Wettability Of Shale And Its Effect On Methane Adsorption

The marine-continental transitional facies shale-gas in China has huge resource potential,and the transitional shale has stronger heterogeneity compared with marine and continental shales.The initial migration and micro-accumulation conditions of shale-gas are unique and complex,and the geochemical characteristics,lithology,petrophysical properties,wettability,and adsorptiondesorption properties of the reservoir jointly restrict the gas-bearing and recoverability of shale.The thesis takes the Carboniferous-Permian Shanxi-Taiyuan Formation transitional shale as the research object in the Southern North China Basin(SNCB).Based on the pore structure,wettability and high-pressure methane adsorption/desorption experiments,it focuses on theoretical analysis and covers the shale wettability and its main controlling factors,the effect of wettability on water content and pore parameters,and the control mechanism of wettability on methane adsorption,etc.,which are all key issues influencing the preservation and migration of shale gas.Key conclusions and awareness are as follows:(1)The wettability characteristics and its main controlling factors of the Shanxi-Taiyuan Formation transitional shale in the SNCB were clarified,and a three-terminal element evaluation model for the shale wettability was established based on the parameters of mineral composition,pore structure,and organic matter(OM).The results obtained by different methods show that the transitional shale has the characteristics of both hydrophobic and hydrophilic.The water-air contact angle of argon ion polishing is between 49.7°-88.2°(average 71.4°),showing relatively strong water wetness.Larger surface roughness and higher contents of OM(vitrinite and polar functional groups),illite-smectite mixed layer(I–S ML),carbonate minerals and feldspar are conducive to water-wet,while higher saprolite content and salinity promotes oil-wet.Also,temperature and pressure have a nonlinear effect on wettability,and in-situ buried shale has stronger water wettability.(2)The “dual effect” of the wettability on the pore space and methane adsorption in shale was revealed,reflecting the control mechanism of wettability on methane adsorption.The variation in wettability will affect the distribution of water content in the shale reservoir.On the one hand,water molecules can damage the porosity through the capillary condensation water in the micropores and monolayer-multilayer adsorbed water in the non-micropores.Meanwhile,water molecules inhibit the adsorption capacity by competing with methane.On the other hand,the swelling and pore expansion of shale clay after being wet-wet can make methane obtain additional adsorption space,and the adsorption capacity will increase instead of decreasing.(3)Two shale-gas desorption hysteresis modes,“water-wet–water-block–hysteresis " and" water-wet–pore-expansion–hysteresis” are proposed,revealing the hysteresis mechanism of methane desorption in water-wet shale.The desorption hysteresis is essentially controlled by the wettability of shale components,pore humidity and pore size.Mineral swelling and pore expansion may also cause desorption hysteresis.Regardless of the presence or absence of the ink-bottle pore,the desorption hysteresis may occur for hydrophilic OM and swelling minerals.(4)The response relationship between shale wettability and methane adsorption-desorption is proposed,and the wettability–gas-bearing evolution geological model is further established.Also,the favorable geologically intervals of shale-gas are selected based on the vertical variation of wettability–gas-bearing properties of shale in typical drilling profiles.It indicates that the heterogeneous wettability and pore availability of shale determine the adsorption-occurrence and desorption-migration of methane,and the sequential changes of shale wettability are of great significance to the occurrence and reservoir evaluation of shale gas in the transitional shale.The research results can provide reference for revealing the occurrence and migration mechanism of transitional shale gas,improving the shale gas reservoir evaluation system,and boosting the geological theory and exploration-development practice of marine-continental transitional facies shale-gas in China.