Assessing Different Formation Evaluation Methods to Distinguish Low Resistivity Pay in Carbonate Reservoirs

Abstract

Abstract Low resistivity pay is one of the main challenges that exist in carbonate reservoirs with complex pore structure. When hydrocarbon charging occurs, it displaces water in larger pores with lower entry pressure, leading to a complex fluid distribution. As resistivity logging is based on how easily electrical current tends to flow through formation, a rock made up of micro pores filled with water will exhibit a misleading low resistivity reading that can highly influence fluid saturation and formation evaluation when using simple logging tools such as resistivity. The objective of this paper is to provide an assessment of two different formation evaluation methods: resistivity logs and capture cross-section (formation sigma) across low resistivity pay intervals. The workflow includes a thorough evaluation of water salinity across the targeted field. Then, candidates were selected to compare results between the different methods to evaluate hydrocarbon and water saturations, as well as bound fluid volume. These results were then benchmarked and validated against formation testing and sampling (FTS) and surface production data to identify level of accuracy and limitation of each technique. Results of this study show that sigma-based saturation can be considered as reliable across LRP intervals in simple rock mineralogy carbonate reservoirs, and it is recommended to compliment triple-combo logs to flag such challenging zones. The computed hydrocarbon and water saturation is based on set sigma values for each fluid type and lithology, which are inputted into the volumetric equation. On the other hand, resistivity log tends to underestimate hydrocarbon saturation using conventional Archie's equation. NMR logs and core analysis provide an additional knowledge of porosity distribution and existence of micro-porosity in the formation. Fresh water injection, as well as any variation in formation salinity would highly impact both saturation computation methods. Combining sigma and triple combo logs would lead to increasing engineers’ confidence in precise horizontal well placement in hydrocarbon-bearing low resistivity intervals, as well as accurate identification of fluid contacts in vertical wells. Additionally, this could lead to a significant optimization in unnecessary sampling jobs and completion designs and its associated costs.