Abstract
Deep shale gas reservoirs are a significant alternative type of shale gas reservoir in China. The productivity of deep shale gas wells is lower than that of shallow shale, and the imbibition characteristics of deep shale have a significant effect on the retention and backflow of fracturing fluid and the productivity of shale gas wells. In this study, the pore structure characteristics of organic-rich deep shale in the Lower Silurian Longmaxi Formation of Weiyuan-Luzhou play were analyzed by low-temperature nitrogen adsorption experiments, and then the imbibition characteristics and factors influencing deep shale were extensively investigated by spontaneous imbibition and nuclear magnetic resonance experiments. The results show that mainly micropores and mesopores are growing in the deep organic-rich shale of the Longmaxi Formation. The spontaneous imbibition curve of deep shale can be divided into an initial spontaneous imbibition stage, an intermediate transition stage, and a later diffusion stage, and the imbibition capacity coefficient of deep shale is lower than that of shallow shale. The transverse relaxation time (T2) spectrum distributions suggest that clay hydration and swelling produce new pores and microcracks, but then some pores and microfractures close. Deep shale reservoirs have an optimal hydration time when their physical properties are optimal. The increasing pore volume and the decreasing TOC content can enhance the imbibition capacity of shale. An inorganic salt solution, especially a KCl solution, has an inhibitory effect on the imbibition of shale. Higher salinity will result in a stronger inhibitory effect. It is crucial to determine the optimal amount of fracturing fluid and soaking time, and fracturing fluid with a high K+ content can be injected into the Longmaxi Formation deep shale to suppress hydration. These results provide theoretical guiding significance for comprehending the spontaneous imbibition and pore structure evolution characteristics of deep shale and enhancing methane production in deep shale gas reservoirs.
Highlights
Shale gas resources in China are abundant, the proven accumulated shale gas reserves were 1:7865 × 1012 m3 in China by the end of 2019, and shale gas production is expected to reach 5:0 × 1010 m3 in 2030 [1]
The mineral composition is dominated by quartz and clay minerals, and the quartz content varies between 33.4% and 56.0%, with an average of 48.89%
The petrophysical properties of deep shales were measured in the Longmaxi Formation of the southern Sichuan Basin, China, and the spontaneous imbibition and T2 spectral characteristics of deep shales were evaluated with high-precision electronic balance and low-field nuclear magnetic resonance
Summary
Shale gas resources in China are abundant, the proven accumulated shale gas reserves were 1:7865 × 1012 m3 in China by the end of 2019, and shale gas production is expected to reach 5:0 × 1010 m3 in 2030 [1]. The exploitation of shallow marine shale gas reservoirs The production of shale gas wells has experienced a sharp decline in three years, and it is necessary to find new blocks and layers for production. Deep shale gas reservoirs (deeper than 3500 m) account for more than 65% of the total resources and are a hotspot and alternative for shale gas research in China [2]. Similar to deep shale gas reservoirs in North America, the gas production of deep shale gas wells in the Weiyuan, Fuling, and Fushun-
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