Mapping Geomechanical State of Unconventional Shale - A More Robust, Faster Lab Characterization Method

Abstract

Abstract Unconventional shale mechanical parameters are important for many applications such as reservoir stress-state determination, horizontal drilling and hydraulic fracturing design among other engineering design parameters. These parameters include Young's modulus, Poisson's ratio, cohesion, angle of internal friction, and unconfined compressive strength. Their determination is commonly performed on single-stage triaxial (SST) compression tests using three or more core samples at various confining pressures. For unconventional shale, this has been a practical bottleneck - it is very difficult to drill multiple plugs with good quality from a whole core because of its brittleness and complexity. An alternative procedure is the multi-stage triaxial (MST) compression test, which requires only one plug to be tested. However, its main problem lies in the practical difficulty in determining the failure envelope of the sample. Judgment must be made regarding the stress-strain state "immediately prior to failure". It is not uncommon that a wrong estimation of the failure state occurs when interpreting these stress-strain curves. The MST test uses one plug, which in itself could be a significant advantage. This paper presents a robust modified method of the MST compression test that accurately determines the imminent rock failure through continuously monitoring the radial deformation. To test and validate the method, Berea sandstone and Mancos shale plugs are selected to perform a series of the SST and MST tests. For each rock type, at least four plugs are used, which are cut from the same whole core or have similar lithofacies. Experimental results show that the Mohr-Coulomb failure envelope can be generated perfectly from both the SST and MST tests. However, the failure parameters derived from the MST tests are lower than those from the SST tests. The difference may be attributed to the intrinsic heterogeneity of rock samples or the damage accumulated from the early stages in a MST test. The proposed MST method is an efficient alternative to generate the Mohr-Coulomb failure envelope when the availability of core samples with good quality is limited, especially in the exploration and development of unconventional reservoirs.