Stabilizing Sensitive Shales With Inhibited, Potassium-Based Drilling Fluids

Abstract

Laboratory and field studies have demonstrated that potassium cations at sufficient concentrations in water-based drilling fluids can effectively reduce the swelling and dispersive tendencies of clay-containing shales. Here is a unified classification method for relating the amount of water-sensitive clay in a shale to the occurrence and severity of wellbore problems. Introduction Proper selection of the drilling fluids to be used on Proper selection of the drilling fluids to be used on a particular wellsite is an essential phase of any carefully planned drilling operation. When this drilling is expected to encounter water-sensitive shale zones, the selection of the fluid becomes even more important. To maintain a stable borehole through such zones, an inhibitive drilling, mud will often be required. The design of successful fluids for this type of application depends largely on a knowledge of the physical and mineralogical characteristics of the shale and its behavior when contacted by drilling mud. We shall discuss here procedures and experimental results of laboratory tests in which a wide variety of fluids and shale types were used. The data should aid in designing effective inhibitive muds.Sensitive shale formations are encountered in most major oil fields throughout the world. These formations, referred to by a variety of names, including mud-making shale, caving shale, heaving shale, and sloughing shale, frequently cause wellbore instabilities. Drilling and completion problems associated with such instabilities include the accumulation of fill on the bottom after trips, tight holes, stuck drill pipes, solids buildup in the mud, hole enlargement pipes, solids buildup in the mud, hole enlargement and washouts, and poor primary cement jobs. Stable, gauge boreholes, reduced rig time, and lower total costs could be realized if efficient inhibitive systems were employed. Presently the only way to insure success in many problem formations is to use balanced-activity oil muds. Cost factors and handling considerations, however, suggest that water-based fluids be designed to perform the same function.In recent years many new water-based mud systems have been devised. Most of these rely on polymer or combinations of polymer and salt to polymer or combinations of polymer and salt to achieve a given level of inhibition. Low-solids drilling, using total or selective polymeric flocculants to achieve controlled clay yields, has been an important step forward. The new directions that drilling fluids in general and inhibitive fluids in particular are taking promise still further advances. promise still further advances. Causes of Shale Problems To investigate properly the nature of problems in drilling shales one must first study the various related physics-chemical processes. The general class of physics-chemical processes. The general class of shale problems presented in the introduction results from the following interrelated factors:shale hydration and swelling,dispersion of shale cuttings, andabnormal pressure.In addition, other processes and borehole conditions tend to aggravate the instability. These includetime spent in the open shale zone,in-situ stresses and formation characteristics, andmechanical and erosive action. Shale Hydration and Swelling Shales are basically sedimentary rocks that have been laid down over geologic time in marine basins. JPT P. 1089