Contribution And Its Prediction Model Of Production Strata For Joint CMG Mining

Objective determination of the gas production contribution from different producing strata is the primary basis for improving the benefits of the joint multi-type coal measures gas(CMG)development.Facing the needs of this industry,relying on the National Science and Technology Major Special Demonstration Project,the paper analyzed the geological characteristics of coal measure gas production in the Upper Paleozoic of Linxing-Shenfu area in the northeastern margin of the Ordos Basin.The gas-water distribution law and interlayer interference mechanism during joint mining were explored and a prediction model of joint mining production was established.Based on the analysis of logging response,combined with the temperature and pressure change resistivity experiment,water-bearing acoustic wave and isothermal adsorption experiment,the interpretation models of physical properties and fluid properties of coal measure reservoirs were established.And the joint mining geological characteristics of coal measure tight sandstone gas and coalbed methane were analyzed.The joint mining water production model of coalbed methane and coal measure tight gas was created.Combined with sensitivity experiments,the sensitivity evaluation method was constructed for the coalbed methane and tight gas reservoirs under the condition of joint mining.It was considered that the critical production pressure difference(upper limit)and the critical mineralization degree(lower limit)are 5~8 MPa and 30000 ppm when No.4+5 coal seam is mined with tight sandstones from Shanxi formation to Benxi formation,respectively.For the joint mining among No.8+9 coal seam and tight sandstones from Shanxi formation to Benxi formation,the critical production pressure difference(upper limit)and the critical mineralization degree(lower limit)are 5 MPa and 30000~40000 ppm,respectively.The physical simulation experiments of different inlet pressure and permeability were carried out to reveal the dynamic variation law of gas flow rate and the mechanism of interlayer interference during the joint mining process.The numerical relationship between pressure contribution rate,permeability contribution rate and interlayer interference intensity of single-layer reservoir was established.It was found that during the joint mining process of coalbed methane and coal measure tight sandstone gas,only when the reservoir pressure contribution rate of the low-pressure layer exceeds 28% and the permeability contribution rate of the low-permeability layer exceeds 16%,the interlayer interference will not occur.Four types of gas production curves were identified: desorption type,desorption free type,free desorption type and free type.Then,a gas production splitting method based on peak fitting was eastablished,which was used to determine the contribution rate of free gas and adsorbed gas in typical commingled wells.On this basis,the productivity prediction model of joint mining well of coalbed methane and coal measure tight sandstone gas was established.Based on this,the dynamic change of gas production in the joint mining process was analyzed under the different spatial superposition combinations.It was found that the difference in permeability and discharge radius between the coal seam and sandstone layer and the critical desorption pressure of the coal seam are the key factors affecting the single layer contribution rate for the joint mining well.