Summary
Optimization of hydraulic fracture treatments in tight gas reservoirs requires an in-depth understanding of the reservoir conditions. Elastic properties estimated with standard isotropic amplitude variation with offset (AVO) inversions can offer insight into areas that may be more susceptible to brittle deformation, but do not provide information regarding natural fracture networks or differential stress fields in the rock mass. A more complete characterization therefore requires estimates of anisotropy to account for these phenomena.
Azimuthal AVO inversion makes use of changes in reflection amplitudes with angle and azimuth to extract estimates of anisotropic parameters from seismic data. This case study of the Lower Triassic Montney Formation in NE British Columbia, Canada, showcases an anisotropic extension to the isotropic AVO inversion to include estimates of the fast and slow shear wave velocities. The corresponding ratio of fast and slow shear wave velocities then provides an indication of anisotropy, from which inferences can be made regarding the presence of natural fractures or differential stress fields within the reservoir.
Introduction
The Lower Triassic Montney formation is a gas bearing, 250-300m thick succession of very fine grained sandstones, siltstones and shales located in NE British Columbia, Canada, with porosities ranging from 3-6% and permeabilities ranging from 0.001-0.05mD (Norton et al., 2011). Due to its low permeability, hydraulic fracture stimulation is often necessary to maximize reservoir contact and increase permeability. As such, areas that are more prone to brittle deformation need to be identified. Furthermore, in these tight gas reservoirs, fractures in the rock mass act as primary conduits for fluid flow and play a large role in storage capacity. Optimization of hydraulic fracture treatments therefore requires knowledge of both elastic and anisotropic parameters in the formation to determine regions that are most susceptible to fracturing.
Previous studies of the Montney formation were carried out by by Norton et al. (2011), Maxwell et al. (2011) and Close et al. (2010). These studies demonstrated the utility of elastic properties estimated from AVO inversion, which determine a rock’s susceptibility to undergo either brittle or ductile deformation in order to predict the success of hydraulic fracture treatments. In general, higher values of the quartz-clay ratio and porosity are desired for increased reservoir potential. Through rock physics studies, the relationship between petrophysical and elastic properties can be determined. Figure 1 shows cross-plots of the porosity and volume of quartz in acoustic impedance and Vp/Vs space. The points corresponding to both a high value of porosity and volume of quartz then represent the optimal reservoir parameters. Estimates of elastic properties from AVO inversion can then be used to identify the most perspective zones within the reservoir.
