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Determining the Effect Mechanical Rock Properties Have on Variability in Fracture Gradients


Subsurface injection of large-volume, high-pressure fluids is integral to fluid disposal. As these technologies develop, societal impacts and safety considerations require that subsurface storage scenarios proceed with site-specific risk assessments that include evaluation of the impacts of mechanical rock properties, stress orientations and an understanding of geo-tectonic history. Mechanical failure in rocks is a function of cohesive, tensile, and frictional strengths and the current and historic in situ stress. We examine mechanical rock properties from analog clastic rocks to determine their modified Coulomb-Griffith failure envelopes and apply these modeled failure envelopes to understand the rock failure potential at depth under conditions of increased pore pressure. Laboratory derived tensile rock strength data have a lithologic dependence; indirect tensile strength ranges from 2.3 MPa in siltstone to 11.5 MPa in calcareous shale. When combined with results from triaxial compressive rock strengths, which range from 6 MPa to 33 MPa, the shape change of the Coulomb-Griffith failure envelopes predict variability in the maximum pressures each intact rock type can withstand in the subsurface. The failure envelopes constrain the predicted mechanical behavior of the disposal reservoir and its seals. Associated top and bottom seal intervals respond to increased pore fluid pressure, and this response can be used in risk assessments for the reactivation of existing fractures or faults or creation of unintended hydrofractures within the injection reservoir or over/underlying seal lithologies. We combine rock material properties and the failure envelope variability with pressure data derived from evaluation of active injection wells where deep injection (0.5–6 km) occurs at maximum permitted injection pressures that range from 0.7 to 41 MPa. Incorporation of rock strength data helps predict the effect changes in intact rock strength have on fracture gradients and the potential for mechanical failure in the subsurface. Changes in cohesive and tensile strengths due to lateral and vertical anisotropy including lithological changes or the presence of fractures will result in variations in their associated failure envelopes and the fracture gradient. These types of analyses can be used to better constrain the conditions under which rocks fail and provide improved risk assessment of sequestration systems.