Methane Adsorption Behavior In Organic-rich Shale At HTHP

The existence form of shale gas mainly includes free gas,adsorbed gas and dissolved gas.The initial production of shale gas reservoirs is mainly dependent on free gas,and the later stage production depends mainly on the adsorbed gas in shale gas reservoir.Behavior and its influencing factors are of great significance to exploration and development.At present,the research on the adsorption behavior of shale gas is mainly carried out by macroscopic experiment and theoretical method,it is necessary to study the adsorption behavior of gas molecules on shale surface by microscopic scale.Taking the Longmaxi underground shale as subject,this paper focus on the adsorption behavior of isothermal experiment under the conditions of particle size,TOC,water content and temperature as the main factor at high temperature(38 MPa)and high pressure(82?),which based on the characteristics of shale mineral and organic matter,as well as the analysis of shale pore structure.The adsorption model of methane in quartz,illite and kerogen slit hole was studied based on the Monte Carlo molecular simulation method using methane molecules and water molecules as fluid adsorption model.It is of great significants to describe,explain and verify the ehavior of methane adsorption from the molecular point of view.The results show that the target layer is a high temperature and high pressure gas reservoir with temperature of 85.99? and formation pressure of 38MPa.Its mineral composition is mainly quartz(the average is 43.2%)and clay minerals(the average is 33.16%),clay minerals in the highest content of illite(the average is 63.68%).The average porosity of the rock sample is 3.39%and the average permeability is 0.036 mD.Reservoir organic nanopores are well developed(average of TOC is 3%),the pore diameter is mainly distributed in the pore space of 3?50nm,3?10nm of pore diameter controlled the 50?80%of the pore space.It has been analysed the influence of high temperature and high pressure isothermal adsorption curve "off" behavior mechanism.Organic carbon content,moisture content and temperature as the main control factors of a series of isothermal adsorption curve have existed a common characteristic:with the increase of the pressure gradually to 38 MPa,methane adsorption on the shale content showed a trend of decrease the increase then.In the low pressure stage with pressure less than 5 MPa,the adsorption capacity increases with the increase of pressure.When the pressure reaches the vicinity of 12MPa,the adsorption capacity appears at the peak value.When the pressure is greater than 12MPa,the pressure continues to rise to promote the adsorption state to the free state conversion,making the adsorption gas content decreased,free gas content increased.High temperature and high pressure promote the adsorption state to free state transition.The quantitative relationship between the adsorption gas volume and the free gas volume conversion based on the high temperature and high pressure isothermal adsorption experiment and the molecular adsorption simulation.The particle size decreased from 16 to 20 mesh to 120-150 mesh,and the adsorption gas content increased by 52.8%.When the TOC content increased from 1.39%to 4.4%,the adsorption capacity of methane increased by 101.6%and the adsorption gas content was 70%?80%under the simulated conditions.The water content increased from 0 to 30%,the adsorption capacity decreased by 24.79%;it decreased by 37.92%and adsorption gas content accounted for 50%to 60%at simulated conditions.And the adsorption capacity decreased by 17.67%with the temperature from 42? to 82?;under simulated conditions,it decreased by 17.59%,11.43%and 13.31%respectively in quartz,illite and kerogen slit hole,correspondingly,adsorption gas content accounted for 55%?75%,60%?80%and 65%?82%,respectively.Revealing the shale on the influence factor of methane adsorption mechanism.Organic carbon content is higher,the stronger the aromatization of organic matter,kerogen has stronger affinity to the methane under the condition of high heat evolution of pore structure and the development the microporous.Water molecules with electrostatic forces dominate adsorption of hole wall,and exist with methane molecules adsorption reduces methane adsorption quantity.Shale powder particle size decreased to improve the pore connectivity,reducing the adsorption,improved the adsorption mass transfer rate,by providing high energy absorption and increase the specific surface area increased methane adsorption capacity.