Abstract

In recent years, the economic viability of large-scale shale-gas exploration and development projects has been impacted directly by improvements in horizontal-well drilling and hydraulic-fracturing technologies. However, results from multiple shale-gas projects indicate that accurate reservoir descriptions and resulting predictions of sweet spots are also crucial to maximize production from those projects. Accurate reservoir description results in the most appropriate placement of well locations and trajectories, hydraulic-fracture design, and reservoir engineering. The successful experience in shale-gas exploration and development outside China can aid in the search for the most favorable geologic characteristics and rock-physical properties and the examination of appropriate well-log interpretation and seismic-reservoir-prediction technologies for a shale-gas study in the eastern area of Sichuan Basin, China. Favorable parameters in a shale-gas reservoir are mid- to high kerogen content, low clay volumes, high brittleness, high effective porosity, and high permeability, along with well-developed microfractures. Of those parameters, mid- to high total organic carbon (TOC) is favorable for shale-gas accumulation, and high brittleness is beneficial to hydraulic-fracture propagation. Hence, the aim is to find sweet spots with high TOC and high brittleness. Starting with petrophysical analyses that combine laboratory measurements, core data, and well logs, a statistical model is used to estimate relative volumes and compositions of TOC and minerals and to establish a qualitative evaluation criterion for shale-gas reservoirs in this area. Petrophysical modeling and rock-property analyses establish the relationship among petrophysical properties (such as TOC, effective porosity, and gas saturation) and elastic properties (such as P-impedance, VP/VS ratio, Poisson's ratio, and Young's modulus) of the formation, which are related to brittleness. Rock-property analysis, prestack simultaneous inversion, and facies-fluid probability-analysis technologies help to predict favorable development zones for the shale-gas reservoir. Finally, the values of brittleness, TOC, and shale-gas lithology volume are combined to predict the sweet spots of the gas shale and to guide subsequent developments.

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