The Method Of Permeability Calculation For Moisturized Shale Using The Dynamic Process Data Of Methane Adsorption

Shales are extremely tiny rock and have complex pore structure,so the permeability of shale has always been a focus of scholars.Many methods have been provided for permeability measurement and calculation.Because shales have initial water content in natural state,the study of permeability for moisturized shale is important.Carboniferous shale samples in this study were taken from the eastern Qaidam Basin.Permeability of dry and wet samples were calculated and compared.Rock characteristics of the shale samples,including mineral content,organic geochemical characteristics,pore structure and pore size distribution,were measured before the permeability analysis.The samples were crushed into particles of a diameter for 0.25 mm.3 groups of the samples were moisturized at a temperature of25?,relative humidity of 65%?85%?95%.Methane adsorption experiments were conducted on dry and moisturized crushed samples over a pressure range up to 10MPa and at a temperature of 40?.Then the permeability of dry and moisturized shale samples were calculated by the dynamic data of methane adsorption.In this study,the methane adsorption capacities of the shale samples were reduced averagely by 48.88%after being wetted,and the trend of the methane adsorption isotherm curves of moisturized samples were gentler than that of dry samples.For dry samples,the curves of the permeability and the methane adsorption rate?with pressure have a similar“w”shape.The permeability were found to have the same variation trend with?at experimental pressures before an inflection point.We believe the permeability and methane adsorption rate represents the small pores property before the inflection point and the bigger pores after the point.Combining the curves variations and analysis,when the experimental pressures reach infinity,we assume that the methane adsorption rate will decrease to infinitesimal and the permeability value will stop fluctuating and become a fixed value,which we call kf.The parameter k_f is regarded as the intrinsic permeability of the sample.Permeability was reduced by 74.4%-89.3%after samples were wetted,and the relationship between permeability and water saturation fitted the exponential equation y=a0)well.Porosity changes and poor connectivity of pores caused by water were considered the main reason for the permeability reduction.No significant difference in permeability was found among the wet samples.After calculating the water film thickness and pore size distribution of the moisturized samples,water was found to only have important influence for small pores,which was considered as the reason of the unobvious difference in permeability of the samples under different water content.In addition,clay minerals are hydrophilic,and larger specific surface areas cause stronger water adsorption on shales.Correspondingly,a positive relationship was found between the permeability reduction and total clay content and surface area.