The New Look in Oil-Mud Technology

Abstract

Abstract Liquids having oil as the continuous phase long have been recognized as desirable for drilling and completing under certain conditions. Performance limitations, high costs and inconvenience of handling, however, have kept these oil muds from becoming accepted for widespread, general use. As previously used, the muds consisted of a body in g agent for oil, or primary emulsifier of water in the oil, and a series of supplementary additives to modify that principal component. In recent years a new approach to oil muds has been taken. Oil-dispersible colloids have been developed to perform specific functions in oil without dependence upon thickening of the oil or emulsification of water. Such colloids are typified by an organophilic clay for suspension of solids and an organic filtration control agent. These colloids are effective and stable at well temperatures presently encountered and do not require supplementary additives. They also are compatible with wetting agents and emulsifiers commonly used to provide tolerance for water and solids in oil muds. With the development of independently effective additives, oil muds now have much of the control and variability achieved with water-base muds. Field examples demonstrate that oil systems are being used successfully for drilling, coring and completing under extreme conditions of both low and high temperature and pressure. Field personnel utilizing ordinary rig equipment can prepare and control the oil muds. In areas of concentrated use, liquid mud facilities are providing further economy and convenience. Introduction Drilling fluids having oil as the continuous phase were first conceived and used in the middle 1930's. The value of these oil systems was immediately recognized, particularly in providing quicker and easier well completions and in increasing the initial rate of oil recovery. Oil systems have not been used extensively in the past because of performance limitations and high costs that can be attributed to the general inflexibility of the systems. This inflexibility resulted primarily because one basic component was used in an attempt to arrive at all mud properties. These oil systems can be compared with the early water-base muds. Water-base muds in the early years of drilling depended upon bentonite for rheological properties, suspension of solids and filtration control. As was learned with these earlier clay-water systems, one basic component cannot by itself provide all of ideal drilling-fluid properties. Today each water-base drilling-fluid property can be controlled with an exactness that was not believed possible even 10 years ago. Water-base muds are prepared to have certain properties to serve specific purposes, some examples of which include the following: the low- solids, low-viscosity, low-filtration fluids for hard-rock drilling; the high-solids, low-viscosity, low-filtration, inhibitive muds for shale drilling in areas of abnormal pressure; and the high-viscosity high- filtration muds that are left in wells between casing and tubing. Properties of these muds can be varied over wide ranges, and the muds can be converted from one type to another as the need arises. The control and variability of water-base muds are possible because of the large number of additives currently available for performing specific functions. With the development of new additives and new technology, oil muds now have achieved a similar variability in performance, cost and convenience. This paper describes the development and performance mechanisms of materials that independently control specific properties of oil muds. Also discussed in the paper are field applications of this new oil-mud technology. The Need for Oil Systems Oil systems have been used for many years to protect pay zones having productivities that can be impaired by water-base muds. Completions using oil muds with good filtration control consistently have resulted in greater initial production rates than could be obtained with water-base muds--often saving the time and cost of fracturing, acidizing, or extensive swabbing. Cores often must be obtained for use in reservoir-evaluation studies. JPT P. 1177^