Progress in Three-Dimensional Network Fracturing for Ultra-Deep Shale Gas and Suggestions for Further Improvements

To identify the research direction of three-dimensional (3D) network fracturing for ultra-deep shale gas in China, a review was conducted on the geological and engineering characteristics, fracturing techniques, and post-fracturing flowback and gas testing performance of reservoirs in shale gas blocks with vertical depths exceeding 4500 m worldwide. The development practices of typical blocks, including the Woodford, Mancos, and Sichuan Basin, were analyzed. The adaptability of fracturing techniques under different reservoir conditions and the differences in post-fracturing production performance were summarized. Leveraging well-developed natural fractures, small stress differences, and high brittleness, a 3D network fracturing model is developed in the Woodford block, integrating small cluster spacing, medium-to-high fluid intensity, high-intensity of proppant, fine-grained proppants, and temporary plugging and divergence. In contrast, ultra-deep shale gas reservoirs in China generally exhibit high temperature, high stress, large stress differences, high closure pressure, and multiple interbeds. These result in high pumping pressure, narrow proppant-loading windows, and limited complexity of networks. In some wells, high flowback ratios and high liquid production are exhibited after fracturing. The analysis indicates that natural fractures or micro-faults are key geological factors for the formation of complex fractures and high gas productivity, while high net pressure and effective far-field fracture propping are engineering guarantees for high and stable production. The existing evaluation criteria for geological and engineering sweet spots, fracturing techniques, and monitoring methods are insufficient to fully meet the requirements for efficient stimulation of various types of ultra-deep shale gas reservoirs in China. It is recommended to establish dual sweet spot evaluation criteria suitable for ultra-deep shale gas, conduct classified research on differentiated 3D network fracturing technologies, soaking and flowback management strategies, and far-deep fracture monitoring and detecting technologies, and construct an integrated technology system of “evaluation-fracturing-production” suitable for ultra-deep shale gas in different regions of China.