Determining Water Saturation in Permian Basin Intercalated Reservoirs Using NMR Log Data

Abstract

Alternating conventional and unconventional reservoir layers in the Permian Basin challenge the acquisition, processing, and interpretation of water saturation (Sw) using nuclear magnetic resonance (NMR) log data. A new-generation NMR wireline tool addresses these challenges using a specially designed conventional-unconventional activation sequence to enable construction of optimized maps of Longitudinal–Transversal Relaxation times (T1-T2 maps) at regular depth intervals. T1-T2 maps are used to compute level-by-level Sw based on a multicomponent fluid model with appropriate statistical properties. Each spot in the T1-T2 space represents a fluid component from which a volume fraction is calculated. Integrating the volume fractions gives the total porosity. Because of the diverse relaxation mechanisms in the conventional and unconventional layers, oil spot positions with T1/T2 values greater than two reflect either viscosity (for bulk relaxation) or pore-size distribution (for surface/volume relaxation). Water tends to be close to the 1:1 T1/T2 diagonal line with T1/T2 values less than two. Low permeability means that mud-filtrate invasion does not appear on the T1-T2 maps. NMR porosity matched expected values based on core and density-neutron log analysis. NMR fluid-typing-derived Sw—including clay bound water (CBW), capillary bound water (BVI), and free water—matched values from tested intervals. Results are in good agreement with reference values from production and core data within an uncertainty of one standard deviation. The resolution of fluid components in intervals where the components overlap can be enhanced by changes in the inversion parameters and map-grid dimensions. This methodology for conventional-unconventional data acquisition followed by a multimodel approach for fluid typing will be applied to other wells. It enables a more accurate assessment of water saturation, especially when intercalated layers of conventional and unconventional reservoirs are present.