Geochemical Modeling of Fracture Filling

Abstract

Abstract This paper reports on a mathematical model to simulate hydrodynamics and fluid-mineral reactions in the fracture within a permeable media. Fluid convection, diffusion and precipitation / dissolution (PD) reaction inside a finite space are solved as a simplified representation of natural fracture mineralization. The problem involves mass transfer within the fluid accompanied by chemical reaction at the fracture surface. Mass-conservation equations for components in the fluid are solved in this problem, and these are coupled with chemical reaction at the fracture surface. The intent of this model is to show the time evolution of fracture aperture shrinkage patterns caused by PD reactions. We present the aperture distribution along the fracture with various boundary conditions. Partially cemented fractures are created if cementation fails to completely fill the fracture or if subsequent dissolution leaches out some of the mineral. Introduction A fracture has been recognized as a fluid conduit that has high permeability relative to that of the surrounding rock matrix in a fractured reservoir. To characterize the reservoirs, core and well log data can be investigated. However, only a few fractures can be observed by the direct measurements; determining which particular fractures are controlling fluid flow and by which mechanisms the wells in fractured media produce are difficult tasks. Often, the most reliable way of characterizing the response of a fractured reservoir is through the analysis of reservoir production behavior, such as well productivity and breakthrough data. The reservoir quality of sedimentary rocks is closely related to diagenesis, a process involving post-depositional alteration of previously deposited sediments. Rock properties such as porosity, permeability, pore-size distribution, reservoir heterogeneity and spatial correlation can be the product of diagenetic modification of original properties. One of the most important mechanisms of diagenetic processes is chemical reaction between minerals and migrating fluids. Fluid-mineral reactions are dynamic processes that involve many effects: complex fluid flow, pore space changes, surface chemistry, and the mineral composition. We have developed a simple precipitation / dissolution (PD) model for the flow in a finite parallel plate that represents a single fracture. The idea behind the model is to approximate the description of a variable fracture aperture, surface reaction, and diffusion by a set of parameters and simple rules governing the alteration of fractures. Fracture aperture can either grow or shrink as chemical reaction proceeds at the fracture surface. In this study, we solve mass conservation equations for the components of aqueous and solid phases simultaneously and show the fracture aperture size distribution with time. Calcite cementation is as the form of precipitation considered. Characterization of Fractured Reservoir The spatial variations of fracture properties, such as aperture size and orientation, are complicated and irregular so that the characterization of a fractured reservoir is more difficult than that of an unfractured reservoir. One way to approach the characterization is to start from a local characterization of a single fracture and proceed to fracture systems. Parameters for the characterization of fractures include fracture property distributions, fracture density and the size and the shape of matrix blocks. Especially in low permeability reservoirs, natural fracture permeability is an important issue. In crystalline rocks, the system permeability is almost entirely the result of the fracture network even though the matrix contains most of the reservoir fluid. In general, the two rock surfaces that bound a fracture are rough. The degree of roughness can be a function of the fracture aperture and the fluid properties within the fractures. Fractures can be partially or fully-filled by mineral precipitation. The nature of a fracture is reservoir-specific, depending on mineral composition, tectonic stress history, diagenesis and petrophysical properties. One purpose of this work is to put some of these connections into a quantitative understanding.