Sensitivity Mechanism Of Coalbed Methane Reservoir And Its Impact On Cbm Production

This dissertation focused on the mechanism of coal reservoir damage during the coalbed methane(CBM)development.Based on the conventional core sensitivity test,the micro mechanisms of velocity sensitivity,stress sensitivity and acid sensitivity were systematically investigated,combined with the low magnetic field nuclear magnetic resonance(LF-NMR)technology,field emission scanning electron microscope +energy spectrum analysis technology,X-ray ?-CT scanning technology,quantitative evaluation of minerals by scanning electron microscopy(QEMSCAN)technology and ion concentration testing technology.The relationship between permeability variation and porosity variation caused by the same reservoir sensitivity,the effect of velocity sensitivity and stress sensitivity on the actual production were analyzed.The critical velocity model of coal fines detachment during the single-phase flow,a new method to calculate the effective stress coefficient of coal and the dewatering rate optimization model were established.To better understand these results and achievements,the detailed discussion are as follows:(1)Combined with the LF-NMR technology and conventional core velocity sensitivity experimental equipment,the variation law of porosity and permeability under different flow velocities were for different rank coals were calculated and clarified.The result revealed that the main reason for permeability change caused by different velocities is the seepage porosity variation.By analyzing the stress condition of coal fines under hydrodynamic condition,the critical velocity model of coal fines detachment during the single-phase flow was established to clarify the influence of flow velocity on permeability of coal in different coal ranks.(2)The X-ray ?-CT scanning technology was used to scan the coal under the loading and unloading stress states.Based on the theory of porosity calculation with image gray value and threshold segmentation method,the porosity under the same loading and unloading stress condition was quantitatively calculated,and the pores and fracturs under the same loading and unloading stress condition were extracted.The essential reason why the permeability can't be restored after stress loading and unloading was revealed and clarified.(3)The stress sensitivity of coal reservoir was investigated under constant confining pressure conditions and constant pore pressure conditions.The result revealed that the sensitivity of the permeability on the confining stress is stronger than that on the pore pressure.The response surface method was introduced to calculate the effective stress coefficient of coal,and the importance of the effect of effective stress coefficient of coal on the dynamic change of permeability was emphasized.(4)Combined with the LF-NMR technology,the QEMSCAN technology and ion concentration detection technology,the characteristics of pore size change and mineral composition change before and after different acid treatment were quantitatively analyzed.The result revealed that mineral dissolution,particle migration and new mineral generation are the microscopic mechanism of acid sensitivity.(5)Based on the understanding of microscopic mechanism of different sensitivities,the effects of velocity sensitivity and stress sensitivity on CBM production in Zhengzhuang block were analyzed.Based on the governing equation of pressure propagation of CBM well and optimization conditions of drainage rate,the optimization model of dewatering rate was established.The effects of the stress sensitivity and velocity sensitivity on pressure propagation were clarified,and the importance of reasonable drainage rate on CBM production was emphasized.