Comparative Study of Marine and Lacustrine Shale Reservoirs from the Viewpoint of Rock Mechanics

Abstract

Marine and lacustrine shales are two major important types of organic-rich shales that are usually deposited in the stable ocean basin and deep lake/swamp settings, respectively. In the past decade, commercial development of marine shale gas resources has been achieved. The underdeveloped lacustrine shale also showed great oil and gas potential. Is it feasible to simply copy the development mode of marine shale gas to lacustrine shale? This is what this paper would attempt to answer from the viewpoint of rock mechanics. Using borehole cores from two typical marine and lacustrine shale gas wells, a series of experiments, including X-ray diffraction (XRD), triaxial compression, fracture surface three-dimensional (3D) scanning, and scanning electron microscopy (SEM), were implemented to comprehensively reveal the mechanical difference between marine and lacustrine shales. Postfailure fracture complexity and surface roughness were quantified by fractal dimension and morphology reconstruction, respectively. A new index BInew, considering the influences of the complete stress–strain curve and fracture characteristics, was proposed to evaluate the brittleness of these two types of shales. Results showed that the mechanical properties of marine and lacustrine shales were quite different and should be exploited by following their own characteristics. Mechanical parameters of lacustrine shale, such as compressive strength, Young’s modulus, cohesive strength, and internal friction angle, were much lower than those of marine shale. Different from the typical shear plus bedding splitting failure mode of marine shale, lacustrine shale usually formed only one smooth shear fracture. Based on the newly proposed index, the brittleness of lacustrine shale was significantly lower than that of marine shale. Huge discrepancy in quartz and clay mineral contents and the consequent petrophysical structure divergence were the reasons for the marked mechanical difference between marine and lacustrine shales. Differentiation strategies and effects of hydraulic fracturing for these two kinds of formations were also discussed.