Summary Unconventional reservoirs such as gas shales and tight gas sands require technology-based solutions for optimum development because of the undeveloped matrix pores and poor permeability. Hydraulic fracturing is one of the most critical technologies. The quantitative characterization of hydraulic fractures is of great significance to the stimulation evaluation of the reservoir, but there is still a lack of fine, effective and systematic evaluation methods. 3D optical scanning technology is widely used in the quantitative characterization of rock fracture morphology for its advantages of high speed, convenience, high precision, and nondestructive testing. In this study, after the indoor hydraulic-fracturing simulation experiments, 3D optical scanning was used to visualize the fracture network. On this basis, two aspects of quantitative evaluation methods for stimulation effectiveness were established, including: (1) evaluating the local conductivity (permeability) of different fractures by cutting hydraulic-fracturing samples. Then combining local conductivity of different fractures with the overall stimulated reservoir area, which could be more reasonable to evaluate the stimulation scope of the reservoir; (2) calculating the fractal dimension (FD) of the 3D spatial structure based on the point-cloud processing, which could directly reflect the complexity of the fracture network. Finally, a new evaluation index for stimulation (Es) was established to comprehensively assess the stimulation effectiveness of the reservoir, which was applied and verified through the indoor fracturing simulation experiments of tight sandstone from the Ordos Basin, China.

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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.

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