Drilling Fluids and Reserve Pit Toxicity
Authors: Leuterman Arthur J.J. , Jones Frederick V. , Candler John E. 
Published: December 2007

Drilling fluids are now classified as exempt under the Resource Conservation and Recovery Act (RCRA) hazardous waste laws. Since 1986, however, the U.S. Environmental Protection Agency (EPA) has been studying reserve pit coutents to determine whether oilfield wastes should continue under this exemption. Concerns regarding reserve pit contents and disposal practices have resulted in state and local governmental regulations that limit traditional methods of construction, closure, and disposal of reserve pit sludge and water. A great deal of attention and study has been focused on drilling fluids that eventully reside in reserve pits.

In-house studies show that waste from water-based drilling fluids plays a limited role (if any) in possible hazards associated with reserve pits. Reserve pit water samples and pit sludge were analyzed and collated. Analyses show that water-soluble heavy meta1s (Cr, Pb, Zn, and Mn) in reserve pits are generally undetectable or, if found in the total analysis, are usually bound to clays or organics too tightly to exceed the limitations as determined by the EPA toxicity leachate test. Our experience is that most contamination associated with reserve pits involves high salt content from produced waters and/or salt formations, lead contamination from pipe dope, or poorly designed pits, which could allow washouts into surface waters or seepage into groundwater sources. Our analyses show that reserve pits associated with water-based drilling fluid operations should not be classified as hazards; however, careful attention should be paid to reserve pit construction and closure to help avoid any adverse environmental Impact.

Introduction

Oil and gas drilling activities produce wastes that need to be disposed of in a manner that is both economical and safe for the environment. The traditional method of disposal has been to discharge wastes into a reserve pit located at the drill site. Upon completing the well, the pit waste can be dewatered and then filled, solidified, or treated in any number of ways on location or the waste can be removed for offsite disposal.1 The most economical method is to backfill the pit on site. State and federal agencies, however, have recently questioned this practice. Many states have passed laws regulating pit construction and closure,2 and in 1986 the federal government began a study to characterize reserve pit waste constituents to determine whether oilfield waste should remain an exempt waste under the federal hazardous waste laws.3

With this situation in mind, we decided to investigate reserve pit constituents containing waste from water-based driling operations. Chemical analyses were conducted on reserve pits shortly after drilling operations were completed. Results were collected and collated to determine the range of inorganic constituents present, and concentration of metals and ions were compared to various water-quality parameters to determine whether these levels would be considered a hazard to the environment or to humans.

Literature Review

Few studies are available that investigate the effects of reserve pits on soil, groundwater, or plant productivity. Miller4 summarized studies conducted since 1974 that were sponsored by the American Petroleum Inst. (API). Results indicated that some common constituents of drilling fluids can affect plant growth, but these effects can be reduced or eliminated by proper soil application. The two most detrimental constituents werehigh sodium levels, which resulted in hard soil crusts, andsoluble salts, which made water absorption by plants difficult.

Diesel used as a lubricant in some drilling fluids did show phytotoxic effects (inhibiting germination and/or killing seedlings) and also reduced soil wettability. Alkaline muds (high in sodium salts) caused the least problem in acidic soils having a high organic-matter content. The most difficult problem occurred when alkaline muds were added to alkaline soils in areas with low rainfall. Suggesed reclamation procedures includedaiding soluble calcium salts or gypsum.adding water, orallowing enough time for microbial decomposition.

Moseley5 summarized three other studies funded by API that substantiated Miller's work. Heavy metals from reserve pits were found to migrate slowly and were below standards set by the EPA for hazardous wastes. Some plants can and do take up heavy metals, but not at high levels and not at concentrations that can endanger wildlife.5 Nelson et al.6 postulated that low-grade barite might be one of the major sources of trace metals in soils. However, soluble levels of low-grade barite in the soil and uptake by plants, even in greenhouse pot studies, were minimal. All these studies agreed that mixtures of 1:4 mud to soil were adequate to safeguard the environment and groundwater sources.

Younkin and Johnson7 discussed the methods and effects of drilling waste-soil disposal in Canada. Their findings also showed that high-salt muds (KCl systems) caused the highest amount of damage to the natural plant cover, seed germination, and productivity. Salt content above 34,000 ppm was damaging to plant health and soil chemistry. Diesel fuel at or below 4,500 ppm had only a slight effect on plants, while lignosulfonate (165 ppm or less) and polymers (67,000 ppm or less) had little effect. No long-term-effects were noted for any of the reserve pits investigated and no damage was evident after 3 years.

Mikesell8 found that muds added to soils increased the total concentrations of zinc, copper, lead, chromium, and barium, but the metals were in a form that was not available for plant uptake. Murphy and Kehew9 found that shallow groundwater obtained from the unsaturated zone beneath reserve pits exceeded the recommended concentration limits for trace elements and major ions; however, levels were greatly reduced as the depth increased. The reduced levels beyond the immediate area of the pit suggested that very little impact was expected on usable groundwater resources.

Methods and Materials

As waste settles in a reserve pit, it splits into a water phase and a sludge phase, which is called the mud phase. Samples were collected from both the water and mud phases from 125 reserve pits dating from 1979 through 1986. The majority of the samples were from reserve pits located in west Texas, Oklahoma, and Louisiana. Samples were also collected in California and the Rocky Mountain states.