An Advanced Discrete Fracture Methodology for Fast, Robust, and Accurate Simulation of Energy Production from Complex Fracture Networks

Abstract

Fracture networks are abundant in subsurface applications (e.g., geothermal energy production, CO 2 sequestration). Fractured reservoirs often have a very complex structure, making modeling flow and transport in such networks slow and unstable. Consequently, this limits our ability to perform uncertainty quantification and increases development costs and environmental risks. This study provides an advanced methodology for simulation based on Discrete Fracture Model approach. The preprocessing framework results in a fully conformal, uniformly distributed grid for realistic 2D fracture networks at a required level of precision. The simplified geometry and topology of the resulting network are compared with input (i.e., unchanged) data to evaluate the preprocessing influence. The resulting mesh-related parameters, such as volume distributions and orthogonality of control volume connections, are analyzed. Furthermore, changes in fluid-flow response related to preprocessing are evaluated using a high-enthalpy two-phase flow geothermal simulator. The simplified topology directly improves meshing results and, consequently, the accuracy and efficiency of numerical simulation. The main novelty of this work is the introduction of an automatic preprocessing framework allowing us to simplify the fracture network down to required level of complexity and addition of a fracture aperture correction capable of handling heterogeneous aperture distributions, low connectivity fracture networks, and sealing fractures. The graph-based framework is fully open-source and explicitly resolves small-angle intersections within the fracture network. A rigorous analysis of changes in the static and dynamic impact of the preprocessing algorithm demonstrates that explicit fracture representation can be computationally efficient, enabling their use in large-scale uncertainty quantification studies.Plain Language Summary Fractured rocks occur naturally and are abundant in the Earth's subsurface, especially in rocks that host a variety of resources, from geothermal energy to clean water. Modeling fluid flow in such systems is complex and time-consuming, increasing environmental and economic risks. We attempt to tackle this problem by introducing an advanced modeling technique that simplifies the fractures' representation while maintaining the main characteristics. The method's performance is analyzed based on changes in the geometry of the fractures and fluid-flow patterns. The framework manages to significantly speed up the required time for fluid flow calculations while remaining close to the highfidelity solution (i.e., solution of unchanged fracture configuration). Because most of the parameters in subsurface-related energy applications are uncertain, many simulations have to be carried out to quantify these uncertainties. Since our framework reduces the computational time, more simulations could be executed, reducing the risks associated with the development of subsurface energy resources.

Velocity-dependent heat transfer controls temperature in fracture networks

“…Whitby Mudstone is the exposed counterpart to Posidonia Shales present in a potential unconventional gas reservoir in the Netherlands 18 . The outcrop data has been scaled up to cover a range of approximately 700-1000 m and used for geothermal reservoir simulations in the past 19 . In the reservoir, fractures are the dominant flow paths in the system due to the low intrinsic permeability and porosity of the Mudstone varying between 1 × 10 −18 m 2 to 1 × 10 −21 m 2 and 0.5% to 2.5%, respectively 18 .…”

“…The accurate representation of fracture networks is subjected to required simplifications while preserving the network characteristics. In this work, we compare three representations of the network with 34, 66 and 116 fractures 19 . Therefore, all networks share a mutual geometry but fracture number and fracture length vary.…”

“…Locations of production and injection boreholes are chosen to achieve the longest possible flow distance in the diagonal direction while providing sufficient connection to the network (Fig. 5b) 19 . The calculated apertures in combination with an injection pressure of 50 MPa result in flow velocities in the range of 1 × 10 −4 m/s to 3 × 10 −1 m/s in the individual fractures.…”