Abstract:

Rock anisotropy is important in many reservoirs geomechanics problems, but is it often challenging to characterize rock anisotropy In this work, a testing program is developed to determine the orientation of material bedding planes through a hydrostatic compression test and to reveal the nature of the rock anisotropy. Also, static and dynamic measurements are carried out on a whole core to measure the coefficients of elastic anisotropy. The core used is from Utica Shale, a Middle Ordovician formation in the Appalachian Basin, underlying much of the northeastern United States and adjacent parts of Canada. The geomechanical properties of this shale are determined using multiple methods at multiple scales. The measurements are first made on a 4” diameter core using three strain rosettes to completely characterize a 3D hydrostatic strain tensor, which then is used to identify the type of anisotropy (transversely isotropic vs orthotropic), as well as the strike and the dip of the core bedding planes with respect to the locations of the strain rosettes. Dynamic measurements on the whole core are also made using multiple transducers placed on the sample surface. The sample is then sub-cored and plugs are extracted for deviatoric compression tests, which allow for complete determination of all the anisotropic elastic constants. The results from static and dynamic measurements are presented and compared, for whole core as well as the sub-cored plugs. Scale effects are observed to play a role in the measurement of mechanical properties.

Introduction

Geomechanical anisotropy of reservoir rocks plays an important role in wellbore stability analysis (Cui and Abousleiman, 1998) as well as the design of hydraulic fracture treatments (Sesetty and Ghassemi, 2016). Laboratory characterization of anisotropy is typically performed, by extracting core plugs from at least three orientations with respect to the bedding planes (Aoki et al 1993) – this can be done only if the orientation of the bedding planes in the core sample is completely known. Gonzaga et al. (2008) proposed a technique to determine the elastic coefficients for transversely isotropic rock from a single core sample. However, the technique requires a sample with obliquely dipping bedding planes with respect to the cylindrical sample axis (i.e., a standard parallel or perpendicular plug cannot be used since only one material axis is loaded under a deviatoric compression test for either of these orientations).

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