Integration of Digital Rocks, MICP, Petrography, and Petrophysical Logs to Understand the Rationale Behind a Productive Low-Resistivity-Pay Carbonate Reservoir

Carbonate reservoirs pose significant challenges for a quantitative understanding of water saturation (Sw). This is primarily due to their complex and heterogenous pore structures. In many cases, the abundance of micropores in carbonate reservoirs can make a potential hydrocarbon bearing reservoir rock appear water bearing, a typical low-resistivity-pay (LRP) phenomenon. In such cases, the capillary-bound formation brine in the micropores provides a continuous path for the electric currents thus masking the presence of hydrocarbons and overlooking their true potential. Heterogenous carbonate reservoirs with complex pore structures can cause low resistivity pay due to multiple reasons including the presence of micropores, fractures, and conductive reservoir minerals. The computation of a realistic water saturation from such reservoirs requires alternative resistivity independent techniques. In this study, high resolution digital rock techniques were incorporated with scanning electron microscopy (SEM), petrography, and mercury injection capillary pressure (MICP) to understand pore structures, pore types, and their relative proportions. The work highlights the significance of understanding the distribution of pore types at plug scale and how this would highlight the true potential of a carbonate reservoir with high water saturation. Carbonate reservoirs can yield poor productivity due to the abundance of micropores making flow paths highly tortuous. In this study, we showcase an LRP looking carbonate reservoir exhibiting excellent reservoir quality. Due to the heterogeneity at plug scale, the entire reservoir looked homogenous on the log responses. The high fraction of micropores and its impact on resistivity log resulted in extremely high water saturation. The subsequent reservoir fluid sampling followed by the well test produced dry hydrocarbons. A series of measurements including digital rocks, SEM, thin sections, X-ray diffraction, and MICP were conducted on sidewall core plugs across the reservoir. The various independent measurements highlighted the presence of multiple pore systems at plug scale. The sedimentological investigations on thin sections revealed calcite cementation drastically reducing the pore volume. SEM results also indicated widespread and heterogeneously distributed calcite cementation. High resolution digital rock images showed both micropores and macropores co-existing at plug scale with micropores showing a higher percentage distribution. The non-resistivity based Sw measurements from digital rock and MICP helped in quality control of the final saturation model. This paper discusses the computation of water saturation using resistivity and non-resistivity based techniques. The integration of various subsurface, laboratory, and dynamic data enabled identification of the source of LRP in this oil-bearing carbonate reservoir. The study outcome significantly contributed to the uncertainty quantification of water saturation and development of a robust saturation model. In the development phase, this workflow would help reservoir simulation teams for accurate history matching, production forecasting and in optimized field development plan.