A Method for Determining 3-D Anisotropy of Electrical Resistivity in Multifaceted Shale and Mudstone Samples: Application to the Horn River Basin, British Columbia
The analysis of anisotropy especially in unconventional reservoirs is becoming increasingly important in modelling reservoir response particularly to hydraulic fracturing. As the electrical response in shales and mudstones depend on microstructure and intrinsic anisotropy, an understanding of resistivity anisotropy may provide insights into their stress response to loading. Although multi-component induction logging data for some shales show that electrical resistivities perpendicular to bedding are greater than electrical resistivities parallel to bedding, an understanding of 3D electrical response of shales and mudstones is lacking. Moreover, logging tools that measure both horizontal and vertical resistivities are not commonly used. In this study, a method for determining the anisotropy of electrical resistivity of the Devonian Horn River Group of the Horn River Basin, British Columbia, is detailed and factors controlling the electrical fabric are determined by relating results to elements of petrofabric observed in thin section. The formations of the Horn River Group are an ideal sample set with three distinct lithofacies identified. Siliceous clay rich mudstone intervals are observed in the Muskwa and Otter Park, argillaceous laminated shales dominate the Otter Park and clay rich calcareous mudstones make up the Evie formation. Traditional laboratory methods of measuring anisotropy require that core plugs be taken in a number of desirable directions. A 3 dimensional look at electrical resistivity is difficult with these methods since multiple plugs have to be taken for one sampling interval/depth and the available sample size may not allow for this. Moreover, the fissile nature of shales makes it challenging to retrieve multiple usable plugs from a sampling interval. In the method used in this study, for each interval sampled, 18 different directional electrical resistance measurements are made on a single eight sided prism shaped sample saturated in a potassium chloride electrolyte solution. To convert directional resistances to resistivities, integration in the direction of current flow is used. From the directional resistivities, the ellipsoid describing the electrical resistivity tensor is determined. Results show that all three formations in the Horn River Group are transversely anisotropic with an axis of symmetry perpendicular to the bedding plane. All formations also exhibit varying, but strong electrical anisotropy. Mineralogy, especially the distribution and alignment of clays and carbonate grains as well as the presence of fine laminations appear to be the primary control on the anisotropy of electrical resistivity. The presence of pyrite streaks and lenses in the Otter Park and Muskwa may also contribute to their anisotropy of electrical resistivity.
AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015