The shale gas “sweet window”: “The cracked and unbroken” state of shale and its depth range

Abstract

Abstract To assist in the progression of shale gas exploration in southern China, we aimed to clarify the burial depth of the “sweet window” of shale gas reservoirs. In this study, we statistically explored the relationship between burial depths and shale gas production of the Wufeng-Longmaxi Formations in the Sichuan Basin, Southwest China, analyzed the development morphology of fractures under different stress fields and fracturing effects of high-yield strata, conducted a series of triaxial mechanical experiments on shales under the formation conditions, and summarized the characteristics of the shale gas “sweet window” as well as its main affecting factors. The results show some interesting conclusions. First, a “sweet window” of shale gas reservoir burial depths does exist. Second, it is found that the bedding cracks and artificial fracture network formations are affected by burial depth. Third, shale mechanical properties in the “sweet windows” generally have the characteristics of transitioning from brittleness to ductility. Based on the above results, a method for determining the top and bottom depth of the shale gas “sweet window” is proposed. The top boundary of the “sweet window”, i.e., the depth at which shale suffers from brittle fractures, can be determined using the shale's over consolidation ratio (OCR). The bottom boundary of the “sweet window”, i.e., the depth at which shale transitions to a ductile state, can be estimated using triaxial stress experiments. The depth range of the “sweet window” top boundary is 2000–2700 m for the Longmaxi shale in the Sichuan Basin, and the depth range of the bottom boundary is 4200–4400 m. In areas with tectonic stability and low geo-stress differences, the top boundary of the “sweet window” can be shallower than the brittle boundary depth as determined by OCR, and the shale gas buried shallower than 2000 m may be preserved effectively and allow for higher production capacity. In areas with higher brittle mineral content and greater geo-stress differences, the bottom boundary of the “sweet window” may reach 4500–5000 m. The area of the Wufeng-Longmaxi shale burial depth between 3500 m and 4500 m is up to 5.5 × 104 km2 in the Sichuan Basin, which is a potential area for shale gas exploration and development in the future.