Sca 2001-34.pdf

Low resistivity in argillaceous sandstone reservoirs may be attributed either to the effect of microporosity, or to specific effects due to clays and/or to conducting minerals. In the frame of a study on pore lining chlorite bearing sandstones, from different hydrocarbon reservoirs, measurements of cation exchange capacities and specific surface areas have been performed, on whole rocks as well as on size separated fractions. Both sets of values are low, in agreement with the structural and textural observations. The dielectric properties, including intrinsic conductivity, of these chlorites have been investigated, on size separated fractions, as a function of temperature, and compared to that of reference clays. The results show a large influence of the moisture, current frequency, and temperature on the electrical properties. The chemical composition and structure of the clays has an obvious influence as well. However, the range of conductivities is such that the intrinsic influence of clays on the conductivity of argillaceous sandstone is lower than that related to the presence of water by several orders of magnitude. INTRODUCTION In the frame of a Consortium study on pore lining chlorite bearing sandstones, formation resistivities have been found in the range 0.3 to 20 Ω.m, at depth, in the different hydrocarbon reservoirs made available by the partner companies. Laboratory experiments performed at room conditions on plugs selected from these reservoirs have shown that saturation exponents n drawn from resistivity index measurements are lower than the usual 2 value (Durand et al, 2000). The conclusions of the laboratory study were that the microporosity created by the texture of the chlorite pore lining had a major effect on the values of resistivity index, and that the cation exchange capacities (CEC) and specific surface area (SSA) effects were subordinated, due to the low values of these parameters. However, the question of the specific contribution of chlorite to low resistivity in sandstones was still open, due to poor documentation in literature. Moreover, the possible effect of temperature on these properties was not studied. According to Worthington (2000), low resistivity in argillaceous sandstone reservoirs may be attributed either to the effect of microporosity, or to specific effects due to clays and/or to conducting minerals. Effect of CEC of usual clays, like smectite, illite and kaolinite, on resistivity measurements of sandstones has been widely studied in literature, and has contributed in the building of well known log interpretation models (Waxman and Smits,1968, Waxman and Thomas, 1974, Clavier et al, 1984, Raiga-Clemenceau et al., 1984, Revil et al., 1998 ...). Even the models which do not deal explicitly with CEC (Poupon and Leveaux, 1984, Givens, 1991) assume a “clay conductivity”. However these 1 Corresponding author: claudine [email protected] 2 Now with EGID, Bordeaux, France.