Brittleness estimation and anisotropy analysis of lower Silurian Qusaiba Shale, Saudi Arabia

Extraction of hydrocarbon from shale gas reservoirs always presents challenges due to their extremely low permeability and hence production depending totally on natural and induced fractures. Geomechanics is of vital importance for shale gas prospectivity evaluation in terms of brittleness and anisotropy. The research objective is to evaluate the prospectivity of Qusaiba Shale by delineating brittle zones/layers. This study also quantifies the anisotropy leading to accurate determination of hydraulic fractures orientation. The log data, including sonic (P and S-waves), density, and mineralogy logs for about 950 feet thick shale interval, were used to calculate the brittleness index. Brittleness index was determined based on two approaches: elastic parameters and mineralogy. Thomsen parameters and Schoenberg model were used to investigate the anisotropy. Both approaches for brittleness estimation revealed that Qusaiba Shale is composed of alternating brittle and ductile layers. The comparison of results revealed that the quartz rich layers exhibit high Young’s Modulus and low Poisson’s ratio in terms of high brittleness index. Anisotropy analysis revealed high degree of P and S wave anisotropy. The study emphasizes the importance of the role of brittleness and anisotropy analyses in prospectivity evaluation of Qusaiba Shale and assist in optimum designing of completion stages.