Study On Coalbed Methane Adsorption&Desorption Mechanism Influenced By Coupling Of Water Injection And Temperature

The high efficiency exploitation and utilization methods of coalbed methane, not only the mine gas disasters were decreased but also energy utilized efficiencies were improved.For the industry project and technology of coalbed methane exploitation, superheating water or vapour was adopted to inject into coal seam afterwards so many fractures were shaped. The paper was stated theoretical study, meso-permeation of two phase fluid mechanics experiments, macro-large scale coal samples in laboratory on different temperature and water pressure of methane adsorption or desorption experiments, and micro-small scale coal specimen corresponding with laboratory ones in numerical studies. Finally, the adsorption or desorption law of coalbed methane influenced by coupling of water and temperature was studied systematically for the probable technology. And the results showed that: 1) The meso two phase fluid permeation testing machine of coal-gas-water was also successful manufactured. And the viewings of coal adsorbed methane and high pressure water injection experiments under micro CT were observing. It is presented for the coal specimen of2mm×2.5mm×10mm and showed that once coal specimen adsorbed methane, the porosity was decreased about3%at the same zone and the tendency was not obvious followed by gas pressure increased. Once after water injection, the porosity was also decreased about4%at the same zone. If external water unloaded, the porosity was still decreased.2) The adsorption-water injection-desorption testing machine was manufactured by own for room temperature (20℃) desorption characteristics experiments of high pressure water injection after adsorption methane using direct cylinder raw coal sample, the results presented that:the natural desorption law of experimental coal sample is the same as actual coal mine. Desorption percentages (PD) in different water injection conditions are obeyed one time effect formula, the value is determined by critical value of desorption time effect t0, the law is more water pressure, larger the value. Desorption characteristics of methane bearing coal sample are mostly influenced by water, PD are only50～70%times to natural at equality water pressure with gas pressure. Followed by water injection pressure increased, final PD is relative with water pressure. Combining porous distribution law of coal samples, the critical pore size-scale of water into coal sample was calculated in different water pressure. The smaller is critical value, the lower is PD. And PD of coal sample is related with equilibrium adsorption methane pressure and porous distribution law of coal.3) For the temperature raised (30～110℃) desorption characteristic experiments of coal sample in high pressure water injection after adsorbed methane, the results presented that:desorption capacity of methane bearing coal sample after water injection is improved as a function of temperature rose. Desorption will reach saltation once up to or exceed boiling point of water. And the PD in90℃after water injection is still larger than natural PD. There is a function relationship between PD and temperature; they are related with liquid activity and surface adsorption potential.4) For the methane adsorption experiments of water bearing coal sample and the results presented that:the methane adsorption capacity will decreased after block coal sample adsorbed water. It is existed functional relationship between adsorption capacity and moisture rate.5) For the high temperature (30～270℃) desorption characteristic experiments of methane bearing coal sample, the results presented that:the decreasing velocity of adsorption in fixed volume experiments is decreased with temperature rose; increasing velocity of equilibrium adsorption gas pressure is increased with temperature rose. The decreasing velocity of adsorption in fixed pressure experiments is increased with temperature rose. In experimental temperature (30～270℃) and gas pressure (<7MPa), the adsorption is still the first one of coal adsorb methane and would be reach maximum value once up to definite gas pressure at the constant temperature. It is defined the single molecular adsorption model. The adsorption parameter a and b in single molecular adsorption is decreased in minus exponential law as a function of temperature. The actual mechanism is adsorption activity at the surface of coal is decreased with temperature rose.6) In theoretical studies, coal base single capillary pipe which one side sealing the other opening is adopted as research object and pore size critical formula of water injecting into coal is deduced. There are two conditions in actual use:the first is after water pressure unloaded, the gas in pipe is move out so gas pressure was more than capillary force due to water; the second is there is a new equilibrium between gas and liquid if water still exists in capillary pipe. The inequality of phase change law coupling of liquid and gas influenced by temperature is deduced. It is presented that the phase change point of water is a critical one, there was two phase fluid state which less than it and become single phase state once more than it. In that case gas and liquid fluid has become gas fluid, and could be used as permeation and desorption features.7) The temperature and coupling fluid mathematical model of methane-water in coal has been deduced. It is composed of methane bearing coal sample equation as a function of temperature after water injection, kinematics and continuity equations of single gas and water.8) The corresponding numerical studies of micro porous coal specimen in different size-scale and porosity were preceded, and the results presented that:at the same initial external water pressure, the more inner gas pressure of specimen, the smaller seepage zone of water. The zone of water divide gas equal2is about2/3times of no gas. The seepage velocity is related with specimen size-scale for sheet model of same porosity at constant temperature, seepage velocity is related with model porosity of same size-scale. But the seepage velocity is not related with specimen size-scale for cube model.9) Porous units of former water become gas followed by temperature rose, and once up to definite value, all of the units are full of gas. Water pressure divide gas equal10will at240～270℃for all gas units, but only120～150℃with water divide gas equal5. Beyond results simulation, percentage of gas seepage area is displayed good linear correlation as a function of temperature. The linear correlation is related with porosity and size-scale for sheet model. And the correlation is not only related with porosity and size-scale, but also seepage condition before warming up for cube model.