Resistivity Index and Capillary Pressure Hysteresis for Rock Samples of Different Wettability Characteristics

Abstract

SPE Members Abstract This paper presents the results of an experimental investigation into the effects of hysteresis (saturation history) on resistivity index and capillary pressure characteristics for some sandstone rocks of different wetting character. The drainage and imbibition resistivity index and capillary pressure measurements were carried out at equilibrium conditions using oil and brine with semi-permeable membranes. The displacing-cycle dependent hysteresis was also investigated experimentally. The resistivity profiles along the core length obtained from six potential electrodes enabled the assessment of equilibrium conditions, core and saturation heterogeneity and the end effects. All test samples (except for an oil-wet sample) were initially saturated with brine and placed in a multi-sample coreholder with a water-wet membrane at one end of the sample and an oil-wet membrane at the other end. A mineral oil was injected through the oil-wet membrane in incremental capillary pressure steps while a fixed back pressure in the water phase was maintained. At the end of the oil-displacing-water cycle, brine was then injected back through the water-wet membrane by decreasing the oil-phase pressure in steps until reaching the maximum entry pressure of the oil-wet membrane. To study the displacement-cycle dependent hysteresis the second desaturation cycle was performed on some samples from the oil saturation remaining after a water-flood at the end of primary saturation. Our results indicate that both the displacing-phase and displacing-cycle dependent hysteresis should be taken into consideration for relevant resistivity index and capillary pressure curves. The hysteresis effects were also found to depend on the wettability of the rock samples. A parametric study was presented to highlight quantitatively the petrophysical effects. Introduction For partially saturated, hydrocarbon-bearing rock, Archie proposed the following equation: where I is the resistivity index defined by the ratio between the resistivity of the formation when partially saturated by brine (Rt) and the resistivity when fully saturated by the same brine (Ro). Sw is water saturation fraction, and n is known as the saturation exponent. Laboratory measurements on core plugs obtained from the formations of interest remain the only reliable source for determining the saturation exponent n. The great variability of n values due to a range of factors including inappropriate laboratory procedures, can be a considerable source of error in the calculation of water saturation from resistivity logs. 2. BACKGROUND Different displacement processes may occur during the history of hydrocarbon reservoirs. For example the displacement of water by oil during the migration period; P. 519