Research On Digital Fracture Modeling And Seepage Property Calculation

Natural fractures generally occur in a wide range type of porous media(e.g.rocks,soils)with multiple length-scales and different intensities,thus it is of great importance in geology,mining,material,soil,environment and petroleum engineering to understand the impact of morphological structures on the flow characteristics through fractures.With the rapid development of X-ray CT scanning,image processing,and computer technology,it has become a hot and leading research field to quantitatively characterize the morphology of fractures and to establish the seepage model of fractures.Therefore,starting from a series of CT scan images of samples that contain fractures,this thesis studies the fracture morphology with various quantitative parameters,as well as investigate the methodology and algorithms for modelling fracture using the presented parameters.Further,research in-depth was carried out on the influence of various fracture properties on fluid flow,an effective and accurate calculation of fluid transport has been proposed successfully,which is based on the simplified structure(medial surface)of fractures.The major contributes of this thesis can be summarized as follows:1.Regarding the challenging measurement and characterization of the internal void space of fractures of complex morphology,an accurate method for describing the spatial geometry and topology of fractures has been proposed in the thesis,which is based on a new finding on the fracture structure,i.e.,fracture void space can be equivalently regarded as the combination of the fracture trend surface and its aperture field.From this observation,an approach for measuring five morphological quantities(i.e.,fracture mean aperture,roughness of aperture,spatial correlation length of aperture field,fluctuation coefficient of fracture trend surface and spatial correlation length of fracture surface)has been developed as well,aiming to provide new quantitative features for in-depth researches.2.Considering both the extreme difficulty of acquiring natural fractures and the high cost of CT scans without destroying the physical samples,this thesis presents a novel and effective mean-filtering method for constructing the three-dimensional model of single fractures of rough walls.The resultant apertures of the generated models satisfy not only the Gauss distribution but also display the consistent spatial correlation,meanwhile,the two fracture walls show a bulk similarity in fluctuation and a local synchronization in coordination.Consequently,the introduced structural modelling of fractures is able to provide a parameter-controllable and unlimited number of fracture models for having insight into the fluid flow through fractures.3.Inspired by the conventional methodology on the search for the factors that control the fluid flow through fractures,starting with the estimation of correction coefficient for the modified cubic law,this thesis analyses and discusses the impact of various morphological features on the absolute permeability.My investigation indicates that the fracture aperture parameter is the major controlling factor,while the fracture trend surface is the secondary.On the premise of the specified mean aperture,the absolute permeability of the fracture is a power-law function of the fracture roughness,but linearly increases as the spatial correlation length of the fracture aperture does.Moreover,the fracture permeability decreases as the increase of the fluctuation coefficient of the fracture trend surface,and increases linearly with the increase of the spatial correlation length of the trend surface.This study provides a mechanistic framework for improving energy(oil & gas)recovery in fractured reservoirs.4.Aiming at addressing the problem of time-consuming and resource-intensive calculations of the conventional fracture seepage properties,a fast and accurate method based on the medial surface of the fracture is proposed.As a dimensionality reduction process for the complex void space of three-dimensional fractures,the medial surface not only captures the essential elements of seepage(such as apertures and connectivity)but also reduces the complexity of the discrete calculation domain.Therefore,the numerical calculation method from the medial surface shortens significantly the time to simulate the fracture permeability,as well as achieve obtaining fracture morphological data that is proved difficult to measure in laboratory.This research provides an important tool for simulating fluid flow in fractures.