For greenhouse gas control and environment protection, CO₂ emission reduction has become a hot spot in global research. CO₂ injection in developed oil reservoirs to enhance oil recovery is widely regarded as one of the most economical and promising measures for reducing anthropogenic CO₂ emissions into the atmosphere. In this paper, a three-dimensional embedded discrete fracture model is proposed and applied to simulate CO₂ flooding and geological storage processes in hydraulically fractured reservoirs. The gas injection is simplified as a two-phase two-component mathematical model with the varying pressure-related fluid physical properties, and its accuracy is verified by commercial software tNavigator^®. The advantage of this model is that it can deal with the complex geological conditions of three-dimensional arbitrary-inclined fracture networks and accurately assess the effects of CO₂-EOR and geological sequestration in real reservoirs. Two application cases of CO₂ huff-and-puff with a single well and inter-fracture asynchronous injection and production are demonstrated and explained in detail. The optimized technological parameters and CO₂ saturation distribution can provide key technical parameters for field operations.

The mismatching between the multi-scale feature of complex fracture networks (CFNs) in unconventional reservoirs and their current numerical approaches is a conspicuous problem to be solved. In this paper, the CFNs are divided into hydraulic macro-fractures, induced fractures, and natural micro-fractures according to their mode of origin. A hybrid model coupling various numerical approaches is proposed to match the three-dimensional multi-scale fracture networks. The macro-fractures with high-conductivity and wide-aperture are explicitly characterized by a mimetic Green element method-based hierarchical fracture model. The induced fractures and natural micro-fractures that have features of low-conductivity and small-openings are upscaled to the dual-medium grid and enhanced matrix grid through the equivalent continuum-medium method, respectively. Subsequently, some benchmark cases are implemented to confirm the high-precision and high-robustness of the proposed hybrid model that indeed accomplishes accurate modeling of fluid flow in multi-scale CFNs by comparing with commercial software tNavigator^®. Furthermore, an integrated workflow of simulation modeling for multiscale CFNs combined with a field example in Sichuan from China is used to analyzing the production information of fractured horizontal wells in shale gas reservoirs. Compared with the field production data from this typical well, it can be proved that the hybrid model has strong reliability and practicability.

Water injection is often used to supply formation energy in low permeability reservoirs. A lot of field data shows that fractures nearby the wellbores may extend hundreds of meters because of the poor injectability which is called water-induced fractures. The presence of these fractures significantly changes the flow of injected water thus affecting the development effects. However, few studies of reservoir numerical simulation methods have been presented to simulate the dynamic changes of fractures in the long-term waterflooding process. This study is based on the embedded discrete fracture model (EDFM) in which the fracture system has less dependence on the grid system. By improving the preprocessing algorithm, fracture growth can be considered in EDFM which is called the dynamic embedded discrete model (dEDFM). This new method provides an innovative idea for fracture propagation simulation by attaching new fracture elements to the original fractures once the failure criterion is satisfied. Meanwhile, there is no need to calculate the stress field once the in situ stress is provided, so the computational efficiency is greatly improved compared with the fluid-solid coupling method. Besides, dEDFM has a flexible way of handling fracture elements, so it is suitable for water-induced fracture simulation. Results show that fracture propagation speed is significantly influenced by matrix permeability, in situ stress, and injection intensity, and if the water-induced fractures propagate faster than the original waterfront movement velocity, changing law of the water content rate rising of production wells in different directions will be different. The contribution of this work lies in that it provides a suitable and efficient way for reservoir engineers to deal with water-induced fractures since the propagation of fractures can be considered when it comes to reservoir numerical simulation, and it is helpful for production data prediction and development adjustment in low permeability reservoirs.

In this study, we present a hybrid model coupled with two-set nodes Green element method (GEM) and embedded discrete fracture model (EDFM) for capturing the effect of transient flow in inhomogeneous fractured porous media. GEM is an excellent advanced algorithm, which can solve nonlinear problems in heterogeneous media. That is also an obvious advantage of GEM against the original boundary element method (BEM). The novel GEM has double nodes of pressure and flux and it is an improvement of classical GEM, which has a second-order precision and fits for triangle structured grids. In the place of adopting the linear flow approximation for original EDFM, the interflows between local triangle matrix grids and fracture elements are derived using the novel GEM, which has higher accuracy than those in previous EDFMs. Consequently, the modified hybrid model can indeed calculate the pressure and flux distribution of transient flow in multifracture porous media. Three numerical cases are presented to show the practicability of our novel model which include (i) multistage fractured horizontal well, (ii) heterogeneous fractured porous media, and (iii) complex fracture networks (CFNs) in an unconventional reservoir.

There are currently numerous types of water-flooding characteristic curves, most of which are derived from fundamental theories such as material balance, relative permeability, along with experimental results. A single exponential or power function expression cannot accurately characterize the complex flow characteristics of different types of reservoirs, and the equivalent relationships corresponding to production wells and entire oilfields remain unclear. Consequently, practical applications often encounter issues such as curve tailing, difficulty in determining linear segments, inability to identify anomalous points, and inaccuracies in dynamic fitting and prediction. This paper derives a novel water-flooding characteristic curve expression based on fractal theory, incorporating the fractal characteristics of two-phase oil–water flow in reservoirs, as well as the micro-level pore–throat flow features and macro-level dynamic laws of water flooding. The approach is analyzed and validated with real oilfield cases. This study indicates that fitting with the novel water-flooding characteristic curve yields high correlation coefficients and excellent fitting results, demonstrating strong applicability across various types of oilfields and water cut stages. It can more accurately describe the water-flooding characteristics under different reservoir conditions and rapidly predict recoverable reserves, offering significant application value in the dynamic analysis of oilfields and the formulation of development strategies.