The effect of clay, pressure and temperature on the electrical and hydraulic properties of real and synthetic rocks

Abstract

This thesis presents the methods and results of a systematic study of the effect of clay, pressure and temperature on the electrical and hydraulic properties of natural and synthetic rocks. A better understanding of the mechanisms of the effect of pressure and temperature on rock petrophysical characteristics has been achieved in this study. An improved shaley sand interpretation technique based on the experimental data and the numerical simulation results has been developed. A novel multi-sample high pressure and high temperature rock testing system connected to an automatic data acquisition unit has been developed to measure the electrical and hydraulic properties of 5 core plugs of 1.5" in diameter simultaneously. The use of this novel experimental system increases the speed of testing and, since all the samples are placed in identical conditions, eliminates experimental comparison errors caused by the fluctuations of pressure and temperature. A new technique of making synthetic shaley rock samples with desired clay type, content and distribution mode has been developed in order to study systematically the effect of clay minerals on the electrical properties of shaley sands at both room and reservoir conditions. The synthetic shaley samples made are tested in the multi-sample high pressure and high temperature experimental rig. Two 3D pore space network models (NETSIM and CLAY) have been developed to interpret and predict the effect of pressure, temperature and clay on the electrical and hydraulic properties of porous rocks. NETSIM relates the electrical resistivity and permeability of porous rocks to their microscopic pore structures and, therefore, provides an insight into the mechanisms of the effect of pressure and temperature. CLAY simulates the effect of clay content and clay distribution on shaley sand conductivities, and provides a means for quantifying the clay distribution coefficient defined in this study. Significant effect of pressure and temperature on both the electrical resistivity and the absolute permeability of sandstone rocks has been observed. This effect is more pronounced for less porous and permeable rocks compared with more porous and permeable rocks. The combined effect of pressure and temperature on the electrical properties of sandstone rocks is found to be approximately the sum of their individual effect. In addition to the actual effect of pressure and temperature, a significant hysteresis effect has been observed in both pressure and temperature tests. In order to ensure the accuracy of the estimation of in situ rock porosity and hydrocarbon saturation from electrical logs, the Archie cementation factor and saturation exponent need to be quantified in the laboratory at simulated reservoir conditions accounting for the effect of coupled pressure and temperature and the hysteresis. An improved Weixman-Smits model is proposed by including a temperature coefficient (w) for the equivalent clay counterion conductivity, which is a function of temperature and clay content, and a clay distribution coefficient (r) for the clay content [Qv). The values for w as a function of temperature and clay content have been determined experimentally. The clay distribution coefficient (r) as a function of clay distribution modes has been quantified for identified clay distributions based on the numerical model (CLAY). The use of this modified W-S model enables an improvement in the accuracy of the estimation of in situ porosity and hydrocarbon saturation of shaley formations.