A new material balance equation for saturated coalbed methane reservoir
For saturated CBM reservoirs where the critical desorption pressure is equal to the original formation pressure,the classical material balance equation proposed by King still has some shortcomings in practical application,as it has not considered the pore space occupied by adsorbed phase.Ambrose modified the free gas pore volume and established a new material balance equation,which has been effectively applied in practice.On the basis of King and Ambrose,adsorbed phase apparent porosity,adsorbed phase volume change with pressure and adsorbed phase density are considered simultaneously for the first time in this paper,which provides a new method for the calculation of saturated CBM reserves.As indicated by the comparative analysis of examples,considering the adsorbed phase porosity,the calculated value of adsorbed gas reserves increases by 59.92％,the free gas reserves decreases by 82.33％,and the calculated total reserves increases by 31.08％.The total reserves calculated by the new equation are closer to the reserves calculated by the volume method.
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