Occurrence And Gas Volume Prediction Model Of Deep C0oalbed Methane

Several examples show that the gas production curve of deep CBM wells decays slowly after the gas production peak period,and the stable production period is extremely long,which is almost no similar phenomenon in shallow CBM wells.This implies that the occurrence state of deep CBM may have particularities,and the selection theory and drainage control measures are different from those of shallow CBM.Therefore,taking the Yushe-Wuxiang block in the Qinshui Basin as the research object,based on coal sample test analysis,combined with the characteristics of the geothermal field and in-situ stress field in the area the characteristics of the occurrence of deep CBM are studied,and the deep CBM is discussed.Some new understandings have been gained on the existing geological control factors.Firstly,the coupling effect of temperature and stress causes the difference of sensitivity of different physical properties of coal samples.The pore has very strong stress sensitivity and strong temperature sensitivity,and the coal structure is also an important factor leading to the temperature-stress sensitivity differentiation;temperature has an effect on pore stress sensitivity and confining pressure on pore temperature sensitivity.The permeability has a strong temperature-stress sensitivity;the permeability stress sensitivity has no substantial relationship with temperature,and the permeability temperature sensitivity gradually strengthens with the increase of the confining pressure.Secondly,the applicability of existing gas density models to deep temperature and pressure conditions is different.From the perspective of the density of free phase methane,the fitting result of the RK equation has the smallest error.From the analysis of the adsorption phase density,the supercritical adsorption model based on the extension of the DR model has the strongest applicability.The thermodynamic characteristics of adsorption indicate that the supercritical adsorption of methane in deep coal reservoir is still physical adsorption,and the absolute adsorption isotherm belongs to type I adsorption curve;the maximum adsorption capacity V₀ and adsorption phase density decrease decrease linearly with increasing temperature,and the constant D increases linearly with the increase of temperature.Thirdly,under deep formation conditions,coalbed methane occurs as a transition state between free gas and adsorbed gas.For the study area,the occurrence state of coal seam shows obvious zonation under the condition of buried depth.Free-phase methane of coal seam exists in the form of free gas in the shallow zone with a burial depth of less than1149m,in the form of compressed free gas in the deep zone with buried depth between1149～4152m,and in the form of transitional state in the ultra deep zone with buried depth more than 4152m.Corresponding to the transition free gas zone,methane in the adsorbed phase also transforms into the transition free gas.Based on this relationship,an in-situ coal reservoir adsorbed gas and free gas content estimation model was established,and the adsorption model is used for the first time to confirm that there is a"critical depth"of adsorbed gas content in coal seam and that the importance of free gas increases below this depth.Fourthly,the transformation of the occurrence state of deep CBM is the result of the coupling of internal and external geological factors.The external factors are formation temperature and formation pressure.The adsorbed gas has a positive pressure effect in the shallow part,and a negative temperature effect in the deep part;free gas has an obvious response to pressure and a weak response to temperature.The internal factors are coal reservoir pores and cracks and coal seam water.The greater the porosity,the stronger the free gas storage capacity,the higher the water content,the less the content of adsorbed gas and free gas.The paper includes 77 diagrams,30 tables and 170 references.