Testing method and controlling factors of specific surface area of shales

• The influencing factors on shale specific surface area were investigated.
• Particle size of shale samples has little effect on shale specific surface area.
• Moisture content has important effect on shale specific surface area.
• The quartz content and specific surface area show a good positive correlation.

Shales contain abundant organic matter, and have complex mineral composition and developed nanopores. Shale porosity has large surface area and a strong gas adsorption capacity. The specific surface area (SSA) of shales is an important factor for the adsorption of shale gas, while there are several factors which can affect the test results of SSA. In this study, shales collected from Longmaxi formation in the south of Sichuan Basin were taken as the research object. A set of testing methods were established for determining shale SSA through low-pressure N2 adsorption-desorption method. Following this, the key factors affecting shale SSA were investigated. The results show that particle size of shale samples has little effect on the SSA, and the recommended particle size for SSA measurement is about 3 mm. After 3-h heating treatment at 200 °C, most of adsorbed water and part of the interlayer water of clay minerals are removed, and the tested SSA is highest. Therefore, the recommended optimum degassing temperature is 200 °C. The content of total organic carbon and the SSA have positive linear relationship. The quartz content and the SSA show a good positive correlation. No positive correlation is found between the SSA and other mineral components. The SSA is of great significance for the evaluation of shale gas resources potential.