«Paper #2-11 SUBSEA DRILLING, WELL OPERATIONS AND COMPLETIONS Prepared by the Offshore Operations Subgroup of the Operations & Environment Task Group ...»
to the limits on the number of casing strings that can be run in any one well, often riserless drilling with water-based, weighted drilling fluids is used to drill to a depth where the formations have the required strength. This practice is critical to the development of reservoirs in ultradeepwater between the continental shelves and deep oceans but it also discharges large volumes of weighted water-based muds at the seafloor.
In the past 10 years, mechanical subsea systems have been developed which allow deepwater riserless drilling with weighted mud and with fluid returns to the drilling rig (Gordon et al., 2010). Those systems allow a dual-gradient hydrostatic pressure to be applied, thereby more closely matching the natural deepwater pressure profile. While those systems have been used on a number of offshore wells, there is a limited supply of the necessary equipment and other wellcontrol issues must be carefully considered for each particular application.
DRILLING WASTE MANAGEMENT
Waste generated during drilling falls into four primary categories:
• Residual drilling fluids and cuttings which constitute the largest volume of waste produced during drilling operations.
• Different types of wastewater produced during the drilling process.
• Air emissions generated from the drilling equipment and support vessels and aircraft.
• Industrial or solid waste including paint, spent solvents and packing materials.
The approach to handling each type of waste depends on the volumes and worksite circumstances and can involve treatment and disposal, waste reduction, recycling and re-use options to reduce environmental impacts. Efforts in recent years have been increasingly toward more environmentally friendly outcomes.
A. Drilling Fluids and Cuttings
There are two primary types of drilling fluids for offshore: water-based fluids (WBFs) and nonaqueous drilling fluids (NAFs) that often also are called synthetic-based fluids (SBFs). The selection of the drilling fluid to be used depends on many variables including geologic formation conditions, wellbore stability, temperature and pressure, lubricity required, mud density required, gas-hydrate prevention, logistics, and overall drilling and completion plan -- all factors to be considered to make the drilling operation safe and environmentally sound.
NAFs reduce drill solids and liquid waste volumes, are more recyclable than WBFs, allow faster drilling rates, reduce drilling problems, allow greater extended-reach drilling to access more resources with fewer offshore installations, and overall result in fewer rig days which means reduced overall emissions and health and safety risks to personnel (Bernier et al., 2003;
Pettersen, 2007). Those features and the pollution-prevention aspects of SBFs were cited by the US EPA (Code of Federal Regulations, 2011b) when guidelines were established for the water
discharge of NAF drill cuttings:
“In these final regulations, EPA supports pollution prevention technology by encouraging the appropriate use of synthetic-based drilling fluids (SBFs) based on the use of base fluid materials in place of traditional: (1) Water-based drilling fluids (WBFs); and (2) oil-based drilling fluids (OBFs) consisting of diesel oil/or and mineral oil. The appropriate use of SBFs in place of WBFs will generally lead to more efficient and faster drilling and a per well reduction in non-water quality environmental impacts (including energy requirements) and discharged pollutants.
Use of SBFs may also lead to a reduced demand for new drilling rigs and platforms and development well drilling though the use directional and extended reach drilling.”
However, NAFs have limitations as compared to WBFs including higher costs (especially if lost circulation is anticipated), increased disposal and logistical issues, more difficult displacement and clean-up, issues of cement compatibility, and possible logging incompatibilities (Jacques Whitford Environment Limited, 2001). Often WBFs and NAFs are used in drilling the same well wherein the WBF is used to drill the shallow section and the NAF is used for the deeper horizons.
WBFs consist primarily of water (~ 75%) mixed with a variety of chemical additives and barite to obtain the desired properties and density. WBFs have been demonstrated to have only limited effect on the environment. The US EPA has evaluated the environmental issues with regard to WBFs and established effluent guidelines for the discharge of WBFs and cuttings (Code of Federal Regulations, 2011b). Other countries and the IFC World Bank Group also provide for effluent guidelines and discharge of WBF and cuttings with toxicity and mercury and cadmium limits (Code of Federal Regulations, 2011b). The clay and bentonite are chemically inert and non-toxic and the heavy metals (Ba, Cd, Zn and Pb) are bound in minerals and therefore have limited bioavailability. Ocean discharges of WBFs have been shown to affect benthic organisms by smothering to a distance of approximately 100 feet from the discharge and to affect species diversity to 300 feet from the discharge. However those impacts normally are temporary in nature.
The NAFs are further grouped according to their aromatic hydrocarbon content and include the
Group I NAF (high aromatic content). These were the first NAFs used and include diesel and conventional mineral oil-based fluids. The polycyclic aromatic hydrocarbon (PAH) content of the diesel-oil fluids is typically 2 to 4%. Because of concerns about toxicity, diesel-oil cuttings are not discharged.
Group II NAF (medium aromatic content). These fluids, called Low Toxicity Mineral Oil-Based Fluids (LTMBF), were developed to address the concerns of the potential toxicity of diesel-based fluids. The PAH content of the diesel-oil fluids is reduced to less than 0.35%.
Group III NAF (low to negligible aromatic content). These fluids are the newest generation of drilling fluids that include highly processed mineral oils and syntheticbased fluids produced by chemical reactions of relatively pure compounds and include synthetic hydrocarbons (olefins, paraffins and esters). These synthetic fluids are stable in high-temperature downhole conditions and are adaptable to deep water drilling environments. The PAH content is very low (0.001%).
Group III NAFs have the lowest acute toxicity. Group III cuttings discharges have produced far fewer effects on benthic communities than the early generation oil-based mud cuttings discharges and the effects are rarely seen beyond 750 to 1500 feet from the discharge. Studies have shown that in most cases, but not all, benthic communities start to recover within one year of the drilling discharge. The development of these more sophisticated NAFs was required to
meet the technical challenges of directional, extended-reach and deepwater drilling and to deliver high performance yet also environmentally sound operations.
Technical developments with regard to drill cuttings relate to the volume generated and processing techniques prior to disposal. Drilling improvements which can reduce the volume of cuttings generated include closer spacing of successive hole sizes and casing strings, increased casing sizes, expandable casing, increased bit sizes, bi-centered bits, and reaming-while-drilling, plus advanced casing-while-drilling technologies.
For NAF drill cuttings, thermal processing equipment has been developed which can reduce the base fluid retained on cuttings to very low levels, below 1% total petroleum hydrocarbons (TPH). The most compact of these thermal units are Hammermill-process (impact friction-based) thermal desorption types (Murray et al., 2008). Although that type of equipment has seen limited use in offshore drilling, its size is too large to be widely applicable for retrofitting onto most existing offshore drilling units or production installations. Such equipment is used most frequently for land-based centralized processing stations where NAF waste is processed and the resulting solids are disposed into landfills.
There are several options for disposal of drilling fluids and cuttings and all have their advantages and disadvantages with regard to environmental impact. The primary considerations in selecting a waste-management option are the characteristics of the environment, operational circumstances and costs. The three principal options are offshore discharge, re-injection and onshore discharge.
Offshore Discharge. Offshore discharge is the least expensive, operationally uncomplicated and safest of the three options (Jacques Whitford Environment Limited, 2001). WBFs and cuttings have been discharged offshore for 50 years with minimal impact to the environment (Neff, 2005). The recent development of more environmentally friendly NAFs has been undertaken to reduce the environmental impact associated with discharge of NAF drill cuttings and make this option more broadly acceptable. After separation from entrained solids, NAF liquids are not discharged but are reused or recycled. Offshore discharge is often critical for efficient deep water exploratory drilling due to the long distance from shore, lack of land-based disposal facilities and technical limitations on use other disposal options, such as subsurface re-injection.
Offshore discharge often results in the least overall environmental impact. Alternatives to offshore discharge come with an additional environment impact plus associated environmental and personnel safety risks. The additional impacts pertain to the increased level of handling as well as the energy required to perform the other disposal options (James and Rørvik, 2002;
Pettersen and Hertwich, 2008).
The US EPA noted the extended impacts when guidelines were established for the water discharge of NAF drill cuttings in 2001 (Code of Federal Regulations, 2011b):
Compliance with this rule is estimated to reduce the annual discharge of priority and non-conventional pollutants by at least 7.82 million pounds per year and result in the reduction of 2,927 tons of air emissions and reduce energy use by 200,817 barrels of oil equivalent (BOE).
Drilling Fluid / Cuttings Re-injection. Another option for drilling waste disposal is on-site cuttings reinjection. This process involves pumping fluids and seawater-diluted cuttings, which have been ground into small particles, into an underground formation that has been fractured.
Care is taken to make the slurry particles sufficiently small that they do not readily settle or plugup the fractures in the receptor formation. Injected fluids are confined in the receiving formations, which are selected for their geological isolation, and by cementing the injection-well casings. Cuttings may be injected via the annulus of a well being drilled or through a dedicated or dual-use disposal well.
Injection is a complicated process which requires assessment of several issues. First, a geologic formation is required that is suitable for sealing the cuttings and will not allow them to migrate into other formations or to the surface. Also, the types and quantities of waste, surface equipment and well design and integrity must be considered before injection is performed.
Research is continuing to make improvements for cuttings injection to be a more successful application.
Subsurface re-injection has been used about 20 years. Industry best practices have been developed (Nagel and McLennan, 2010), improvements to fracture modeling and monitoring have been made, and specialized companies have become established for designing and executing subsurface injection projects with greater reliability and operational monitoring (Redden, 2009).
Onshore Disposal. The third option for disposal of drill fluids or cuttings is to capture and transport to shore for disposal. Consideration of any onshore disposal option must also include consideration of the offshore operations associated with getting the drilling waste to shore.
Bringing cuttings to shore requires extensive use of support vessels which produce air emissions (James and Rørvik, 2002; Jacques Whitford Environment Limited, 2001). Safety and environmental risks (potential for a spill) are increased over those of other options, particularly in areas of harsh weather conditions. There may be operational or business-continuity issues with handling large volumes of cuttings if transport operations are shutdown due to inclement weather. The baseline zero-discharge operation uses “cuttings boxes” which hold 15 to 20 barrels of solid or liquid waste and must be lifted with a crane 10 to 15 times during each filland-disposal cycle. Recent advancements in bulk handling of drilling waste can become feasible where the drilling unit is large enough to justify the bulk handling vessels.
Once onshore there are several options for treatment, recycling and disposal of drilling waste.
Those options include landfill disposal (if WBFs were used), stabilization/solidification, bioremediation and thermal treatment technologies such as thermal desorption and incineration if NAFs are used. The viability of each of those options will depend on an assessment of the environmental conditions, components of the drilling waste, regulations, operational limitations and economic factors. As with other options, onshore disposal may not be a technically or economically viable option and selection must be evaluated on a case-by-case basis.
Liquid discharges from offshore drilling include domestic and sanitary wastewater, deck drainage water, once-through fire water, non-contact cooling water, bilge water, and ballast water. Any effluent discharges are regulated and monitored according to the applicable permit.
In general the quantity of those wastewater streams is small and has less environmental impact as compared with the discharges of drilling fluids and drill-cutting wastes.