Fractured Reservoirs

A large percentage of the world’s oil and natural gas is contained in rocks, such as carbonates, in which it is often difficult to extract fluids unless the rocks contain natural or induced fractures. Although the resources are extensive, the production of fluids from these reservoirs is hampered by our inability to predict the location, orientation, and permeability of the fractures in the rocks. As a result, the recovery rates, that is the amount of oil in place that is extracted, tends to be very low for these reservoirs (5-25%). If we can detect spatial variations in the fracture distribution and develop methods for estimating the permeability of these fracture zones using surface geophysical methods, the recovery rates for known reservoirs could be increased significantly, having a potentially major impact on the world’s oil and gas supplies. Such methods would also be directly applicable to the development of geothermal reservoirs, which are generally dominated by fracture systems for the extraction of heat energy from the earth. Likewise, the safe sequestration of CO2 requires a high degree of confidence about the caprock seal that keeps the CO2 in place in the subsurface reservoirs. Natural fracture systems are one potential escape pathway that needs to be characterized effectively before injection of CO2.

Our work in fractured reservoir characterization primarily involves the use of scattered seismic energy. In most conventional seismic imaging applications, scattered signals are a source of coherent noise that is removed to allow for better imaging and analysis of reflected, refracted, and transmitted waves. As a result we tend to use only a small percentage of the actual recorded seismic wavefield. However, scattered waves contain important information about the subsurface, particularly about heterogeneities such as fractures. Our methods seek to exploit the azimuthal variations in seismic scattering to provide information about fracture orientation. We also note that variations in the spectral content of scattered signals can provide estimates of fracture density, and variations in the total scattered wave energy can be used to estimate fracture compliance. With these data it may be possible to make estimates of the spatial variability of fracture permeability in the reservoir.

The fractured reservoir problem can only be solved with an integrated approach. The seismic scattering measurements can provide information about spatial variations in some mechanical properties of the subsurface, but to connect these data to permeability estimates we need to interpret the seismic data in conjunction with dynamic fluid flow data and modeling. In addition, the use of GPS and InSAR data with geomechanical modeling allows production-related surface deformation to provide additional constraints on fracture compliance estimates that are the critical link between seismic and fluid flow properties in the reservoir. Borehole measurements are used as important calibration data in this process.