxides||44(for barite)||Trace metal|
|Lost Circulation Material||Calcium carbonate, graphite||variable||negligible|
Prevalent Conventional Solutions
Environmental regulations and economic pressures have caused the offshore drilling industry to respond with improved solids control methods and more efficient cuttings disposal options. Many offshore areas restrict discharge of oil-contaminated cuttings to zero or near-zero offshore disposal. Because of these discharge regulations, different systems and solutions have been developed for the collection and transportation of drill cuttings. These solutions include, and are not limited to, skip/cuttings boxes, screw conveyors/augers, vacuum units and dense phase blower systems.
Skip and cuttings boxes are a traditional solution for transporting waste to a disposal facility onshore. These boxes contain drill cuttings waste moved by gravity-fed delivery systems such as screw conveyors or discharge tanks/ hoppers or pneumatically from vacuum units known as dense phase blower units. Each box has a capacity of 5 to 11 tons. The number of cuttings boxes kept on the drilling platform is determined by deck space. Cranes are needed to move cuttings boxes. A box is normally fitted with slings attached to the pad eyes using shackles and moved from one stage to another stage by the crane. Multiple movements of the cutting boxes present a major safety hazard to rig personnel. Cranes are not permitted to operate when wind speeds exceed maximum wind speed guidelines. The wind speed limitation for cranes can lead to nonproductive time when drilling operations are shut down because all cuttings boxes are filled.
A screw conveyor/auger is a traditional low-cost method of transporting drill cuttings on drilling locations. A screw conveyor is a screw that rotates on its axis surrounded by a tubular casing. It is rotated by electric or hydraulic power motor with a gear box arrangement. The cuttings are fed into the gutter and transported along its length by rotation of its curved metal blades known as flights. Single or multiple auger systems may be used. Although screw conveyors are very effective and well-proven technology for drill cuttings transportation, they operate at high torque to convey materials with high densities such as drill cuttings. This creates a potential safety risk to operators involving severe pinch and crush points.
Vacuum units are used offshore to transport liquid waste for general cleanup duties. They are commonly used in drill cuttings collection. The cuttings generated while drilling are collected from a cuttings ditch or customized collection point and then conveyed by vacuum lines. These lines are normally flexible hoses or polyvinyl chloride (PVC) or steel piping that transport the waste stream to the containment or treatment area. The waste can be carried out to a variety of collection vessels including bulk tanks, vacuum rated skips/cuttings boxes and supply boats. These systems have fewer movable parts and are generally safer than other mechanical systems. The major disadvantages of the vacuum systems are its limited ability to transport materials effectively over long distances. Rig deck modification is often required to enable closer location of these systems to the cuttings ditches. Vacuum systems also require more than one unit to support higher volumes of cuttings generated during periods of high drilling rates.
The dense phase blower system is a recent solution. Typically, it is mounted under a vibrating feed hopper to receive cuttings by gravity from a feeder system such as a screw conveyor or auger. This type of pneumatic conveying system is arranged to provide cycles of continuous conveyance by providing an air stream through the auxiliary air connection. While the cuttings are being conveyed, the valve is closed and the vessel is refilled with cuttings as part of the next conveying cycle. This type of blower is described in more details in the next section.
Components and Operational Steps for a New Cuttings Transfer System
The unique drill cuttings storage with pneumatic transfer technology provides a means of storing drill cuttings in non-pressurized bulk storage tanks at an offshore rig. Cuttings are transferred in dense phase over long distances and heights to the boat storage tanks arrangement, enabling continued drilling over the duration of the well section.
The drill cuttings transport system begins when the drill cuttings are separated by the shale shaker unit and removed to the ditch storage. Next, cuttings are blown by a pneumatic transfer unit that is fed by gravity using a relatively small screw conveyor for a short distance to minimize the safety risk.
The system consists of a cuttings storage intermediate system that is operated by a single remote control panel unit, a dual-pod pneumatic transfer unit, boat tanks with a logic control system, air compressors and an onshore hydraulic tipping mechanism.
The system works by transferring the drill cuttings pneumatically. The dual-pod pneumatic transfer unit is fed by a screw conveyor that transports the cuttings to the collection feed hopper placed above the unit. When the unit is filled, it is sealed and pressurized. Then, the outlet valve opens and the positive pressure carries drill cuttings to either rig storage tanks or to the bulk tanks located on the supply vessel. There are two pneumatic transfer units: one to transport the drilled materials from collection points at the solids control equipment to rig tanks or to the boat tanks and a second pneumatic unit to move the cuttings from the rig storage tanks to the boat tanks. These pneumatic units can be operated either manually or fully automatically. Drilling rates, weather conditions and the supply vessel availability determine the choice of the operational mode. Fig. 1 shows the two optional flow lines; one is to the rig tanks and the other one directly to the boat tanks.
The rig tank system is operated from a remotely placed control panel. The control panel is used to adjust the discharge rate of each tank, open/close valves, adjust the multi-screw conveyor rotation speed and monitor the cuttings level. These tanks operate on a self-operating emptying system, which means there is no need to apply positive or negative pressure to assist unloading of the cuttings.
The self-emptying mechanism allows cuttings to be discharged by means of rotating vanes that allow for drill cuttings to flow radially towards the discharge chambers. The discharge rates range up to 60 tons per hour per tank. Discharge rates vary depending on material properties, such as density, viscosity and clay content. The typical rig storage tanks configuration consists of eight tanks with 320 tons total capacity using 84 m² deck space. Fig. 2 shows the rig tanks set up on a jack-up rig.
Typically, 16 bulk tanks (otherwise known as ‘boat tanks’) can be installed aboard a vessel, providing 320 tons of containment volume (20 tons rated per each tank). The pneumatic unit can have a total discharge length up to 656 ft (200 m). The flow line manifold modification has multiple feed lines. Each line can feed two tanks, with the ability to direct feed from one tank to opposite tank.
The cuttings are diverted to each tank by an assembly of pneumatically operated diverter valves placed in the middle of a walkway as shown in Fig. 3, offering safe access for personnel working on the supply boat. The bulk tanks are short design 16-ft length (4.8 m) and are mounted on the support vessels using frame which uses International Standards Organization (ISO) locks to secure containers to the boat as shown in Fig. 4.
The bulk tanks are shipped to an onshore waste management facility for treatment or disposal of content. Fig. 5 shows a waste tank being emptied at the waste pit by utilizing a hydraulic lifting frame mechanism designed to tilt the tanks up to 85 degrees for efficient cuttings discharge. The mechanism is powered by a hydraulic power pack unit to open the gate at the side.
Bulk Transfer System Performance-First Well
The first offshore use of the complete system began in 2015 for a North Sea drilling operation. The drill cuttings were pneumatically conveyed to the bulk storage tanks from a single collection point. The collection point could be the shale shaker or cuttings dryers. The system was rigged up to receive cuttings from the shale shaker collection point at this trial.
A12¼-in. hole section was drilled, and it generated nearly 440 tons of cuttings from the entire section. A portion of cuttings were transferred to the bulk tanks on a boat and the rest were stored on the rig. Cuttings were handled with specific gravities of 1.7to 2.0. The cuttings were pumped over 180 meters to the first tank on the boat and 200 meters to the last tank through 4-in. piping without dilution or slurrification at nearly 6 bar (90 psi) pressure. These cuttings were transferred to 23 boat tanks onboard the supply vessel. All tanks were efficiently emptied at the onshore waste facility without incidents.
The rig used 560 fewer crane lifts compared to normal operations. The rig was able to continue drilling the section during a bad weather when no boat loading or unloading was possible, without crowding the deck with cuttings skips in advance.
At the end of the well, the rig tanks were opened, and it was confirmed that there was almost no residue left in the tanks caused by cuttings bridging or accretion. Nearly nine wells were drilled by using this system, with significant reduction of crane usage and skips.
The bulk transfer technology presented above is a combination of multiple technologies, resulting in a safe and efficient solution for handling drill cuttings. This system offers several advantages over other existing technologies:
- Reduces number of rig to boat crane lifts;
- Reduces upload and offload time on supply vessel;
- Continued drilling regardless of weather conditions and over the duration of the operations;
- Requires less rig space for storage;
- Eliminates the needs of vacuum cuttings from storage tanks;
- Eliminates the need for slurrification or dilution because of its ability to handle cuttings up to 2.0 S.G.;
- Improves the auto operation by using single remote control panel;
- Provides safer navigation and faster turnaround time because the boat tanks are low profile, secured by ISO locks, and empty quickly;
- Improves HSE by eliminating skips and using cranes.