Rock fracture morphology influences two-phase fluid flow in rock fractures. However, the role of the surface morphology of rock fractures in two-phase flow remains to be fully quantified. In this study, an analytical model for gas and water relative permeability in rough rock fractures was developed, incorporating surface roughness factor, hydraulic tortuosity, and surface tortuosity to quantify fracture morphology. The performance of the proposed model was evaluated through a comparison with empirical permeability models and experimental data. The proposed model accurately characterized two-phase fluid flow in rough fractures and examined the impact of surface morphology and flow dynamics on the relative permeability of two-phase fluids in rock fractures, demonstrating that increased surface roughness shifts the equal-permeability point toward higher water saturation and lower relative permeability. To investigate the influence of the flow dynamics on relative permeability, the proposed model was extended to express the Reynolds number of two-phase fluids as a function of water saturation and relative permeability, enabling a detailed exploration of the evolution law of relative permeability with Reynolds number.

Changes in rock fracture permeability due to fine particle migration affect gas production and stress distribution within a reservoir. Under stress, engineering disturbances, fluid scouring and erosion, and fracture protrusions are often sheared off, forming cavity structures within fractures. These cavities influence particle migration and aggregation within fractures. Numerical simulations of particle-laden fluid flow in fractures with circular, triangular, and rectangular cavity shapes were performed using the computational fluid dynamics–discrete element method. The results showed that the fluid velocity decreased with increasing cavity area, whereas increasing the inlet velocity of the suspension flow promoted fines aggregation and adhesion to fracture walls and reduced fine particle dispersion. A Belehradek function relationship between the fines velocity and quantity was attained when the inlet velocity of the suspension flow was greater than 0.25 m/s, the fines particle residence time was inversely correlated with the particle velocity. As the particle concentration increased, contact and aggregation within the channel increased, forming larger agglomerates that adhered to the walls, particularly in the cavity domains. Notably, compared with circular and triangular shapes, rectangular cavities showed reduced aggregation and adhesion.

The hydro-mechanical coupling in fractures plays a significant role in fluid transport through fracture networks. However, current studies still exhibit certain limitations in the multi-parameter characterization of fracture permeability under stress conditions. To address this, a hydro-mechanical coupling model was developed to investigate the coupled hydro-mechanical behavior of fractures under different stress states and shear displacements. The results show that fluid flow patterns within fractures exhibit notable heterogeneity and anisotropy, influenced by aperture distribution and the connectivity of preferential flow paths. High normal stress significantly reduces the mechanical aperture while enhancing its anisotropy, as the normal stress increased from 2 MPa to 8 MPa, the average mechanical aperture of the fractures decreased by 61% to 65%. With increasing shear displacement, both the mechanical aperture and its standard deviation increase, and the aperture distribution shifts from a sharply peaked pattern to a more flattened one, the maximum aperture increased by 23–38%, reflecting enhanced variability in fracture structure. Increased surface roughness amplifies the effect of shear displacement on the evolution of fracture architecture. Under low normal stress conditions, the mechanical aperture increases gradually with higher roughness, the mechanical aperture decreased more significantly in high-roughness (JRC = 17.94) fractures (28–31% greater reduction) compared to low-roughness ones (JRC = 2.01). To assess fracture permeability, a predictive model was developed and validated against further data, confirming its effectiveness in evaluating permeability. This study highlights the mechanisms by which shear displacement and normal stress influence fracture permeability.

Understanding particle transport in rough-walled fractures is essential for predicting flow behavior, clogging, and permeability evolution in natural and engineered subsurface systems. This study develops a fully coupled CFD–DEM framework to investigate how self-affine fractal roughness, represented by the Joint Roughness Coefficient (JRC), governs fluid–particle interactions across multiple scales. Nine fracture geometries with controlled roughness were generated using a fractal-based surface model, enabling systematic isolation of roughness effects. The results show that increasing JRC introduces a hierarchy of geometric perturbations that reorganize the flow field, amplify shear and velocity-gradient fluctuations, and enhance particle–wall interactions. Particle migration exhibits a nonlinear response to roughness due to the competing influences of disturbance amplification and the formation of preferential high-velocity pathways. Furthermore, roughness-controlled scaling relations are identified for mean particle velocity, residence time, and energy dissipation, revealing JRC as a fundamental parameter linking geometric complexity to transport efficiency. Based on these findings, a unified mechanistic framework is established that conceptualizes fractal roughness as a multiscale geometric forcing mechanism governing hydrodynamic heterogeneity, particle dynamics, and dissipative processes. This framework provides new physical insight into transport behavior in rough fractures and offers a scientific basis for improved prediction of clogging, proppant placement, and transmissivity evolution in subsurface engineering applications.