U.S. Department of the Interior Minerals Management Service Gulf of Mexico OCS Region

MMS Releases Report on
Environmental Impacts of Synthetic Based Drilling Fluids

The Minerals Management Service (MMS), Gulf of Mexico OCS Region, has just released a new scientific report Environmental Impacts of Synthetic Based Drilling Fluids (OCS Report MMS 2000-064).  This study discusses the recently developed synthetic-based drilling fluids (SBF’s) and reviews the literature concerning the environmental impacts on the marine environment from the discharge of muds and cuttings derived from the drilling of wells with SBF’s.  The SBF’s are used in drilling situations where water-based (WBF’s) drilling fluids are not suitable.  The SBF’s were developed as a more environmentally friendly option to oil-based drilling fluids.

Drilling fluids are required for drilling oil and gas wells.  During drilling, drilling fluid is circulated down the hollow drill pipe, through the bit and up the annulus between the drill pipe and the borehole.  The fluid carries drill cuttings (crushed rock produced by the drill bit) to the surface.  The drilling fluid is processed on the platform to remove the cuttings and is then recycled back down the well.  The separated cuttings are, in most cases, discharged to the ocean.  The discharged drill cuttings contain 5-15 percent adhering drilling fluids.

There are two major types of drilling fluids, WBF’s and nonaqueous-based fluids (NABF’s).  The WBF’s, used in most offshore drilling operations in U.S. waters, consist of fresh- or saltwater, barite, clay, caustic soda, lignite, lignosulfonates, and/or water-soluble polymers.  The NABF’s are emulsions.  A base fluid consisting of a liquid hydrocarbon or other water insoluble organic chemical forms the continuous external phase, while calcium chloride brine forms the discontinuous internal phase.  The NABF’s also contain barite, clays, emulsifiers, water, calcium chloride, lignite, and lime.  The two most important types of NABF’s are oil-based fluids (OBF’s), in which the base fluid is diesel fuel or a petroleum-based mineral oil, and SBF’s, in which the base fluid is a well characterized synthetic organic chemical.  The SBF’s include linear-a -olefins, poly-a -olefins, internal olefins, linear alkyl benzenes, ethers, esters, or acetals.  The SBF’s used most frequently in U.S. waters are linear-a -olefins and internal olefins.

Although WBF’s are used for drilling most wells in offshore waters of the U.S., use of SBF’s is increasing.  The SBF’s have several technical advantages over WBF’s, particularly during drilling in very deep water or drilling a highly deviated well.  The OBF’s have many of the same technical advantages as SBF’s, but because of their adverse environmental impacts, the OBF’s and OBF cuttings are not permitted for offshore disposal in U.S. waters.  The WBF’s and WBF cuttings are currently permitted for disposal in Federal waters.  The SBF cuttings, but not SBF’s, are currently permitted for offshore disposal in the Gulf of Mexico.

The objectives of the study were (1) to review and summarize the available information in the scientific literature about the composition, environmental fates, and environmental effects of SBF’s in the marine environment; (2) to incorporate the available information into a screening-level ecological risk assessment for discharge of SBF cuttings to deep offshore waters of the Gulf of Mexico; and (3) to produce an annotated bibliography of publications concerning the environmental fates and effects of SBF’s.

A computer-based literature search was performed for all available published documents dealing with SBF’s and additives.  Technical contacts within the oil and gas and drilling mud industries of the United States, North Sea countries, and Australia were contacted for unpublished reports and information about marine environmental fates and effects of SBF’s and SBF cuttings.  All the potentially relevant reports and publications were accessed and reviewed.  An annotated bibliography of SBF publications was prepared.  All the relevant information was synthesized and interpreted in a technical review and in the form of an ecological risk assessment for SBF discharges to deep offshore waters of the Gulf of Mexico.

The SBF’s used most frequently for drilling in the Gulf of Mexico are linear-a -olefins (LAO) and internal olefins (IO).  Bulk SBF’s are not permitted for discharge to waters of the Gulf of Mexico, but SBF cuttings are.  Discharged SBF cuttings usually contain 5-15 percent adhering SBF’s.  The SBF cuttings form clumps in the receiving waters and quickly settle to the bottom following discharge to the ocean.  The SBF’s are biodegradable.  Esters degrade most rapidly.  LAO and IO biodegrade at moderate rates under aerobic and anaerobic conditions, but poly-a -olefins and acetals do not biodegrade at a rate significantly greater than that of mineral oils in OBF’s.  Biodegradation often causes depletion of oxygen in sediments.  The SBF’s have a low toxicity to water column and sediment-dwelling marine organisms.  However, high concentrations of SBF’s in sediments produce adverse effects in benthic communities.  The usual response is the loss or decrease in abundance of several species of benthic fauna and an increase in the abundance of a few opportunistic species, leading to an overall decrease in species diversity.  This effect is probably caused by organic enrichment of the sediments with biodegradable SBF, leading to sediment hypoxia.  Direct burial may also contribute to effects of SBF cuttings in benthic communities, particularly in deep continental slope waters.  Recovery of the benthic community occurs slowly as SBF’s are biodegraded or advected from the site by sediment transport.  Recovery may require three to five years.  Adverse effects of SBF cuttings discharges have not been observed in the water column and are not expected because of the low toxicity of SBF’s and the rapid settling of cuttings to the bottom.

The MMS is the Federal agency that manages the Nation’s oil, natural gas, and other mineral resources on the OCS; and collects, accounts for, and disburses about $4 billion in yearly revenues from offshore Federal mineral leases and from onshore mineral leases on Federal and American Indian lands.

-MMS-GOM-
MMS's Website Address:  http://www.mms.gov