| The CBM(coalbed methane)reservoir belongs to the typical unconventional gas,which is mainly stored in adsorption state.The tremendous reserve,shallow depth and high gas content enable CBM to become a high efficient energy in China.The commercial production of CBM has been of great significance to ameliorate the unbalanced distribution and supply of natural gas in China.Compared with conventional gas,the production of most CBM reservoirs experiences two unique stages including water drainage and critical desorption period.This indicates that the water drainage has to be conducted to reduce pore pressure before the gas begins to desorb and starts to flow.Both stages illustrate that the storage state of most CBM reservoirs is undersaturated.Conventionally,the adsorption studies on CBM reservoirs attribute the undersaturated state of storage to the undersaturated state of adsorption.Nevertheless,the gas adsorption is an equilibrium process and the undersaturated state of adsorption can hardly occur.Hence,there are limitations when gas adsorption model is applied to explain the production in undersaturated CBM reservoirs.According to these limitations in traditional CBM adsorption study,the possibility of liquid-phase adsorption relation is explored between the CBM,water and coal seam.Then,the liquid-phase adsorption model is applied to account for the adsorption state,the storage state and the critical desorption phenomenon during production.The liquid-phase adsorption of CBM couples two physical processes including the gas solution in water and the liquid-phase adsorption.Due to this coupling characteristic,adsorption chamber is designed to load the free gas and water-saturated coal sample.To measure the pressure drop of free gas induced by solution and liquid-phase adsorption,the reduction of free gas and the solution of gas can be calculated to obtain the liquid-phase adsorption volume,which can be employed to validate the CBM liquid-phase adsorption model.Following the liquid-phase adsorption theory of CBM,the compound desorption model is constructed considering the effect of tight porous media on gas desorption in coal.Meanwhile,the gas escaping problem is proposed during CBM production period.To investigate the effect of gas escaping on recovery,an analytical recovery model is presented by considering factors including control degree of well patten,adsorption characteristics,escaping process and the effect of two-phase flow.According to the presented CBM recovery model,the main controlling factor is studied in each production stage and methods are discussed to reduce the methane escaping.Finally,considering the compound desorption,the two-phase flow model is established for undersaturated CBM reservoirs and the effect of numerical dispersion is expounded on CBM simulated production history.Then a field case is studied using the simulation model and considering the effect of gas escaping process.During the study,conclusions can be drawn as follow.(1)In undersaturated CBM reservoirs,the gas in adsorption and solution state follows the Langmuir liquid-phase adsorption model.The undersaturated storage state of CBM reservoirs results from the undersaturated solution state of gas in water.The gas desorption in coal is a compound process controlled by both gas-phase and liquid-phase desorption mode.(2)Owning to the gas storage state in liquid-phase adsorption model,the gas can escape from coal at the early production period of CBM,leading to the loss of gas recovery.(3)The main controlling factors on CBM recovery are the sweeping efficiency of effective pressure drop,the degree of desorption and the escaping ratio.When there is no trap condition for free gas in coal seam,both the improvement of sweeping efficiency of effective pressure drop in early production period and the reduction of abandon pressure can decrease the escaping volume and enhance the CBM recovery.(4)During numerical simulation study,the numerical dispersion can accelerate the calculated pressure diffusion,and increase the simulated the sweeping efficiency of effective pressure drop and the degree of desorption.This leads to the overestimate of simulated water drainage and gas production in undersaturated CBM reservoirs.(5)The flow model,which is based on compound desorption process,can well satisfy the request of numerical simulation study in field case.This study has broken through the conventional recognition of CBM adsorption,makes a preliminary exploration on CBM liquid-phase adsorption model and provides a better understanding for the production process in undersaturated CBM reservoirs. |
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