Deep shale gas in the Ordovician-Silurian Wufeng–Longmaxi formations of the Sichuan Basin, SW China: Insights from reservoir characteristics, preservation conditions and development strategies. (1st April 2023) - Nie, Haikuan
Graphical abstract: The variation of burial depths and distribution of main shale gas fields related to the Ordovician-Silurian Wufeng–Longmaxi formations in the Sichuan Basin. Sites of wells are also shown on the map. Highlights: Deep shale gas reservoirs are characterized by complex formation conditions and relatively poor reservoir properties. Siliceous shale of the Dicellograptus complexus-Cystograptus vesiculosus biozone presents excellent reservoir performance. Geological and engineering factors were analyzed for explaining difficulties in effective hydraulic fracturing and stable gas production. Strategies involving hydraulic fracturing, length of the horizontal well and production system were proposed for deep shale gas reservoirs. Abstract: Shale gas at a burial depth greater than 3500 m is an important potential strategic target for exploration and development in China. Due to the complex geological and engineering settings, our understanding of the enrichment and retention mechanisms of deep shale gas is limited, and large-scale commercial development has not yet been realized. In this study, deep shale gas in the Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation of the Sichuan Basin is systematically studied in terms of the spatial distribution of black shale, reservoir types and properties, preservation conditions, and gas content as well as exploration and development practices. The deep siliceous shale of the Dicellograptus complexus-Cystograptus vesiculosus biozone exhibits high quartz content, which is conducive to the formation and preservation of effective reservoirs, thus it is often recognized as the optimal interval for deep shale gas development. Compared with the shallower reservoirs, microfractures in deep shale gas reservoirs are not developed. Also, overburden pressure and high temperature bring about a 40 %–50 % reduction in porosity and an 80 %–90 % decrease in permeability. High formation temperature, large in-situ stress and high-stress difference between the horizontal stress and vertical stress in the deep shale gas reservoirs lead to elevated shale plasticity, making it difficult to form and extend fracture. Moreover, due to the high fracture closure pressure, proppant tend to easily break in addition to the reduction of effective hydraulic fractures and fracture conductivity, which explains why it is difficult to effectively fracture deep shale and to achieve high and stable gas production. It is recommended to establish a volume fracturing plan (such as a long horizontal well with an increasing number of stages and perforations) for certain deep shale gas reservoirs. Creating and optimizing reasonable production systems with excellent pressure management procedures are also crucial to maximize deep shale gas production and ultimately realizing the commercial development of deep shale gas.