A Dual-Porosity-Stokes Model and Finite Element Method for Coupling Dual-Porosity Flow and Free Flow

In this paper, we propose and numerically solve a new model considering confined flow in dual-porosity media coupled with free flow in embedded macrofractures and conduits. Such situation arises, for example, for fluid flows in hydraulic fractured tight/shale oil/gas reservoirs. The flow in dual-porosity media, which consists of both matrix and microfractures, is described by a dual-porosity model. And the flow in the macrofractures and conduits is governed by the Stokes equation. Then the two models are coupled through four physically valid interface conditions on the interface between dual-porosity media and macrofractures/conduits, which play a key role in a physically faithful simulation with high accuracy. All the four interface conditions are constructed based on fundamental properties of the traditional dual-porosity model and the well-known Stokes-Darcy model. The weak formulation is derived for the proposed model, and the well-posedness of the model is analyzed. A finite element semidiscretization in space is presented based on the weak formulation, and four different schemes are then utilized for the full discretization. The convergence of the full discretization with the backward Euler scheme is analyzed. Four numerical experiments are presented to validate the proposed model and demonstrate the features of both the model and the numerical method, such as the optimal convergence rate of the numerical solution, the detail flow characteristics around macrofractures and conduits, and the applicability to the real world problems

A Data Assimilation Enabled Model for Coupling Dual Porosity Flow with Free Flow

Coupling of dual porosity flow and free flow arises in many important applications, e.g., groundwater system and industrial filtrations. Existing Stokes-Darcy types of models cannot accurately describe this type of coupled problem since they only consider single porosity media. With the support of lab experiment data we are developing a new coupled multi-physics, multiscale model and an efficient numerical method to solve it. Furthermore, both the lab and field data provide the possibility to improve the accuracy of the model prediction through data assimilation

D-Optimal Design for Rapid Assessment Model of CO₂ Flooding in High Water Cut Oil Reservoirs

Most of major oilfields in China have reached high water cut stage, but still, they contribute to more than 70% of domestic oil production. How to extract more oil from mature oilfields has become a hot topic in petroleum engineering. Carbon dioxide flooding is a win-win strategy because it can enhance oil recovery and simultaneously reduce CO2 emissions into the atmosphere. In order to evaluate the potentials of CO2 flooding in high water cut oil reservoirs, various 3-D heterogeneous geological models were built based on Guan 104 fault block in Dagang Oilfield to perform reservoir simulations. The D-optimal design was applied to build and verify the Rapid Assessment Model of CO2 flooding in high water cut oil reservoirs. Five quantitative variables were considered, including average horizontal permeability, permeability variation coefficient, ratio of vertical to horizontal permeability, net thickness of formation and percentage of recoverable reserves by water flooding. The process of weighting emphasized the contributions of linear terms, quadratic terms and first-order interactions of five quantitative parameters to improved recovery factor and Net Present Value of CO2 flooding. Using the Rapid Assessment Model of CO2 flooding in high water cut oil reservoirs, significant first-order interactions were sorted out and type curves were established and analyzed for the evaluation of technical and economic efficiency of CO2 flooding in high water cut oil reservoirs. Aimed at oil reservoirs with the similar geological conditions and fluid properties as Guan 104 fault block, the Rapid Assessment Model and type curves of CO2 flooding in high water cut oil reservoirs can be applied to predict improved recovery factor and Net Present Value of water-alternating-CO2 flooding at different conditions of reservoir parameters and development parameter. The approach could serve as a guide for the application and spread of CO2-EOR projects

Co₂P@N,P-codoped carbon nanofiber as a free-standing air electrode for Zn-air batteries : synergy effects of CoNₓ satellite shells

Here, a free-standing electrode composed of cobalt phosphides (Co₂P) supported by cobalt nitride moieties (CoNₓ) and an N,P-codoped porous carbon nanofiber (CNF) in one-step electrospinning of environmentally friendly benign phosphorous precursors is reported. Physiochemical characterization revealed the symbiotic relationship between a Co₂P crystal and surrounding nanometer-sized CoNₓ moieties embedded in an N,P-codoped porous carbon matrix. Co₂P@CNF shows high oxygen reduction reaction and oxygen evolution reaction performance owing to the synergistic effect of Co₂P nanocrystals and the neighboring CoNₓ moieties, which have the optimum binding strength of reactants and facilitate the mass transfer. The free-standing Co₂P@CNF air-cathode-based Zn-air batteries deliver a power density of 121 mW cm⁻² at a voltage of 0.76 V. The overall overpotential of Co₂P@CNF-based Zn-air batteries can be significantly reduced, with low discharge-charge voltage gap (0.81 V at 10 mA cm⁻²) and high cycling stability, which outperform the benchmark Pt/C-based Zn-air batteries. The one-step electrospinning method can serve as a universal platform to develop other high-performance transition-metal phosphide catalysts benefitting from the synergy effect of transition nitride satellite shells. The free-standing and flexible properties of Co₂P@CNF make it a potential candidate for wearable electronic devices.This work was supported by the National Natural Science Foundation of China (Grant No. 51502135) and the Natural Science Foundation of Fujian Province of China (Grant No. 2017J01005)

Effect of Polymer on Disproportionate Permeability Reduction to Gas and Water for Fractured Shales

Large volumes of fracturing fluid are required in shale slickwater fracs, and a considerable amount of polymer friction reducer would remain in microfractures if the polymer has not been broken before gas production. It is of major interest to evaluate the effect of polymer on water/gas flow behavior in the microfractures of shale reservoirs. We fabricated six shale fracture models with different fracture widths and set up a core flooding apparatus to conduct brine/gas-injection experiments before and after polymer treatment. A method by which to calculate the residual resistance factor for gas (Frr,gas) was defined. The experimental results illustrate that polymer can reduce the permeability to water more than to gas. In the first cycle of brine/gas injection experiments after polymer treatment, the residual resistance factor for brine (Frr,water) and Frr,gas exhibited power-law characteristics through their shear rate and superficial gas velocity, respectively. The Frr,water and Frr,gas tended to decrease as the fracture width grew. Surprisingly, polymer treatment does not impair gas flow in wider fractures, and may even improve it. The mechanisms responsible for disproportionate permeability reduction (DPR) in the fractured shales were proposed in this paper

One-step coaxial electrodeposition of Co0.85Se on CoNi2S4 nanotube arrays for flexible solid-state asymmetric supercapacitors

A one-step electrochemical method is applied to coaxially deposit highly conductive Co0.85Se nanosheets composed of ultrasmall Co0.85Se nanocrystals on three-dimensional (3D) CoNi2S4 nanotube arrays supported on graphene foam (GF) (Co0.85Se@ CoNi2S4/GF) as an electrode for flexible solid-state supercapacitors. The Co0.85Se@CoNi2S4/GF electrode has an areal capacitance of 5.25 F cm−2 at 1 mA cm−2 and good rate capability (2.65 F cm−2 at 20 mA cm−2). A solid-state asymmetric supercapacitor with Co0.85Se@NiCo2S4/GF as the cathode and hollow carbon spheres (HCSs) as the anode in a PVA/KOH electrolyte was assembled which shows an energy density of 46.5 W h kg−1 at a power density of 750 W kg−1, and 89.0% capacity retention after 10 000 cycles over a potential window of up to 1.55 V. These results demonstrate that the electrodeposition method is applicable for engineering of 3D microstructured electrodes and also provides an efficient strategy for fabricating transition metal selenide based nanodevices