Practical Petrophysical Evaluation of Thin and Heterogeneous Carbonate Reservoir with Resistivity Simulation and Variable Archie's Cementation Factor (m) Analysis in an Oil Field Offshore UAE and Qatar

Abstract Resistivity tools have been regularly run in an oil field and is one of the essential measurements for petrophysical analysis. However, due to the resistivity contrast in the formation (e.g., laminated reservoirs with tight resistive zones) and borehole geometry, it is not always possible to accurately measure the true formation resistivity. In addition, carbonate reservoir's heterogeneity adds a complexity in petrophysical analysis. This paper demonstrates a successful approach used to overcome these challenges. In Well-A of a studied field located in the Arabian Gulf, a laterolog resistivity tool was run as an open hole log and the water saturation was estimated using the Archie equation with its standard parameters normally used in the field. However, while the water saturation calculated from the wireline logs was low, the actual production results yielded a much higher water cut, causing a large discrepancy between the two results. To investigate the cause of this discrepancy, various investigations were conducted including resistivity simulation and variable Archie's cementation factor (Variable-m) analysis. The purpose of resistivity simulation was to understand the factor of resistivity curve separation, including the effect of the bounding of high resistive anhydrite layers and the relative dip of the layers with respect to the borehole. The main objective of the Variable-m processing is to obtain a continuous cementation factor of the Archie equation using the nuclear magnetic resonance data. The resistivity simulation results suggested that the resistivity curve with shallow depth of investigation represents the true formation resistivity better, unlike the standard petrophysical analysis that typically uses deep-reading curve. Based on the Variable-m analysis, it was found that there are intervals where the cementation factor is higher than the standard parameters. By combining these results, it was now possible to reproduce the high water-saturation results from electrical log data, which is more consistent with actual production data. This paper demonstrates a practical petrophysical evaluation method in a well with limited data set, i.e., no core data and limited resistivity or reservoir saturation logging tools run. The lessons learnt from this study will also add value to the formation evaluation planning of future wells, especially in the selection of logging tools and assumption of Archie parameters to be used.