A Systematic Reservoir Simulation Study on Assessing the Feasibility of CO2 Sequestration in Liquid-Rich Shale Gas Reservoirs with Potential Enhanced Gas Recovery
Authors: Zhan Jie , Soo Eric , Fogwill Allan , Cheng Shiqing , Cai Hua , Zhang Kai , Chen Zhangxin 
Published: March 2018

The application of horizontal well drilling coupled with the multistage fracturing technology enables commercial development of shale gas formations, which launches the energy revolution from conventional resources to unconventional resources. Some of the shale reservoirs, particularly the Eagle Ford shale, contain a wide range of hydrocarbon fluids covering from low GOR black oil and volatile oil to the rich and lean gas-condensate. With the progress of understanding the nature of shale reservoirs, we find that some hydrocarbons are stored as an adsorbed phase on surfaces of organic carbon. Meanwhile, laboratory and theoretical calculations indicate that CO2 has significantly greater sorption capability compared with some lighter hydrocarbons like CH4 and C2H6. Shale gas reservoirs are recently becoming the promising underground target for CO2 sequestration. In the paper, systematic numerical simulations will be implemented to investigate the feasibility of CO2 sequestration in Eagle Ford liquid-rich shale gas reservoirs and quantify the associated uncertainties.

First, a multi-continua porous medium model will be set up to present the matrix, nature fractures and hydraulic fractures in shale gas reservoirs. Based on the Eagle Ford gas condensate data, 14 components will be simulated in the compositional model. Meanwhile, we will investigate a three-stage flow mechanism which includes convective gas flow mainly in fractures, dispersive gas transport in macro pores and multi-component sorption phenomenon in micro pores. To deal with this complicated three-stage flow mechanism simultaneously, analytical apparent permeability which includes slip flow and Knudsen diffusion will be incorporated into a commercial simulator CMG-GEM. In addition, multicomponent adsorption/desorption lab data will be included in the model. A mixing rule is introduced to deal with the competitive adsorption phenomenon between the different components.

In the paper, an integrated methodology is provided to investigate the CO2 sequestration process. Simulation results indicate that the Eagle Ford liquid-rich shale gas reservoir is an ideal target for the CO2 sequestration. To some extent, the average reservoir pressure is maintained due to injection of CO2. But most of the pressure is trapped around an injector due to the tight formation. That is why the reservoir productivity is enhanced by the injection process. But the increment is very small. Hydraulic fracking which creates freeways for gas flow is the key to improve the reservoir performance. The pressure maintenance around the injector reduces the effect of the liquid blockage, which is a good sign to implement the cyclic inert gas injection to reduce the effect of the liquid blockage and enhance the reservoir performance ultimately. The multicomponent desorption/adsorption is a very important feature in a shale gas reservoir, which should be fully harnessed to benefit the CO2 sequestration process. Meanwhile, the multicomponent desorption/adsorption process will increase the condensate production, which will lead to severer liquid blockage. In addition, it will limit the gas production. Furthermore, we cannot ignore the contribution of slip flow and diffusion to the reservoir performance during the sequestration process. Based on the methodology provided in this paper, we can easily deal with the apparent permeability effect based on a commercial simulator platform.