Modeling, simulation and interpretation of spontaneous potential logs to quantify hydrocarbon saturation

Abstract

The Spontaneous Potential (SP) log has served for decades as a borehole correlation log and, under favorable circumstances, for the reliable in-situ assessment of water resistivity in rock formations of interest. Nevertheless, it is known from laboratory and field measurements that SP logs are sensitive to the presence of hydrocarbons. This report introduces the principles of SP log modeling using a mechanistic approach and describes their implementation in a SP log numerical simulator. Various synthetic and field cases verify the capabilities and improvement due to numerical modeling in the interpretation of SP logs. Quantification of hydrocarbon pore volume from SP logs is currently being validated with laboratory experiments. Those results and any modification to the models introduced in this document will be reported in a future doctoral dissertation. The main contribution and conclusions from this thesis originate from the presence of the electrical double layer (EDL) on the surface of mineral grains, its impact on SP measurements in the presence of hydrocarbons, and the utility and limitations it poses for the calculation of hydrocarbon pore volume and porosity. These petrophysical properties are independent of size. Thus, pore-size distribution and the volume of influence of the EDL in the pore space both determine whether SP logs will capture valuable information about hydrocarbon pore volume. Field cases are presented in which quantification of hydrocarbon pore volume is possible. Simulations made using the mechanistic principles presented in this work show consistency both in modeling and in comparison to measurements at the borehole scale. These field cases consider both water- and hydrocarbon-bearing formations in distinct petrophysical and geological environments. Calculations of hydrocarbon saturation and porosity are verified by the use of porosity and resistivity logs. The theory and results reported in this research highlight the importance of the EDL and the strong impact EDL has on improving interpretation of SP logs. Petrophysicists benefit from the possibility of mechanistically simulate SP logs that indicate the presence of hydrocarbon pore volume. This capability is useful in cases such as fresh-water environments where interpretation of resistivity logs may be problematic, or in mature hydrocarbon fields where only SP logs are available to the interpreter. The ability to simulate SP logs, particularly in mature hydrocarbon fields, offers a faster and less expensive way to evaluate new or overlooked gas or oil reservoirs.