In wells nearing the end of their productive lives, water can comprise 98% of the material brought to the surface. The American Petroleum Institute estimates that over 15 billion barrels of water are produced annually. This is nearly eight barrels of water for every barrel of oil produced. Natural gas wells typically produce much lower volumes of water than oil wells, with the exception of certain types of gas resources such as Coal Bed Methane (CBM) or Devonian/Antrim shales.
Although, many petroleum components are separated from the water easily, some components and impurities are water-soluble and difficult to remove. Some substances may be found in high concentrations, including chloride, sodium, calcium, magnesium and potassium. Others found are:
- Organic compounds: benzene, naphthalene, toluene, phenanthrene, bromodichloromethane, and pentachlorophenol;
- Inorganics: lead, arsenic, barium, antimony, sulfur, and zinc;
- Radionuclides: uranium, radon, and radium.
It should be noted that concentrations of these pollutants vary considerably depending on the location of the well and the extent of treatment of the water. Geography can be a key factor in whether a substance may exist in produced water. For example, radionuclides are found only in some areas.
Hydrostatic testing waters
Hydrostatic testing of offshore equipment and marine pipelines involves pressure testing with water (typically filtered seawater, unless equipment specifications do not allow it) toverify 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.
In managing hydrotest waters, the following pollution prevention and control measures should be considered:
- Minimizing the volume of hydrotest water offshore by testing equipment at an onshore site before the equipment is loaded onto the offshore facilities;
- Using the same water for multiple tests
- Reducing the need for chemicals by minimizing the time that test water remains in the equipment or pipeline
- Careful selection of chemical additives in terms of dose concentration, toxicity, biodegradability, bioavailability, and bioaccumulation potential
- Sending offshore pipeline hydrotest water to shore facilities for treatment and disposal, where practical.
If discharge of hydrotest waters to the sea is the only feasible alternative for disposal, a hydrotest water disposal plan should be prepared that considers points of discharge, rate of discharge, chemical use and dispersion, environmental risk, and monitoring.
Hydrotest water disposal into shallow coastal waters should be avoided.
Anti-foulant chemical dosing to prevent marine fouling of offshore facility cooling water systems should be carefully considered.
Available alternatives should be evaluated and, where practical, the seawater intake depth should be optimized to reduce the need for use of chemicals. Appropriate screens should be fitted to the seawater intake if safe and practical. The cooling water discharge depth should be selected to maximize mixing and cooling of the thermal plume to ensure that the temperature is within 3oC of ambient seawater temperature at the edge of the defined mixing zone or within 100m of the discharge point.
Operators should consider mixing desalination brine from the potable water system with the cooling water or sewage water discharge. If mixing with other discharge waste streams is not feasible, the discharge location should be carefully selected w.r.t. potential environmental impacts.
Wastewater routinely generated at offshore facilities is listed below:
- Sewage: Gray and black water from showers, toilets, and kitchen facilities
- Food waste: Organic (food) waste from the kitchen
- Storage displacement Water: Water pumped in and out of storage during loading and off-loading operations
- Bilge water: Bilge water from machinery spaces in offshore facilities and support vessels
- Deck drainage water: Drainage water generated from precipitation, sea spray, or routine operations, such as deck and equipment cleaning and fire drills.