Effect And Mechanism Of Controlled Source Microwave Field Irradiation On CH₄ Adsorption/desorption Performance Of Marine Shale

The sustainable development of human society depends on adequate energy supply.The traditional fossil energy is dominant in energy consumption structure in China.However,the resultant environmental issues require promotion of clean energy.As an alternative energy to the traditional fossil energy,the exploration and production of nonconventional natural gas including shale gas,coal-bed methane,and tight gas are of great significance to optimize the current energy consumption structure and alleviate en vironmental issues in China.Among these,the shale gas has been gaining great attention in recent years because of its abundant reserves in China.At present,the shale gas has been achieved commercial production.China still adopts hydraulic fracturing technology for shale gas production.Unlike North American countries,the shale gas resources with burial depth greater than 3500 m account for 65%of the total resources in China,thereby greatly increasing the cost of hydraulic fracturing.In addition,the implementation of hydraulic fracturing in China is limited due to the following factors,i.e.poor reservoir fracturing,scarce water resources,and complex terrain,etc.Therefore,in consideration of gas-bearing shale reservoir conditions of China,the research and development(R&D)on clean,efficient and low-operating cost shale gas production technology is particularly meaningful to the implementation of energy development and environmental protection strategies in China.The shale gas within shale reservoir typically presents as adsorbed state.Its production incorporates desorption,diffusion and flow of shale gas within gas-bearing shale reservoirs.Unlike hydraulic fracturing technology,the increase in temperature is also capable of desorbing shale gas.Therefore,a novel technic proposal,i.e.controlled microwave field radiation technology is recommended in this paper.Particularly,the Lower Silurian Longmaxi Formation shales in Jiaoshiba area,Fuling,Sichuan Basin were selected.The influences of controlled microwave field radiation on adsorption and desorption capability of marine shales were mainly addressed.The following aspects were investigated as follows,i.e.the heating behavior of marine shales under controlled microwave field radiation,the effects of controlled microwave field radiation on physicochemical property of shales,and the effects of controlled microwave field radiation on adsorption and desorption capability of methane on shales.The main conclusions derived from this study are as follow s:(1)Under controlled microwave field radiation with frequent of 2450 MHz,power of 400 W and duration of 4 min,four shale samples exhibits remarkable microwave-absorbing and heating capa bility.Specifically,the bulk temperature of marine shales rises with the increasing controlled microwave field radiation duration.Furthermore,the heating curve of all shale samples consists of rapid heating stage and slow heating stage.Moreover,after radiation duration of 240 s,the bulk temperature of two shale samples can reach up to 320?and 235?,respectively.In addition,the microwave-absorbing capability of four shale samples exists notable differences,which is associated with the content of pyrite and marcasite of shales.Concretely,shale with high p yrite and marcasite content has strong microwave-absorbing capability.(2)Controlled microwave field radiation remarkably change s the functional groups and pore structure of marine shales.On the one hand,microwave radiation can change surface chemical propert ies related to adsorption and desorption capability of shales.Specifically,it increases the total content of oxygenic func tional groups including highly-conjugated C=O,conjugated C=O and COOH and reduces aromaticity of shales.The abovementioned changes can weaken the force strength between CH₄ and the shale matrix,thereby promoting the desorption behavior of in-situ CH₄ within shale reservoirs.On the other hand,controlled microwave field radiation alters the adsorption pores and seepage pores of shale samples.Particularly,the amount of pores with a pore diameter less than 10 nm decreases while that with a pore diameter larger than 10 nm increases after microwave radiation.Furthermore,the cracks and permeability of plunger shale sample with diameter of 25 mm and height of 27 mm significantly increase after microwave radiation.In addition,the fractal dimension of adsor ption pores and seepage pores decrease s after microwave radiation.In other words,the controlled microwave field enhances the pore surface smoothness of shale samples.The abovementioned change s in pore structure parameters are beneficial to desorption,diffusion and flow of shale gas within gas-bearing shale reservoirs.(3)Controlled microwave field radiation affects the adsorption equilibrium,adsorption kinetics,and adsorption and desorption hysteresis of CH₄ on marine shales.Concretely,controlled microwave field radiation decreases the CH₄ maximum adsorption capability of four shale samples by up to 12.70-47.83%.Moreover,controlled microwave field radiation increases the diffusion rate of CH₄ within shale micropores.Additionally,controlled microwave field radiation weakens the CH₄adsorption and desorption hysteresis.In summary,controlled microwave field radiation is beneficial to improving desorption and diffusion capability of CH₄ within shale reservoirs,thereby enhancing shale gas recovery.This research well demonstrates the feasibility of controlled microwave field radiation technology for enhancing shale gas production.The derived conclusions are not only conducive to remedying the existing shortcomings regarding current shale gas production technology in China,but also can provide novel ideas for researchers and engineer engaged in the field of shale gas exploration and production.