Discrete Fracture Network Modeling to Design Hydraulic Fracturing to Develop a Sedimentary Enhanced Geothermal System
Authors: Porlles J. W. , Tomomewo O. , Afari A. S. , Gosnold W. 
Published: September 2023

The Williston Basin, located in northwestern North Dakota, is one of the most prolific basins in the US, with a particular characteristic; some depth reservoirs have more than 140°C and lie between 3.0 km to 4.0 km in depth (Red River formation). Those kinds of reservoirs are cataloged as Sedimentary Enhanced Geothermal systems. Heterogeneity is an inherent component of these rocks and may be present in different forms, including mineral heterogeneity, geometrical heterogeneity, and micro-defects. This study develops a 3D numerical model of natural fracture reservoir with dual porosity/dual permeability and discrete fracture network, investigating how affect the intensity and abundance of natural fracture in the heat extraction accumulation, and developing a comprehensive statistical analysis identifying that natural fracture spacing is the most significant parameter that affect the heat extraction in comparison of porosity, horizontal permeability, and vertical permeability.

INTRODUCTION

This study observed the intensity and abundance of natural fractures. A scaled-up characterization of dual porosity/dual permeability and discrete fracture network was used and implemented to evaluate how the natural fracture affects heat extraction. Log interpretation results indicate that the physical properties of rocks, such as secondary porosity and production information, are influenced by the intensity of the microcrack. The tornado chart shows that natural fracture spacing is the most critical parameter affecting recoverable energy. The results show that natural fracture spacing changes affect more than 62% of the recoverable energy. Natural fracture spacing constitutes an essential parameter, hence a relationship between natural fracture spacing and recoverable energy; while natural fracture spacing increases, the recoverable energy decreases due to poor natural fracture connections to extract the hot fluid from the reservoir to the wellbore.

The Red River (Ordovician) formations in the Williston Basin of Northwestern North Dakota are reservoirs with a particular characteristic; these reservoirs have more than 140°C. Hence those kinds of reservoirs are cataloged as Sedimentary Enhanced Geothermal Systems (SEGS).

We used temperature well logs from some wells in the Red River formations and geomechanical data to develop a geothermal reservoir model. The Red River Formation, which lies between 12790 and 13380 ft depth and the permeability is 0.1-38 mD, is a viable site for installing SEGS. Some rock properties, such as natural fractures, matrix porosity, permeability, rock thermal conductivity, and heat capacity, should be analyzed to determine the optimal reservoir model using data analysis, statistical, and process improvement tools to generate a SEGS. This study develops an integrated reservoir model of the Red River formation, using statistical data analysis to determine which parameter affects the enthalpy that the Red River formation will provide as a significant SEGS power system.