Numerical Simulation On Field Test Of CO₂ Injection Into Shallow Low-rank Coal

CO₂-ECBM(CO₂-Enhanced coalbed methane)is one of the effective means of CCUS(Carbon Capture,Utilization and Storage).China's carbon emissions rank first in the world for many years and there are abundant reserves of low rank coal in China.However,the research on CO₂-ECBM in shallow low rank coal is still lacking.Therefore,this thesis will take the shallow low-rank coal field test as the background,aiming at two engineering subjects:reservoir and the wellbore.The numerical simulation is carried out to study them respectively.The main research contents are as follows:(2)Based on CMG-GEM numerical simulation software,a typical single well injection and production numerical model was established to obtain the dynamic evolution process of pressure,gas adsorption and desorption and permeability in the process of injection and production.Sensitivity analysis of permeability,porosity,gas desorption time constant,volumetric strain constant and other parameters was also conducted.(3)Then the numerical model under the background of shallow low rank coal engineering is established.Numerical simulation is carried out for the whole process of multi-well production and multiple injection in single well.A relatively accurate set of reservoir parameters was obtained by historical fitting of field measured data with numerical simulation,and the relevant process of this project was predicted numerically.(4)Finally,a CO₂ injection well dynamic model of vertical wellbore was established,and the phase change process caused by CO₂ short-term transient flow heat transfer was numerically simulated.The history match of the transient pressure fluctuation after well opening in actual engineering is carried out.The effects of injection temperature and surrounding rock heat transfer were studied.According to the research results,the main conclusions are as follows:(1)After CO₂ injection,After CO₂ injection,it mainly concentrates near the bottom of the well,and the pressure decreases rapidly as the distance from the bottom of the well increases.CO₂ injection can make CH₄ desorption,and the amount of desorption is much greater than that of conventional production.At the CO₂ injection stage,the permeability firstly increased briefly and then decreased,while in the production stage,the permeability firstly decreased and then increased.Permeability and other parameters have certain influence on the process of injection and recovery.(2)The numerical model can accurately simulate the actual engineering process.The numerical prediction shows that nearly 100%CO₂ can be stored in the coal seam by the shallow low-rank coal,and the CO₂ concentration and adsorption capacity in the matrix are affected by the injection time.(3)The lower the injection temperature,the easier it is for CO₂ in the wellbore to be converted into liquid;When the injection temperature is higher than the formation temperature,the CO₂ may be liquefied temporarily,resulting in a wide range of bottomhole temperature changes.According to the prediction of adiabatic model and heat transfer model,CO₂ has not reached the thermal equilibrium with the wellbore and surrounding rock in the short term after well opening,and the heat transfer effect is significant,and there is a complex phase state transition in the wellbore.It is predicted that the bottom hole temperature will exceed the injection temperature for a long time,and there may be supercritical CO₂ in the bottom hole.In short,this paper,based on the field test of CO₂ injection into shallow low-ranking coal,explores the quantitative analysis method for the change of wellbore fluid phase state in the process of CO₂-ECBM on the basis of the characteristic historical fitting numerical simulation research,which lays a foundation for the realization of the historical fitting method for the interaction between wellbore and reservoir.