ACCESS NUMBER: 31179

STUDY TITLE: Degradation of Synthetic-Based Drilling Mud Base Fluids by Gulf of 

Mexico Sediments 

REPORT TITLE: Degradation of Synthetic-Based Drilling Mud Base Fluids by Gulf of 

Mexico Sediments, Final Report 

CONTRACT NUMBER: 1435-01-01-CA-31179 

SPONSORING OCS REGION: Gulf of Mexico 

APPLICABLE PLANNING AREAS: Eastern Central and Western Gulf of Mexico 

FISCAL YEARS OF PROJECT FUNDING: 2001-2005 

COMPLETION DATE OF REPORT: August 2006 

COSTS: FY 2001: $148,664; FY 2002 $105,810: FY 2003: $64,924, FY 2005: 56,430 

(projected) 

CUMULATIVE PROJECT COST: $391,847 

PROJECT MANAGER: D. J. Roberts 

AFFILIATION: Dept. of Civil and Environmental Engineering, University of Houston. 

ADDRESS: N107 Engineering Building 1, University of Houston, Houston, TX 77204-

4003 

PRINCIPAL INVESTIGATOR: D. J. Roberts 

KEY WORDS: synthetic base fluids, biodegradation, kinetics, fate model 

BACKGROUND: An important consideration in the environmental assessment of deepsea drilling operations in the Gulf of Mexico is the discharge of cuttings coated with 

synthetic-based fluids (SBF). Synthetic-based fluids are one type of nonaqueous 

drilling fluid and are used in drilling mud to lubricate the drill bit, control reservoir 

pressure, and bring rock chips, or cuttings, to the surface. Synthetic-based fluids, which 

can be composed of linear alpha olefins, internal olefins, esters or paraffins, are 

released into the marine environment as a residue on the cuttings as they are 

discharged. This study addresses the fate of the synthetic base fluid portion of the 

drilling mud in Gulf of Mexico sediments by determining the potential of marine 

sediment microbes to degrade representative SBF under deep-sea conditions. This 

study will also examine the effect of the discharge on the microbial ecology of the 

sediments. The Environmental Protection Agency (EPA) has chosen the anaerobic 

Closed Bottle Testing (CBT) as the method of choice for determining the 

biodegradability of synthetic drilling mud base fluids. This test was chosen over other 

tests because it is more environmentally relevant. However, the CBT currently relies on 

the use of a limited number of near-shore sediment samples as representative inocula. 
ACCESS NUMBER: 31179

The use of more representative sediments, specifically deep-sea sediments obtained 

from "pristine" and impacted areas of the Gulf of Mexico will help to improve the 

understanding of how microbial populations react to the addition of SBF. A model to 

predict how fast the sediment will recover under realistic conditions will form a scientific 

basis for evaluating impacts from the discharge of SBF in the deep-sea. 

OBJECTIVES: The objectives of this study are to 1) establish the relationship between 

chemical structure and the rate of removal of synthetic base fluids from deep-sea 

sediments incubated under relevant temperature and pressure conditions; 2) 

characterize microbial populations involved in the removal of SBF and determine the 

response of the microbial community; and 3) develop a model which incorporates 

microbial response, environmental conditions, and base fluid structure to estimate the 

response and recovery of impacted deep-sea sediments.

DESCRIPTION: Sediments collected from three locations in the Gulf of Mexico, 66 m 

to 1135 m depth, near field and far field locations were used in a modified CBT method 

to measure degradation rates of surrogate SBF tetradecene and ethyl oleate. The 

sediments were incubated at 4°C and under a hydrostatic pressure equal to that of the 

depth they were taken from, or at atmospheric pressure. The microbial population in 

the sediments was characterized by fluorescent in-situ hybridization (FISH) and mostprobable number (MPN). A model that can be used to predict the fate of SBF deposited 

to the floor of the Gulf of Mexico was developed.

SIGNIFICANT CONCLUSIONS: The significant conclusions of the study are that there 

are cold tolerant, anaerobic microorganisms that can degrade the surrogate SBF 

components tetradecene and ethyl oleate in sediments from the Gulf of Mexico. Sulfate 

was determined to be the major electron acceptor involved in the degradation process. 

Analyses of the microbial ecology of the sediment were not as useful as had been 

hoped due to interference of natural sediment components with the methods used for 

analyses. The results did show an increase in the number of sulfate reducing bacteria 

(SRB) present in sediments that had been exposed to SBF (comparing impacted 

samples with unimpacted) or incubated in the presence of surrogate SBF tetradecene 

or ethyl oleate. The study also showed that the EPA CBT could be adapted to reflect 

deep sea pressure. The anaerobic incubations revealed that the removal of ethyl oleate 

from a contaminated site could be described using a first order k value of -0.22 ± 0.02 

week-1. The removal of ethyl oleate from an uncontaminated site would occur at a rate 

of -0.11 ± 0.02 week-1. A lag time for ethyl oleate removal is predicted to be between 0 

and 11 weeks. The average first order decay coefficient (k value) for the removal of 

tetradecene linked to sulfate reduction was -0.05 ± 0.01 week-1. Tetradecene 

degradation typically required a much longer lag period, (4-28 weeks). 

STUDY RESULTS: Sediment samples were collected from the Gulf of Mexico. The 

sediments were spiked with surrogate SBF (tetradecene and ethyl oleate) and 

incubated under anaerobic conditions. An incubator which reproduces the pressure at 

more than 1000 meter water depth was constructed for testing SBF biodegradation in 

sediments from the deepest sampling location. Bacteria in the sediments have the 

capability to degrade SBF. A mathematical model to predict SBF biodegradation has 
ACCESS NUMBER: 31179

been constructed. This study received a 12-month extension and additional funding to 

extend the sediment incubation period. 

STUDY PRODUCTS: The written products of this study include three masters theses, 

a Ph. D. dissertation, numerous quarterly status reports, a final report, and at least two 

peer reviewed publications (planned for submission). The results of the research have 

been presented in the MMS ITM conferences in 2002, 2003 and 2005, at the 

International Petroleum Environmental Conference (IPEC) in 2003, the American 

Society for Microbiology Annual meeting in 2003 and 2005, and the Applied and 

Environmental Microbiology Gordon Research Conference in 2005. 

Roberts, D.J. and A.H. Nguyen. 2006. Degradation of synthetic-based drilling mud 

base fluids by Gulf of Mexico sediments: final report. U.S. Dept. of the Interior, Minerals 

Management Service. Gulf of Mexico OCS Region, New Orleans, LA. OCS Study MMS 

2006-028. 140 pp.
ACCESS NUMBER: 31179

Map Showing area of study