Flow Simulation of Artificially Induced Microfractures Using Digital Rock and Lattice Boltzmann Methods

Abstract

Abstract:Microfractures have great significance in the study of reservoir development because they are an effective reserving space and main contributor to permeability in a large amount of reservoirs. Usually, microfractures are divided into natural microfractures and induced microfractures. Artificially induced rough microfractures are our research objects, the existence of which will affect the fluid-flow system (expand the production radius of production wells), and act as a flow path for the leakage of fluids injected to the wells, and even facilitate depletion in tight reservoirs. Therefore, the characteristic of the flow in artificially induced fractures is of great significance. The Lattice Boltzmann Method (LBM) was used to calculate the equivalent permeability of artificially induced three-dimensional (3D) fractures. The 3D box fractal dimensions and porosity of artificially induced fractures in Berea sandstone were calculated based on the fractal theory and image-segmentation method, respectively. The geometrical parameters (surface roughness, minimum fracture aperture, and mean fracture aperture), were also calculated on the base of digital cores of fractures. According to the results, the permeability lies between 0.071-3.759 (dimensionless LB units) in artificially induced fractures. The wide range of permeability indicates that artificially induced fractures have complex structures and connectivity. It was also found that 3D fractal dimensions of artificially induced fractures in Berea sandstone are between 2.247 and 2.367, which shows that the artificially induced fractures have the characteristics of self-similarity. Finally, the following relations were studied: (a) exponentially increasing permeability with increasing 3D box fractal dimension, (b) linearly increasing permeability with increasing square of mean fracture aperture, (c) indistinct relationship between permeability and surface roughness, and (d) linearly increasing 3D box fractal dimension with increasing porosity.

An Analytical Solution for Transient Productivity Prediction of Multi-Fractured Horizontal Wells in Tight Gas Reservoirs Considering Nonlinear Porous Flow Mechanisms

“…(4) Mutual interference exists between the fractures and the pressure loss in the wellbore is neglected. (5) Contamination of the fracture wall is neglected. (6) Pressure loss in the horizontal wellbore is neglected.…”

“…Energies 2020, 13, x FOR PEER REVIEW 6 of 20 Combining Equations (4) and (5), the relationship between p and y under Cartesian coordinates can be transformed to the relationship between p and  under elliptical coordinates.…”

“…where q is the gas flow rate, 3 m /s ; h is the reservoir thickness, m; and m v is the gas velocity, m/s. By combining Equations (4) and (5), an isobaric elliptical equation and hyperbolic streamline equations can be expressed as Equation (6) and (7), and its flow field is illustrated in Figure 3:…”

“…The results illustrate that threshold pressure gradient has the most significant influence on well productivity, followed by stress sensitivity, turbulence flow, and slippage flow. To summarize, the proposed model has demonstrated a potential practical usage to predict the productivity of multi-stage fractured horizontal wells and to analyze the effects of certain factors on gas production in tight gas reservoirs.Energies 2020, 13, 1066 2 of 20 of tight gas reservoirs has made considerable breakthrough and become the primary growth point for oil and gas production [1][2][3][4][5][6]. Because of its strong heterogeneity, complicated microscopic pore structure, and poor connectivity of the effective sand bodies, the porous flow mechanism of tight gas reservoirs is obviously distinct from those of conventional reservoirs.…”

“…Energies 2020, 13, 1066 2 of 20 of tight gas reservoirs has made considerable breakthrough and become the primary growth point for oil and gas production [1][2][3][4][5][6]. Because of its strong heterogeneity, complicated microscopic pore structure, and poor connectivity of the effective sand bodies, the porous flow mechanism of tight gas reservoirs is obviously distinct from those of conventional reservoirs.…”