Revisiting the methods for gas permeability measurement in tight porous medium

Permeability is a key parameter to describe fluid transport properties of porous medium; however, the permeability measurement is extremely difficult for tight porous medium, e.g. fine-grained rock or dense soil. In this paper, three methods for gas permeability measurement, i.e. steady state method, pulse decay method(PDM) and pressure oscillation method(POM), are first reviewed and then their advantages and drawbacks are discussed. Both analytical and numerical solutions of gas permeability are presented for the tight porous medium. The results show that the analytical method is relatively simple but only valid under certain conditions, whilst the numerical method is more robust and generic, which can take into account several factors such as porosity, saturation, gas leakage, and unconventional boundary conditions. The influence of the effective porosity on the permeability determination is further analyzed using the proposed numerical method. In this study, new pressure data interpretation procedures for PDM and POM are proposed, and the obtained results can serve as a guidance to define a proper method for permeability measurement of the tight porous medium.

Evaluation of hydro-mechano-chemical behaviour of bentonite-sand mixtures

Bentonite-sand mixtures are widely used in engineering barrier of deep geological disposal of high-level radioactive nuclear waste and anti-seepage barrier of civil geotechnical engineering.Under the action of groundwater solution infiltration and external stress,the hydro-mechanical(HM)behaviour of bentonitesand mixtures,i.e.the swelling characteristics and permeability,will change.Once the anti-seepage and filtration effect is weakened or lost,the pollutants will spread to the biosphere.Therefore,it is necessary to study the swelling characteristics and permeability of bentonite-sand mixtures under coupled mechanochemical(MC)effect and to establish corresponding prediction model.For this reason,swelling tests under salt solution with different concentrations are conducted on pure bentonite and its mixtures with 30%,70%and 90%sand contents,the compression tests are carried out on saturated samples,and the saturated permeability coefficient k of the sample under each load is calculated by Terzaghi’s one-dimensional consolidation theory.The concepts of true effective stress pe,montmorillonite void ratio em and critical sand content as are introduced to determine the em-pe relationship and finally the k-em relationship of bentonite-sand mixtures.It is found that when the sand content aas,the em-pe relationship of the mixture is linear and independent of the salt solution concentration,and when a>as,the em-pe relationship of bentonite-sand mixture is bi-linear with the true effective deviatoric stress pesa as the intersection.In addition,the em-k relationship also shows the linear trend when aas,and the slope of the line increases with the increase of the salt solution concentration.When a>as,the k-em relationship will deviate from the linear relationship.Moreover,the larger the sand content is,the farther the deviation is.On the basis of summing the regularity,a model for predicting the HM behaviour of bentonite-sand mixture under the coupled MC effect is proposed.By comparing the swelling and permeability test results with model prediction results of different types of bentonite and its sand mixtures,the predictive model is verified.The study on the HM behaviour of bentonite-sand mixtures under salt solution infiltration and the model establishment can provide experimental and theoretical basis for the design and construction of anti-seepage engineering by bentonite-sand mixtures.

Characterization of microstructural features of Tamusu mudstone

Tamusu mudstone formation, located in the Alxa area in western Inner Mongolia, is considered a potential host formation for high-level radioactive waste(HLW) underground disposal in China. In this study, complementary analyses with X-ray diffraction(XRD), field emission scanning electron microscopy(FE-SEM), mercury intrusion porosimetry(MIP), and N_(2) physisorption isotherm were conducted on the Tamusu mudstone to characterize its physical characteristics and microstructural features, such as mineral compositions and pore structure. Several minerals, including carbonates, feldspar, clays and analcime, were identified in Tamusu mudstone by XRD. Images from FE-SEM show that pores in the Tamusu mudstone were dominantly on nanometer scale and generally located within their mineral matrix or at the interface with non-porous minerals. The combination of the MIP and N_2 physisorption curves indicated that the Tamusu mudstone has diverse pore sizes, a porosity varying from 2.34% to 2.84%, and a total pore volume in the range of 0.0065—0.0222 cm^(3)/g with the average pore diameter ranging from 9.6 nm to 19.23 nm. The specific surface area measured by MIP(2.572—5.861 m^(2)/g) was generally higher than that by N_(2) physisorption(1.29—3.04 m^(2)/g), due to the pore network effect, pore shape(e.g. ink-bottle shape), or technique limits. The results related to pore information can be applied as an input in the future to model single-or multi-phase fluid flow and the transport of radionuclides in porous geomedium by migration and diffusion.

Multi-scale modelling of gas flow in nanoscale pore space with fractures

In this work,a multi-scale pore network with fractures is developed against experimental data in a wide range of degrees of water saturation.The pore network is constructed based on the measured microstructure information at several length scales.The gas transport is predicted by different gas transport equations(e.g.Javadpour,dusty gas model(DGM),Civan and Klinkenberg),which can consider the fundamental physics mechanisms in tight porous media,such as Knudsen diffusion and viscous flow.Then,the model is applied to simulating the gas permeability of the Callovo-Oxfordian(COx)claystone.The predicted gas permeability is basically in good agreement with the experimental data under different degrees of water saturation.Then the effects of micro-fissures are studied.The results suggest that this model can predict the gas flow in other tight porous media as well and can be applied to other fields such as carbon capture and storage.