| Unsaturated seepage in fractured sandstone is widely used in mining,oil and gas,geology,geotechnical engineering and so on.In particular,it is embodied in green mining and ecological restoration in arid and semi-arid areas of western China,water injection softening and weakening rock burst in hard coal strata,hydraulic fracturing technology and water flooding oil and gas technology.Spontaneous imbibition is an important macroscopic phenomenon for unsaturated seepage.However,the hydrodynamic mechanism of microscopic structure affecting on macroscopic transport is still unclear.Therefore,the fractal geometry models of spontaneous imbibition for porous matrix,single fracture,fracture network and dual media are constructed by using seepage mechanics and fractal geometry theory considering various geometric factors in this thesis.The neutron imaging and gravity experiments on fractured sandstone at core scale have been carried out to reveal the dynamic wettability behavior of their spontaneous imbibition.A variety of geometric characterization methods(MIP,SEM,X-ray CT)were used to accurately measure the microstructure of various fractured sandstones from various regions.Meanwhile,permeability test and XRD mineral composition test were also carried out.Finally,the reliability of the theoretical model is verified based on the experimental data,and the influence mechanism of the microscopic geometric characteristics of fractured sandstone on the macroscopic unsaturated hydraulic transport is quantitatively revealed by using the model and experimental data.There are a few of important research results and findings as follows.(1)The topological characteristics,pore throat characteristics,tortuous characteristics,pore geometry and fractal geometry of microscopic pore structure have important influence on the macroscopic hydraulic transport mechanism of sandstone matrix,and their influence sensitivity decreases in turn.Their influence on permeability is stronger than that on water absorption.The comprehensive geometric parameter tends to decrease with the increase of porosity,which comprehensively explains that the matrix with large porosity may have weak water absorption.There is a linear relationship between the square of hydroscopicity characteristic parameter(capillary and sorptivity coefficients)and permeability which shows great difference for different permeability rock.The linear relationship of K～S~2is more obvious for high permeability rocks and K～C~2is more obvious for low permeability rocks.(2)The two-dimensional fractal dimension of the scale distribution of the fracture,the two-dimensional fractal dimension of the tortuous seepage path of the fracture,and the three-dimensional fractal dimension of the roughness characteristics of the fracture wall are introduced into the multifractal model of fracture network considering the influence of the fracture width on the wettability.The model reveals that the stronger the heterogeneity and anisotropy of fracture network and the roughness of fracture wall,the weaker the water absorption.The contribution of porous matrix and fracture network to hydroscopicity is considered in the fractal geometry model of dual media,and a dimensionless index is derived to describe the relative contribution of the two,and is related to the fractal dimension,maximum size,volume porosity,area porosity and tortuosity of pore or fracture.(3)Neutron imaging intuitively shows the piston displacement process of macroscopic wetting fronts for 8 different types of sandstone substrates,and quantitatively reveals the linear evolution law of the wetting front with the square root of time.The wettability evolution behavior of sandstone can be accurately described by the multi-factor geometric model,which reveals the water imbibition kinetics mechanism by the coupling multiple geometric factors.The fractal dimension of tortuosity based on the inverse prediction of nonlinear fitting demonstrates an increasing trend with the increase of porosity,indicating that the rock with large porosity displays strong heterogeneity.There is a strong linear relationship between the square of sorptivity coefficient and permeability for the eight kinds of sandstones,especially for high-permeability specimens.(4)The wettability model for single fracture considering the fissure-matrix interaction shows the fracture wettability process going through three distinct stages,namely,the first stage-rapid imbibition,second stage-matrix influence,and third stage-stable expansion.The wetting matrix in the three stages exhibits three typical zones.In the model,the three-dimensional fractal dimension is introduced into the time index for the rough wall imbibition.The neutron imaging data of fractured sandstone also confirms that the fracture wettability front has experienced three stages.The inverse prediction results of 3D fractal dimension based on the fitting neutron data has the characteristics of partition,which is related to anisotropic failure mode caused by sandstone heterogeneity.(5)Neutron imaging of heavy water flooding purified water in fractured sandstone directly shows the wetting interaction between fracture and matrix in the later stage.The wetting fronts of heavy water and pure water drive each other in the matrix region to form an obvious arc wetting interface.Due to the displacement of wetting front in the matrix region,the heavy water front in the fracture presents a complex multi-stage evolution pattern,and the vertical wetting front in the matrix region also experiences a segmented evolution behavior.Based on the measured microscopic geometric parameters and neutron imaging experimental data,the fractal pore-throat model is verified and compared with existing models available from literature.Further,the influence mechanism of sandstone’s complex micropore structure on hydraulic transport is analyzed from the perspective of geomechanical origin.In this thesis,a series of research achievements have been made in the method about dynamic identification of wetting front and wanter content,revealing the micro response mechanism of water imbibition kinetics,constructing the fractal geometric model of imbibition,and 3D fine characterization of real pore and fracture geometry for the fractured sandstones.It can provide theoretical guidance and experience reference for coal,oil and gas,geotechnical engineering,etc. |
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