Shale gas formation stress sensitivity

With the production of water and gas from shale formation, the effective stress acting on shale rock changes, leading to variation of its porosity and permeability. As the effective stress increases, the bedding fractures and pore spaces in shale tend to close, reducing permeability. This phenomenon, known as compaction, is a key mechanism governing shale permeability behavior under varying stress conditions, which has been confirmed by the back flow tests after hydraulic fracturing and the lab-scale experiments. Many studies have attempted to establish the relationship between permeability of different cores and effective stress through lab-scale experiments. The porosity and permeability stress sensitivity experiments were conducted on different marine- continental transitional shale samples.Different types of marine-continental transitional shale samples were selected for this study, including four ORDMS and four siliceous shale cores (Fig. 1). In order to study the effect of natural micro fractures on the stress sensitivity of shale permeability, core plugs with and without natural fractures were specifically selected. Two parallel core plugs from Well D6-5 (2071.50-2073.28 m) and two from D3-4 (2165.52-2165.82 m) were defined as ORDMS type, with an average clay mineral content of 53.9%, quartz - 31.6%, pyrite - 8.2%, and small portions of plagioclase, K-feldspar and siderite. The permeability of these cores ranges from 0.0026 to 0.0085 mD, except for the core plug #1-2 (0.3767 mD) with natural fractures. Four parallel core plugs from Well D6-5 (2073.28- 2075.21 m) were defined as siliceous type, with an average clay mineral content of 61.4%, quartz - 36.5%, and small portions of siderite and plagioclase. This type of shale is characterized by the presence of siliceous laminae. The permeability of these cores ranges from 0.0093 to 0.0457 mD, except for the core plug #2-2 (0.5566 mD) and #2-4 (0.1652 mD) with natural fractures.