This document provides guidelines on full-depth reclamation using a combination of foamed asphalt and an active filler, typically portland cement or lime. The following aspects are discussed: Project selection, Mix design, Structural design, Construction. The references provide a list of other commonly used guidelines.

Energy extraction from subsurface reservoirs is important for addressing the increasing energy demand and environmental concerns such as global warming. However, the characterization of subsurface reservoirs, particularly reservoirs dominated by fracture networks remains a challenge due to the lack of means to directly observe subsurface processes. This study explores the feasibility and efficacy of characterizing fracture flow and transport processes in an enhanced geothermal system (EGS) testbed through stochastic tracer modeling. There are two enabling factors that allow application of stochastic modeling to characterize a subsurface reservoir. First, an abundance of geological and geophysical measurements enables the development of a high-fidelity and well-constrained fracture network model. Second, high-performance computing (HPC) allows running massive realizations efficiently. Six conservative tracer tests were stochastically modeled and produced satisfactory realizations that successfully reproduce field tracer recovery data from each tracer test. The evolution of flow and transport processes in the fracture network was then analyzed from these satisfactory realizations. The present study demonstrates that stochastic tracer modeling on a high-fidelity fracture network model is feasible and can provide important insights regarding flow and transport characteristics in subsurface fractured reservoirs.

This document provides a distillation of learnings of the EGS Collab (Collab) project for several topics of interest requested by the Utah Frontier Observatory for Research in Geothermal Energy (FORGE) project. This document was written following the completion of Collab Experiment 1, but prior to beginning Collab Experiment 2. Recommendations offered herein are perspectives offered by Collab scientists in support of FORGE. The United States has an enormous indigenous renewable energy potential from enhanced geothermal systems (EGS). To realize this potential, the US Department of Energy (DOE) Geothermal Technologies Office (GTO) has made significant investments into research to eliminate impediments to developing EGS. Two major current projects are Collab initiated in 2017, and the FORGE project initiated with a site selection process in 2015. Collab is a collaborative multi-national-lab, university, and commercial entity research endeavor bringing together a team of skilled and experienced subsurface process modeling, monitoring, and experimentation researchers and engineers to focus on intermediate-scale EGS reservoir creation processes and related model validation in crystalline rock. Collab is utilizing readily accessible underground facilities to refine the understanding of rock mass response to stimulation using experiments on the order of 10 m scale under EGS-relevant stress conditions. Experimental results from Collab stimulation, flow, tracer, and thermal tests are being used to validate coupled thermal-hydrological-mechanical-chemical (THMC) modeling approaches applicable to EGS. Collab is also testing and improving conventional and novel field monitoring tools. The project focuses on understanding and predicting permeability enhancement and evolution in crystalline rock. This focused research includes creating sustained and distributed permeability for heat extraction by generating new fractures that complement existing natural fractures. FORGE has the mission of establishing an EGS field test site that enables cutting-edge research and testing for EGS technology to identify a replicable, commercial pathway to EGS. The FORGE team is developing the EGS field test site near Milford, Utah. The two projects differ in some attributes, including spatial scale (Collab / FORGE - 10-m / reservoir), access to the rock (short boreholes and nearby instruments / deep wells and standard field geophysical equipment), environmental conditions (cool rock at reasonable stress / hot rock at reasonable stress), focus (direct investigation / development of a testbed and management of a research program), and project structure (integrated team / science and engineering framework supporting many individual research teams). The specific topics of this report requested by FORGE are: 1) data processing, annotation, and integration; 2) processing seismic data, 3) connecting geophysics, fractures, and flow systems, and 4) challenges of modeling fractured flow systems. Each of these topics is briefly discussed in this executive summary and covered in substantially greater detail in the main document. Numerous conference and journal papers are cited in the report that provide additional information. Many lessons have been learned. Some are not directly transferable to FORGE, but the underpinnings of the lessons may be applicable.

Heat recovery from an enhanced geothermal system (EGS) is a complex process involving heat transport in both fracture networks and rock formations. A comprehensive understanding of and the ability to model the underlying heat transport mechanisms is important for the success of EGS commercialization but remains challenging in practice due to the generally insufficient characterization of EGS reservoirs. In the present study, we analyze an extensively monitored intermediate-scale EGS field experiment performed in a well-characterized testbed. The high-resolution, high-quality measurements from the field experiment enable the development of a high-fidelity model incorporating a well-constrained fracture network. Based on the field experiment, we investigate the complex heat transport processes in an EGS-relevant environment and validate the capability of a numerical approach in simulating these inherently coupled heat transport processes. A series of numerical simulations were performed to study the effects of different heat transport mechanisms, including thermal convection with fracture flow, thermal conduction in rock formations, and the Joule-Thomson effect. The agreement of thermal responses between field measurements and simulation results indicates that our numerical approach can appropriately model the heat transport processes pertaining to heat recovery from EGS reservoirs.

The final version of the above article was posted prematurely on 16 July 2021, owing to a technical error. The final, corrected version of record will be made fully available at a later date.

Fracture injection/shut-in tests are commonly used to measure the state of stress in the subsurface. Injection creates a hydraulic fracture (or in some cases, opens a preexisting fracture), and then the pressure after shut-in is monitored to identify fracture closure. Different interpretation procedures have been proposed for estimating closure, and the procedures sometimes yield significantly different results. In this study, direct, in-situ strain measurements are used to observe fracture reopening and closure. The tests were performed as part of the EGS Collab project, a mesoscale project performed at 1.25 and 1.5 km depth at the Sanford Underground Research Facility. The tests were instrumented with the SIMFIP tool, a double-packer probe with a high-resolution three-dimensional borehole displacement sensor. The measurements provide a direct observation of the fracture closure signature, enabling a high-fidelity estimate of the fracture closure stress (ie, the normal stress on the fracture). In two of the four tests, injection created an opening mode fracture, and so the closure stress can be interpreted as the minimum principal stress. In the other two tests, injection probably opened preexisting natural fractures, and so the closure stress can be interpreted as the normal stress on the fractures. The strain measurements are compared against different proposed methods for estimating closure stress from pressure transients. The shut-in transients are analyzed with two techniques that are widely used in the field of petroleum engineering – the ‘tangent’ method and the ‘compliance’ method. In three of the four tests, the tangent method significantly underestimates the closure stress. The compliance method is reasonably accurate in all four tests. Closure stress is also interpreted using two other commonly-used methods – ‘first deviation from linearity’ and the method of (Hayashi and Haimson, 1991). In comparison with the SIMFIP data, these methods tend to overestimate the closure stress, evidently because they identify closure from early-time transient effects, such as near-wellbore tortuosity. In two of the tests, microseismic imaging provides an independent estimate of the size of the fracture created by injection. When combined with a simple mass balance calculation, the SIMFIP stress measurements yield predictions of fracture size that are reasonably consistent with the estimates from microseismic. The calculations imply an apparent fracture toughness 2-3x higher than typical laboratory-derived values.