52nd U.S. Rock Mechanics/Geomechanics Symposium,
2018. American Rock Mechanics Association
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ABSTRACT: In this study, we investigate through lab measurements, the mechanical anisotropies of a sample of calcareous shale from Duvernay Formation, Alberta. The elastic properties of the materials are measured acoustically with ultrasonic pulses transmitted through the samples. Samples are cut into prisms allowing piezoelectric (PZT) ceramic wafer to be directly attached to measure P and S wave velocities at a set of strategic angles to the material's symmetry in order to obtain the full set of material constants concurrently. The beam ‘skew’ effect in the anisotropic material is a recurrent issue in the analysis of such data. Particular efforts are taken to account for this through beam modeling and the additional receiving transducers. The measurements are conducted on ‘dry’ jacketed samples to confining pressures of 180 MPa with no attempt at control pore pressure. For reference, the maximum in-situ stress in the Duvernay Formation is estimated to be less than 150 MPa at 3000 m depth. The five stiffness essential to describe the engineering behaviors of these materials are subsequently obtained and evaluated.
Shale is the most common rock present in the sediments of the crust of the earth. Shale rocks deposited over the carbon platforms of sedimentary basin often consist high organic content and are considered source rock for hydrocarbon reservoirs nearby. Additionally, due to its low permeability, hydrocarbon formed within shale is often ‘locked’ within shale’s rock matrix and cannot be exploited through conventional methods. In the recent decades, the rapid development of hydraulic fracturing techniques allows extraction of hydrocarbon from shale rocks known as ‘shale oil’ or ‘shale gas’ brings up needs to quantify the mechanical properties of shale, in addition to more traditional needs of reflection seismic image processing. A hydraulic fracturing operation requires injection of high-pressure fluid into the reservoir to fracture the reservoir rocks and allows extraction of hydrocarbons through these created pathways. Rocks’ response to subsurface fluid injection relies on many critical geomechanical parameters (i.e., rock strength, in-situ stress, etc.), which needs to be a constrained by the mechanical properties of the formation rocks. Additionally, constraining the in-situ stress, predicting wellbore stabilities and better location of micro-seismic events all require detailed knowledge of the formation rocks’ response to such external loadings.
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