Drilling Offshore for Oil and Gas—Muddy Issues

Our reliance on offshore gas and petroleum resources has increased as relatively easily accessible stocks have been developed and depleted. Today, oil and gas resources are being tapped in coastal oceans throughout the world, with largest reserves in the Arabian Gulf, Gulf of Mexico, North Sea, and Caspian Sea.

Drilling for oil and gas offshore is a technically challenging operation that produces wastes that must be treated and disposed of in an environmentally protective manner to avoid pollution. The largest-volume wastes associated with drilling are drilling muds and drill cuttings. Drilling mud is pumped down into the hole and returns to the surface laden with cuttings. The cuttings are separated from the mud on the platform and usually are discharged to the ocean; the mud usually is recirculated down-hole.

Drilling muds are mixtures of natural clays and heavy minerals (usually barium sulfate), and small amounts of several other chemicals suspended in water or an organic liquid. Three types of drilling muds are used offshore: water based drilling muds (WBM), oil based drilling muds (OBM), and synthetic based drilling muds (SBM). OBM work the best for drilling, but are harmful to marine communities if the oily cuttings are discharged. WBM and cuttings are much less toxic and are the only types of drilling wastes permitted for ocean discharge in U.S. waters until recently. However, WBM do not work well, particularly when drilling a very deep or deviated well. As a result, SBM were developed to combine the favorable drilling properties of OBM with the low toxicity of WBM. The liquid phase of SBM is a synthetic organic chemical (usually an ester or olefin).

SBM have been in use for the last decade for drilling, particularly in the North Sea and Gulf of Mexico. Only the cuttings produced during use of SBM are discharged to the ocean; the drilling mud itself is separated from the cuttings and recycled or regenerated. Battelle has participated in several laboratory and field studies in the United States, North Sea, and Mexico to determine the long-term effects of cuttings discharges on the ocean environment. The results of these studies will be used to develop strategies for minimizing environmental impacts of cuttings discharges to the ocean.

For more information about impacts of offshore oil and gas development on the marine environment, contact Dr. Jerry Neff at (781) 952-5229, neffjm@battelle.org.