Study On Gas Adsorption And Desorption-Flow Mechanisms In Moist Shale

Water,which is omnipresent in some gas shales,exerts profound influence on gas migration characteristics.However,the exact controls and mechanisms for effect of moisture content(M_c)on gas adsorption and desorption–flow behaviours are to date far from clarity.This paper firstly evaluates several petro-physical parameters of underground and outcrop shale collected from Fuling shale play.Then,A series of experiments are conducted to measure gas migration behavoirs in shale,including moisture isotherm,evaporation rate,CH₄adsorption and dynamic production,and several models are established to fit the experimantal data.Finally,the gas migration mechanisms in moist shale are revealed and analysed.The major achievements of this study follows as:Results of petro-physical measurements indicate that carbonate content and TOC of the underground shale are about 40%and 2.81%,respectively,which meets the criterion of shale sweet spots.Bidisperese model is sufficient to respresent petro-structure of mosit shale,consist of hydrophobic organic micropores smaller than 10 nm and hydrophilic inorganic macropores larger than 20 nm.These two types of pores are parallel connection in the shale.Moisture uptake isotherms of shale are measured and experimental results indicates that the isotherm of shale is of S type with measured data well-fitting the GAB adsorption model when the relative pressure(RH)is less than 60%.The monolayer capacity of shale corresponds to a relative pressure of approximately 20%,which indicates that monolayer water molecules are adsorbed on the surface of shale pores when RH?20%,and multilayer adsorption occurs when RH is larger than 20%.Experimental results of water evaporation measurements indicate that vaporization rate incerases with the increase of average pore pressure and the rate is much smaller than gas flow rate in the pore space.Experimental results of CH₄ adsorption tests on moist shale indicate that the methane sorption capacity versus moisture content curves exhibit three stages separated by two threshold moisture contents.The first critical moisture content threshold(M_fc)represents coverage of the entire hydrophilic surface by a monolayer of water at the water monolayer capacity(W_m)of the shale.The second critical moisture content threshold(M_sc)is the point at which no methane is adsorbed on the surface of the clay pores and adsorption capacity is reduced as moisture content is increased.The M_fc is affected by both pressure and temperature while the M_sc is independent of these factors.Methane adsorption capacity decreases linearly with an increase in M_c to a maximum of M_fc and is mainly dominated by the completion between water and methane for adsorption sites on the surface of clay pores.Moisture content has negligible effect on shale adsorption capacity for M_c in the range M_fc to M_sc.Methane adsorption capacity decreases convexly above M_sc,suggesting water condensation in organic pores as the surface area for methane adsorption is reduced by water blocking.On the basis of experimental data,a conceptual Bi-Langmuir model is presented to represent the crucial effects of moisture content on methane adsorption capacity.Gas dynamic production tests on moist shale sample are conducted to investigate the gas migration mechanisms.The results indicate that gas production equilibrium time decreases with the incease in M_c when M_c is less than the M_fc.When the M_fc is exceeded and up to the M_sc,the larger the M_c,the greater the time taken to reach equilibrium.A variable pressure bidisperse pore(VPB)model is presented to decribe gas flow characteristics in both in-organic and organic pores of shale.Compared to the traditional small pressure bidisperse pore(SPB)model,the VPB model is considerably more applicable to actual gas production by using pseudo-pressure and pseudo-time to accommodate the effect of gas pressure dependent PVT parameters.Permeability for both in-organic and organic pores of moist shale can be eatimated by matching the semi analytical solution of the VPB model with the dynamic gas production data.The permeability for these two type of pores are calculated in the range of 3.89×10^-7-5.97×10^-7?m² and 3.08×10^-8-5.24×10^-8?m²for undergound shale in 100-120 mesh under pressure drop of 10MPa to 6 MPa,respectively.