An Experimental Investigation of Shale Mechanical Properties Through Drained and Undrained Test Mechanisms

Abstract

Shale mechanical properties are evaluated from laboratory tests after a complex workflow that covers tasks from sampling to testing. Due to the heterogeneous nature of shale, it is common to obtain inconsistent test results when evaluating the mechanical properties. In practice, this variation creates errors in numerical modeling when test results differ significantly, even when samples are from a similar core specimen. This is because the fundamental models are based on the supplied test data and a gap is, therefore, always observed during calibration. Thus, the overall goal of this study was to provide additional insight regarding the organization of the non-linear model input parameters in borehole simulations and to assist other researchers involved in the rock physics-related research fields. To achieve this goal, the following parallel activities were carried out: (1) perform triaxial testing with different sample orientations, i.e., 0°, 45°, 60°, and 90°, including the Brazilian test and CT scans, to obtain a reasonably accurate description of the anisotropic properties of shale; (2) apply an accurate interpretative method to evaluate the elastic moduli of shale; (3) evaluate and quantify the mechanical properties of shale by accounting for the beddings plane, variable confinement pressures, drained and undrained test mechanisms, and cyclic versus monotonic test effects. The experimental results indicate that shale has a significant level of heterogeneity. Postfailure analysis confirmed that the failure plane coincides nicely with the weak bedding plane. The drained Poisson's ratios were, on average, 40 % or lower than the undrained rates. The drained Young's modulus was approximately 48 % that of the undrained value. These mechanical properties were significantly impacted by the bedding plane orientation. Based on the Brazilian test, the predicted tensile strength perpendicular to the bedding plane was 12 % lower than the value obtained using the standard isotropic correlation test. The cyclic tests provided approximately 6 % higher rock strength than those predicted by the monotonic tests.

Investigation of Fracture Tip Behaviour in Visco-Elastic/ Visco-Plastic Shale Rocks and Its Effect on Fracture Propagation

“…Elastic moduli and poisson's ratio are two common measures of shale strength and deformation. Experimental studies have been carried out in observing those mechanical behaviours in different shale plays, including Barnett [5,6], Woodford [7], Tournemire [8], Pierre-1 [9] and North Sea Shales [10]. The effect of stratigraphy on mechanical properties has also been studied [7,11].…”

Geomechanical Characterization of Marcellus Shale

“…Firstly, available shale specimens were already room-dried, and the re-saturation process could damage the rock. Secondly, due to the low permeability of shales, a single drained test could take several weeks, even months (Dewhurst and Siggins 2006;Islam and Skalle 2013).…”

“…Sample scarcity and variability is one of the main problems in reservoir geomechanics laboratory testing (Fjaer et al 2008;Yang 2012;Islam and Skalle 2013) (a) Elastic Multi-Stage triaxial (MSE): Shale gas rocks are known to be non-linear materials, and the characterization of their static properties requires performing unloading-reloading cycles at different stress levels (Fjaer et al 2008). This test consisted of nine stages at different confinement levels ranging from 0 to 70 MPa (Fig.…”

“…They include borehole stability modeling, lithology identification, exploitation of shale-oil and shalegas reservoirs, and geological storage of CO 2 , to name a few (Islam and Skalle 2013). This growing interest is largely driven by their notable contribution to the world's gas reserves (EIA 2013).…”

“…Geomechanical characterization of shales is somewhat incomplete if reservoir conditions are not considered. Also, operational procedures in these reservoirs induce THCM environmental changes that may play an important role in shale mechanical response (Horsrud 2001;Eseme et al 2007;Ghassemi and Suarez-Rivera 2012;Islam and Skalle 2013;Mohamadi et al 2013;Hu and Hueckel 2013;Bauer et al 2014). …”