These waste materials should be segregated offshore into non-hazardous and hazardous wastes at a minimum, and shipped to shore for re-use, recycling, or disposal.
A waste management plan for the offshore facility should be developed that contains a clear waste tracking mechanism to track waste consignments from the originating location offshore to the final waste treatment and disposal location onshore. Efforts should be made to eliminate, reduce, or recycle wastes at all times.
Significant additional waste streams specific to offshore development activities include:
- Drilling fluids and drilled cuttings
- Produced sand
- Completion and well work-over fluids
Drilling fluids and drilled cuttings
The primary functions of drilling fluids used in oil and gas field drilling operations include removal of drilled cuttings (rock chippings) from wellbore and control of formation pressures. Other important functions include sealing permeable formations, maintaining wellbore stability, cooling and lubricating the drill bit, and transmitting hydraulic energy to the drilling tools and bit. Drilled cuttings removed from the wellbore and spent drilling fluids are typically the largest waste streams generated during oil and gas drilling activities.
Various drilling fluids are available, but they can generally be categorized into one of two fluid systems:
- WBDF: Fluids where the continuous phase and suspending medium for solids is seawater or a water miscible fluid. There are many WBDF variations, including gel, salt-polymer, salt-glycol and salt-silicate fluids;
- NADF: The continuous phase and suspending medium for solids is a water immiscible fluid that is oil-based, enhanced mineral oil-based, or synthetic-based. Diesel-based fluids are also available, but the use of systems that contain diesel as the principal component of the liquid phase is not considered current good practice for offshore drilling programs and should be avoided.
Typically, the solid medium used in most drilling fluids is barite (barium sulfate) for weight, with bentonite clays as a thickener. Drilling fluids also contain a number of chemicals that are added depending on the downhole formation conditions.
Drilling fluids are either circulated downhole with direct loss to the seabed along with displaced cuttings, particularly while drilling well sections nearest to the surface of the seabed, or are recirculated to the offshore facility where they are routed to a solids control system. In the solids control system, the drilling fluids are separated from the cuttings so that they may be recirculated downhole leaving the cuttings behind for disposal. These cuttings contain a proportion of residual drilling fluid. The volume of cuttings produced depends on the depth of the well and the diameter of the hole sections drilled.
The drilling fluid is either replaced when its rheological properties or density of the fluid can no longer be maintained or at the end of the drilling program. These spent fluids are then contained for reuse or disposal. Disposal of spent NADF by discharge to the sea must be avoided at all cost. Instead, they should be transferred to shore for recycling or treatment and disposal.
Feasible alternatives for the disposal of spent WBDF and drilled cuttings from well sections drilled with either WBDF or NADF should be evaluated. Options include injection into a dedicated disposal well offshore, injection into the annular space of a well, containment and transfer to shore for treatment and disposal and, when there is no other option available, discharge to sea.
When discharge to sea is the only alternative, a drilled cuttings and fluid disposal plan should be prepared taking into account cuttings and fluid dispersion, chemical use, environmental risk, and necessary monitoring. Discharge of cuttings to sea from wells drilled with NADF should be avoided. If discharge is necessary, cuttings should be treated before discharge.
Pollution prevention and control measures to consider prior to the discharge of spent drilling fluids and drilled cuttings should include:
- Minimizing environmental hazards related to residual chemicals additives on discharged cuttings by careful selection of the fluid system. WBDFs should be selected whenever appropriate
- Careful selection of fluid additives taking into account their concentration, toxicity, bioavailability and bioaccumulation potential
- Use of high efficiency solids control equipment to reduce the need for fluid change out and minimizing the amount of residual fluid on drilled cuttings
- Use directional drilling (horizontal and extended reach) techniques to avoid sensitive surface areas and to gain access to the reservoir from less sensitive surface areas
- Use of slim-hole multilateral wells and coiled tubing drilling techniques, when feasible, to reduce the amount of fluids and cuttings
Drilling fluids to be discharged to sea (including as residual material on drilled cuttings) are subject to tests for toxicity, barite contamination, and oil content. All discharges should be made via a caisson at least 15 m below the sea surface.
Produced sand originating from the reservoir is separated from the formation fluids during hydrocarbon processing. The produced sand can be contaminated with hydrocarbons, but the oil content can vary substantially depending on location, depth, and reservoir characteristics. Well completion should aim to reduce the production of sand at source using effective downhole sand control measures.
Whenever practical, produced sand removed from process equipment should be transported to shore for treatment and disposal, or routed to an offshore injection disposal well if available. Discharge to sea is not considered to be current good practice as it is a great risk to the environment. If discharge to sea is the only demonstrable feasible option then the discharge should meet some guideline values.
Completion and well work-over fluids
Completion and well work-over fluids (including intervention fluids and service fluids) can typically include weighed brines or acids, methanol and glycols, and many other chemical systems. These fluids are used to clean the wellbore and stimulate the flow of hydrocarbons, or simply used to maintain downhole pressure. Once used these fluids may contain contaminants including solid material, oil, and chemical additives.
Feasible disposal options should be considered, where practical, including:
- Collection of fluids if handled in closed systems and shipping to shore to the original vendors for recycling
- Injection in an available injection disposal well, where available
- Shipping to shore for treatment and disposal
- If discharge to sea is the only demonstrated feasible option
- Chemical systems should be selected in terms of their concentration, toxicity, bioavailability and bioaccumulation potential
- Consideration should be given to routing these fluids to the produced water stream for treatment and disposal, if available Spent acids should be neutralized before treatment and disposal
- The fluids should meet pre-set and approved discharge levels.
Naturally occurring radioactive materials
Naturally Occurring Radioactive Material, or “NORM,” is produced in solution with oil field brines and deposited as scale on the inside of oil field vessels and piping. Its radioactivity (rated as “low specific activity” by the Nuclear Regulatory Commission) poses a hazard if it is inhaled as a dry dust during vessel and pipe cleaning or repair.
Recycling of this material is impeded by the lack of safe exposure levels.
Depending on the field reservoir characteristics, NORMs may precipitate as scale or sludge in process piping and production vessels.
Where NORM is present, a NORM management program should be developed so that appropriate handling procedures are followed. If removal of NORM is required for occupational health reasons, disposal options may include: canister disposal during well abandonment; injection into the annular space of a well; shipping to shore for disposal to landfill in sealed containers.
Depending on the type of NORM and when there is no other option available, discharge to sea with the facility drainage. Sludge, scale, or NORM-impacted equipment should be treated, processed, or isolated so that potential future human exposures to the treated waste would be within internationally accepted risk-based limits. Recognized industrial practices shall be used for disposal. If waste is sent to an external onshore facility for disposal, the facility must be licensed to receive such waste.
Hazardous materials management
There are many hazardous materials used in offshore oil and gas operations. The following additional principles should be followed for offshore chemicals:
- Use of chemical hazard assessment and risk management techniques to evaluate chemicals and their effects
- Prior test of selected chemicals for environmental hazards
- Offshore drilling and production chemicals should be selected based on the OSPAR4 Harmonized Offshore
- Chemical Notification Format (HOCNF) or similar internationally recognized system
- Chemicals with least hazard and lowest potential environmental impact, and lowest potential health impact, should be selected, whenever possible
- Use of chemicals suspected to cause taint or known endocrine disruptors should be avoided
- Use of Ozone-depleting substances should be avoided
- Chemicals known to contain heavy metals, other than in trace quantities, should be avoided