Introduction

Over the last decade, development of new conceptual models of fluid flow and transport in fractured rock aquifer systems has been received substantial scientific attentions. Various flow mechanisms occur in an unsaturated rock matrix-fracture system. For example, the fracture may act as a capillary barrier, or it may have flow in relatively thick films along the fracture surface. Broadly applicable quantitative models for predictive treatment of unsaturated flow in fractured media, analogous to those based on Darcy's law and Richards' equation for non-preferential unsaturated flow, are yet to be developed. Basic knowledge of these mechanisms is critical to the development of the quantitative models needed for research and management issues concerning fractured media. Modeling multiphase flow through fractured systems on the field scale requires estimates of average properties for the ability of the fractures to transmit each phase (the phase relative permeability) and for the ability of the surrounding matrix blocks to imbibe wetting phase and emit non-wetting phase (matrix/fracture transfer). These properties must be representative of the average behavior within a grid block that may have fractures that have different orientations and matrix block sizes. Groundwater flow in a rock can occur both in the rock matrix and in the fractures. Dominating part depends on the porosity and properties of the fractures. For low porosity rock types, unlike highly permeable rock types, the fracture flow is the most important factor. The presence of fractures in a reservoir plays a major role in the fluid flow patterns and the fluids transport.

In this study the Brazilian test technique was employed to induce an extensional fracture with dimensions of about in a layered Berea (calcite-cemented) sandstone sample. High-resolution X-ray micro-tomography (CT) imaging was used to determine the geometry of the fracture. A post-processing code was developed and used to computationally model the fracture geometry; Gambit was then used to generate an unstructured grid of about 1,000,000 cells. Single-phase and two-phase flows through the fracture were studied using FLUENT™ code. The Volume of Fluid (VOF) model was employed for the case of two-phase flow. Flow patterns through the induced fracture were analyzed. In geological flow simulations, flow through fractures is often assumed to occur between parallel plates. The combination of CT imaging of real fractures and computational fluid dynamic simulations may contribute to a more realistic and accurate description of flow through fractured rocks. Two and three dimensional models of the fracture are built using the raw data from CT images. A FORTRAN code is developed to create an AutoLISP script to generate the model in AutoCAD® package, and the model is then meshed in Gambit™ pre-processor. Using FLUENT™ CFD package, the flow field inside the fracture is solved for laminar flow. For the cases of single phase flow (water) and two phase flow of (water and oil) the flow inside the fracture is evaluated.

Copyright © Kambiz Nazridoust, 2005.