High temperatures and reactive fluids in sedimentary basins dictate that interplay and feedback between mechanical and geochemical processes could significantly influence evolving rock and fracture properties. In this paper, we propose an integrated methodology of fractured reservoir characterization and show how it can be incorporated into fluid flow simulation. In recent years, there have been a number of important discoveries regarding fundamental properties of fractures, in particular related to the prevalence of kinematically significant structures (crack-seal texture) within otherwise porous, opening-mode fractures, and the presence of an aperture size threshold below which fractures are completely filled and above which porosity is preserved. Significant progress has been made as well in theoretical fracture mechanics and geomechanical modeling, allowing prediction of spatial distributions of fractures that mimic patterns observed in nature. Geomechanical modeling shows the spatial arrangement of opening mode fractures (joints and veins) is controlled by the subcritical fracture index of the material. Fluid flow simulation of representative fracture pattern realizations shows how integrated modeling can give new insight into permeability assessment in the subsurface. Using realistic, geomechanically generated fracture patterns, we propose a methodology for permeability estimation in non-percolating networks.
The continuity of fracture-porosity is fundamental to how fractures conduct fluids. One approach to predicting the spatial arrangement of opening mode fracture networks is through geomechanical modeling. We utilize a model based on subcritical crack growth to generate fracture trace patterns and mechanical opening distributions for various boundary conditions and material properties [1,2]. An important capability of such modeling is the ability to predict the presence or absence of fracture clustering, as well as the shape of the fracture length distribution. Another aspect of the problem is how diagenesis modifies fracture porosity and effective length distribution and may affect the dynamics of fracture propagation. Cements may also alter the compliance of fractures and host rock, tending to preserve fracture pore space under changing load conditions. Although most recent literature emphasizes Earth stress orientation [3,4], cementation in fractures and host rock is likely a critically important control on porosity, fluid flow attributes, and even sensitivity to effective stress changes [5-7].
Little is known of the evolution of fracture networks in the context of the diagenetic pathway followed by the host rock or of the influence on fracture growth of diagenetic processes within fractures. Yet the high temperatures and reactive fluids in sedimentary basins suggest that interplay and feedback between mechanical and geochemical processes could have significant influence on evolving rock and fracture properties. In this paper we show how coupling fracture mechanics and diagenesis considerations can lead to improved predictions of flow performance in fractured reservoirs.
2. CEMENT IN SANDSTONE FRACTURES
Here we focus on the effects of quartz cement because diagenetic modeling can be used to predict the distribution and abundance of this phase [8,9]. Quartz is the most abundant and widespread cement in sandstones exposed to temperatures in excess of ~90 °C for geologically significant periods [10,11]. It is therefore not surprising that virtually all transgranular fractures in such sandstones show at least some degree of porosity loss due to quartz cementation.