Combined resistivity and capillary pressure measurements using micropore membrane technique

Abstract

Capillary pressure measurements using water and oil wet micropore membranes can be combined with four electrodes resistivity measurements in a simple apparatus. Such a device can improve significantly resistivity log interpretation while providing data for reservoir simulations. The originality of the device is that resistivity is measured using current and potential electrodes located around the sample as in electrical tomography. To speed up capillary pressure measurements with thin semi-permeable membranes, the sample must be short (e.g. 2.5 cm) while having a large diameter (e.g. 4 cm) to minimise saturation measurement errors. Hence, the classical implementation of the electrodes on the faces and around the sample is technically very difficult for various reasons. In the design presented, the electrodes do not interact with the membranes and the determination of resistivity is possible in any capillary pressure cycle, whether or not one face is saturated with oil. The method is validated using results from experiments and numerical simulations. Combined capillary pressure and resistivity measurements on a water-wet outcrop sandstone indicate a single value of saturation exponent in drainage and imbibition and a very good correlation of the data point in log–log scale. When a 2-electrode technique is used, experimental data indicate also that contact resistance can lead to severe artefact of measurements such as the appearance of a hysteresis between drainage and imbibition. Numerical simulations of the electrical field were performed to test the design of the electrodes and optimise their size. It is shown that a large part of the sample contributes to the measurements and short cuts due to conductive end pieces do not affect the results.