Correction of source-rock permeability measurements owing to slip flow and Knudsen diffusion: a method and its evaluation

Source-rock permeability is a key parameter that controls the gas production rate from unconventional reservoirs. Measured source-rock permeability in the laboratory, however, is not an intrinsic property of a rock sample, but depends on pore pressure and temperature as a result of the relative importance of slip flow and diffusion in gas flow in lowpermeability media. To estimate the intrinsic permeability which is required to determine effective permeability values for the reservoir conditions, this study presents a simple approach to correct the laboratory permeability measurements based on the theory of gas flow in a micro/nano-tube that includes effects of viscous flow, slip flow and Knudsen diffusion under different pore pressure and temperature conditions. The approach has been verified using published shale laboratory data.The ＇＇corrected＇＇（or intrinsic） permeability is considerably smaller than the measured permeability. A larger measured permeability generally corresponds to a smaller relative difference between measured and corrected permeability values. A plot based on our approach is presented to describe the relationships between measured and corrected permeability for typical Gas Research Institute permeability test conditions. The developed approach also allows estimating the effective permeability in reservoir conditions from a laboratory permeability measurement.

Numerical simulation of gas transport mechanisms in tight shale gas reservoirs

Due to the nanometer scale pore size and extremely low permeability of a shale matrix,traditional Darcy＇s law can not exactly describe the combined gas transport mechanisms of viscous flow and Knudsen diffusion.Three transport models modified by the Darcy equation with apparent permeability are used to describe the combined gas transport mechanisms in ultra-tight porous media,the result shows that Knudsen diffusion has a great impact on the gas transport and Darcy＇s law cannot be used in a shale matrix with a pore diameter less than 1 μm.A single porosity model and a double porosity model with consideration of the combined gas transport mechanisms are developed to evaluate the influence of gas transport mechanisms and fracture parameters respectively on shale gas production.The numerical results show that the gas production predicted by Darcy＇s law is lower than that predicted with consideration of Knudsen diffusion and the tighter the shale matrix,the greater difference of the gas production estimates.In addition,the numerical simulation results indicate that shale fractures have a great impact on shale gas production.Shale gas cannot be produced economically without fractures.

Adsorption of benzene, cyclohexane and hexane on ordered mesoporous carbon

Ordered mesoporous carbon（OMC） with high specific surface area and large pore volume was synthesized and tested for use as an adsorbent for volatile organic compound（VOC）disposal. Benzene, cyclohexane and hexane were selected as typical adsorbates due to their different molecular sizes and extensive utilization in industrial processes. In spite of their structural differences, high adsorption amounts were achieved for all three adsorbates, as the pore size of OMC is large enough for the access of these VOCs. In addition, the unusual bimodal-like pore size distribution gives the adsorbates a higher diffusion rate compared with conventional adsorbents such as activated carbon and carbon molecular sieve. Kinetic analysis suggests that the adsorption barriers mainly originated from the difficulty of VOC vapor molecules entering the pore channels of adsorbents. Therefore, its superior adsorption ability toward VOCs, together with a high diffusion rate, makes the ordered mesoporous carbon a promising potential adsorbent for VOC disposal.