Study On The Mechanism Of Regime Evolution And Fluid Properties On Nonlinear Flow In Rock Fractures

Fractures serve as important channels for fluid migration and solute transport in fractured bedrocks,and are thus relevant to many geophysical processes and industrial activities,such as hydrocarbon extraction,geological carbon sequestration,groundwater remediation,hazardous waste isolation,and hydraulic engineering construction.It is challenging to accurately describe the flow behavior of geofluids in fractured media due to the heterogeneous structure of the fractured medium,the complicated and changeable flow regime,and the diverse physical properties of the geofluids.In this thesis,a thorough investigation of the rational evaluation of the nonlinear permeability parameters of nonlinear flow under different flow regimes,as well as the flow regime discrimination and permeability characterization for various geofluids flowing in rock fractures,were carried out using laboratory experiments,theoretical analysis,and numerical simulation.On one hand,the intrinsic physical mechanism of the impact of flow regime evolution on the inertial permeability was revealed.A new concept of global inertial permeability was proposed,and criteria for discriminating global inertial permeability were put forward.Meanwhile,practical and mechanistic criterion models for determining global inertial permeability are both established.On the other hand,the microscopic impact mechanism of fluid properties on flow regime evaluation and permeability parameters characterization was revealed,and the critical Reynolds number parameterization characterization model for various geofluid nonlinear flows was constructed.Moreover,the single-factor(fluid property)parameterization characterization model and the dual-factor(fracture geometry and fluid property)parameterization characterization model of inertial permeability were established.The major achievements of this study are summarized below:(1)The experimental platforms of fluid flow under different flow regime and various fluids were built respectively,and their macroscopic nonlinear flow behavior were analyzedThe fluid flow experiments of different rough fractures under different flow regime were carried out.The analysis found that with the evolution of the flow regime,the normalized apparent permeability first showed a downward trend and then tended to a stable value.The Forchheimer equation is used to fit the monitored pressure and flow data under different flow regimes.It is found that the inertial permeability k_i fluctuates initially and then stabilizes with flow regime evolving.A regression parameter model is established for the minimum Reynolds number(that is,when k_i first reaches stability)and the initial hydraulic aperture.Furthermore,various types of fluid flow tests of fractures are carried out.The macroscopic behavior of nonlinear flow in fractures under different types of fluids was compared and analyzed.Furthermore,parametric characterization model of the inertial permeability,fracture geometry and fluid properties was established.(2)The evolution of inertial permeability with flow regime was studied,and a new concept of global inertia permeability was proposedBy substantial DNS simulations of fluid flow in rock fractures with a wide range of different hydraulic gradients and diverse surface roughnesses,the variation of inertial permeability with fitting flow range is manifested which is mechanistically ascribed to eddy evolution process.This variation is found to reach a stable value with constantly enlarging the fitting flow range.A concept of global inertial permeability(i.e.,the stable value)is subsequently proposed which can predict the whole flow regime in rock fractures.(3)The criteria for discriminating global inertial permeability was proposed,and practical and mechanistic criterion models for determining global inertial permeability were both establishedThe minimum ranges of Reynolds number and eddy volume ratio for fitting this global inertial permeability are quantitatively determined.Two criteria,for determining global inertial permeability with the lowest cost,are successfully parameterized using hydraulic aperture and roughness parameter.Further discussion indicates that the proposed global inertial permeability is meaningful for many relevant scientific issues such as non-Darcian flow prediction and flow regime assessment.Although the parameterized criterion model for determining global inertial permeability is based on2D fracture simulations,a few 3D fracture simulations and in situ packer tests in fractured rock aquifers prove its great potential to apply to more complex 3D and practical situations.This work provides a set of guidelines for reasonably determining inertial permeability,and is of great significance for non-Darcian flow parameterization and modeling in fractured rocks.(4)Critical regime parameterized model was proposed to quantify the emergency of nonlinear flow,and permeability characterization model was proposed for different types of fluid flowThe effects of fluid properties on the evolution of microscopic flow structures as well as on macroscopic nonlinear flow behavior are systematically investigated through extensive 3D fracture numerical simulations,considering fluids with different physical properties,including non-Newtonian and Newtonian fluids(both liquid and gas).The results show that the fluid properties have a significant effect on the macroscopic flow regime assessment,while the fluid properties hardly affect the viscous permeability but significantly affect the inertial permeability.And it is found that inertial permeability is positively related to viscosity and inversely related to density.Further analyzed from the perspective of microscopic eddy evolution,the results show that the increase of density in fluid properties has a facilitating effect on eddy growth,while the increase of fluid viscosity has an inhibiting effect on eddy evolution.Further,the parametric characterization model of critical Reynolds number for different types of fluid flow was constructed,and the single-factor(fluid properties)parametric characterization model and the two-factor(fracture geometry and fluid properties)parametric characterization model of inertial permeability were established.The main novelties of this thesis:(1)The physical mechanisms underlying the effects of regime evolution and fluid properties on key parameters of nonlinear flow in rock fractures are elucidated.The microscopic flow structure during fluid flow in fractures is quantified by the automated eddy detection technique.The physical mechanism of the anomalous dependence of inertial permeability on the fitted range of flow data is clarified,and the mechanism of the impact of fluid properties on the flow discrimination and permeability parameters in nonlinear flow of rock fracture is revealed.(2)Quantitative characterization models for the acquisition at key parameters of nonlinear flow in rock fractures were established.Experimental flow tests and extensive numerical simulations of fluid flow in rock fracture were carried out.Practical and mechanistic criterion models for determining global inertial permeability were both established.Further,the parametric characterization model of critical Reynolds number for different types of fluid flow was constructed,and the single-factor(fluid properties)parametric characterization model and the two-factor(fracture geometry and fluid properties)parametric characterization model of inertial permeability were established.