Robert Podgorney

Challenges for numerical modeling of enhanced geothermal systems.

A recent guest editorial by Wood (2009) pointed out the potential of enhanced geothermal systems (EGS) as a future source of “green” energy and suggested that EGS offers research opportunities for hydrogeologists seeking to become involved in the world’s energy future. Although EGS may have a bright future as a sustainable, low-carbon emission energy source, significant technical challenges must be overcome before this promising energy resource can be commercially viable. Because pilot EGS projects in the United States face very different economic constraints than current European projects, there is a real need to make technological advances to improve the return on capital investment. In this article, we amplify on Wood’s excellent editorial by describing some of the challenges that exist for the simulation of EGS.

Unsaturated Flow Through a Small Fracture -- Matrix Network: Part 2. Uncertainty in Modeling Flow Processes

Simulations of flow and transport in variably saturated fractured rock generally assume equilibrium conditions between the fractures and the porous matrix, leading to predictions that are dominated by a diffusive process. Contrary to these predictions, an increasing body of evidence suggests that fracture-dominated flow, under nonequilibrium conditions between the fractures and porous matrix, occurs frequently in field and laboratory settings. Flow processes, such as fluid cascades and flow path switching, are often observed in laboratory experiments, but are generally not captured by diffusion-based conceptual and numerical models. Many of these processes are assumed to be averaged out at some representative elemental volume scale; however, anecdotal evidence from field experiments conducted at various scales of investigation suggest that this may not be the case. Comparison of experimental observations with numerical simulations illustrates at least two potential problems with standard equivalent continuum and discrete fracture conceptual models of unsaturated fractured and porous media flow. First, such models tend to overestimate the strength of interaction between the fracture and matrix domains. Second, model calibration may allow diffusion-based models to accurately reproduce experimental observations without providing a complete description of the physics governing the system. Failure to incorporate convective transport, reduced fracture–matrix interaction, and other sub-grid-scale processes in models of flow in fractured porous media may result in erroneous descriptions of system behavior.

Physical constraints on geologic CO2 sequestration in low-volume basalt formations

Deep basalt formations within large igneous provinces have been proposed as target reservoirs for carbon capture and sequestration on the basis of favorable CO 2 -water-rock reaction kinetics that suggest carbonate mineralization rates on the order of 10 2 –10 3 d. Although these results are encouraging, there exists much uncertainty surrounding the influence of fracture-controlled reservoir heterogeneity on commercial-scale CO 2 injections in basalt formations. This work investigates the physical response of a low-volume basalt reservoir to commercial-scale CO 2 injections using a Monte Carlo numerical modeling experiment such that model variability is solely a function of spatially distributed reservoir heterogeneity. Fifty equally probable reservoirs are simulated using properties inferred from the deep eastern Snake River Plain aquifer in southeast Idaho, and CO 2 injections are modeled within each reservoir for 20 yr at a constant mass rate of 21.6 kg s –1 . Results from this work suggest that (1) formation injectivity is generally favorable, although injection pressures in excess of the fracture gradient were observed in 4% of the simulations; (2) for an extensional stress regime (as exists within the eastern Snake River Plain), shear failure is theoretically possible for optimally oriented fractures if S h ≤ 0.70S V ; and (3) low-volume basalt reservoirs exhibit sufficient CO 2 confinement potential over a 20 yr injection program to accommodate mineral trapping rates suggested in the literature.

Unsaturated Flow through a Small Fracture–Matrix Network

Simulations of flow and transport in variably saturated fractured rock generally assume equilibrium conditions between the fractures and the porous matrix, leading to predictions that are dominated by a diffusive process. Contrary to these predictions, an increasing body of evidence suggests that fracture-dominated flow, under nonequilibrium conditions between the fractures and porous matrix, occurs frequently in field and laboratory settings. Flow processes, such as fluid cascades and flow path switching, are often observed in laboratory experiments, but are generally not captured by diffusion-based conceptual and numerical models. Many of these processes are assumed to be averaged out at some representative elemental volume scale; however, anecdotal evidence from field experiments conducted at various scales of investigation suggest that this may not be the case. Comparison of experimental observations with numerical simulations illustrates at least two potential problems with standard equivalent continuum and discrete fracture conceptual models of unsaturated fractured and porous media flow. First, such models tend to overestimate the strength of interaction between the fracture and matrix domains. Second, model calibration may allow diffusion-based models to accurately reproduce experimental observations without providing a complete description of the physics governing the system. Failure to incorporate convective transport, reduced fracture–matrix interaction, and other sub-grid-scale processes in models of flow in fractured porous media may result in erroneous descriptions of system behavior.

Deep geothermal: The ‘Moon Landing’ mission in the unconventional energy and minerals space

Deep geothermal from the hot crystalline basement has remained an unsolved frontier for the geothermal industry for the past 30 years. This poses the challenge for developing a new unconventional geomechanics approach to stimulate such reservoirs. While a number of new unconventional brittle techniques are still available to improve stimulation on short time scales, the astonishing richness of failure modes of longer time scales in hot rocks has so far been overlooked. These failure modes represent a series of microscopic processes: brittle microfracturing prevails at low temperatures and fairly high deviatoric stresses, while upon increasing temperature and decreasing applied stress or longer time scales, the failure modes switch to transgranular and intergranular creep fractures. Accordingly, fluids play an active role and create their own pathways through facilitating shear localization by a process of time-dependent dissolution and precipitation creep, rather than being a passive constituent by simply following brittle fractures that are generated inside a shear zone caused by other localization mechanisms. We lay out a new theoretical approach for the design of new strategies to utilize, enhance and maintain the natural permeability in the deeper and hotter domain of geothermal reservoirs. The advantage of the approach is that, rather than engineering an entirely new EGS reservoir, we acknowledge a suite of creep-assisted geological processes that are driven by the current tectonic stress field. Such processes are particularly supported by higher temperatures potentially allowing in the future to target commercially viable combinations of temperatures and flow rates.

CO2 sequestration in basalt: Carbonate mineralization and fluid substitution

Geological sequestration of carbon dioxide in deep reservoirs may provide a large-scale option for reducing the emissions of this gas into the atmosphere. The effectiveness of sequestration depends on the storage capacity and stability of the reservoir and risk of leakage into the overburden. Reservoir rocks can react with a CO2-water mixture, potentially resulting in the precipitation of minerals in the available matrix pore space and within pre-existing fractures. This induced mineralization may form internal seals that could help mitigate the leakage of CO2 into the overburden. For basaltic host rocks, carbonic acid partially dissolves minerals in the host rock, such as the calcium plagioclase mineral, freeing various cations (e.g., Ca2+ and Mg2+) for later precipitation as carbonate cements (Gislason et al., 2010).

Assessment of a Hybrid Continuous/Discontinuous Galerkin Finite Element Code for Geothermal Reservoir Simulations

FALCON (Fracturing And Liquid CONvection) is a hybrid continuous/discontinuous Galerkin finite element geothermal reservoir simulation code based on the MOOSE (Multiphysics Object-Oriented Simulation Environment) framework being developed and used for multiphysics applications. In the present work, a suite of verification and validation (V&V) test problems for FALCON was defined to meet the design requirements, and solved to the interests of enhanced geothermal system modeling and simulation. The intent for this test problem suite is to provide baseline comparison data that demonstrates the performance of FALCON solution methods. The test problems vary in complexity from a single mechanical or thermal process, to coupled thermo-hydro-mechanical processes in geological porous medium. Numerical results obtained by FALCON agreed well with either the available analytical solutions or experimental data, indicating the verified and validated implementation of these capabilities in FALCON. Whenever possible, some form of solution verification has been attempted to identify sensitivities in the solution methods, and suggest best practices when using the FALCON code.

Benchmark Problems of the Geothermal Technologies Office Code Comparison Study

............................................................................................................................................. iii Summary ............................................................................................................................................. v Acknowledgments ............................................................................................................................. vii Acronyms and Abbreviations ............................................................................................................. ix 1.0 Introduction .............................................................................................................................. 1.1 1.1 Approach ......................................................................................................................... 1.3 1.1.1 Study Objectives .................................................................................................. 1.3 1.1.2 Study History and Structure ................................................................................. 1.3 1.2 Participants and Codes .................................................................................................... 1.5 1.3 Benchmark Problems ...................................................................................................... 1.9 1.3.1 Benchmark Problem 1: Poroelastic Response in a Fault Zone (PermeabilityPressure Feedback) ............................................................................................... 1.9 1.3.2 Benchmark Problem 2: Shear stimulation of randomly oriented fractures aby injection of cold water into a thermo-poro-elastic medium with stress-dependent permeability ........................................................................................................ 1.10 1.3.3 Benchmark Problem 3: Fracture opening and sliding in response to fluid injection .............................................................................................................. 1.11 1.3.4 Benchmark Problem 4: Planar EGS fracture of constant extension, pennyshaped or thermo-elastic aperture in impermeable hot rock .............................. 1.12 1.3.5 Benchmark Problem 5: Amorphous Silica dissolution/precipitation in a fracture zone .................................................................................................................... 1.13 1.3.6 Benchmark Problem 6: Injection into a fault/fracture in thermo-poroelastic rock1.14 1.3.7 Benchmark Problem 7: Surface deformation from a pressurized subsurface fracture ............................................................................................................... 1.15 1.4 Comparison Standard .................................................................................................... 1.16 2.0 Governing and Constitutive Equations .................................................................................... 2.1 2.1 Heat Transfer Modeling .................................................................................................. 2.1 2.2 Fluid Flow Modeling ....................................................................................................... 2.2 2.2.1 Fracture Transmissivity ........................................................................................ 2.2 2.3 Rock Mechanics Modeling .............................................................................................. 2.3 2.3.1 Continuum Geomechanics ................................................................................... 2.4 2.3.2 Discrete Fracture Geomechanics .......................................................................... 2.5 2.3.3 Joint Models ....................................................................................................... 2.10 2.4 Geochemical Reaction Modeling .................................................................................. 2.12 2.4.1 Aqueous Reaction Rates ..................................................................................... 2.14 3.0 Numerical Solution Schemes ................................................................................................... 3.1 3.1 Sequential Schemes ......................................................................................................... 3.1 3.2 Iterative Schemes ............................................................................................................ 3.1

A field sampling strategy for semivariogram inference of fractures in rock outcrops

The stochastic continuum (SC) representation is one common approach for simulating the effects of fracture heterogeneity in groundwater flow and transport models. These SC reservoir models are generally developed using geostatistical methods (e.g., kriging or sequential simulation) that rely on the model semivariogram to describe the spatial variability of each continuum. Although a number of strategies for sampling spatial distributions have been published in the literature, little attention has been paid to the optimization of sampling in resource- or access-limited environments. Here we present a strategy for estimating the minimum sample spacing needed to define the spatial distribution of fractures on a vertical outcrop of basalt, located in the Box Canyon, east Snake River Plain, Idaho. We used fracture maps of similar basalts from the published literature to test experimentally the effects of different sample spacings on the resulting semivariogram model. Our final field sampling strategy was based on the lowest sample density that reproduced the semivariogram of the exhaustively sampled fracture map. Application of the derived sampling strategy to an outcrop in our field area gave excellent results, and illustrates the utility of this type of sample optimization. The method will work for developing a sampling plan for any intensive property, provided prior information for a similar domain is available; for example, fracture maps or ortho-rectified photographs from analogous rock types could be used to plan for sampling of a fractured rock outcrop.

Introduction to Selected Contributions from GeoProc, The 5th International Conference on Coupled Thermo-Hydro-Mechanical-Chemical Process in Geosystems Held in Salt Lake City, Utah, from February 25–27, 2015

This special issue of Rock Mechanics and Rock Engineering contains papers that are a representative sample of contributions from GeoProc, The 5th International Conference on Coupled Thermo-Hydro-Mechanical-Chemical Process: Petroleum and Geothermal Reservoir Geomechanics and Energy Resource Extraction. This triennial multidisciplinary international meeting sponsored by the International Society for Rock Mechanics (ISRM) was designed to bring scientists and engineers together from different backgrounds to address common scientific issues for a wide range of coupled natural and engineering phenomena, largely centered on energy resource extraction. GeoProc is a focal point for professionals and students from a wide variety of backgrounds, such as civil, geological, mining, geophysical and petroleum engineering, who to seek to understand and find solutions for coupled thermal-hydro-mechanical-chemical problems. The manuscripts presented address a range of topics ranging from shale plasticity to hydraulic fracturing, and high-performance computing approaches, to simulate such phenomenon. The papers presented include field-relevant study, laboratory experiments, and investigations empowered by high-performance computing. The papers contained in this special issue were selected by members of the GeoProc organizing committee based on the quality of the technical content of the symposium papers. All papers were expanded and rewritten, then re-reviewed for this special issue.