Low-Viscosity Base Fluid for Low-Toxicity Oil-Mud Systems

Abstract

Summary The use of a new low-toxicity oil for drilling-fluid systems is presented. The low-viscosity, low-aromatic oil has undergone extensive presented. The low-viscosity, low-aromatic oil has undergone extensive laboratory and toxicity testing with favorable results. In addition, a number of field tests have been conducted to evaluate the use of this oil in oil-based muds, water-based muds, and spotting fluids. These tests were very successful and showed excellent performance of the oil for each application. Evidences of improved penetration rates (ROP's), reduced mud-treatment costs, and better borehole stability are all characteristic of this low-viscosity, low-aromatic oil in low-toxicity, oil-based mud systems. Introduction Over the past several years, the use of oil-based drilling fluids in the petroleum industry has increased significantly. The increased use of these petroleum industry has increased significantly. The increased use of these invert-emulsion systems has occurred as a result of their proven advantages over water-based drilling fluids in many difficult drilling situations. Oil muds are favored over water muds for many reasons, including (1) excellent thermal stability when deep, high-temperature wells are drilled; (2) lubricating characteristics that aid in drilling deviated wells offshore; and (3) ability to drill thick, water-sensitive, active shales with little or no stuck-pipe or hole-washout problems. The primary concern when conventional, diesel-based oil-mud systems are used is their potential for adverse environmental impact. This is especially true during drilling in such ecologically sensitive locations as offshore or in wetlands near shore. Sec. 311 of the U.S. EPA Clean Water Act prohibits the discharge of any material that will result in a sheen or discoloration of the surface water or adjoining shoreline. Such environmental regulations have forced the use of expensive cuttings washers and special oil-mud-cuttings handling equipment. In most cases where conventional, diesel-based oil-mud systems are used, the drill cuttings must be collected and transported to shore for proper disposal. Among the many components that make up a whole oil mud, the base fluid, No. 2 diesel oil, is the component of major concern because of the known toxic effect of diesel on marine and freshwater organisms and because diesel oil creates a "polluting" sheen on the water surface. The diesel replacement-i.e., an environmentally safe base fluid-should be an oil that will not create a sheen or discoloration on the water surface and will show no or low toxicity to marine organisms. The oil must also be compatible with current-market oil-mud additives and be capable of producing a stable invert-emulsion oil mud with performance properties producing a stable invert-emulsion oil mud with performance properties equal to or superior to diesel-oil-mud systems. Mineral-oil muds were introduced as drilling-fluid systems in 1980–81 to offer a low-toxicity, nonpolluting alternative to diesel-mud systems. In an effort to understand and to improve low-toxicity oil-mud systems, especially with regard to the high-viscosity mineral oils being used, our Production Research Div. (in conjunction with other divisions) has recently Production Research Div. (in conjunction with other divisions) has recently tested and proved a new low-viscosity, low-aromatic base oil that offers some advantages over both diesel and other commercial mineral-oil muds. Laboratory Study In the search for an oil that would show viscosity improvements (i.e., lower viscosity) over the current-market commercial mineral oils, yet retain the low-toxicity environmental advantages of these mineral oils, we sought an oil that had the following properties:water-white and clear,causes no sheen or discoloration on water,shows low toxicity to marine organisms (with approved EPA acute-toxicity test procedures),has a low aromatic content (especially low polar and polynuclear aromatics), nonfluorescent,has a low viscosity (i.e., diesel oil),has an aniline point >160F [71C] and flash point >150F [66C],is chemically compatible with current-market oil-mud additives, andis available and reasonably priced. An oil having all of these properties was identified during the second quarter of 1982. This low-viscosity, low-aromatic oil was found to differ from most other "mineral oils" in that it is composed of >99% cyclic and branched paraffins with an average carbon number of C . It contains less than 1% normal paraffins and less than 0.1% aromatic compounds. Its viscosity is 2.2 cSt [2.2 mm2/s] at room temperature compared with 3.8 cSt [3.8 mm2/s] for diesel oil and 7.4 cSt [7.4 mm2/s] for a currently marketed commercial mineral oil. Additional property comparisons are shown in Table 1. SPEDE p. 218