Study On Coal-water-methane Interaction In Dafosi Mine Field

Binchang mining region is located in the northern margin of Weibei uplift in the south of Ordos Basin,whose main coal seam is the typical low rank coal reservoir of Jurassic Period.The No.4 coal reservoir of Dafosi mine field was selected as the research object,and based on the knowledge of coalbed methane(CBM)geology,interface physical-chemistry,surfactant chemistry and other disciplines theory,this paper mainly conducted the method of combining large amount of coal samples of adsorption/desorption experiment with prediction of adsorption potential theory.Through the thermodynamic quantitative and qualitative analysis,it studied the three phase coupling system in the presence of coal-water-methane and the interaction between phases from the microcosmic angle of coal reservoir.Finally,the model of coal-water-methane interaction was revealed.The main results are as follows:(1)Based on Polanyi adsorption potential theory and the measured data at reservoir temperature,the isothermal adsorption/desorption parameters of different types of coal samples(such as samples of air dried,equilibrium moisture and different surfactants),under different temperature and pressure,were predicted by using the temperature-pressure comprehensive adsorption model,which contains the effect of gas fugacity.And it is found that,under the experimental conditions and the limit conditions respectively,the change of adsorption capacity and adsorption heat are different,which may be caused by the difference of the process of water-methane under the influence of temperature,pressure and surfactant.(2)The higher moisture content and ash content,high oxygen content,high inertinite and low vitrinite content,and abundant species of oxygen-containing functional groups and development of pore and fracture are the inherent reasons to promote the good water quality of low rank coal in Dafosi mine field.As for external factors,the properties of different surfactants have different effects on wettability,the order of wetting is from strong to weak:positive wetting agent,distilled water,production water,and negative wetting agent.(3)The effects of"water" on methane include displacement and desorption.Temperature,pressure and surfactant influence together affect the effect of displacement-desorption.As for displacement effect,the increase of pressure is favorable to displacement effect,but the increase of temperature is unfavorable.Samples of equilibrium moisture and positive wetting agent cause the positive displacement effect,but samples of negative wetting agent lead to the negative displacement effect.As for desorption effect,in the relatively low pressure,the unit pressure drop is more favorable for methane desorption,and the increase of pressure causes the reduction of desorption parameters,and the desorption hysteresis rate is high in the low pressure,and tends to be gentle while in the high pressure;increase of temperature causes the increase of desorption parameters,and the desorption hysteresis rate is weaker while in the higher temperature.(4)The calculation of surface free energy and wetting heat indicated that the wetting of liquid water(with the addition of surfactant)can effectively promote the desorption of adsorbed methane.(5)The formation of gaseous water in coal matrix is related to the pore structure,water migration is mainly in two forms:wetting of liquid water and diffusion of gaseous water(evaporation),the gaseous water can displace and desorb the adsorbed methane and liquid water may be beneficial to methane adsorption to some extent.The desorption of CBM of low rank coal in Dafosi mine field is the result of the joint effect of pressure drop transfer function(Energy effect)and vaporization of water vapor replacement of methane(displacement effect).