Evaluation of the effectiveness of shale bedding fractures: a case study of the Longmaxi Formation in the Sichuan Basin

Abstract

Bedding fractures are among the key factors affecting the production efficiency of shale oil and gas, but relatively little research has been conducted on the effectiveness of bedding fractures. Based on field outcrops and drill cores from the Fuling area in the Sichuan Basin, this work discusses the development, filling, and opening characteristics of bedding fractures and their quantitative impact on physical properties. Multiple methods were employed, e.g., immersion testing, wet illumination, high-power microscanning, imaging logging identification and experimental measurement. The results indicate that the highest density reaches 437 fractures per meter, with apertures less than 0.5 µm being the majority. The average permeability of the shale samples with vertical bedding is 44.6 times that of the shale samples with parallel bedding, while the porosity exhibits less anisotropy. Many open bedding fractures with gas outlets in the core are shown by on-site immersion experiments and electron microscopy scanning experiments. Each dark stripe on the imaging logging map corresponds to a bedding fracture, and the thickness of the dark stripes corresponds to the aperture of the bedding fracture. There is no need to consider unfilled bedding fractures, as fractures filled with calcite veins and pyrite crystals can also become effective seepage channels because of the pores within the calcite veins and between the pyrite crystals. The utilization and transformation of bedding fractures during fracturing is one of the key steps in producing shale oil and gas. It is necessary to combine the in situ stress field, bedding fracture characteristics, and fracturability of shales to reasonably utilize bedding fractures to transform oil and gas reservoirs.

Graphical abstract

Keywords

bedding fractures / complex fracture network / formation of bedding / Fuling area

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Higher Education Press