Spontaneous Water Imbibition Into Methane-Saturated Shales: Experiment And Numerical Simulation

With the development of hydraulic fracturing,shale gas resources have been rapidly exploited in recent decades.However,millions of liters of hydraulic fracturing fluids are injected into shale gas reservoirs,while the fracturing fluid recovery is usually lower than 40%.Most commonly,9099%of hydraulic fracturing fluids are water.Public concern regarding the potential environmental impacts has increased owing to the large volumes of water consumed.Spontaneous imbibition of hydraulic fracturing fluid is known as the primary mechanism for fracturing fluid loss and inefficient water recovery.Besides,a deep understanding of hydraulic properties and water movement mechanism of the shale is of great significance for engineering applications and natural science fields,such as groundwater cycle,CO2 geological storage,nuclear waste disposal,and so forth.In this work,the Carboniferous shale samples were taken from the eastern Qaidam Basin,the deionized(DI)water was adopted as the wetting phase,and methane(CH4)was employed as the nonwetting phase.A new experimental approach was developed to perform a series of spontaneous water imbibition experiments on methane-saturated shale samples under different methane pressure conditions(0.1,1,and 5 MPa).The process of spontaneous imbibition and the controlling factors of imbibition were studied.Then,according to the imbibition data,the shale sample water movement parameters were obtained by using the parameter inversion method in the Hydrus-1D software.Then,the water movement process of the shale under different conditions was simulated.It provides a theoretical basis for the rational utilization of groundwater resources and the exploration of shale gas.The detailed studies are presented as follows:(1)The dominant minerals of the shale samples are quartz,clay,and calcite.The samples are dominated by nanoscale pores with an average pore radius of 6.04-10.74 nm.The proportion of the mesopores to the total pore volume is the largest.The gas(N2 and CO2)adsorption and highpressure mercury intrusion(MICP)method can be combined to approximately obtain the pore volume of the sample matrix,while the helium expansion method can be used to obtain the pore volume of the sample matrix and microfractures.(2)Owing to gas slippage,the methane permeability of the shale sample decrease with increasing the average pore pressure.To eliminate the effect of gas slippage,the Klinkenberg permeability of the shale sample is calculated.However,it is found that the Klinkenberg permeability of the sample is two orders of magnitude higher than its measured water permeability.It is likely that the interaction between water and the surface of nanopores of the sample forms a bound water film,which changes the pore structure of the sample.(3)The imbibition rate and the imbibed volume of the shale sample decrease with increasing the methane pressure.The imbibed volume of the shale sample versus square root of time under the different methane pressures shows a strong linear relationship.Besides,with the increase of the mesopores volume,quartz,clay,and TOC content,the imbibition rate and imbibed volume of the sample tend to increase.(4)The capillary pressure of the shale sample rapidly decreases with increasing the methane pressure from 0.1 to 1 MPa and then decreases with a smaller slope with increasing the methane pressure.There may be a logarithmic relationship between the capillary and methane pressures.(5)Experimental and simulation results indicate that more methane molecules are adsorbed onto the pore surface of the sample with increasing the methane pressure,resulting in the wettability of the pore surface changing from water-wet to oil-wet.When spontaneous imbibition occurs under higher methane pressures,more methane gas may be trapped in the pore spaces of the sample,which reduces the effective saturated water content and the effective permeability coefficient.(6)The simulation results show that when the pressure difference is less than 104 cm H2O,the influence on the invasion depth of the sample is relatively small,while when the pressure difference is greater than 105 cm H2O,it has a significant effect on the invasion depth of the sample.