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SPE Members Abstract Methods currently being used to measure the electrical conductivity of fluid saturated porous media are, highly sensitive to various capillary and electrical effects. If these effects are not taken into account when making electrical measurements on cores, large errors in the computed values of the resistivity index (I) and the Archie saturation exponent (n) can occur. Resistivity was monitored at 1 inch [25.4 mm] and 2 inch [50.8 mm] intervals along 6 inch [152.4 mm] Berea cores in order to observe electrical and capillary end effects. Computerized Axial Tomography (CAT Scan) was utilized to examine fluid distributions within the sandstone cores under water-wet primary drainage conditions. When the CAT Scan data was combined with electrical resistivity data, the magnitude of electrical contact resistances between the current electrodes and the core can be estimated as well as the saturation variation due to the capillary end effect. It was found that the contact resistances are substantial at low water saturations and in oil-wet cores. Introduction Researchers have been measuring the electrical properties of fluid saturated rocks for many years. Numerous techniques have been devised for measuring the electrical resistivity of rocks under varying conditions of stress, temperature and wettability. Methods for measuring electrical resistivity can be separated into two general categories, which are: Two-electrode and four-electrode measurements. Previous works have shown that two-electrode and four-electrode electrical measurements produce differing values of resistivity in rocks which have been saturated with both conductive and non-conductive phases. Dunlap indicated that contact resistances which develop between the current/voltage electrodes and the core could lead to resistivity measurement errors with two-electrode systems. He advocated using a four-electrode system with a very high internal impedance voltmeter, where voltage electrodes which are separate from the current electrode are used for measuring voltage drops along the core. Also, saturation gradients caused by capillary boundary effects can lead to uncertain water saturations between the electrodes used to make the voltage drop measurements. This uncertain water saturation between the electrodes can lead to erroneous calculations of the saturation exponent (n). Not all two-electrode data is unreliable. When water-wet cores with a high water saturation are used, particularly with electrodes with large surface areas (platinum black, etc.) two and four-electrode data are often quite similar. However, with low water saturations, and especially with oil-wet cores, the differences can be substantial. In order to better understand the effects of electrical contact resistances, electrical resistivity measurements were made on Berea sandstone cores which were saturated with petroleum naptha and a sodium chloride brine. The cores varied from strongly water-wet to oil-wet. Both two and four-electrode resistivity measurements were made to determine the differences between the two electrical systems. Water saturation profiles along water-wet cores under drainage were determined using data obtained from Computerized Axial Tomography. Using the CAT Scan data, four-electrode resistivities could be combined with corrected water saturations to calculate the saturation exponent. The significance of electrical measurements on cores is a function of how the measurements are made. Much of the previous work in the area of electrical conductivity of porous media has been subject to various electrical and capillary effects which have not been taken into consideration when the results of the experiments were analyzed. Large errors can be made when applying petrophysical parameters obtained under less than ideal conditions. Since many reservoir models use the electrical data derived from core analysis, it is critical that the information obtained from electrical studies be both accurate and consistent. EXPERIMENTAL METHOD Berea sandstone was used for all of the electrical measurements in this work. The Berea was a relatively homogeneous, arkosic sandstone with a small fraction of clay, primarily kaolinite and chlorite. P. 697 |
