Three Dimensional Numerical Analysis of Fluid Flow through Fractured Rock Core Using X-Ray Computed Tomography

Abstract

Abstract A three dimensional numerical modeling coupled with X-ray Computed Tomography (CT) for fracture flow was applied to fractured granite core samples. One of the samples had an artificial single fracture, and the others had natural multiple fractures. A relationship between CT value and fracture aperture (fracture aperture calibration curve) was obtained by X-ray CT scanning for a fracture aperture calibration standard with varying the aperture from 0.1 to 0.5 mm. As a result, a linear relationship was obtained between CT value and fracture aperture. With the fracture aperture calibration curve, three dimensional distributions of CT values of the samples were converted into fracture aperture distributions to obtain fracture models of the samples. Porosities of the fracture models could provide good agreement with experimentally determined porosities for all the samples. By using the fracture models, a fluid flow simulation was also performed with a local cubic law-based fracture flow model. Numerical permeabilities by the flow simulation were much higher than experimentally determined permeabilities of the samples. It was however possible to match the numerical permeabilities with the experimental permeabilities for all the samples, by using a unique modification coefficient of 0.5 for the fracture aperture in the fracture flow model. With the modified flow model, it was possible to obtain detailed information of heterogeneities in fracture flow as well as permeabilities of the samples. Although the present X-ray CT scanning was performed at room temperature and pressure, it was expected that the numerical modeling had possibility to provide insights into heterogeneous nature of fracture flow in fractured reservoirs, such as channeling, as well as porosity and permeability, when the CT scanning was performed at reservoir conditions. Further studies on this kind of numerical modeling should provide simple and easy way to address heterogeneous fracture flow in reservoirs that can have impacts on productivity of wells, efficiency of recovery by water/gas flooding, and so on. Introduction Effective developments of oil/gas fractured reservoirs need well understanding of fracture flow characteristics in those reservoirs. Field and laboratory studies have suggested that fluid flow through a rock fracture is far from that of smooth parallel plates, due to channeling flow in a heterogeneous aperture distribution by rough surfaces [1–9]. When channeling flow occurs in a single fracture of granite, the area where flowing fluid exists is expected only 5–20% at confining pressures of up to 100 MPa, with various features in the preferential flow paths [9]. Against this background, we have recently developed a discrete fracture network model simulator, GeoFlow, in which fractures can have aperture distributions with natural heterogeneities [10]. Three dimensional fluid flow simulations for fracture networks by GeoFlow have demonstrated developments of three dimensional preferential flow paths in those fracture networks, and have suggested that performance of production well can be strongly affected by the heterogeneous fracture flow. Consequently, it should be considerably important to evaluate the heterogeneity and its impact for effective developments of fractured reservoirs.