Mechanisms Of Shale Gas Recovery With Enhanced Desorption By Electrical Heating

The percolation mechanism and the mining method of shale gas reservoir are different from conventional gas reservoirs.First of all,the permeability of the shale gas is extremely low,and the size of pore is less than micro/nano orders of magnitude generally.In the micro-nano pore,the Darcy’s Law is no longer applied.Secondly,the content of the adsorption state of shale gas is between 20%~85% of total gas.Therefore,Shale gas desorption is a key factor to affect shale gas production.Study shows that temperature and pressure are the important factors.The conventional method of mining the shale gas is reducing the pressure to desorption the gas,but the production is low and the period is long.because reducing the pressure is limited,and the large part of gas cannot be desorbed.Based on considering the reservoir’s characters,migration and production mechanisms of shale gas,an electrical heating method is proposed to enhance shale gas desorption.According to the electro-thermal conversion principle,we will transform the electrical energy into heat energy.To describe the mechanism of shale gas desorption,Langmuir adsorption model is introduced to the mass-conservation equation.Knudsen number is used to describe the flow regime of shale gas in reservoir,the molecular diffusion and slippage effect are considered.According to the desorption,diffusion and slipping effect on the dual porosity media,the mathematical model of shale gas for the coupling field of current and temperature and flow is established,Using finite difference method for the numerical solution of the mathematical model,the electrical efficiency of heating the shale gas is deeply discussed and analyze the factors of effecting shale gas production capability The main results are as follow:(1)The adsorption experiment data of methane is matched well with the curve of the Langmuir and Bi-Langmuir adsorption model.The adsorption index and the adsorption quantity decrease with temperature increasing,showing that increasing temperature can increase the shale gas desorption.(2)Compared with the vertical well pattern,horizontal well pattern can heat the reservoir more efficiently and can make the energy distribution equally.So in the same geological and production conditions,horizontal well drilling should be the first choice.The effect of resistivity is small;The length and the distance of the horizontal well will affect the efficiency of the electrical heating.Under the permission of economy,we can increase the heating power to increase the shale gas desorption ratio.(3)Using horizontal well pattern heating fractured shale gas reservoir,the accumulated production increases with the heating power increasing;Because of the temperature and pressure,the effect of the distance of the horizontal well on the shale gas production is small;The total production increases with the length of the horizontal well,but the gas production rate of unit length decreases;the accumulated production increases with the heating time increasing while the increasing speed of accumulated production is not obvious and the cost is high.