Fracture Network Modeling of a Hot Dry Rock Geothermal Reservoir

Fluid flow and tracer transport in a fractured Bot Dry Rock (HDR) geothermal reservoir are modeled using fracture network modeling techniques. The steady state pressure and flow fields are solved for a two-dimensional, interconnected network of fractures with no-flow outer boundaries and constant-pressure source and sink points to simulate wellbore-fracture intersections. The tracer response is simulated by particle tracking, which follows the progress of a representative sample of individual tracer molecules traveling through the network. Solute retardation due to matrix diffusion and sorption is handled easily with these particle tracking methods. Matrix diffusion is shown to have an important effect in many fractured geothermal reservoirs, including those in crystalline formations of relatively low matrix porosity. tracer behavior are matched for a fractured HDR reservoir tested at Fenton Hill, NM. INTRODUCTION AND MOTIVATIONS Reservoir engineers and groundwater hydrologists have long recognized the importance of fractures on fluid flow and solute transport in underground porous media. Many analytical and numerical models exist to predict flow behavior for various fracture geometries ranging from a single fracture to multiple, interconnected fractures. Steady state or pressure transient responses can often be predicted using these models, which provide a macroscopic description of the flow process in terms of parameters suitable for use by hydrologists and engineers. Solute transport is not so easily simulated using these models, however. The typical approach of employing the convectivedispersion equation with the adjustable parameter of dispersion coefficient usually fails in several important ways. In one dimension, a good match between model and field data is often difficult to achieve, since field data are seldom if ever perfect Gaussian distributions of residence times about a mean value. Multi-dimensional forms Pressure drop and of the convective-dispersion equation can provide better fits, but at the expense of more adjustable parameters of questionable physical significance. Fracture network modeling is a different approach to simulating flow and transport in fractured rock. The flow system is comprised of a network of interconnected fractures. A pressure difference imposed in such a system due to fluid injection or a natural hydraulic gradient results in a flow of water through the fractures. This flow field can be calculated assuming a fracture geometry, appropriate boundary conditions, and a relationship between pressure drop and flow rate within each fracture. Once the flow field is determined, the transport of a conservative, reacting, or adsorbing chemical component can be calculated using particle tracking techniques, which follow the progress of a representative sampling of tracer molecules through the network. Fracture network modeling has been used extensively to model groundwater flow (see, for example, Castillo et al. (1972), Schwartz (1977), Smith and Schwartz (1980), Schwartz et al. (1983), Long et al. (1982), Anderson and Thunvik (1983), and Hopkirk et al. (1985). Long and Billaux (1987)). The primary focus of most previous work has been to determine the conditions under which a fractured rock could be treated as an equivalent porous medium. With the fracture network approach, one can assess the effect of fracture size, spacing, aperture, and orientation on the fluid flow, permeability distribution, and tracer behavior. Typically, Monte Carlo techniques are used, in which a large number of realizations of different fracture geometries, all with identical fracture statistics, are performed to determine the average and variability of behavior. The latter is a measure of the inherent uncertainty of flow behavior in the fracture network, given the measured statistical parameters. In most cases, these studies have assumed the flow to be within a rectangular grid in two dimensions, with constant-head boundary conditions at opposite ends of the plane and no-flow or linearly-