Digital SCAL for Improved Determination of Water Saturation in a Low Resistivity Pay Ooidic Middle East Formation

Abstract Digital rock analysis was used to develop a transform of deep resistivity to water saturation in a Low Resistivity Pay (LRP) Ooidic formation. These reservoirs often exhibit low resistivity which lead to pessimistic hydrocarbon volumes using traditional Archie's relationships. The paper presents the causes of LRP condition. The scope of this analysis was to obtain digital drainage capillary pressure and resistivity index (PCRI) data in rock types with significant volume fraction of micro porosity where physical experiments are difficult to perform. The digital PCRI data yielded variable Archie saturation exponents leading to new resistivity-water saturation transforms. The Ooidic formation had three main rock types characterized by the volume of calcite cement in the macro pore system. The representative digital rocks were used to generate drainage PCRI data. Physical lab measurements included helium poro-perm tests, mercury injection, thin sections, and mineralogy. The outer porous layers of the ooids were partially saturated with high salinity water that created the LRP. The nano-environments translate to low rock quality that requires long experimental times, especially using porous plate drainage capillary pressure experiments. Representative three-dimensional digital rocks were generated using a combination of X-ray computed tomography (CT) and focused ion beam – scanning electron microscopy (FIB-SEM) methods. Digital rocks were used to simulate drainage PCRI experiments followed by a new resistivity-water saturation transform. The digital rocks representing the outer layers of the ooids were used to generate PCRI data that matched well the MICP responses. The PCRI data for the micro porosity were integrated with the macro porosity data to create unique upscaled RI responses. The resulting Archie's saturation factor, "n", was variable with water saturation and unique for each rock type. Texture analysis of whole core CT data provided the location of the rock types in the logs. The new correlations between deep resistivity and water saturation were applied to generate an updated water saturation log that showed a significant decrease in water saturation in the LRP zone. The novel aspect of this project is the generation of digital PCRI data for the porous outer layer of the ooids that explains the presence of a low resistivity pay zone. Obtaining such data with physical laboratory experiments is difficult and time consuming. Digital upscaling of the micro and macro PCRI data provided the means to map Archie's "n", construct a new transform equation, and generate an updated water saturation log for the well. The digital/physical integrative workflow was used for the first time in an Ooidic formation. A streamlined workflow may rapidly produce updated logs to increase the certainty in quantifying the original hydrocarbon in place in an exploration campaign or field development.