| Different from conventional reservoirs, the main object of coalbed methane (CBM) recovery is adsorbed gas which stores in matrix pores and occupies70%～90%of total CBM content, migration and production of CBM need to go through coupled process of desorption-diffusion-seepage, so diffusion plays the role of bridge between desorption and seepage. Large numbers of production practices and research results of numerical simulation have revealed that diffusion rate is an important effect factor of CBM recoverability, when other reservoir conditions are the same, the faster CBM diffusion rate is, the earlier CBM well reaches the peak and the higher development benefit is. Therefore, it’s important to research gas diffusion behavior in coal under different conditions, analyse mechanism of affecting factors, and seek for effective ways of strengthening CBM desorption-diffusion rate.Coal from9#coals of Taiyuan formation in Ningwu Basin was selected as research object, coal pore structure was described by scanning electron microscope, liquid nitrogen adsorption and mercury penetration method. Methane desorption kinetics experiments were carried out by self-designed apparatus, effects of water content, working fluid filtrate action, desorption temperature, coal sample grain and heat treatment on methane desorption behavior were studied. Experimental data were fitted by use of Origin software, in order to quantitatively reflect effects, methane diffusion coefficient (diffusivity) under different conditions was obtained by Crank diffusion model. Mechanism of affecting factors was analysed in this paper and new thought of strengthening CBM desorption-diffusion rate was proposed.Methane desorption kinetics experiments under different conditions were designed and carried out, methane diffusion coefficient of each experiment was obtained. The results showed that, methane diffusion coefficient decreased with the increase of water content of coal sample, when water content was between0and equilibrium water content, methane diffusion coefficient decreased by a large margin as water content increased. However, when samples had become equilibrium water coal samples, external liquid water has a relatively small effect on methane diffusion coefficient. The higher desorption temperature was, the greater methane diffusion coefficient was, selecting adsorption equilibrium temperature (38℃) as reference, compared with desorption of lowering temperature, sensitivity of diffusion coefficient on desorption temperature change was stronger when desorption temperature rose. Coal sample grain directly reflects how long diffusion path is and how big diffusion resistance is, the greater coal sample grain is, the longer methane diffusion path in matrix pores is and the bigger diffusion resistance is, leading to a slower desorption-diffusion rate.Effects of different working fluid filtrate action on methane desorption-diffusion behavior were evaluated in this paper. Compared with simulated formation water, after working fluid filtrate action, methane desorption-diffusion abilities were in sequence as follows:clear fracturing fluid B filtrate, simulated formation water, drill-in fluid filtrate, clear fracturing fluid A filtrate, activated water fracturing fluid filtrate, ion balance fracturing fluid filtrate, guar gum fracturing fluid filtrate. Solid and liquid phase of working fluid filtrate were easy to be adsorbed by the coal samples, moreover, compared with simulated formation water, working fluid filtrate has a stronger wetting degree on coal samples, these made the amount of micropores increase and pore structure become more complex, so methane diffusion coefficient decreased. The wetting degree of clear fracturing fluid B filtrate on coal samples was closed to simulated formation water. Besides, it was faintly acid and could dissolve natural filling substance in coal samples, and then widening methane diffusion passageway and strengthening diffusion ability.A new idea of strengthening CBM desorption-diffusion rate was proposed. By use of heat treatment, a series of high temperature physical and chemical reactions are caused in the near-wellbore CBM reservoir, leading to the change of coal components and pore structure, so the amount of macropores increases significantly and pore connectivity is improved. Heat flux generated from heat treatment also can elevate local reservoir temperature and relieve liquid phase trap effectively. In addition, heat treatment creates favorable conditions for gas injectiorrthrough lowering fracture pressure of CBM reservoir. On the basis of reasonable injection-production pattern deployment, thermal CO2/N2with over fracture pressure are injected intermittently after heat treatment, this can induce micro-fractures in CBM reservoir, cause nonisothermal desorption of CBM, improve drive energy and weaken stress sensitivity damage. |
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