Research On Gas Transport Mechanism In Shale By Experimental And Modeling Evaluation

Shale gas is becoming an increasingly important energy resource in recent years to compensate the shortage of the fossil energy.Identifying the gas transport process in shale matrix is therefore of great importance in designing development strategies and in formulating the appropriate predictive mathematical models.Experimental and numerical investigations were conducted to study the gas transport in shale.This paper presents the experimental work and develops a numerical model considering both the effect of matrix and kerogen on diffusion.Of great relevance to field development and management is knowing the contribution of each gas source to gas transport history and to ultimate gas recovery.Based on the results of this paper,the contributions of free gas and adsorbed gas could be determined.Shale samples from Sichuan Basin in China were studied and most basic parameters were measured and discussed,i.e.permeability,wettability,microstructure,specific surface area and pore distribution,XRD analysis,and the total organic carbon.Due to the effect of Klinkenberg,gas permeability increases with decreasing fluid pressure;the samples tested are water-wet;shale sample is constructed with nano-to micro-pores thus the specific surface areas are as high as 24 m2/g for shale rock and 84 m2/g for kerogen.The total organic carbon(TOC)varies for two different samples,4.67% for samples from Longmaxi and 1.59% for Da'anzhai.Steady-state flow method and the flow under constant outlet pressure were conducted to investigate the gas transport mechanism at room temperature.Based on the comparison between gas flow test of shale core and tight core,two stages were divided and it is analysed that the first stage is dominated by the free gas flow and the second stage is dominated by the desorbed gas diffusion.To make the gas flow mechanism clear,120 tests with three shale particles were conducted at designed conditions to test the effect of boundary pressure,temperature,particle size,and total organic content(TOC)on the dynamic desorption-diffusion process.The tests for crushed shale samples were conducted at 35oC,40oC,and 45oC,and at a series of test pressures.By plotting a curve describing the gas volume change per unit mass at standard condition over time,the desorption-diffusion process at isothermal and constant boundary pressure was investigated and interpreted.Coincidence with the conclusion of flow test with cores,the two-stage gas flow process is assured by the test results of shale particles.These particle test results indicate that both higher pressure and higher temperature could promote a faster desorption-diffusion rate,thus could promote a greater desorption constant rate.Higher temperature caused less gas to be desorbed from the shale particles.Under the same experimental conditions,a difference in particle size showed no influence on the amount of gas adsorbed,but had a significant effect on the dynamic desorption-diffusion process: the processing time extended linearly with the diameter of the particle size.Due to the fact that organic matter such as kerogen widely distributes in shale,it plays an important role in gas transport in shale.Although many mathematical models considering gas diffusion and desorption have been studied,few studies consider the effect of kerogen on gas transport process or combine experiment with mathematical models.A gas desorption-diffusion method is presented to analyze transport processes,considering the effect of gas diffusion in kerogen.This mathematical model is built based on variable-volume volumetric method and compared with the dynamic adsorption/desorption diffusion experiments with constant pressure condition.The numerical result of the model has been proved to be accurately reveal the gas transport process under isothermal and constant boundary pressure condition.