Effect of Nanopore Confinement on Fluid Phase Behavior and Production Performance in Shale Oil Reservoir

Abstract

The confinement effects including capillarity and adsorption play a significant role in phase behavior and transport of shale fluids. The effect of capillary pressure has been widely studied and is fully understood. However, the investigation of the adsorption effect on reservoir fluid properties and production performance is still lacking. In this work, an efficient model is proposed to fill this gap and investigate the phase behavior and well performance in the Bakken shale oil reservoir. First, an improved phase equilibrium model is developed based on an adsorption-dependent Peng−Robinson equation of state and a modified Young−Laplace equation. The effect of adsorption and adsorption induced critical property shifts in nature is considered. Second, the phase equilibrium model is used to calculate and compare the black-oil properties of the Bakken oil under different pressures and confinement conditions. Finally, the fluid properties results are combined with the reservoir simulator to assess the confinement effects on the well performance of the Bakken shale reservoir. The result indicates that fluid adsorption makes a more significant contribution to the suppressed bubble point pressure than capillarity. The change in the black-oil properties shows that the confinement effect exhibits a great impact on the physical properties of the oil phase including solution gas−oil ratio, oil formation volume factor, and oil viscosity, but presents a small impact on the physical properties of the gas phase including gas formation volume factor and gas viscosity. In terms of production performance, the presence of confinement increases the cumulative oil production, first increasing, and then decreasing cumulative gas production, which leads to a flatter producing gas−oil ratio. Overall, the effect of adsorption is more significant under high-pressure conditions, and the effect of capillary pressure is more significant under low-pressure conditions.

Carbon Dioxide Flow Behavior through Nanopores: Implication for CO2 Sequestration in Unconventional Gas Reservoirs

“…In this work, distinguishing from the existing research, efforts are devoted to investigating the CO 2 flow capacity in unconventional reservoirs. Meanwhile, complexity to achieve a good understanding of CO 2 sequestration stems from three aspects, including the nanoconfined space, , surface wettability variation, , and inherent CO 2 properties. , Also, CO 2 critical properties shall shift under the nanoconfinement effect. , The presence of nanopores in typical unconventional reservoirs, encompassing shale gas/oil reservoirs, coal-bed methane (CBM) reservoirs, and tight gas reservoirs, is revealed based on the advanced experimental approaches. , Notably, although the horizontal multifractured technology enables the creation of a microscale fracture network in unconventional reservoirs, the stimulated area is far less than the whole reservoir for CO 2 sequestration, , and the investigation on the CO 2 flow mechanism in nanoconfined space is still meaningful. Other than that, the surface wettability has a relatively wide variation range in unconventional reservoirs, as a result of its complex composition.…”

“…76,77 The presence of nanopores in typical unconventional reservoirs, encompassing shale gas/oil reservoirs, coal-bed methane (CBM) reservoirs, and tight gas reservoirs, is revealed based on the advanced experimental approaches. 19,20 Notably, although the horizontal multifractured technology enables the creation of a microscale fracture network in unconventional reservoirs, the stimulated area is far less than the whole reservoir for CO 2 sequestration, 21,22 and the investigation on the CO 2 flow mechanism in nanoconfined space is still meaningful. Other than that, the surface wettability has a relatively wide variation range in unconventional reservoirs, as a result of its complex composition.…”

Phase Behavior of CO2-CH4-Water Mixtures in Shale Nanopores Considering Fluid Adsorption and Capillary Pressure

“…The smaller the R p , the more pronounced the above trend. This can be attributed to the fact that adsorption and capillary force can boost the condensation, causing the molecules of the vapor phase to transfer to the liquid phase, 30 which has the same effect with the increasing pressure. In general, the gas solubility in the aqueous phase increases due to the effect of nanopore confinement, which facilitates carbon sequestration in shale gas reservoirs.…”

“…Based on the calculation results obtained by molecular simulation in small spaces, the improvement of the traditional equation of state (EOS) is considered as a promising method to provide a coherent description of fluid phase equilibrium from the bulk phase to nanopores . For example, some scholars introduced capillary pressure into conventional EOSs to compute the thermodynamic property of the fluid in micropores and nanopores. − However, these models fail to reflect the nature of confinement; thus, some recent works focus on the development of an EOS that can embody nanopore confinement. , Yang et al upgraded the Peng–Robinson EOS (PR-EOS) to characterize the phase equilibrium of three shale oils by considering wall–molecule interaction.…”

Comprehensive Outlook into Critical Roles of Pressure, Volume, and Temperature (PVT) and Phase Behavior on the Exploration and Development of Shale Oil

“…Solid knowledge of the PVT behavior of fluids is key to the successful design of production facilities in unconventional resources, including shale oil and gas. 25 The phase envelope boundary corresponds to the saturation pressure of the hydrocarbon fluid. The saturation pressure changes rapidly under thermal maturation conditions in the late stage of the oil generation window, and the volatile oil is replaced by retrograde condensate.…”