Gas Flow Characteristics Of Unsaturated Shale

Shale is usually either as source socks,but also can serve as reservoir and seal.The study of gas flow characteristics in shale is of great practical significance for the performance evaluation of geological storage of the harmful gas such as greenhouse gas(C02? CH4)and the exploration and development of shale gas.Due to the characteristices of secondary porosity,small pore throats,strong heterogeneity,high water saturation,and complex relation between gas and water,gas-water two-phase flow mechanism of shaleis very complex.Therefore,it is a great challenge to fully understand the gas-water two-phase flow mechanism in shale.In this paper,the Carboniferous shales of the Qaidam Basin are studied by permeability test(pressure pulse–decay and the steady state method,gas adsorption method(low temperature N2 and low temperature CO2)and high pressure mercury intrusion test.The porosity and permeability characteristics of Carboniferous shale are studied.Effects of water saturation,effective stress,anisotropy,mineral composition,pore pressure and micropore structure on flow characteristics of unsaturated shaleare analyzed.Based on the DGM model,a new simple shale gas mass flow model is established considering the complex pore structure of shale,the influence of slip effect of nanopores and the diffusion in the matrix system.Gas flow mechanism in unsaturated shaleis studiedby low pressure methane seepage simulation experiment.The mineral composition of the Carboniferous shale varies significantly from sample to sample.It mainly includes clay and detrital minerals.Shale permeabilities range from 1.69×10-21 to 1.27×10-17m2,mostly distributein 10-20m2.Mineral composition controls the shale permeability.Shales have strong heterogeneity,the permeability anisotropy of the enriched clay samples is more pronounced than that of the enriched quartz samples.The correlation between TOC and micropore volume is positively relative,which indicates that the microporesarethe main space for hydrocarbon enrichment.The surface area is positively correlated with the clay content,but negatively correlated with the quartz content.The micro-nanopores developed in the clay can provide places for the gas adsorption,while the quartz belongs to the brittle minerals and the shale samples with high quartz content are easy to form microfractures which provide access for gas transport.The stress sensibility of permeability is controlled by the shale microscopic pore system and mineral composition.The clay-rich samples are mainly occupied by easily-compressed nanoscale slit-like pores,which are more sensitive to changes in effective stress.The stress sensitivity of shale samples containingwateris higher,and the helium permeability is an exponential decline with the water saturation.Based on the microscopic model of gas-water two-phase flow in shaleproposed in this paper,the relationship between Klinkenberg slip factor(b)and water saturation is analyzed.With the increase of water saturation,bfirstdecreases and then increases.When the water is mainly present in the micropores,the channel flow occurs and the slip factor decreases as the water saturation increases;with the increase of water saturation,the water is attached to the pore wall in the form of water film.As the water phase is movable,the funicular flow occurs,resulting in an increase of slip factor.The correctness of the relationship between the apparent permeability and the reciprocal of the average pressure is verified by the physical simulation experiment of methane seepage in unsaturated shale under low pressure.The shale gas flow in the low pressure is the transitional flow,in which the slip flow and the gas diffusion coexist.The Darcy permeability is independent of the pressure,and the contribution of the slip flow to the apparent permeability is inversely proportional to the pressure.The contribution of the diffusion flow to the apparent permeability is inversely proportional to the pressure squared.The sensitivity of the diffusion flow to the pressure is significantly greater than the slip flow.