ACCESS NUMBER: 15240 

STUDY TITLE: A Literature Review of Environmental Impacts of Synthetic Based 

Drilling Fluids 

REPORT TITLE: Environmental Impacts of Synthetic Based Drilling Fluids 

CONTRACT NUMBER: Purchase Order No. 15240 

SPONSORING OCS REGION: Gulf of Mexico 

APPLICABLE PLANNING AREA: Gulfwide 

FISCAL YEARS OF PROJECT FUNDING: 1998, 1999, 2000 

COMPLETION DATE OF REPORT: August 2000 

COSTS: FY 1998: $0; FY 1999: $52,069.77; FY 2000: $ 47,852.23; CUMULATIVE 

PROJECT COST: $99,922 

PROJECT MANAGER: R.C. Ayers, Jr. 

AFFILIATION OF PROJECT MANAGER: Robert Ayers & Associates, Inc. 

ADDRESS: 6329 Rutgers St., Houston, TX 77005 

PRINCIPAL INVESTIGATORS: J.M. Neff, Battelle Ocean Sciences; 

S. McKelvie, Rudall Blanchard Associates; R.C. Ayers, Jr., Robert Ayers & Associates 

KEY WORDS: synthetic based drilling fluid; water based drilling fluid; oil based drilling 

fluid; drilling mud; drill cuttings; linear alpha olefin; poly alpha olefin; internal olefin; 

linear alkyl benzene; ether; ester; acetal; Gulf of Mexico; North Sea; Australia; outer 

continental slope; benthic; toxicity; bioaccumulation; biodegradation; anaerobic; anoxia; 

hypoxia; ecological recovery; ecological risk assessment. 

BACKGROUND: 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 recycled back down the well. The separated 

cuttings are in most cases, discharged to the ocean. The discharged drill cuttings 

contain 5 to 15% adhering drilling fluids. 

There are two major types of drilling fluids, water based fluids (WBFs) and nonaqueous 

base fluids (NABFs). WBFs, used in most offshore drilling operations in U.S. waters, 

consist of fresh or salt water, barite, clay, caustic soda, lignite, lignosulfonates, and/or 

water-soluble polymers. NABFs, are emulsions. A base fluid consisting of a liquid 

hydrocarbon or other water insoluble organic chemical forms the continuous external 
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phase while calcium chloride brine forms the discontinuous internal phase. NABFs also 

contain barite, clays, emulsifiers, water, calcium chloride, lignite, and lime. The two 

most important types of NABFs are oil based fluids (OBFs), in which the base fluid is 

diesel fuel or a petroleum-based mineral oil, and synthetic based fluids (SBFs), in which 

the base fluid is a well characterized synthetic organic chemical. SBF base fluids 

include linear-α-olefins, poly-α-olefins, internal olefins, linear alkyl benzenes, ethers, 

esters, or acetals. The SBF base fluids used most frequently in U.S. waters are linearα-olefins and internal olefins. 

Although WBFs are used for drilling most wells in offshore waters of the U.S., use of 

SBFs is increasing. SBFs have several technical advantages over WBFs, particularly 

when drilling in very deep water or when drilling a highly deviated well. OBFs have 

many of the same technical advantages as SBFs, but, because of their adverse 

environmental impacts, OBFs and OBF cuttings are not permitted for offshore disposal 

in U.S. waters. WBFs and WBF cuttings are currently permitted for disposal in Federal 

waters. SBF cuttings, but not SBFs are currently permitted for offshore disposal in the 

Gulf of Mexico. 

OBJECTIVES: 1) To review and summarize the available information in the scientific 

literature about the composition, environmental fates, and environmental effects of 

SBFs 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 SBFs. 

DESCRIPTION: A computer-based literature search was performed for all available 

published documents dealing with SBFs and SBF base fluids 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 SBFs 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. 

SIGNIFICANT CONCLUSIONS: The SBF base fluids used most frequently for drilling 

in the Gulf of Mexico are linear-α-olefins (LAO) and internal olefins (IO). Bulk SBFs are 

not permitted for discharge to waters of the Gulf of Mexico, but SBF cuttings are. 

Discharged SBF cuttings usually contain 5 to15% adhering SBF. The SBF cuttings 

form clumps in the receiving waters and quickly settle to the bottom following discharge 

to the ocean. SBF base fluids are biodegradable. Esters degrade most rapidly. LAO 

and IO biodegrade at moderate rates under aerobic and anaerobic conditions, but polyα-olefins and acetals do not biodegrade at a rate significantly greater than that of 

mineral oils in OBF. Biodegradation often causes depletion of oxygen in sediments. 

SBFs and SBF base fluids have a low toxicity to water column and sediment-dwelling 

marine organisms. However, high concentrations of SBF in sediments produce adverse 
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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 probably is caused by organic enrichment of the sediments with biodegradable 

SBF base fluid, 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 SBFs 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 SBFs and the rapid settling of cuttings to the 

bottom. 

STUDY RESULTS: Cuttings containing 5 to 15 percent adhering SBF are discharged 

to the ocean during drilling with SBFs. During drilling of each of 21 wells in the UK 

Sector of the North Sea in the late 1990s, 150 to 3000 metric tons of cuttings containing 

11 to 300 tons of SBF were discharged to the ocean. SBF cuttings tend to clump 

together in large particles that settle rapidly to the sea floor. Because of the rapid 

settling, SBF cuttings discharges do not increase water column turbidity and drilling fluid 

ingredients do not reach concentrations in the water column high enough to cause 

toxicity to water column organisms. The cuttings settle in a very heterogeneous pattern 

on the bottom near the platform, the direction of deposition depending on water column 

currents. Several field studies have shown that the highest concentrations of SBF 

cuttings usually are located in sediments within about 100 m of the platform. However, 

some SBF cuttings may be deposited out to 1 to 2 km from the discharge point. SBF 

base fluid concentrations as high as 200,000 mg/kg dry wt have been observed in 

sediments. Concentrations in sediments usually are less than 10,000 mg/kg at 

distances greater than 200 m from the discharge. SBF base fluid concentrations in 

sediments usually decrease with time due to resuspension, bed transport, mixing, and 

biodegradation. 

Sediment dwelling bacteria and fungi are able to use SBF base fluids as a source of 

nutrition, releasing simple non-toxic metabolic degradation products. Biodegradability of 

SBF base fluids under different conditions in sediments varies widely. Esters are most 

biodegradable; poly-α-olefins degrade slowly. Biodegradation of SBF base fluids in 

sediments results in depletion of oxygen concentrations in the sediments. Oxygen 

depletion slows biodegradation and adversely affects resident benthic fauna. 

SBF base fluids have a low bioavailability and toxicity to water column and sediment 

dwelling marine organisms. Their toxicity is much less than that of OBF base oils. The 

toxicity of SBFs is comparable to that of WBFs. 

Several field studies have been performed to monitor the biological effects of SBF 

cuttings discharges on the benthic environment. The studies show that where SBF 

base fluids accumulate to high concentrations in sediments, adverse effects in benthic 

communities are evident. The usual pattern of response to SBFs in sediments is a 

decrease in the number of taxa of marine animals in the sediments, accompanied by 
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little change or even an increase in the number of individuals present. This change 

results in a decrease in species diversity. The change in benthic community structure is 

considered an organic enrichment effect. Microbial degradation of SBF in sediments 

results in oxygen depletion and hypoxia or anoxia. Sensitive species are eliminated and 

are replaced by tolerant, opportunistic species. Population size of the opportunists may 

increase to a high biomass as the fauna use the elevated microbial production as food. 

Recovery of affected benthic substrates occurs when SBF concentrations decline to 

levels that are low enough that the sediments can become re-oxygenated. 

It is probable that within three to five years of cessation of SBF cuttings discharges, 

concentrations of SBF base fluids in sediments will have fallen to low enough levels and 

oxygen concentrations will have increased enough throughout the previously affected 

area that complete recovery will be possible. The rate of deep-water benthic ecosystem 

recovery will depend on the rate of recruitment and recolonization by the benthic fauna 

characteristic of the area. Because some species of deep-water benthic animals 

reproduce and grow slowly, complete recovery may require many years. However, 

ecological succession toward recovery is likely to begin shortly after completion of 

cuttings discharges and will be well advanced within three to five years when SBF base 

fluids have degraded to low concentrations. 

STUDY PRODUCTS: Neff, J.M., S. McKelvie, and R.C. Ayers, Jr. 2000. Environmental 

Impacts of Synthetic Based Drilling Fluids. A final report for the U.S. Department of the 

Interior, Minerals Management Service, Gulf of Mexico OCS Region, New Orleans, LA. 

OCS Study MMS 2000-064. Contract No. 15240. 132 pp. 

*P.I.’s affiliation may be different than that for Project Manager(s).