Simulation study on the CO2-driven enhanced gas recovery with sequestration versus the re-fracturing treatment of horizontal wells in the US unconventional shale reservoirs

It is proposed that very low permeability formations are possible candidates for CO2 sequestration. Further, experimental studies have shown that shale formations have huge affinity to adsorb CO2, the order of 5 to 1 compared to the methane. Therefore, potential sequestration of CO2 in shale formations leading to enhanced gas recovery (EGR) will be a promising while challenging target for the oil and gas industry. On the other side, hydraulic re-fracturing treatment of shale gas wells is currently gaining more attention due to the poor performance of shale gas reservoirs after a couple years of production. Hence, investigating and comparing the performance of CO2-EGR with the re-fracturing treatment is essential for the future economic viability of depleted shale gas reservoirs. This paper presents a systematic comparison of the effect of these two processes on improving gas production performance of unconventional reservoirs, which is not well understood and has not been studied thoroughly in the literature. In this paper, a shale gas field data has been evaluated and incorporated in our simulations for both CO2-EGR and re-fracturing treatment purposes. Numerical simulations are performed using local grid refinement (LGR) in order to accurately model the non-linear pressure drop. Also, a dual-porosity/dual-permeability model is incorporated in the reservoir simulation model. Further, the uncertainties associated with inter-related set of geologic and engineering parameters are evaluated and quantified for refracturing treatment through several simulation runs. This comprehensive sensitivity study helps in understanding the key reservoir and fracture properties that affect the production performance and enhanced gas recovery in shale gas reservoirs. The results showed that re-fracturing treatment outperforms CO2-EGR due to the pronounced effect on cumulative methane gas production. Moreover, the sensitivity analysis showed that the characteristics of reservoir matrix including permeability and porosity are the most influential parameters for re-fracturing treatment The findings of this study recommend hydraulic re-fracturing of shale reservoirs at first for enhancing gas production followed by CO2 injection at a later time. This work provides field operators with more insight into maximizing gas recovery from unconventional shale gas reservoirs using re-fracturing stimulation, CO2 injection, or a combination of both methods. (C) 2014 Elsevier B.V. All rights reserved.