Water Saturation from NMR, Resistivity and Oil-Base Core in a Heterogeneous Middle-East Carbonate Reservoir

Abstract

Accurate water saturation calculation is one of the requirements for estimating oil in place. Typically, this is performed using resistivity logs and the Archie equation. In this heterogeneous Middle East carbonate field, the resistivity technique is subject to large uncertainties related to values of the Archie exponents m and n and their unpredictable variability. We propose deriving water saturation directly from NMR logs. Using a water-free oil-base mud, we demonstrate that sufficient contrast exists in the invaded zone between oil mud filtrate and water to deduce irreducible water saturation (S wirr ) directly from the NMR signal when the filtrate completely displaces the native crude. Various laboratory experiments have been performed to demonstrate that no water is moved in the invaded zone and to determine the NMR signature at representative conditions. These include whole mud invasion experiments to determine the mud leak-off rate, determining NMR T 2 distribution at S wirr with reservoir brine and oil filtrate, flooding with oil filtrate at a representative flow rate, assessing the effect of oil filtrate surfactants, and investigating the effect of temperature on the T 2 distribution. To test the technique and provide a saturation benchmark, a well was cored with water-free oil-base mud (confirmed by laboratory analysis of mud samples gathered during the coring process). Both induction resistivity and pad NMR logs were run. For resistivity log interpretation, m and n were measured at reservoir temperature and overburden pressure using resistivity measurements on as received plugs from the rig and with Fast Resistivity Index Measurement (FRIM) on gently cleaned, preserved plugs. The NMR and resistivity logs of the pilot well were interpreted for water saturation using in-house software and the results were compared to Dean Stark saturations from the oil-base core plugs. Water saturations determined through NMR and resistivity interpretations agree with core saturations in some reservoir layers and disagree in others. The possible reasons for this are explored. We conclude that NMR logs have the potential to provide accurate S wirr information.