Abstract

The lower Cotton Valley-upper Bossier tight gas sand benches in North Louisiana offer significant challenges in identifying the best section to drill a horizontal well or for vertical reservoir stimulation. These rocks are typically characterized by low resistivity pay and varying pore throat size. Determining correct saturation and a reasonable permeability estimation in these rocks has always been a challenge. A wireline logging suite comprised of gamma spectral elemental, nuclear magnetic resonance (NMR), and dielectric tools were deployed to acquire data for characterizing reservoir properties such as a quantitative mineral volumetric, varying Archie’s cementation exponent used for water saturation estimation, permeability based on pore size distribution, etc. Water saturation model based on standard resistivity tool response cannot account for a true water saturation estimation in these type of laminated sand-shale reservoirs because of poor vertical resolution of the standard resistivity tool. An advanced resistivity tool which can resolve the resistivity measurement into vertical and horizontal component to have a better estimate of resistivity of the sand was not part of the logging tool string. Instead, a computational approach from the standard resistivity tool data in developing a laminated sand-shale analysis model was used to compute water saturation at an enhanced vertical resolution. Understanding vertical hydraulic fracture height growth, which may connect productive zones, is important in the lower Cotton Valley which has multiple target sections. A dipole sonic tool was added to the logging suite to provide measurements of rock mechanical properties for a reservoir stimulation model. A complete petrophysical and rock mechanical model was built using “TightGasXpert^TM” workflow, which integrates the enhanced saturation model with the pore size variation dependent permeability and 3D rock mechanical properties so as to take into account the frequent layering nature of the rocks and help in identifying the key parameters in delineating the best target section. This workflow offers a very robust methodology in characterizing low resistivity, frequently interbedded tight shale-sand formation.