New Base Oil Used in Low-Toxicity Oil Muds
Authors: Boyd P.A. , Whitfill D.L. , Carter T.S. , Allamon J.P. 
Published: December 2007

The use of a new and different low-toxicity, low-pollutant oil-mud base fluid is presented. This low-polynuclear-aromatic (LPNA) oil differs from most other white mineral oils in that it is composed of 99+ % cyclic and branched paraffins with an average carbon number of C13. It contains less than 0.1 % aromatics and less than 1 % n-paraffins. This base oil composition provides for some interesting oil mud properties, which are discussed. Toxicity studies by an independent laboratory show the LPNA oil and the oil-mud cuttings to be substantially less toxic than diesel oil and the diesel-mud cuttings, respectively. Oil retention on the cuttings was shown to he less than for a comparable diesel mud. Laboratory formulation data, toxicity testing, and field test results are presented and discussed. These studies show the LPNA-oil-mud system to be an acceptable substitute for diesel mud systems and especially applicable to environmentally sensitive drilling locations.

Introduction

Advantages of Oil-Based Muds

In recent years, there has been a marked increase in the use of oil-based drilling fluids. The widespread usage of these fluids has come about as a result of the proved advantages of oil-based muds over water-based muds in many difficult drilling situations. Oil-based drilling fluids have many favorable characteristics, including:excellent thermal stability for drilling deep hot holes;inherent protection against acid gases (e.g., CO2 and H2S) and corrosion caused by the presence of the continuous oil phase and the alkalinity control reagent (lime);capability of drilling water soluble formations (e.g., salt, complex salts, gypsum, and anhydrite) with little or no hole washout problems;improved lubricity, which aids in drilling deviated holes and reduces occurrences of "stuck pipe" problems;protection of producing formations from water intrusion, protection of producing formations from water intrusion, thereby eliminating clay swelling, which could result in reduced permeability of the pay zone; andability to drill thick, water-sensitive shale sections with relative ease, thus providing for gauge-hole drilling.

This last characteristic is accomplished through the design of the internal brine phase of the oil-based mud. Such a system can be structured to provide an osmotic dehydration of the formation shale, thus preventing hole sloughing and providing for the stabilization of the borehole. providing for the stabilization of the borehole. When considering a drilling fluid program for a given well, the economics of a mud system is certainly of prime importance. In many cases, oil muds are not considered (although the drilling conditions warrant their use) because of their high initial mud cost (two to four times greater than most water-based muds). However, if the overall drilling costs are considered, the costs accompanying the use of an oil mud are usually comparable with or less than those of a water-based fluid. This is because most of the drilling problems are eliminated, resulting in substantial time savings. In addition, oil muds can be reused or resold following the drilling operation.

Environmental Concerns

The major concern in the use of oil-based drilling fluids revolves around their potential for adverse environmental impact, especially in potential for adverse environmental impact, especially in ecologically sensitive areas (e.g., active reefs or wetlands). In U.S. gulf coast drilling, current environmental regulations prohibit discharge to the sea of oil-based muds or any oil-mud cuttings that will cause a sheen. This is covered in Sec. 311 of the U.S. Federal Water Pollution Control Act (Clean Water Act) where Pollution Control Act (Clean Water Act) where Environmental Protection Agency (EPA) regulations disallow the discharge of any oil that will "cause a sheen or film or discoloration of surface water or adjoining shorelines - … "(In the context of this paper, the lack of a sheen, film, or surface discoloration on the water is referred to as "nonpolluting.")

Sec. 402 of the Clean Water Act also regulates the discharge of pollutants by National Pollution Discharge Elimination System (NPDES) permits. Such permits allow for the discharge of drill cuttings, provided no oil is released that will cause a sheen or discoloration of the surface water. Whole oil mud discharges are prohibited in every case. Interim environmental regulations by the U.K. Dept. of Agriculture and Fisheries allow for the discharge of oil-based drill cuttings in North Sea waters, provided that the cuttings have less than 5 wt% oil (measured by retort) if diesel oil is used as the base fluid, or less than 15 wt% oil if a mineral oil is used as the oil-mud base fluid. Again, no whole mud discharge is permitted in any case.

Oil Mud Components.

JPT

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