Breakthrough Pressure And Permeability Characteristics Of Unsaturated Shales From Eastern Qaidam Basin, China

Breakthrough pressure and permeability play an important role in shale gas flow,mining,and caprock evaluation.Carboniferous shale from eastern Qaidam Basin is considered to have great potential for shale gas exploitation;thus,investigations of breakthrough pressure and permeability are significant.Six shale samples taken from two different boreholes in this study area were chosen to conduct breakthrough and permeability experiments to analyses the effects of water saturation and carbon dixode mole fraction.Prior to the experiments,relevant geochemical tests and micro structural characteristics?low pressure N2 adsorption,low pressure CO2 adsorption,mercury intrusion porosimetry and FE-SEM experiments?of micro pores were also conducted.Breakthrough pressures of the six shale samples 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.Breakthrough pressure was found to 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.The situation for permeability is just the opposite.Pore size distribution measurements indicate that there are many micropores?at the nanoscale?that can easily be blocked by water molecules.This result in the reduced connectivity of gas pathways;thus,permeability decreases with water saturation.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 only the TOC content exhibits a positive correlation with breakthrough pressure.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 volume of macropores and the porosity.The breakthrough pressure decreases with the CO₂ mole fraction,and situation for permeability is just the opposite.The breakthrough pressure decreases with the CO2 mole fraction because the interfacial tension?IFT?of the CO2-water system is smaller than that of the CH₄-water system.The viscosity of the CO₂-CH₄ mixture was found to increase with the CO₂ mole fraction by fitting a series of values under the same temperature and pressure conditions.In addition,this leads to an increase in permeability.Furthermore,CO₂ molecules are smaller than CH₄ molecules,which makes it easier for CO₂ to move across pathways.After each breakthrough experiment,the CO₂ mole fraction in the effluent was less than that in the injected gas,and it increased over time until reaching the initial injected gas composition.This is because a considerable amount of CO₂ dissolves in the water and was adsorbed at the surface of rock particles and the adsorption and solubility of CO₂ are greater than that of CH₄.