Seismic methods are beginning to be used in hydrocarbon recovery assessment, in view of the growing need to better understand recovery, especially the compleity of reservoirs, leading to large uncertainties in estimated total recovery, recovery rates, and recovery method. Reservoir complexity is due to the spatial heterogeneity in porosity, permeability, clay content, fracture density, etc. These spatial variabilities cannot be inferred from well test data, logs, or cores. They may only be obtained from remote geophysical measurement, especially seismic measurement. Such measurements will assume a major role in helping to solve production and recovery problems. To achieve that it is necessary to better understand what it is that seismic waves can tell us about reservoir rocks, and how to extract the desired information. This review provides a brief summary of what we know at present about the relation between velocities in rocks and reservoir properties.
Seismic methods, notably reflection methods in exploration geophysics have been used in the past mostly to delineate rock interfaces in the earth's shallow crust, to delineate structures which might bear hydrocarbons or to describe pre-existing faults. Relatively little use has been made of seismic waves for the determination of the rock properties such as porosity, permeability, compressibility, saturation, pore pressure, fracture density, etc. Seismic methods are only beginning to be used in hydrocarbon recovery assessment, in view of the growing need to better understand recovery. The major problem is the complexity of reservoirs, leading to large uncertainties in estimated total recovery, recovery rates, and recovery method. Reservoir complexity is typically related to the significant spatial heterogeneity in porosity, permeability, clay content, fracture density, etc. These spatial variabilities may only be obtained, hopefully, from remote geophysical measurement, especially seismic measurement. A direct consequence of the heterogeneous nature of reservoirs is the complexity of their recovery processes, ranging from problems like the migration of the gas cap in reservoirs with discontinuous shales, overpressure zones, and the tracking of steam or temperature in thermal recovery in reservoirs with large spatial variations of permeability. For seismic methods to help solve production and recovery problems it is necessary to better understand what it is that seismic waves can tell us about reservoir rocks, and how to extract the desired information. This review provides a brief summary of what we know at present about the relation between velocities in rocks and porosity, saturation, pore pressure, fluid phase behavior, and hydrocarbons and temperature.
Shaly sandstones and shales are a major component of sedimentary basins and are of foremost relevance to hydrocarbon reservoirs. When both Vm and Vf are fixed, the only variable in the equation is porosity. To a first order this simple equation appears adequate for clean sandstones in the middle range of porosity (10% < φ < 25%). It is well known however that acoustic velocities of sandstones are also related to mineralogy, pore geometry, degree of consolidation, cementation, confining pressure, pore fluid, pore pressure, and temperature.