Environmental Aspects of Drilling Fluids

Much attention has been given to the environmental aspects of the drilling operation and the drilling fluid components. Well-deserved concern about the possibility of polluting underground water supplies and of damaging marine organisms, as well as effects on soil productivity and surface water quality, has stimulated widespread studies on this subject.

Drilling Fluid Toxicity There are three contributing mechanisms of toxicity in drilling fluids: chemistry of mud mixing and treatment, storage and disposal practices, and drilled rock. The first group conventionally has been known the best because it includes products deliberately added to the system to build and maintain the rheology and stability of drilling fluids.

Petroleum, whether crude or refined products, needs no longer to be added to water-base muds. Adequate substitutes exist and are economically viable for most situations. Levels of 1% or more of crude oil may be present in drilled rock cuttings, some of which will be in the mud.

Common salt, or sodium chloride, is also present in dissolved form in drilling fluids. Levels up to 3,000 mg/L of chloride and sometimes higher are naturally present in freshwater muds as a consequence of the salinity of subterranean brines in drilled formations. Seawater is the natural source of water for offshore drilling muds. Saturated-brine drilling fluids become a necessity when drilling with water-base muds through salt zones to get to oil and gas reservoirs below the salt. In onshore drilling, there is no need for chlorides above these background levels. Potassium chloride has been added to some drilling fluids as an aid to controlling problem shale formations. Potassium acetate or potassium carbonate are acceptable substitutes in most of these situations.

Heavy metals are present in drilled formation solids and in naturally occurring materials used as mud additives. The latter include barite, ben-tonite, lignite, and mica (sometimes used to control mud losses downhole). There are background levels of heavy metals in trees that carry through into lignosulfonate made from them.

Attention has focused on heavy metal impurities found in sources of barite. Proposed U.S. regulations would exclude many sources of barite ore based on levels of contamination. European and other countries are contemplating regulations of their own.

Chromium lignosulfonates are the biggest contributions to heavy metals in drilling fluids. Although studies have shown minimal environmental impact, substitutes exist that can result in lower chromium levels in muds. The less-used chromium lignites (trivalent chromium complexes) are similar in character and performance, with less chromium. Nonchromium substitutes are effective in many situations. Typical total chromium levels in muds are 100-1000 mg/L.

Zinc compounds such as zinc oxide and basic zinc carbonate are used in some drilling fluids. Their function is to react out swiftly sulfide and bisulfide ions originating with hydrogen sulfide in drilled formations. Because human safety is at stake, there can be no compromising effectiveness, and substitutes for zinc have not seemed to be effective. Fortunately, most drilling situations do not require the addition of sulfide scavengers.

Indiscriminate storage and disposal practices using drilling mud reserve pits can contribute toxicity to the spent drilling fluid. The data in Table 1.6 is from the EPA survey of the most important toxicants in spent drilling fluids. The survey included sampling active drilling mud (in circulating system) and spent drilling mud (in the reserve pit). The data show that the storage disposal practices became a source of the benzene, lead, arsenic, and fluoride toxicities in the reserve pits because these components had not been detected in the active mud systems.

TABLE 1.6 Toxicity Difference between Active and Waste Drilling Fluids









Rate, %






















The third source of toxicity in drilling discharges are the cuttings from drilled rocks. A study of 36 cores collected from three areas (Gulf of Mexico, California, and Oklahoma) at various drilling depths (300 to 18,000 ft) revealed that the total concentration of cadmium in drilled rocks was more than five times greater than the cadmium concentration in commercial barites. It was also estimated, using a 10,000-ft model well discharge volumes, that 74.9% of all cadmium in drilling waste may be contributed by cuttings, but only 25.1% originate from the barite and the pipe dope.

Mud Toxicity Test for Water-Base Fluids The only toxicity test for water-base drilling fluids having an EPA approval is the Mysid shrimp bioassay. The test was developed in the mid-1970s as a joint effort of the EPA and the oil industry. The bioassay is a test designed to measure the effect of a chemical on a test population of marine organisms. The test is designed to determine the water-leachable toxicity of a drilling fluid or mud-coated cutting. The effect may be a physiological or biochemical parameter, such as growth rate, respiration, or enzyme activity. In the case of drilling fluids, lethality is the measured effect. For the Mysid test, all fluids must exceed a 30,000 concentration of whole mud mixed in a 9:1 ratio of synthetic seawater.

Nonaqueous Fluid (NAF) and Drilling Fluid Toxicity Until the advent of synthetic-based invert emulsion fluids in the early 1990s, the discharge of NAF was prohibited due the poor biodegradability of the base oils. In 1985, a major mud supplier embarked on a research program aimed at developing the first fully biodegradable base fluid. The base fluid would need to fulfill a number of criteria, regarded as critical to sustain drilling fluids performance while eliminating long-term impact on the environment:

  • Technical performance — the fluid must behave like traditional oil-base muds and offer all of their technical advantages
  • The fluid must be nontoxic, must not cause tainting of marine life, not have potential to bioaccumulate, and be readily biodegradable.

Research into alternative biodegradable base fluids began with common vegetable oils, including peanut, rapeseed, and soybean oils. Fish oils such as herring oil were also examined. However, the technical performance of such oils was poor as a result of high viscosity, hydrolysis, and low temperature stability. Such performance could only be gained from a derivative of such sources, so these were then examined.

Esters were found to be the most suitable naturally derived base fluids in terms of potential for use in drilling fluids. Esters are exceptional lubricants, show low toxicity, and have a high degree of aerobic and anaerobic biodegradability. However, there are a vast number of fatty acids and alcohols from which to synthesize esters, each of which would have unique physical and chemical properties.

After 5 years of intensive research, an ester-based mud that fulfilled all of the design criteria was ready for field testing. This fluid provided the same shale stabilization and superior lubricity as mineral oil-based mud but also satisfied environmental parameters. The first trial, in February 1990, took place in Norwegian waters and was a technical and economic success. Since then, over 400 wells have been drilled world wide using this ester-base system, with full approval based on its environmental performance. This history of field use is unrivalled for any synthetic drilling fluid on a global basis, and no other drilling fluid has been researched in such depth. The research program included

  • Technical performance testing using oil-base mud as a baseline
  • Toxicity to six marine species, including water column and sediment reworker species
  • Seabed surveys
  • Fish taint testing
  • Aerobic and anaerobic biodegradability testing
  • Human health and safety factors

The release of ester-base fluids onto the market marked the beginning of the era of synthetic-base invert drilling fluids. Following the success of esters, other drilling fluids were formulated that were classed as synthetics, these fluids included base oils derived from ethylene gas and included linear alpha olefins, internal olefins, and poly-alpha olefins.

Summary of Flashpoint and Aromatic Data With the introduction of synthetic-base muds into the market, the EPA moved to provide guidelines on the quality and quantity of the synthetic oils being discharged into the Gulf of Mexico. In addition to the water column aquatic testing done for water-base fluids, the EPA set forth guidelines for examining toxicity to organisms living in the sediments of the seafloor. ALeptocheirus sedimentary reworker test was instituted in February of 2002 for all wells being drilled with synthetic-base muds to examine how oil-coated cuttings being discharged into the Gulf of Mexico would impact the organisms living on the seafloor.

Two standards were set forth to govern the discharge of synthetic-base muds. A stock standard test is required for the base oil looking at the biodegradability of a synthetic base oil and as well as a new test for the Leptocheirus sedimentary reworker. This stock standard is done once per year to certify that the base oils being used are in compliance with the regulation.

Brine Drilling Fluids
FIGURE 1.11 Base fluid type.

When a well is drilled with a synthetic-base mud, monthly and end-of-well tests are required for the Mysid and Leptocheirus, organisms to ensure that the synthetic-base oil being used meets a certain standard of environmental performance. There are two standards that can be used for these annual and well-to-well tests: an ester standard and a C 1618 Internal Olefin standard.

The test used as a standard is based on the type of base oil being tested against a similarly approved standard. Base oils that are less toxic and highly biodegradable would be compared with esters, while all others would have to meet or exceed the C1618 IO standard (Figure 1.11 and Table 1.7).

With synthetic-based muds being widely used in the Gulf of Mexico, especially in deep water, controlling the quality of these materials is extremely important to the environment.

From US-EPA2001NPDES General Permit for New and Existing Sources in the Offshore Subcategory of the Oil and Gas Extraction Category for the Western Portion of the Outer Continental Shelf of the Gulf of Mexico (GMC290000) 66 Fed. Reg. No. 243, p. 65209, December 18, 2001.

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