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In this paper, the influence of micro-fractures on the flow of tight reservoirs is studied on the microscopic scale. Three-dimensional digital cores of fractured tight sandstone with varying fracture apertures, lengths, and dip angles are constructed using computed tomography (CT) scans. Pore-network models are built using the three-dimensional digital cores to simulate the flow in tight oil reservoirs. The effects of the micro-fracture aperture, length and dip angle on the pore-throat structure, single-phase flow, and two-phase flow for fracture surfaces with/without roughness are studied. The study demonstrates different influences of micro-fracture characteristics on the flow, and the fracture aperture has the most critical effect. Meanwhile, the roughness of the micro-fracture makes a difference in addition to the three factors of micro-fractures. This paper provides a theoretical basis for the effective development of tight sandstone reservoirs.

Summary Horizontal drilling and hydraulic fracturing are recognized as the most efficient techniques to enhance recovery in shale-gas reservoirs. Because of the exploitation difficulties and complex flow mechanism in shale gas, it is imperative to focus on the optimization of fracturing parameters. However, most of the current heuristic algorithms follow the principle that the variable dimension is constant during iteration, which leads to poor performance when dealing with dimension-varying problems. The optimization of fracturing parameters can be regarded as a typical dimension-varying problem when considering the difference among fracture properties such as half-length and conductivity. Thus an improved algorithm named modified variable-lengthparticle-swarm optimization (PSO) (VPSO) (MVPSO) was proposed to automatically select the optimal fracturing parameters: the number of fractures as well as the corresponding fracture properties. Then, MVPSO was verified and compared with VPSO by several benchmarks. In addition, a gas/water two-phase model considering gas-adsorption and Knudsen-diffusion effects was used to describe the shale-gas flow in matrix and fracture domains. An embedded discrete-fracture model (EDFM) was applied to model the hydraulic-fracture geometries and fractal methods were adopted to generate the fracture networks. The results indicated that MVPSO showed better performance in both convergence speed and accuracy than that of VPSO, which also provided a new perspective for the optimization of fracturing parameters. Besides, the multispindle-shapedfracture-distribution pattern reached a higher net-present-value (NPV) contrast to that of homogeneous fracture distribution. The decrease of gas price leads to smaller and more nonuniform half-lengthdistribution.

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Duo to different transport mechanisms and gas storage in organic and inorganic systems, a new triple-continuum model coupling Discrete Fracture Model (DFM) was established to investigate gas flow in shale gas reservoir. Considering the multi-scale and heterogeneity of shale matrix, fractal theory was used to calculate the apparent permeability of organic and inorganic systems while multiple gas transport mechanisms such as viscous flow, Knudsen diffusion, surface diffusion, gas absorption/desorption effect and real gas effect were incorporated. This coupled mathematical model was solved by Finite Element Method (FEM) and the presented fractal apparent permeability model was validated with the experimental data. The results show that fractal characteristics of shale matrix have great impact on gas reservoir performance. The model without considering the influence of fractal characteristics could lead to underestimate gas production by approximately 17%. Viscous flow is the dominate transport mechanisms of shale gas and Knudsen diffusion has an impact on gas flow when the pressure declines. Surface diffusion should be only considered in organic systems and can be ignored. Then the results of sensitivity analysis show that the characteristic parameters of inorganic matter have a greater impact than those of organic matter and establishing a triple-continuum model with considering comprehensive effect of organic and inorganic matter is necessary. In addition, gas production would decrease as the pore fractal dimension and tortuosity fractal dimension increase, which results from the increasing number of small pores and more tortuous path for gas flow.