The effect of partially dehydrated clays on the adsorption potential of shales is presented, along with the resulting movement of fluids and the generation of swelling pressures. An experimental test cell is described which measures fluid diffusion rates for shale-fluid systems under temperatures to 350°, and confining and well bore pressure to 15,000 psi. Results for three fluids in contact with one shale are presented.
Wellbore instability problems are believed to be caused by mechanical and chemical factors. Previous researchers (Bradley 1979, Aadnoy 1987) have shown that mechanical effects depend on in-situ earth stresses, pore pressure, wellbore pressure, and hole angle. This area is fairly well understood, unfortunately the chemical effects which manifest themselves in shale-mud chemical reactions are not well known, and very few analytic solutions exist. The balanced activity oil muds which surfaced in the 70's (Chenevert 1970) provided solutions which eliminated the aqueous chemical reactions by building muds which had osmotic potentials which counteracted the adsorptive potentials of the shales. A second quality of such oil muds was the providing of membrane films which prevented ionic movements from the internal water phase of the mud into the shales. In this water-in-oil emulsion mud both water and ion movement was prevented, and no chemical reactions occurred. Water muds, on the other hand, have not been able to stop the flow of water and ions in such a total manner mainly because of the absence of semipermeable membranes which would restrict ionic flow. Also, the transfer of both water and ions has made analysis of such systems very difficult.
Shales are sedimentary rocks which usually contain large amounts of clays. As these rocks are dewatered during the geologic compaction process, individual clay platelets within the shale are forced closer together thereby increasing their potential to adsorb polar molecules. The electrical double layers with their exchangeable cautions and interfacial water provide a negatively charged environment which attracts positively charged ions and water (which is dipolar). When such de-watered shales are exposed to certain drilling fluids adsorption and subsequent swelling results. Shown in Figure 1 is the electrical potential on the surface of a completely hydrated platelet as a function of distance from the surface. Such platelets no longer react with fluids because their surface charge is completely satisfied with its adsorbed cautions and water envelope. During geologic compaction, however, water and ions are expelled, adjacent clay platelets are forced closer together, and there is a potential developed midway between the platelets. Figure 2 shows how the adsorptive potential "U" increases with a decrease in spacing between platelets (Mitchell 1976). As the potential between the platelets increases, many changes occur for the clay-fluid system, one being the lowering of the vapor pressure of the water phase. One way to determine a shale's potential to adsorb water is to measure its water vapor pressure, or its relative vapor pressure (p/po)also referred to as its water activity.
