Numerical Simulation On CO₂Injection Wellbore Heat Transfer During Geological Sequestration

Nowadays people are faced with resource shortage and global warming, and CO2sequestration and utilization technology might solve this problem. CO2sequestration and utilization technology is to inject CO2into saline layer, oilfield, natural gas field and coalbed methane field in order to store CO2deep in layers as well as enhancing oil, natural gas and coalbed methane recovery, which will reduce CO2content in the atmosphere and alleviate the greenhouse effect. When CO2reaches bottom of the wellbore, CO2thermal condition will affect CO2storage and oil, natural gas and coalbed methane recovery. So it is necessary to study CO2wellbore heat transfer.Ramey model is a classical and practical wellbore heat transfer model. Based on Ramey model, a modified wellbore heat transfer model is proposed based on energy conservation equation and momentum conservation equation to calculate temperature and pressure distribution. The improved model considers Joule-Thomson effect. In order to improve accuracy of the model, this paper uses PR-EXP method to calculate CO2density, cubic PR state equation to calculate CO2viscidity, a fitting formula to calculate CO2specific heat capacity and BWRS method to calculate CO2Joule-Thomson coefficient.Sensitivity analysis is carried out in this paper, including CO2injection temperature, pressure, time, mass flow and Joule-Thomson effect. The results show that at the beginning of the injection CO2temperature is influenced by injection temperature, after that CO2temperature is mainly influenced by earth temperature. When considering Joule-Thomson effect, temperature results are higher than the condition of that not consider. However temperature change caused by Joule-Thomson effect is really small due to a very small Joule-Thomson coefficient. Along with a longer injection time, CO2temperature and pressure state is more stable. After10days difference caused by injection time becomes really small, and heat transfer becomes stable. Compared with lower mass flow, temperature distribution under higher mass flow condition is more stable. CO2injection pressure influences pressure distribution directly which means higher injection pressure leads to higher wellbore pressure. Pressure distribution can hardly be influenced by CO2injection temperature, time and Joule-Thomson effect. At last, the improved model is used to calculate8th wellbore of Caoshe Oilfield in Jiangsu Province, and this paper compares model results with measured results. The results show that trends of the simulation are consistent the measurements. Absolute average deciation between the model results and measurements is1.37%, while absolute average deciation of pressure results is3.61%, which meets engineering requirement.