Integrated Characterization On Pore Structure And Prediction On Methane Adsorption Capacity Of Mudstones And Shales

With increasing difficulty of conventional oil and gas exploration,shale gas resources get more and more attention around the world.Ascertaining the pore structure characteristics of shale reservoirs,understanding the adsorption mechanism and migration law of methane in shale pores and making Predictions for the methane adsorption capacity of the shale reservoirs have great significance to the exploration of shale gas resources.Dozens of samples of Dongying Depression shale reservoirs were studied by image analysis,low nitrogen adsorption experiments and mercury intrusion porosimetry analysis.Shale pore structure were characterized systematically by these empirical methods.At the same time through the integration of the results of low-pressure nitrogen adsorption and mercury intrusion porosimetry experiments,we found a method for obtaining the full aperture shale pore size distribution.In addition,optimal mercury intrusion porosimetry conditions of shale samples were explored.The results show that 45-60s equilibration time and 20-30 mesh sample grain are found to be the optimal operation conditions.Based on the study of nano-pore structure characteristics of shales,a study on the density distribution,occurrence state,and mobility of pore fluids in nano-pores in shales has been carried out by using methods of the quenched solid density functional theory(QSDFT)calculation in this study.Results show that the methane is distributed as a monolayer or bilayer in nano-pores which are less than 1.0 nm in size.However,the pore methane occurs as multiple adsorption layers and even free methane in 2.0 nm sized pores.Under the same conditions,the calculated density of the methane in small nano-pores is higher than that of the methane in relatively big nano-pores,and the density of the methane in the organic pore is higher than that of the methane in the inorganic one.With the increase of temperatore and pressure(i.e.,buried depth),the confining effects of nano-pores are decreased.This resulted in the decrease of the proportion of confining methane and the increase of the free methane increases in nano?pores.QSDFT was used to study the adsorption behavior of methane in a Model Basin,in which the temperature gradient is 0.03?/km and the hydrostatic pressure gradient is 0.01 MPa/m.The results show that:micro pores(per unit volume)showed greater adsorption capacity than the mesoporous(per unit volume).Adsorption capacity of pores(<1 nm)rapidly increases with depth,and reaches the maximum of 14000mol/m3 at the depth of 500 m.For pores(>5 nm),the adsorption capacity keeps increasing above the depth of 2000m,thus,the adsorption capacity of such pores(>5 nm)may plays a very important role under the depth of 2000m.Whaf's more,at the adsorption equilibrium state,the adsorption capacity at the depth of 100m depth can reach 3 to 11 times the amount of adsorption capacity on the surface.The maximum amount of adsorption comes from the pores with the pore width of 0.5 nm.QSDFT method was also used to do the research on the adsorption behavior of methane in a Model Basin.A simulation was conducted to study the changes of the adsorption amount of methane in organic and inorganic shale pores under different burial depth condition of real geological environment.The results show that:the methane adsorption capacity of organic pores was significantly higher than that of the inorganic ones.At different conditions of temperature and pressure,the adsorption capacity of the pores of different sizes are different.Either organic or inorganic bore hole,the adsorption amount of larger pores increases with the depth increasing while that of the smaller pores increases at the beginning and then decreases with the depth increasing.