The effect of water saturation on methane breakthrough pressure: An experimental study on the Carboniferous shales from the eastern Qaidam Basin, China

Breakthrough pressure plays an important role in shale gas flow, mining, and caprock evaluation. A series of breakthrough experiments were conducted under different water saturation conditions for four shales taken from the Carboniferous Hoit Taria Formation in the eastern Qaidam Basin, China to investigate the influence of water saturation on breakthrough pressure. Relevant geochemical tests (mineral composition, clay content, total organic carbon, thermal maturity and vitrinite reflectance) and micro structural characteristics of micro pores were also conducted. Breakthrough pressures under at least five different water saturations (from 0 to 100%) were obtained and relationship between breakthrough pressure and water saturation was fitted for each sample.We found that breakthrough pressure increases exponentially with water saturation. The decrease in effective pore diameter caused by both the bound water films and the swelling of the clay minerals resulted in the increase in the breakthrough pressure. After water saturation reached about 60%, breakthrough pressure increased rapidly from connectivity reduction, caused by the sealing off of smaller pores and partial water saturation of the macropores. By analyzing the correlation between breakthrough pressure and pore structure characteristics, breakthrough pressure is inversely related to porosity, and is primarily affected by macropores. Because macropores consist of many microfractures with lengths up to dozens of micrometers, they determine the porosity and then affect the connectivity of the rock. Correlation analysis between the mineral compositions and breakthrough pressure showed that TOC content exhibits a positive correlation with breakthrough pressure, but neither quartz content nor the clay mineral content exhibits a correlation. By combining this information with Field Emission Scanning Electron Microscope results, we found that microfractures are easily created where the TOC (total organic carbon) is concentrated, and these microfractures substantially contribute to the number of macropores. However, dissolution pores in the quartz and clay minerals are rare. Therefore, the correlation between TOC content and breakthrough pressure primarily results from the microfractures. This study provides practical information for further studies of shale gas migration as well as shale gas mining and caprock evaluation.