Modeling of CO2 injection considering multi-component transport and geomechanical effect in shale gas reservoirs

Abstract

Although several researches have been conducted for CO2 injection in shale gas reservoirs, there is definite lack of study that concentrates on the multi-component transport and geomechanical effect. This paper presents shale gas model for CO2 injection considering dissolution, molecular diffusion and stress-dependent permeability as well as multi-component adsorption. Based on information of Barnett shale reservoir, a simulation model is constructed for CO2 flooding. Extended Langmuir isotherm is used to simulate the competitive sorption among the molecules involved. Gas solubility is represented by Henry's law. Co- and counter-diffusive transportsare depicted by Sigmund correlation. Stress-dependent permeability in shale reservoir is considered by exponential correlation with the linear-elastic constitutive model. In the proposed model, the mechanisms and parameters which could influence on CO2 injection are studied. A series of reservoir simulations based on these effects are performed to study effective injection of CO2. Mechanisms of CO2 injection in shale gas reservoir contribute to both enhanced CH4 recovery and storage of CO2. Desorption of CH4 in the model with the multi-component adsorption is activated by competitive sorption with the CO2 which is preferentially adsorbed over CH4 with a ratio up to 5:1. Molecular diffusion enables CO2 to displace CH4 in ultra-low matrix permeability condition. The model with molecular diffusion presents wide spread CO2 in the reservoir compared with the model not considering molecular diffusion. Because of ultra-low permeability of shale reservoir, effect of diffusion is higher than conventional reservoirs so that it should be considered in the CO2 injection model of shale reservoir. CO2 injection supports the reservoir pressure so that effect of stress-dependent permeability is mitigated compared with primary recovery model. In this model, CO2 injection improves shale gas recovery about 12%. The CO2 injected in the reservoir exists as a super-critical phase, adsorption trapping, and dissolution trapping of 45.8, 46.5, and 3.6%, respectively. According to reservoir and fracture parameters, amount of each state would be changed. The sensitivity analysis for either enhanced CH4 recovery or CO2 storage is conducted to investigate the critical parameters that control these CO2-EGR process and CO2 storage, respectively. This work presents study for mechanisms of CO2 injection in shale gas reservoir taking account of multi-component adsorption, dissolution, molecular diffusion, and geomechanical effect and sensitivity analysis for uncertain parameters of the shale reservoir. The investigation is important for better understanding and design of CO2 injection in order to enhance CH4 recovery and CO2 storage.