Feasible alternatives for the management and disposal of produced water should be evaluated and integrated into production design. The main disposal alternatives may include injection into the reservoir to enhance oil recovery, and injection into a dedicated disposal well drilled to a suitable receiving subsurface geological formation.
Other possible uses such as irrigation, dust control, or use by other industry, may be appropriate to consider if the chemical nature of the produced water is compatible with these options.
Produced water discharges to surface waters or to land should be the last option considered and only if there is no other option available.
Produced water treatment technologies depend on the final disposal alternative selected and particular field conditions. Technologies to consider may include combinations of gravity and/or mechanical separation and chemical treatment, and may require a multistage system containing a number of technologies in series to meet injection or discharge requirements. Sufficient treatment system backup capability should be in place to ensure continual operation and or an alternative disposal method should be available.
To reduce the volume of produced water for disposal the following should be considered:
- Adequate well management during well completion activities to minimize water production,
- Recompletion of high water producing wells to minimize water production,
- Use of downhole fluid separation techniques, where possible, and water shutoff techniques, when technically feasible,
- Shutting in high water producing wells.
To minimize environmental hazards related to residual chemical additives in the produced water stream where surface disposal methods are used, production chemicals should be selected carefully by taking into account their volume, toxicity, bioavailability, and bioaccumulation potential.
Disposal into evapouration ponds may be an option for produced waters. The construction and management measures for surface storage or disposal pits can also apply to produce water ponds.
Hydrostatic testing waters
Hydrostatic testing of equipment and pipelines involves pressure testing with water to detect leaks and verify equipment and pipeline integrity. Chemical additives (corrosion inhibitors, oxygen scavengers, and dyes) may be added to the water to prevent internal corrosion or to identify leaks. For pipeline testing, test manifolds installed onto sections of newly constructed pipelines should be located outside the riparian zones and wetlands.
Water sourcing for hydro-testing purposes should not adversely affect the water level or flow rate of a natural water body, and the test water withdrawal rate (or volume) should not exceed 10% of the stream flow (or volume) of the water source. Erosion control measures and fish-screening controls should be implemented as necessary during water withdrawals at the intake locations.
The disposal alternatives for test waters following hydro testing include injection into a disposal well, if available or discharge to surface waters or land surface. If a disposal well is unavailable and discharge to surface waters or land surface is necessary, the following pollution prevention and control measures should be considered:
- Reduce the need for chemicals by minimizing the time that test water remains in the equipment or pipeline
- If chemical use is necessary, carefully select chemical additives in terms of dose concentration, toxicity, biodegradability, bioavailability, and bioaccumulation potential
- Conduct toxicity testing as necessary using recognized test methodologies. A holding pond may be necessary to provide time for the toxicity of the water to decrease
- Holding ponds should meet the standards for surface storage or disposal pits as discussed below
- Use the same hydro test water for multiple tests
- Hydrostatic test water quality should be monitored before use and discharge and should be treated to meet the discharge limits
- If significant quantities of chemically treated hydrostatic test waters are required to be discharged to a surface water body, water receptors both upstream and downstream of the discharge should be monitored. Post-discharge chemical analysis of receiving water bodies may be necessary to demonstrate that no degradation of environmental quality has occurred
- If discharged into water, the volume and composition of the test water, as well as the stream flow or volume of the receiving water body, should be considered in selecting an appropriate discharge site to ensure that water quality will not be adversely affected outside of the defined mixing zone
- Use break tanks or energy dissipators (e.g., protective riprap, sheeting, tarpaulins) for the discharge flow
- Use sediment control methods (e.g., silt fences, sandbags or hay bales) to protect aquatic biota, water quality, and water users from the potential effect of discharge, such as increased sedimentation and reduced water quality
- If discharged to land, the discharge site should be selected to prevent flooding, erosion, or lowered agriculture capability of the receiving land. Direct discharge on cultivated land and land immediately upstream of community/public water intakes should be avoided
- Water discharge during cleaning pig runs and pretest water should be collected in holding tanks and should be discharged only after water-quality testing to ensure that it meets discharge criteria
If cooling water is used, it should be discharged to surface waters in a location that allows maximum mixing and cooling of the thermal plume to ensure that the temperature is within 3oCelsius (oC) of ambient temperature at the edge of the defined mixing zone or within 100 m of the discharge point. If biocides and/or other chemical additives are used in the cooling water system, consideration should be given to residual effects at discharge using techniques such as risk based assessment.
Wastewaters routinely generated at onshore oil and gas facilities include sewage waters, drainage waters, tank bottom water, fire water; equipment and vehicle wash waters, general oily water and non-process industrial wastewater like engine coolant and unused
cement slurry. These wastewaters range from ones containing practically no contaminant (snowmelt, for example) to waters like cement slurries which clearly require treatment before they can be discharged or recycled.
Pollution prevention and treatment measures that should be considered for such wastewater should include:
- Sewage: Gray and black water from showers, toilets and kitchen facilities should be treated.
- Drainage and stormwater: Separate drainage systems for drainage water from process areas that could be contaminated with oil (closed drains) and drainage water from non-process areas (open drains) should be available to the extent practical. All process areas should be bunded to ensure drainage water flows into the closed drainage system and that uncontrolled contaminated surface run-off is avoided.Drainage tanks and slop tanks should be designed with sufficient capacity for foreseeable operating conditions, and systems to prevent overfilling should be installed. Drip trays, or other controls, should be used to collect run-off from equipment that is not contained within a bunded area and the contents routed to the closed drainage system. Stormwater flow channels and collection ponds installed as part of the open drainage system should be fitted with oil / water separators.Separators may include baffle type or solid particle escaping plate type and should be regularly maintained. Stormwater runoff should be treated through oil / water separation system able to achieve oil and grease concentration of 10 milligram per litre (mg/L).
- Tank bottom waters: The accumulation of tank bottom water should be minimized by regular maintenance of tank roofs and seals to prevent rainwater infiltration. Consideration should be given to routing these waters to the produced water stream for treatment and disposal, if available. Alternatively they should be treated as a hazardous waste and disposed in accordance with the facility waste management plan. Tank bottom sludges should also be periodically removed and recycled or disposed off as a hazardous waste.
- Firewater: Firewater from test releases should be directed to the facility drainage system.
- Wash water: Equipment and vehicle wash waters should be directed to the closed drainage system.
- General oily water: Oily water from drip trays and liquid slugs from process equipment and pipelines should be routed to the closed drainage system.