Pressure Effects on Porosity-Log Responses Using Rock Physics Modeling: Implications on Geophysical and Engineering Models as Reservoir Pressure Decreases

摘要

Due to changes in fluid properties, porosity log responses change with pressure, especially for gas/water systems. Velocity data (compressional and shear) can be described using the Krief rock physics model. This model involves relationships among compressional and shear velocities, rock bulk density, elastic moduli of the matrix, shale and fluid components, shear modulus of the solids, and Biot coefficients. Density response is governed by changing gas density as a function of pressure. Neutron response is controlled by hydrogen indicies of the fluids and the excavation effect. A complete porosity log suite model is presented whereby pseudo porosity logs are calculated as pressure and gas saturation dependent functions. Matrix and shale properties are included. Additionally mechanical properties with changing pressure and saturation are available. From compressional travel time and density log responses, changes in synthetic seismograms as reservoir pressure is reduced can be calculated. This application has significant implications in the interpretation of 4-D seismic surveys over reservoirs undergoing pressure depletion. Estimates of changing mechanical properties as functions of saturation and pressure have a number of engineering applications, including stimulation design and sand control. Examples from a variety of clastic and carbonate reservoirs are presented, including intermediate depth hard rocks, deep offshore soft rocks, and shallow onshore soft rocks.