Experimental Evaluation Of Coal And Rock Reservoir Damage Based On Multi-scale Mass Transfer Process

As a kind of clean and rich energy, Coalbed methane (CBM) is regarded as one of the important to succeed in the 21st century in China. Mainly existing in coal matrix under adsorption, its source and reservoir are in the same bed, which makes it unconventional natural gas.There are three processes when it outputs:desorption, diffusion and percolation which are mutual influenced and restricted. Therefore, damage evaluation of single process is not enough after the fracturing fluid invades during the multi-scale mass transfer. Moreover the evaluation index and of each process of different is not uniform, as well as physical meaning. Accordingly, the working fluid intrusion impact on the overall mass transfer process can not be reflected.This paper focuses on the No.15 coal seam in Qinshui Basin.The impact of fracturing fluid on each process (desorption, diffusion and percolation) has been studied. Then the damage evaluation experiment of coal seam during multi-scale mass transfer was designed to simulate the output process. Combined with microscopic analysis means, the impact of the microscopic mechanism for fracturing fluid influcing mass transfer process of CBM has been analyzed.The damage of fracturing fluid to desorption, diffusion and percolation of coalbed reservoir has been evaluated respectively. Coal permeability, adsorption rate and diffusion coefficient before and after the contact of the activated water fracturing fluid was obtained:The desorption rate decreased by 10.23%, the diffusion coefficient by 16.67%, the gas and liquid phase permeability damage rate of 76.02%,35.84% for rock samples of low permeability respectively while gas and liquid phase permeability damage rate 70.73%,17.56% respectively for the high permeability.Damage evaluation experiment of multi scale mass transfer process has been designed to simulate coal bed methane production. The method is regarding the production of CBM as the integrated process of desorption, diffusion and percolation.Adsorbed gas in the sample tank and the original free gas together as the source, flowed into the atmosphere with constant pressure to simulate production of constant flowing pressure at bottom. Through the comparison of output rate before and after the reaction of activated water fracturing fluid with powder samples (plunger samples), multi-scale mass transfer ability damage was determined after the effect of activated water fracturing fluid on desorption-diffusion(seepage). The role of fracturing fluid on plunger permeability damage rate was 52.2%, while the rate of gas production decreased by 27.1%. It shows that there are some limitations in the evaluation of coal reservoir damage to the single mass transfer process.The mechanism for the formation damage to the coal reservoir with fracturing fluid has been revealed. By means of scanning electron microscope (SEM), nitrogen adsorption, FTIR, XPS analysis, comparative analysis of the pore structure and surface characteristics before and after reacting with activated water fracturing fluid was given. It is shown that, plugging of the pores and crack occurs and seepage channel radius decreases after imbition of activated water fracturing fluid. Owing to more micropores produced, specific surface area increases therefore desorption ability weaks. The activated water fracturing fluid was stuck to pore wall, leading to hydrophilicity which makes flowback of fracturing fluid in the mesopore as well as macropore hard. These resulted in a decrease of CBM mass transfer ability.