Applications of the Fractured Continuum Model (FCM) to EGS Heat Extraction Problems

Abstract

The major objective of this work was to demonstrate the applications of the Fractured Continuum Model (FCM) to common EGS conditions. In our previous work we conducted a number of reservoir simulations assuming homogeneous and heterogeneous reservoir conditions. This work demonstrated great importance of natural fracture properties on heat extraction. To describe the common EGS conditions, we compiled extensive literature data on natural fractures in granite rocks. These data suggested that a typical EGS reservoir may have 2 to 4 sets of sub-vertical fractures with different dips and orientations. Each set may have different fracture spacing and aperture. The fracture density may also change with depth. The situation in which there is a sequence of intervals with high and low fracture densities is very common. To incorporate these data in the reservoir simulations, we extended our previous simplified FCM. Our new FCM incorporates fully three-dimensional representations of anisotropic permeability, multiple independent fracture sets, and arbitrary fracture dips and orientations. The continuum model generation is flexible such that any parameter (strike, dip, aperture, or spacing) can be independent or spatially correlated and parameters can be drawn from a variety of distributions. The fracture strike, dip and aperture for each grid cell in the model domain are drawn independently from the corresponding probability distributions. Fracture spacing uses correlated random numbers to draw from the corresponding probability distribution. The spatial correlation used to draw numbers for fracture spacing ensures that fracture properties are continuous across multiple contiguous grid cells. This capability is needed to represent long fractures if such fractures are present. Additionally, the continuum model can include multiple zones to account for changing fracture properties as a function of depth. The hydro-fractures can be represented in the same model as a separate fracture set or sets with the corresponding properties. Our goal was to apply this new tool to simulating typical reservoir conditions. Two scenarios were considered. In the first scenario, it was conceptualized that the natural fractures will be enhanced to provide the major heat extraction pathways. In the second scenario, the additional fracture sets were added to the natural fractures to represent hydro-fracturing conditions. A number of different combinations of fracture set numbers, fracture orientations, dips, spacing, and apertures were considered. The major goal of these simulations was to define optimal conditions for heat extraction.