Optimization and application of brittleness evaluation methods for continental organic-rich shale: A case study of the 2nd Member of Kongjiadian Formation, Cangdong Sag, Bohai Bay Basin

Shale brittleness is one of the key indexes affecting dimensions of hydraulic fractures and determining the producibility of trapped hydrocarbons in shale. Therefore, it is one of the major parameters for evaluation of the planar sweet-spot zone, horizontal drilling sweet-spot target, and sweet-spot hydraulic fracturing stage. The shale brittleness geological model was built on the framework of the felsic mineral, carbonate, clay, and organic matter, and five shale brittleness evaluation formulas based on mineral composition were developed, via the X-ray diffraction (XRD) mineral analysis, TOC measurement, and tri-axial rock mechanic tests of 17 shale samples collected from the 2nd Member of the Kongjiadian Formation in the Cangdong sag. The novel brittleness evaluation method that comprehensively incorporates TOC and brittleness of various minerals, and the simplified brittleness evaluation considering two key parameters (namely TOC and clays) were selected via the comparison with the brittleness formula based on rock mechanic parameters. The former applies to intervals with sufficient XRD mineral analysis data, while the latter applies to well logging evaluation. The mineral composition-based method was applied to the C6 interval of the 2nd Member of the Kongjiadian Formation in Well G108-8, which also considered the oil content, and the shale interval with high brittleness, and high or medium oil content was identified as the engineering-oil content dual sweet-spot. The formation testing results of Wells Z1608 and GX5 validated the effectiveness of the developed brittleness evaluation method for shale oil exploration and the vital guidance on sweet-spot stage identification and hydraulic fracturing design.