Hydraulic fracture stimulation is the single most important technology which unlocks vast unconventional resources. Hydraulic fracture design requires a good knowledge of subsurface stress regime, especially the anisotropic minimum in-situ stress profile to determine landing zone and fault type. Field injection tests such as micro-frac and mini-frac provide the best estimates but they are costly and often limited to a few formation layers. It is now common to obtain in-situ stress profile using sonic logs calibrated to field injection test data.
Log based in-situ stress determination assumes knowledge of rock's elastic properties. For conventional sandstone, assumption of isotropy is reasonable, but for highly laminated and heterogeneous shale rock system, isotropic assumption proves inadequate. In finely thin laminated layers, the sonic logs average over large intervals and might not provide full representation of the lithology variations. Laboratory core testing data enhances well logs and provides parameters necessary to illustrate anisotropy behavior. Lately, vertically transverse isotropic elasticity (VTI) is commonly invoked in stress estimation. VTI requires knowledge of 9 parameters compared to only 2 for isotropic elasticity.
Methods to estimate these parameters in VTI model exist. The most challenging aspect is to get determine an off-diagonal parameter (C13) in the elastic matrix. Here, we outline a coherent lab-based, deterministic approach to obtain all these parameters, combined with borehole sonic logs to derived minimum in-situ stress for shale rock systems. Novel acoustic sensors and measurement technique have been developed to obtain velocity profiles on one single representative rock sample at multiple angles (0, 45, 90 degrees). The associated Biot coefficients in the vertical and horizontal directions can now be derived. The resultant stress profile shows great variability due to the rock's heterogeneity. The experimental approach and comparison of results to field data will be presented.
Laboratory methods upon the characterization of stiffness parameters for VTI model is well published and beyond what this paper is intended for. It is mainly focused on the mechanical properties' measurement as a function of orientation vertical, horizontal, and 45. Technologies have been developed in obtaining a vertical and horizontal plug, however the challenges remain in place to obtain a 45 plug in laminated material without possible damage or alteration of the mechanical properties.
Upon a full review of the available literature it was determined that that needs for the estimation of the C13 parameter is associated with the Vp45 compressional velocity. Due to the experienced difficulties with the acquisition of a 45 plug, efforts were focused to develop a 45-degree polarized transduce to measure Vp (45°) during the using a vertical plug. Multiple measurements were conducted using outcrops and reservoir rock from multiple formations across to validate of the measurement accuracy. Final results indicate consistency between the measurement on a carefully machined 45 plug vs. the measured Vp(45°) velocity from a vertical plugs.