Identifying By-Passed Pay in Complex Water Flooded Reservoirs with Advanced LWD Measurements

Abstract This study addresses the problem of fluid characterization in siliciclastic reservoirs where the resistivity contrast between pay and wet sands is low. Our study field contains connate water of extremely high salinity. The pay sands generally have high irreducible water saturation, which brings down the resistivity reading in oil zones. It is typically difficult to identify hydrocarbon zones with resistivity alone. The low resistivity pay poses a significant technical challenge for the development fields, because the operator must locate the by-passed hydrocarbon and understand the distribution of water floods in order to make future development plans. The key to accurate fluid characterization in this challenging environment is a good knowledge of formation water salinity. Unfortunately, due to years of water injection to maintain production pressure, water salinity in these reservoirs is highly variable. In such environments, one cannot assume a fixed salinity value when applying saturation equations to well logs. Thermal neutron cross-section (Sigma) is very sensitive to chlorine in the reservoir rocks and fluids and can be used to distinguish between water and hydrocarbon in a salty water environment. Since both Sigma and resistivity are dictated by water salinity, we can use these two inputs to simultaneously invert for the two unknowns, water saturation (Sw) and water salinity. This process avoids interpretation errors caused by uncertainty in water salinity and does a good job distinguishing bypassed hydrocarbon from water zones. Furthermore, we can use the inverted Sw and salinity to compute continuous hydrocarbon density, hydrogen index (HI) and viscosity downhole. We applied the above technique to a set of LWD logs acquired in 4 horizontal development wells. We validated our computed water saturation with production history of these wells. The hydrocarbon fluids parameters, ie, salinity, density, HI and viscosity, are cross-checked with surface measurements on produced fluids.