SYNTHETIC-BASED DRILLING FLUIDS HAVE MANY ENVIRONMENTAL PLUSES

Christopher J. Burke, John A. Veil
Argonne National Laboratory
Washington, D.C.

Synthetic-based drilling fluids have several technological and environmental advantages over water-based and oil-based drilling fluids and can cut total well costs in many cases.

Spent drilling fluids and drilling cuttings are among the most significant waste streams from exploration and development activities. They pose a serious and costly disposal problem for offshore operators who must barge spent mud and cuttings to shore for land disposal if the material does not meet U.S, Environmental Protection Agency (EPA) discharge limitations or permit requirements.

Suppliers of mud systems have responded to this problem. Since 1990, several nontoxic, biodegradable, synthetic-based muds (SBMs) with desirable performance and environmental characteristics have entered the market.

The EPA regulations, however, are based on mud technology - that is, water-based muds (WBMs) and oil-based muds (OBMs) - that was available when the regulations were developed. Although EPA requirements appear to have been a major driver behind the development of SBM, concern is now focused on the inhibiting effect of discharge limitations on use of alternative mud technologies.

DRILLING FLUIDS

Drilling fluids serve several important functions:

Carry cuttings to the surface for disposal
Cool and clean the bit
Maintain pressure balance between the geological formations and the borehole
Lubricate the bit and drillstring
Reduce friction in the borehole
Seal permeable formations
Stabilize the borehole.

WBMs are by far the most commonly used muds, both onshore and offshore. The Offshore Operators Committee, a group of offshore oil and gas industry representatives, estimates that more than 90% of the drilling rigs operating in the Gulf of Mex- ico are using WBMs. These muds generally consist of more than 90% water, with barite, clays, lignosulfonate, lignite, caustic soda, and other specialty additives depending on specific well conditions. For example, bentonite, a volcanic clay, is used to increase mud viscosity and help lift drill cuttings.

Generally, the use of WBMs generates 7,00013,000 bbl of waste per well. Depending on the depth and diameter of the well, about 1,400-2,800 bbl of that amount are drill cuttings.1

WBMs have limitations for some applications. During the past 30 years, OBMs have been developed and refined to overcome these deficiencies.

The most commonly used type of OBM in the Gulf of Mexico is an oil and brine emulsion containing various additives dispersed in oil. OBMs have traditionally been used to improve lubricity, minimize problems associated with water-sensitive formations, and address other site-specific conditions, such as temperature, for which WBMs are not well suited.

OBMs are used where WBMs are dangerous, technically impossible to use, or are not economical. OBMs are usually rented or sold and repurchased by the supplier.

These muds generally consist of a base oil (usually diesel or mineral oil), barite, clays, emulsifiers, water, calcium chloride, lignite, lime, and other additives.2 OBMs have been the mud of choice for a range of special situations, including high temperatures, hydratable shales, high-angle and extended-reach wells, high-density mud, and drilling through salt.

Because of their enhanced lubricity, oil-based spotting fluids (which may be chemically different from OBMs) are also used as spotting pills during WBM drilling operations if the drill pipe becomes stuck. Stuck drill pipe is an issue for many operators in the Gulf of Mexico, The Offshore Operators Committee reports that in 2,287 wells drilled from 1983 to 1986 in the Gulf of Mexico, 506 instances were identified in which the operator used an oil additive in its attempt to free stuck pipe.3

The inability of WBMs to stabilize effectively some water- sensitive formations can result in hole enlargement or collapse. Frequently, this problem can only be controlled with OBMs or SBMs, which do not hydrate the shale and maintain hole stability.

Wells drilled with OBMs normally produce lower waste volumes than those drilled with WBMs because a nearly gauge hole is drilled, and the mud is reconditioned and reused rather than dis- charged at the end of the well. In some cases, WBMs augmented by costly synthetic-based additives are recycled also, but most WBMs are discharged along with the drill cuttings.

A relatively small volume of WBMs not meeting EPA's discharge limitations are hauled to shore for land disposal; these muds in most cases are contaminated with oil and failed the effluent limitations guidelines' static sheen test.

In contrast, in wells drilled with OBMs, the muds will be recycled, and only the drill cuttings will be disposed of. The average volume of OBM waste (drill cuttings with OBMs adhering to the cuttings) is estimated at 2,000-8,000 bbl per well.5

Because they contain oil, OBM wastes cannot be discharged on site under the effluent limitations guidelines (ELGs). In U.S. offshore areas, most of this waste is hauled to shore for land disposal.

There are two common methods for disposal of cuttings which have been exposed to either OBMs or WBMs contaminated with oil or other substances making them unsuitable for on site disposal in a marine environment. Either the cuttings are transported to shore for land disposal, or they are injected down the cased borehole into the annulus. There are relatively few instances of this type of disposal being required for WBMs.

REGULATORY BACKGROUND

Under the 1972 Clean Water Act (CWA), EPA regulates discharges of all pollutants, including drilling muds, into U.S. waterways and offshore areas. EPA uses technology-based effluent standards (generally ELGs) to issue discharge permits to offshore oil and gas industry operators.

In April 1979, EPA issued final ELGs (44 FR 22069) limiting offshore discharges of drilling mud and other effluents based upon use of best practicable technology (BPT). At that time, the only limitation on drilling mud discharge was a ban on free oil as determined by the visual sheen test. Between 1978 and 1983, EPA allowed discharge of only preapproved low-toxicity generic muds.6

On Aug. 26, 1985, EPA proposed offshore ELGs (50 FR 34592) that included the first end-of-pipe toxicity standard for offshore mud discharges. In the final ELGs issued Mar. 4, 1993 (58 FR 12454), based upon best available technology (BAT, a more stringent level of control than BPT), the EPA placed several additional limitations on discharge of mud and cuttings:

No discharges of free oil as detected by the static sheen test
A 30,000-ppm, 96-hr LC50 toxicity limitation on the suspended particulate phase (A 96-hr LC50 is the concentration lethal to 50% of the test organisms during a 96-hr test.)
Limitations on cadmium and mercury content of barite used in muds
A ban on discharge of muds and cuttings that contain diesel
A ban on discharge of drilling fluids and drill cuttings within 3 miles of shore.

The offshore industry in the U.S. has been operating under these or similar BAT restrictions since 1986 for the toxicity limits and 1992 and 1993 for the other criteria. By discouraging the use of OBMs and requiring the use of less-toxic muds, EPA's regulations, in conjunction with similar developments in the North Sea, created the need for and spurred the development of alternative muds and additives.

EPA's regulations, however, did not include any specific language that would facilitate the use of innovative SBM technology on a significantly larger scale. Although SBMs were not banned or restricted in the EPA regulations, their wide use was effectively limited by uncertainties implicit in the rules.

SYNTHETIC-BASED MUDS

In SBMs, the synthetic liquid forms the continuous phase while a brine serves as the dispersed phase. During drilling operations, the solids in the mud system and the formations are primarily exposed to the synthetic liquid and not to the aqueous phase, preventing swelling and degradation of borehole walls. As with OBMs, drill cuttings are less likely to disperse into the mud, and the system does not need large dilution volumes to control solids. Thus, the volume of spent mud and cuttings is reduced.7

Several SBM systems currently used can be classified according to the molecular structure of their synthetic base compounds:

Esters can be synthesized from fatty acids and alcohols.
Ethers include a range of materials usually synthesized from alcohols.
Polyalphaolefins (PAOs) for drilling are manufactured by the catalytic polymerization of linear alphaolefins such as 1-octene or 1-decene.
Olefin isomers are manufactured by selective isomerization of normal alpha-olefins.

The development and use of SBMs is relatively new and continues to evolve. Other types of base compounds are likely to be developed in the future.

Until recently, few hard data have been assembled on the chemical and toxicity characteristics of SBMs.

In 1994, the U.S. Department of Energy (DOE) organized a Synthetic Drilling Fluids Discussion Group with representatives from the oil producers, drilling fluid supply companies, American Petroleum Institute, National Ocean Industries Association, DOE, EPA, Argonne National Laboratory, and U.S. Department of Interior, Minerals Management Service.

At a December 1994 meeting of the discussion group, oil and gas industry representatives provided data on the toxic chemical constituents and toxicity of SBMs. Chemical analysis of two PAOs and one ester found no detectable priority pollutants in any of the tested SBMs.s

Toxicity data were provided for 61 field samples of SBMs. In all cases, the toxicity easily met the 30,000-ppm ELG limit. Nearly 79% of the samples were sufficiently nontoxic so that a finite LC50 could not be calculated (LC50 1 million ppm).9

USES AND ADVANTAGES

SBMs can perform comparably to, and in some cases may exceed, the performance of OBM systems. SBMs are used where OBMs are commonly used in difficult drilling situations, such as high downhole temperatures, hydratable shales, or salt, where the properties of WBMs would limit performance.

Although the purchase costs of SBMs generally exceed those for OBMs, the cost disadvantage is overcome if SBM drill cuttings can be discharged on site, thus saving transportation and disposal costs.

SBMs are well suited to the high-angle, directional, horizontal, and extended-reach wells which have become commonplace in the multiwell platform sites in the North Sea, Gulf of Mexico, and elsewhere.

SBMs have reduced well completion times compared to WBMs. Like OBMs, SBMs result in significant cost savings compared to WBMs in problem wells because they improve performance (feet drilled per hour) and reduce downtime for common problems like stuck drill pipe. For example, if an operator can drill a well in 3 weeks with SBMs or OBMs, compared to 6 weeks for WBMs, the sav- ings may be significant.

If SBM cuttings can be discharged, additional savings will result compared to the use of OBMs. This advantage should not be underestimated during a period of low oil prices and increasing international competitiveness in the industry.

SBMs have several quantifiable benefits compared to WBMs:

Less waste is produced from a recyclable product.
Increased use of horizontal drilling reduces the areal extent and the environmental impacts of offshore oil and gas operations.
Reduced drilling time results in reduced air emission from drilling power sources.
Improved drilling performance reduces waste-generating incidents such as pipe stuck in the hole and the use of diesel pills, thereby further reducing waste and pollution production.

SBMs have several quantifiable benefits compared to OBMs:

Low toxicity and reduced irritant properties improve worker safety.
Elimination of diesel as a mud base reduces pollution hazard and risks and improves worker safety.

SBMs resolve many of the major environmental problems associated with most OBMs, while achieving comparable drilling performance. SBMs possess some of the advantages of WBMs, such as low toxicity and environmental acceptability. But unlike most WBMs, SBMs are easily reused, which reduces waste discharges.

Except for disposal of drill cuttings, SBMs are more environmentally beneficial than alternatives.

The substantial environmental benefits demonstrated by this new technology appear to justify a regulatory clarification.

Environmental benefits aside, the leading attribute of SBMs is the efficiency they can bring to drilling. Like OBMs, SBMs can provide increased lubricity, reduced friction, and improved per- formance, resulting in faster drilling, more rapid completions, and thus reduced wastes and costs.

TRADITIONAL DRILLING LIMITATIONS

Despite major advances in technology and techniques, many wells are subject to problems that have long plagued operators.

Information provided by Marathon Oil Co. demonstrates the impact various problems have on drilling time on offshore oil wells drilled with WBM.' Total drilling time loss during 871 days at five locations was broken down as follows:

Mechanical problems, including twist offs, loss of stabilizer blades, and other drillstring failures, accounted for 14% of the total drilling days.
Lost circulation, primarily due to insufficient hole cleaning, drilling massive sand sections, and excessive mud weights, accounted for 10% of the total days.
Hole instability, caused by dispersion of the shale and hole enlargement, accounted for 7% of the total drilling days.
Well control, stuck pipe, cementing, and weather accounted for 15% of the total drilling days (Fig. 1)(99272 bytes).

Unproductive downtime on this order can have a severe effect on project economics. Many of the problems causing this lost pro- ductivity may be attributable to the interaction of the drilling mud, the well bore, and the cuttings, among other things.

Mud additives used in response to these types of drilling problems may increase the toxicity of a WBM. In fact, the application of mud additives is viewed as a balancing act between performance and toxicity. The traditional solution to these problems can include changing to an OBM, which presents a new set of environmental concerns.

It is against this backdrop that SBMs have been developed.

COST AND BENEFITS

It is well established that the purchase price of SBMs is greater than that of OBMs and WBMs. In general, SBM per-barrel costs are four to five times greater than that for OBMs.

Compared to WBMs, however, the higher purchase price may be more than offset by SBMs' improved performance in problem wells. Compared to OBMs, the higher purchase price may be more than offset by the elimination of hauling and onshore disposal.

Some of the following information, which documents the substantial economic benefits and cost savings which may result from use of SBMs, comes from preliminary field data submitted by an offshore operator.

For this pilot analysis, a major operator provided detailed, verified, platform cost and drilling data for a series of wells recently drilled under similar conditions and to roughly equal depths in the Gulf of Mexico.n Five of the wells used WBMs, and three of the wells used SBMs. None of the wells were drilled with OBMs; therefore, no comparison with OBMs' drilling effectiveness is possible.

Representatives of the operator and mud supplier for these wells were also interviewed. The data in Table 1 (23736 bytes) show major benefits of SBMs over WBMs in several areas:

As measured by drilled footage per day, SBMs performed with greater overall efficiency than did the WBMs. WBMs averaged 119 ft drilled per day, and SBMs averaged 335 ft per day.
While significantly more costly on a per-barrel basis, SBMs can reduce total mud costs and mud cost per drilled foot, as compared to WBMs. At most of the wells surveyed, the mud cost for SBMs was less than the low-est mud cost for WBM muds.
The WBM total well costs ranged $9.5-18.3 million. The SBM well costs ranged $3.7-7.8 million. The total cost for each of the WBM wells was substantially higher than that for the most expensive SBM wells. SBMs reduce overall costs by reducing downtime and other nonproductive activi- ties.
Improvements in efficiency yield significant increases in productivity. The WBM wells averaged 195 days to completion, whereas the SBM wells were completed on average in 53 days, less than one-third the time.

In these cases, the average total cost of WBM wells was $12 million compared to $5.8 million for SBM wells; the use of SBMs represented a substantial economic benefit. These costs do not include any onshore disposal costs for SBM cuttings, however. It is not known how conventional OBMs would have performed in these wells.

ENVIRONMENTAL IMPACT

Since the first proposed regulations limiting offshore mud discharges, efforts have focused on determining the nature and long-term effect of mud discharges on the marine environment.

The EPA reviewed 23 studies of the field impact of discharges of drilling fluids and drill cuttings.12 The review suggests that localized sea floor impacts may occur depending on the type of mud discharged and the energy level of the sea floor environment, but regional-scale impacts have not been identified. Other calculations provided by EPA estimate that some of EPA's water quality criteria will be exceeded in the water column and in the sediment pore water at a distance of 100 m from the point of discharge.

In studies of OBMs in the North Sea and of WBMs in the U.S., no adverse biological effects in the water column have been observed.13 During a discharge of WBMs from an offshore platform, dispersion occurs rapidly, and concentrations are quickly diluted to below toxic levels. Cuttings and clay particles settle to the sea floor.

WBM cuttings and fluids generally cause only minor impacts to the sea floor. They may cause a local decrease in the abundance levels of immobile bottom-dwelling organisms due to physical burial, but they have little effect on diversity. Abun- dance levels of mobile benthic organisms may actually increase in the rig site area.

In all cases examined with WBM discharge, recovery is rapid and biological effects may not be detectable in high-energy environments.14

Any discharge of OBMs and the associated cuttings poses a potentially greater environmental impact on the sea floor than does discharge of most WBMs and the associated cuttings. OBM cuttings can significantly increase oil content in sediment and decrease biological abundance and diversity in immobile bottom- dwelling organisms in the affected area.

Impacts primarily result from anoxia caused by the heavy organic loading and barrier that the OBM cuttings present to oxygen transport to the sediment.15

In zones severely impacted by OBM discharges, recovery is slow compared to WBM discharges. Recovery is faster in high- energy environments.

At many North Sea drilling sites, cuttings are found in large piles 100-150 ft high and 200 ft across on the sea floor. Large numbers of oil-coated sand and pebble piles on the sea floor smother bottom-dwelling organisms, and leaching of mineral oil has been detected as far as 2.5 miles from one installation.

Growing concern about the persistence of theses cuttings piles and the resulting prolonged pollution of the sea due to migration of the oil has led to ongoing studies on the short and long term peripheral effects. ,3

In addition to water quality, other impacts include environmental and safety aspects associated with use and disposal for different mud types. Impacts include air pollution due to transportation, energy use during transportation, disposal site factors (use of scarce disposal sites, potential site contami- nation, and threat to groundwater), and worker safety from use, loading, and unloading.

Each mud type causes or mitigates a range of indirect environmental impacts associated with its use and disposal. Indirect impacts appear to be mitigated favorably by SBMs. WBMs' indirect impacts are neutral; although most WBMs are discharged on site, significant volumes of WBM waste are still disposed of off site.

Major indirect impacts of off site waste disposal result from use of OBMs and, to a far lesser extent, WBMs. OBM disposal may place toxic hydrocarbons and priority pollutants in landfills, where they can potentially leach into groundwater or otherwise leak.

Another significant indirect impact from such disposal is the air pollution generated by the transportation of large volumes of OBM wastes to shore. The EPA estimates that 298 tons/year of additional air emissions will be released as a result of implementing its final ELGs of the offshore oil and gas industry.1 The leading contributors of emissions are the supply boats used to transport mud and cuttings to shore. Other equipment with significant air emissions contributions include cranes, trucks, tractors, and bulldozers.

The increased use of horizontal drilling techniques is another new practice facilitated by SBMs or OBMs that has notable pollution prevention and reduction impacts. Although data are unavailable to measure the full effect on drilling operations, SBMs or OBMs encourage the drilling of extended reach and horizontal wells, which can reduce the overall development costs and impacts.

U.S. operators would like to use SBMs in certain drilling situations because of favorable economic, environmental, health, and safety considerations. But many operators are hesitant to use SBMs if they will not be allowed to discharge the resulting cuttings on site.

The EPA's ELGs prohibit or greatly restrict the discharge of oil. Many operators are concerned that, based on the current definitions of oil, many SBMs could be considered oils, and therefore would be prohibited from on site discharge. If on site discharge of SBM cuttings is not allowed, SBMs often are no longer economically attractive.

REGULATORY ISSUES

Drilling mud discharge falls under Section 402 of the Clean Water Act, and drilling operators must apply for EPA and state permits for their discharges to surface waters.

Mud disposal is less of a problem for onshore operators who usually dispose of mud by on site burial (which does not fall under the Clean Water Act), off site treatment and disposal facilities, or injection.

Offshore operators have a more serious and potentially more costly problem. An approved onshore disposal site might be hundreds of miles away by barge and truck. Offshore storage of large volumes of mud and drill cuttings until hauling and disposal creates a major cost and logistical problem.

The preferred disposal method of used WBMs and drill cuttings is direct discharge into surrounding seawater. OBMs and their drill cuttings cannot be discharged offshore. The muds are recycled, and the cuttings are hauled to shore.

The excessive SBMs are recycled, and the drill cuttings are discharged offshore whenever possible. The ability to discharge SBM cuttings is the key element in making SBMs economically feasible in many applications.

In its final ELGs regulating discharges for the offshore subcategory issued on Mar. 4, 1993, the EPA determined that it is not technically feasible to control each toxic constituent of drilling muds and cuttings directly. The EPA opted for using a toxicity test requirement and placing limits on drilling mud contaminants that serve as indicators for toxic substances: free oil and diesel oil are indicators of toxic organics, and cadmium and mercury (in stock barite) are indicators of toxic metals.

A toxicity test is a measurement used to determine if pollutant concentrations are at levels that can cause lethal effects to organisms exposed to various concentrations of drilling mud. The EPA's technology basis for this control is product substitution of less toxic additives or, if the toxicity limit cannot be met, transporting drilling mud to shore for disposal.

In its final offshore ELGs, the EPA requires operators to conduct a toxicity test on discharged drilling muds. A mud that passes the test must have an LC50 greater than 30,000 ppm. The 30,000-ppm toxicity limitation is based on EPA's findings on toxicity data received from "round-robin" testing of eight gener- ic mud types sometimes used by the industry.

The EPA has also determined that substitutes are available for most significantly toxic mud additives.16-17 In its justification for the 30,000-ppm toxicity standard, the EPA believes the limit is technologically achievable because industry has operated under Npdes permits imposing such a limit since 1987. These permits were issued by EPA regional offices and established toxicity limits based on best professional judgment.

LIMITATIONS

Technological developments in the drilling industry appear to have outgrown the ELGs' regulatory categories. The ELGs do not recognize the use of pollution prevention systems like SBMs as a control technology for conventional pollutants.4 The ELGs also do not consider the engineering aspects of the effectiveness of a drilling mud as a technology that could be used to reduce overall pollution levels.

The limitations of EPA's regulatory approach may result partially from past conventional wisdom that concentrates on two categories of mud, OBMs and WBMs. During the extended rule-making process, the EPA only addressed current technologies in use, their environmental impacts, and the effects of proposed regu- lations.

The current availability of SBMs as a technical alternative to past and present practices may warrant a regulatory modification. Such a change would recognize the limitations in the past approach and clarify or amend the ELGs so that the use of SBMs is not unnecessarily precluded. Among the specific measures that need clarification are the following:

The term "synthetic-based fluid" should be clearly and simply defined to include low-toxicity SBM products being developed.
The static sheen test, as it applies to SBMs, should be replaced or amended with other analytical procedures to ensure the absence of free oil in SBMs. The static sheen test does not effectively determine presence of conta- minants in SBMs because the continuous phase of SBMs is lighter than water and could cause a film on the water.18 Such a film could result in failure of the sheen test and preclude on site discharge of the associated cuttings.
Implicit in this revised approach is the recognition that EPA regulations must be flexible and responsive to the development of new technologies that reduce environmental impacts. Balancing environmental protection mandates with the need to encourage new techniques and pollution prevention technologies should be an objective of a new approach to limitations on discharges.

RECOMMENDATIONS

While focused on limiting end-of-pipe discharges from offshore platforms, EPA's ELGs have had desirable side effects with major ramifications for offshore operators. Restrictions on offshore mud discharges led to the development of alternative mud systems.

One prominent effect has been the development of innovative SBM systems which can provide major benefits in terms of pollution prevention, operating costs, drilling efficiency, and performance.

SBMs were developed to provide drilling fluids with drilling properties similar to those of OBMs but produce cuttings which could be approved for discharge. Widespread application of this new technology in the U.S. is constrained by regulatory uncertainty over specific definitions and requirements in the ELGs and the resultant Npdes permits intended to curtail discharges of OBMs containing toxic pollutants into the marine environment.

EPA regulations were not drafted with full consideration of the recent development of innovative mud technologies, including SBMs. The EPA should consider revising or clarifying regulations specifically to address appropriate standards for SBMs.

EPA offshore regulations use a command-and-control, end-of- pipe approach that runs directly counter to the source reduction/pollution prevention approach which is an emerging policy priority. Greater regulatory flexibility in encouraging innovations and new technology development can ease the introduc- tion of alternative pollution prevention technology.

The current regulatory wording of controls on offshore discharges suggests, however, that EPA may not be able to exercise the flexibility needed to resolve the present regulatory situation limiting the use of SBMs. EPA must either clarify the present ELGs so that it is clear that SBMs can comply or make a new mud category for SBMs and establish appropriate controls for that category. Either of these approaches would involve substantial changes to EPA's past policies.

This article has identified several areas of research that could assist in sound policy development but that need additional data generation and verification:

A comparative assessment of the sea floor impacts of SBMs as compared with the two other types of mud
A comparative assessment of the status of the North Sea nations' discharge regulations
A comparative environmental impact assessment of SBMs, WBMs, and OBMs, once the primary data gaps have been filled.

REFERENCES

Environmental Protection Agency, "Development Document for Effluent Limitation Guidelines and Standards for the Offshore Subcategory of the Oil and Gas Extraction Point Source Category," EPA 821-R-93-003, January 1993.
McMordie, W.C., "Oil Base Drilling Fluids," Symposium on Research of Environmental Fate and Effects of Drilling Fluids and Cuttings, Lake Buena Vista, Fla., Jan. 21-24, 1980.
Candler, J.E., et al., "Synthetic-Based Mud Systems Offer Environmental Benefits over Traditional Mud Systems," Society of Petroleum Engineers paper 25993, San Antonio, Mar. 7-10, 1993.
Burke, C., "Approaches to Regulating Toxicity in Drilling Mud: Command and Control vs. Pollution Prevention," prepared by Argonne National Laboratory, Washington, D.C., for U.S. Department of Energy, October 1993.
Personal communication with R.C. Ayers, Houston, June 26, 1994.
Ayers, R.C., et al., "The Generic Mud Concept for NPDES Per- mitting of Offshore Drilling Discharges," Journal of Petroleum Technology, March 1985, p. 475.
Park, S., et al., "The Success of Synthetic-Based Drilling Fluids Offshore Gulf of Mexico," Society of Petroleum Engineers paper 26354, 1993.
Unpublished data presented by J. Candler, M-I Drilling Fluids, as the work product of the Analytical Support Group to the Synthetic Drilling Fluids Discussion Group, Washington, D.C., Dec. 15, 1994.
Unpublished data dated Nov. 10, 1994, presented by M. Parker, Exxon Co. U.S.A., as the work product of the Aquatic Toxicity Support Group to the Synthetic Drilling Fluids Discussion Group, Washington, D.C., Dec. 15, 1994.
Unpublished data provided to Chris Burke, Argonne National Laboratory, by Marathon Oil Co., Houston, May 26, 1994.
Environmental summaries and cost data supplied by Wallace White, Marathon Oil Co., to Chris Burke, Argonne National Laboratory, June 6, 1994.
EPA, "Regulatory Impact Analysis of Final Effluent Limitations Guidelines and Standards for the Offshore Oil and Gas Industry," EPA 821-R-93-002, January 1993.
National Research Council, "Drilling Discharges in the Marine Environment," National Academy Press, Washington, D.C., 1983.
Gilmore, R.B, et al., Effects of Exploratory Drilling Discharges on the Benthos, Wastes in the Ocean, Vol. 4, Wylie & Sons, 1985.
Davies, J.M., et al., "Environmental Effect of Oil Based Mud Drilling in the North Sea," presented at the International Conference on Drilling Wastes, Calgary, Apr. 5-8, 1988.
EPA, Fact Sheet for NPDES Permit No. CA0110842 and CA0110851, prepared by EPA Region IX, San Francisco, May 26, 1992.
EPA, Fact Sheet for NPDES Permit No. AK005205-1, prepared by EPA Region X, Seattle, Apr. 9, 1993.
Weintritt, D.I., et al., "How to improve accuracy in the EPA static sheen test," OGJ, May 3, 1993.