Investigation Of The Influences Of The Methane Adsorption Capacity On The Shales

Shale gas as an important sort of unconventional natural gas resource includes the free gas,the adsorbed gas,and the dissolved gas.and the existence form of the gas in the shale gas reservoir is different from the conventional gas reservoirs.The initial potential in the process of development mainly depends on the free gas,whereas the later potential depends on the adsorbed gas,suggesting that it is significant to investigate the adsorption capacity on the shales for the development of the shale gas reservoirs.Because of the investigation of the methane adsorption capacity on the shales was mainly carried out in indoor isotherm adsorption experiment which evaluated adsorption amount at equilibrium pressure,it represented the macro-scale results,which did not deeply reflect the essence of the methane adsorption on shales and was difficult to mirror its microscopic adsorption mechanism.Therefore,we comprehensively studied the methane adsorption capacity on shales and its influencing factors and investigated microscopic adsorption mechanism and various factors that affect the microcosmic effect mechanism for the methane adsorption capacity on shales from the angle of the molecule level,which have important theoretical and practical value in supporting the resource evaluation and development of shale gas reservoirs.Therefore,the pore structure characteristics and adsorption characteristics of shales and a single component in shales were conducted in the indoor experiments.On this basis,the mechanical mechanics of the methane molecules existed in the shales were investigated by the potential energy theory according to the composition characteristics of shales.The organic matter,clay minerals and non-clay minerals was elected as building the hole skeleton object at the same time.Aimed at the pore structure characteristics of shales,the slit-pore models were set up,and the methane,carbon dioxide,nitrogen and water acted as the fluid model.The molecular model needed in the study was build,combined with the parameters of potential energy and potential energy model.The methane micro structure of occurrence in the pores and the microscopic adsorption mechanism in shales from the molecule level were investigated by the molecular simulation method,and the influences of the temperature,moisture and different compositions on the methane adsorption behavior in the pores were investigated.With the main achievements made in the research of understanding is as follows:(1)The Kerogen mainly contained micropore and mesopore with less than 5nm,the montmorillonite mainly contained micropore and mesopore with less than 10 nm,the kaolinite,illite and chlorite mainly contained mesopore and macropore with more than 10nm,and the quartz mainly contained mesopore and macropore with more than 30nm.(2)The absolute adsorption amount of the methane on the Kerogen and mineral compositions increased rapidly with the increase of the pressure at first while increased slowly at last.At the same time,the absolute adsorption amount of the methane on the Kerogen was greater than the mineral compositions.The absolute adsorption amount of the methane on the mineral compositions in the descending order is that:illite>chlorite>quartz.The water molecules will reduce the methane adsorption capacity on the Kerogen and the methane adsorption amount on the mixture of different mineral compositions can be approximately equal to the sum of methane adsorption amount on the single mineral composition according to the mass ratio.(3)The methane molecules in the pores were mainly affected by the dispersion force and repelling force of the van der waals forces and the adsorption energy of the methane molecules in the shales was mainly contributed by the van der waals energy.In addition,the methane isosteric heat in the pores decreased with the increment of the pore size,which was less than the 42 kJ/mol,indicating that the methane adsorption on shales belonged to the physical adsorption.The methane molecules affected by the potential energy of the hole walls gathered near the hole walls to form adsorption phase.However,the area far away from the hole walls were not affected by the potential energy of the hole walls and the methane molecules as the free phase were dispersed in the pores.(4)With the increase of the pressure or the decrease of the pore size,the methane adsorption in the pores transferred from the adsorption sites with higher energy to the adsorption sites with lower energy,resulting in the increase of the methane adsorption amount.In the micropores of shales,the methane adsorption amount increased with the enlargement of the pore size.However,the methane adsorption amount decreased with the enlargement of the pore size in the mesopores.In the same pore size,the methane adsorption capacity on the clay minerals was approximately same,illustrating that the methane adsorption capacity on the different types of clay minerals mainly affected by the factor of specific surface area.(5)In the same pore sizes,the methane adsorption amount decreased in the following order:organic matter pores>clay minerals pores>non-clay minerals pores;The order of the average methane isosteric heat was that:organic matter pores>clay minerals pores>non-clay minerals pores;the interaction energy among the methane molecules decreased in the following order:non-clay minerals pores>clay minerals pores>organic matter pores.Therefore,the order of methane adsorption capacity in the shale was that:organic matter pores>clay minerals pores>non-clay minerals pores.(6)In the same pore size,the proportion of the adsorbed gas amount of methane molecules in the pores showed decreasing trend as the pressure increases.And the proportion of adsorbed gas amount of methane molecules in the pores also decreased with the increase of the pore size under the same pressure.The organic matter pores in the shales made more contribution in the adsorbed gas amount and the clay minerals pores existed part of the adsorbed gas amount made less contribution.However,the non-clay minerals made least contribution,on which the adsorbed gas amount could be ignored.Especially,when the pressure was lower than 15 MPa,the organic matter pores contributed more to the adsorbed gas amount in the shales.(7)With the increasing of the temperature,the isosteric heats of the methane decreased and the adsorption sites of the methane adsorption on the shales transferred from the higher energy to the lower energy,resulting in the methane adsorption capacity decreased.The water molecular in the pore in shales occupied the pore wall in directional way,which were acted upon by the van der Waals force and Coulomb force,resulting in the water molecule in the pore existed in the form of accumulation.With the increasing of the water,the adsorption sites of the methane did not change,indicating that the water molecules occupy only adsorption space of the methane,resulting in the methane adsorption capacity decreased.This was to say that the relationship between the methane molecule and the water molecule in shales was not adsorption site competitive relation but adsorption space competitive relation.(8)The interaction energy between the gas and the quartz decreased in the following order:nitrogen>methane>carbon dioxide,indicating that the adsorption capacity decreased in the following order:carbon dioxide>methane>nitrogen.The mole fraction of the nitrogen or carbon dioxide in the gas phase increased,the mole fraction of the methane in the gas phase would decrease,the adsorption sites of the methane would change and the adsorption space of the methane would deduced,resulting in the methane adsorption capacity decreased.