Phase behavior and fluid properties are governed by molecule-molecule and molecule-pore wall interactions. The effect of molecule-pore wall interactions is negligible in conventional reservoirs because pore sizes are much larger than molecular mean free paths. However, this effect is very important in shale formations because the pore sizes in shale formations are in the order of nano-scale. Reservoir fluids properties and phase behavior under nanoscale confinement exhibit significant deviations from their bulk properties.This work used two methods to investigate the effect of pore proximity on the phase behavior and fluid properties: 1-Developing a new flash calculations algorithm: the influence of difference between oil and gas pressures (i.e. capillary pressure) is neglected in the flash calculations of vapor-liquid equilibrium for conventional reservoirs. However the capillary pressure is very high and cannot be ignored in phase behavior calculations of shale formations. A new mathematical expressions for chemical potentials of pure components and their mixture as a function of capillary pressure is proposed, and 2-Modifying the critical properties of pure components under the effect of confinement: new correlations based on molecular simulation studies are developed for taking into account the effect of pore size (i.e. molecule-pore wall interactions) on critical properties of each component. The modified critical properties are used in phase behavior calculations.Both methods were tested against experimental data of Sigmund et al. (1973) for C 1 -nC 5 mixtures. The relative errors between the prediction of bubble point pressures and gas compositions from the models and the reported experimental data were less than 5 %. Then phase behavior and fluid properties of a mixture of Methane, n-Butane and n-Octane with different compositions were studied under confinement for pore size range from infinity to 2 nm by using both methods. In general, the two-phase envelope shrinked slightly in method 1 and significantly in method 2 with decreasing the pore size. The effect of pore size on two-phase envelope becomes significant when pore radius is smaller than 10 nm. For method 1, critical point does not change and the closer of the temperature to the critical point, the smaller the change in saturation pressures. For method 2, the critical point decreases with the pore radius. Interfacial tension for bulk fluid and confined fluid remain about the same for pore sizes more than 50nm. For pore sizes less than 50nm, interfacial tension from method 1 did not change significantly, but it decreased dramatically especially for pore sizes less than 10nm when method 2 was used. K-values from both methods were almost the same for pore sizes more than 10nm. From method 1, k-value decreases with decreasing the pore radius for all components. But from method 2, it decreases for light component and increases for intermediate and heavy components.The results of this study can have a significant impact on our understanding of the gas ...

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Phase behavior and fluid properties in porous media are governed by not only fluid molecule-fluid molecule interactions but also fluid molecule-pore wall interactions. The current available equations of state consider only fluid molecule-fluid molecule interactions and neglect the interactions between the reservoir fluid molecules and the solid wall of the porous media. For conventional reservoirs, this assumption may be valid because the formation pore sizes are much larger than molecular mean free paths. However, in shale formations that are characterized by nanopores, the fluid molecule-pore wall interactions play such an important role that can change phase behavior and crticial properties of the reservoir fluids. Consequently, the critical temperatures and pressures of multi-component hydrocarbon mixtures under nanopores confinement are influenced strongly by fluid molecule-pore wall interactions. This work investigates the effect of pore proximity in tight and shale formations on phase behavior and fluid properties of the reservoir fluids by modifying van der Waals equation of state. Effects of both fluid molecule-fluid molecule and fluid molecule-pore wall interactions are included in the newly proposed equation of state. Based on molecular simulation studies, correlations are developed to consider the effect of fluid molecule-pore wall interactions for each component required for phase equilibria calculations under nanopore confinement using the proposed equation of state. Phase behavior calculations of a mixture of methane, n-butane and n-octane were studied under confinement effects for pore sizes ranging from 10 to 2 nm. In general, with the decrease of pore size, the two-phase region of the fluid mixture tends to shrink, which makes the fluid mixture behave more like a dry gas. The results indicate that bubble point and dew point pressures of the confined fluids are up to 150 psi and 300 psi higher than their correspondent bulk values. Also n-butane and n-octane tend to evaporate more when pore size dereases. The confinement effects can cause the fluid mixture to behave similar to dry gas, which results in reduction in condensate banking and less near-wellbore permeability impairment in comparison to conventional reservoirs. This has several implications for reservoir and well performances. One is that we can observe increased gas rates and enhanced recoveries over the life of the field by modeling these effects in a numerical reservoir simulation package.

Abstract. Raindrop size distribution (DSD) information is fundamental in understanding the precipitation microphysics and quantitative precipitation estimation, especially in complex terrain or urban environments which are known for complicated rainfall mechanism and high spatial and temporal variability. In this study, the DSD characteristics of rainy seasons in the Beijing urban area are extensively investigated using 5-year DSD observations from a Parsivel2 disdrometer located at Tsinghua University. The results show that the DSD samples with rain rate < 1 mm h−1 account for more than half of total observations. The mean values of the normalized intercept parameter (log 10Nw) and the mass-weighted mean diameter (Dm) of convective rain are higher than that of stratiform rain, and there is a clear boundary between the two types of rain in terms of the scattergram of log 10Nw versus Dm. The convective rain in Beijing is neither continental nor maritime, owing to the particular location and local topography. As the rainfall intensity increases, the DSD spectra become higher and wider, but they still have peaks around diameter D∼0.5 mm. The midsize drops contribute most towards accumulated rainwater. The Dm and log 10Nw values exhibit a diurnal cycle and an annual cycle. In addition, at the stage characterized by an abrupt rise of urban heat island (UHI) intensity as well as the stage of strong UHI intensity during the day, DSD shows higher Dm values and lower log 10Nw values. The localized radar reflectivity (Z) and rain rate (R) relations (Z=aRb) show substantial differences compared to the commonly used NEXRAD relationships, and the polarimetric radar algorithms R(Kdp), R(Kdp, ZDR), and R(ZH, ZDR) show greater potential for rainfall estimation.

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Rainfall exhibits substantial variability, and its temporal resolution considerably affects simulation of hydrological processes. This study aims to investigate the effect of the temporal resolution of rainfall (TRR) on urban flood modeling and to explore how high TRR is required. A routing-enhanced detailed urban stormwater (REDUS) model, which has four layers and accounts for complex urban flow paths, was developed and then applied to the campus of Tsinghua University, Beijing, China. For 30 rainfall events at 1-min resolution, the rainfall accuracy index (RAI) was used to describe the discrepancy of rainfall patterns by upscaling. Through hydrodynamic modelling, the effect of TRR was quantified by the relative error of flood volume and peak in typical areas. The results show that (1) flood peak is sensitive to TRR while flood volume is generally not; (2) with lower TRR, discharge peak is underestimated, and a power function is proposed to express the relationship between the effect of TRR and the characteristics of rainfall and underlying surfaces; and (3) rainfall data of 5-min resolution for urban areas smaller than 1 km 2 , or at least 15-min resolution for larger areas, are required to constrain the relative biases of flood peak within 10%.

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