<p>In tight oil reservoirs, small-scale disconnected cracks, commonly termed as intrinsic cracks, are ubiquitously distributed. These cracks significantly enhance the effective flow conductivity of reservoirs, thereby establishing essential hydrodynamic conditions for waterflooding development. More importantly, they induce heterogeneous frontal advancement characteristics during water-oil displacement processes. This investigation focuses on characterizing intrinsic cracks in tight oil reservoirs. By integrating core-scale unsteady-state flow experimental data with porous media physical modeling methodologies, we developed a dual-continuum mathematical model to describe the coupled crack-matrix flow behavior. Dimensionless governing equations were formulated by introducing characteristic scales, including dimensionless pressure (<InlineEquation ID="IEq1"> <EquationSource Format="TEX">$P_{D}$</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>P</mi> <mi>D</mi> </msub> </math></EquationSource> </InlineEquation>) and characteristic time (<i>t</i>). The analytical solution for upstream/downstream pressure responses was derived through Laplace transformation coupled with Stehfest numerical inversion algorithm. A systematic quantification was subsequently conducted to elucidate the controlling effects of crack permeability (<InlineEquation ID="IEq2"> <EquationSource Format="TEX">$k_{f}$</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>k</mi> <mi>f</mi> </msub> </math></EquationSource> </InlineEquation>), matrix permeability (<InlineEquation ID="IEq3"> <EquationSource Format="TEX">$k_{m}$</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>k</mi> <mi>m</mi> </msub> </math></EquationSource> </InlineEquation>), aperture (<InlineEquation ID="IEq4"> <EquationSource Format="TEX">$h_{f}$</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>h</mi> <mi>f</mi> </msub> </math></EquationSource> </InlineEquation>), and length (<InlineEquation ID="IEq5"> <EquationSource Format="TEX">$l_{f}$</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>l</mi> <mi>f</mi> </msub> </math></EquationSource> </InlineEquation>) on transient pressure behavior. The results demonstrate that the unsteady-state flow in intrinsic crack porous media exhibits triphasic evolution characteristics: (1) crack-matrix dominated flow, (2) matrix-controlled flow, and (3) system equilibration. Results indicate that the influence of crack permeability, crack aperture, matrix permeability, and crack length on the upstream and downstream pressure curves diminishes over time. Higher crack permeability and larger crack apertures shorten the crack-matrix flow stage, leading to an earlier system equilibrium. Increased matrix permeability reduces fluid flow resistance within the matrix, resulting in a more uniform and elevated pressure distribution. Evaluating the impact of crack length on pressure requires a comprehensive consideration of the matrix permeability range. As matrix permeability increases, the effect of crack length on the pressure curve becomes more pronounced. A comparison between the analytical solution of the mathematical model and experimental data demonstrates good agreement. This study develops an asymmetric dual-permeability model to characterize the dynamic coupling mechanism between cracks and the matrix. Additionally, a characteristic scale is introduced, enabling the extension of findings to studies at different scales. This study uses multi-parameter control analysis to examine how crack and petrophysical parameters influence permeability and to provide guidance for optimizing reservoir fracturing.</p>

Study on the Coupling Mechanism of Unsteady-State Flow in Intrinsic Crack Porous Media

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发表日期：

2026年3月13日

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Jingru Wang,
Yuetian Liu,
Yang Song,
Xiankun Song,
Liang Xue

摘要

In tight oil reservoirs, small-scale disconnected cracks, commonly termed as intrinsic cracks, are ubiquitously distributed. These cracks significantly enhance the effective flow conductivity of reservoirs, thereby establishing essential hydrodynamic conditions for waterflooding development. More importantly, they induce heterogeneous frontal advancement characteristics during water-oil displacement processes. This investigation focuses on characterizing intrinsic cracks in tight oil reservoirs. By integrating core-scale unsteady-state flow experimental data with porous media physical modeling methodologies, we developed a dual-continuum mathematical model to describe the coupled crack-matrix flow behavior. Dimensionless governing equations were formulated by introducing characteristic scales, including dimensionless pressure ( $P_{D}$ $P_{D}$ ) and characteristic time (t). The analytical solution for upstream/downstream pressure responses was derived through Laplace transformation coupled with Stehfest numerical inversion algorithm. A systematic quantification was subsequently conducted to elucidate the controlling effects of crack permeability ( $k_{f}$ $k_{f}$ ), matrix permeability ( $k_{m}$ $k_{m}$ ), aperture ( $h_{f}$ $h_{f}$ ), and length ( $l_{f}$ $l_{f}$ ) on transient pressure behavior. The results demonstrate that the unsteady-state flow in intrinsic crack porous media exhibits triphasic evolution characteristics: (1) crack-matrix dominated flow, (2) matrix-controlled flow, and (3) system equilibration. Results indicate that the influence of crack permeability, crack aperture, matrix permeability, and crack length on the upstream and downstream pressure curves diminishes over time. Higher crack permeability and larger crack apertures shorten the crack-matrix flow stage, leading to an earlier system equilibrium. Increased matrix permeability reduces fluid flow resistance within the matrix, resulting in a more uniform and elevated pressure distribution. Evaluating the impact of crack length on pressure requires a comprehensive consideration of the matrix permeability range. As matrix permeability increases, the effect of crack length on the pressure curve becomes more pronounced. A comparison between the analytical solution of the mathematical model and experimental data demonstrates good agreement. This study develops an asymmetric dual-permeability model to characterize the dynamic coupling mechanism between cracks and the matrix. Additionally, a characteristic scale is introduced, enabling the extension of findings to studies at different scales. This study uses multi-parameter control analysis to examine how crack and petrophysical parameters influence permeability and to provide guidance for optimizing reservoir fracturing.