It is proposed that very low permeability formations are possible candidates for CO 2 sequestration. Further, experimental studies have shown that shale formations have huge affinity to adsorb CO 2 , the order of 5 to 1 compared to the methane. Therefore, potential sequestration of CO 2 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 CO 2 -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 CO 2 -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/dualpermeability 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 re-fracturing 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 CO 2 -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 CO 2 injection at a later time. This work provides field operators with more insight into maximizing gas recovery from unconventional shale gas reservoirs using refracturing stimulation, CO 2 injection, or a combination of both methods.
Transport properties and mechanisms as well as phase behavior under nanoscale confinement exhibit significant deviations from their bulk behavior. This is due to the significant effect of molecule-wall interactions as well as molecule-molecule interactions in shale formations which are mainly characterized by nanopores. Consequently, production from shale gas reservoirs is strongly influenced by pore sizes and their effects on phase behavior and transport properties. In this study, we focus on analyzing and determining the effect of phase behavior and transport properties change due to pore proximity on production from a shale gas condensate reservoir. Additionally, the effect of different connectivities between pore sizes on production is studied. The effect of pore size on phase behavior is considered by using modified critical properties for different pore sizes in the phase behavior calculations. A shale gas condensate reservoir with a ternary mixture of methane (80 mol%), n-butane (10 mol%), and n-octane (10 mol%) as the reservoir fluid is modeled. The reservoir pressure and temperature are 5000 psia and 180 °F, respectively. The dew point pressure is 3600 psia. Pore sizes change between 5-150nm. Based on Scanning Electron Microscopy (SEM) studies on shale reservoir rocks, the pore volume of the reservoir was divided into five regions: bulk (stimulated area and pore sizes more than 50nm (17% PV)), 20-50nm (4% of PV), 15-20nm (6% of PV), 10-15nm (45% of PV), and less than 10nm (28% of PV). Three different types of connectivities between pores were considered: 1-completely random distribution 2-pore sizes from smallest to largest connected to the SRV in series, and 3-pore sizes from largest to smallest connected to the SRV in series. Our study has shown that by decreasing the pore size, dew point pressures decrease between 5 to 17%. Also by decreasing pore size, two-phase region shrinks therefore condensate drop-out and near wellbore permeability impairment are reduced. After 10 years of production, condensate saturation around SRV is 6-10% less under confinement effects. Gas and condensate viscosities under confinement decrease 3-16% and 10-45% respectively. Considering effect of confinement did not affect gas production significantly but the liquid production increased significantly and doubled. The effect of different pore size connectivities caused a 20% change in liquid productions. The results of this study can have a significant impact on our understanding of gas condensation and transport in shale formations thereby enabling improved field planning, well placement, completions design and facilities management.
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