The Role of Geomechanics in Geothermal Reservoirs Engineering

Itasca Consulting Group organized a one-day workshop at the 2014 ARMA conference held at the University of Minnesota on June 1st. The workshop was well attended with over 50 participants. By bringing together geothermal reservoir engineers, numerical modelers and rock mechanics experts, the workshop goals were to identify the challenges and issues associated with the engineering of geothermal reservoirs, provide a platform for the exchange of knowledge and communication and aid in the development of innovative ideas and solutions. The workshop was chaired by Dr. Azadeh Riahi and co-chaired by Dr. Branko Damjanac and Dr. Will Pettitt. The topics that were presented and discussed follow.

Perspective on Geothermal Energy and Federal RD&D Strategy

Dan King
AAAS Science & Technology Policy Fellow
U.S. Department of Energy

Dr. King is an AAAS Science & Technology Policy Fellow at the U.S. Department of Energy in the Geothermal Technologies Office within the Office of Energy Efficiency and Renewable Energy. Dan supports the management of a diverse portfolio of geothermal RD&D projects. He is also highly involved with a DOE-wide tech team on subsurface technology and engineering and other cross-cutting initiatives. Prior to joining DOE, Dan was an NSF post-doctoral fellow at Penn State University where his research focused on frictional constitutive behavior of fault gouge. Dan has a Ph.D. in geophysics from the University of Minnesota, a M.Sc. in geology from the University of Vermont and a B.S. in geology from Brown University.

ABSTRACT

The U.S. Department of Energy (DOE) Geothermal Technologies Office (GTO) is committed to developing and deploying a portfolio of innovative technologies for clean, domestic power generation. The Geothermal Technologies Office researches, develops and validates innovative and cost-competitive technologies and tools to locate, access and develop geothermal resources in the United States. Geothermal energy is more vital today than ever-it supplies clean, renewable power around the clock, emits little or no greenhouse gases and takes a very small environmental footprint to develop. By developing, demonstrating and deploying innovative technologies, GTO's efforts are helping stimulate the growth of the geothermal industry within the renewable energy sector and encouraging quick adoption of technologies by the public and private sectors. Our vision is to provide the nation with an abundant, clean and renewable baseload energy source. Our goals are to accelerate near-term hydrothermal growth and to secure the future with enhanced geothermal systems (EGS).

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Geothermal Energy and Associated Geomechanics Challenges

Douglas A. Blankenship 
Manager of Geothermal Research
Sandia National Laboratories

Mr. Blankenship is the manager of the Geothermal Research Department at Sandia National Laboratories, a group that focuses on R&D activities related to geothermal well construction, and reservoir completion and operations. Mr. Blankenship has more than 30 years of experience in the development, testing and monitoring of drilled and mined openings in subterranean environments, about 10 years with Sandia's geothermal program and the remainder in the private sector supporting mining, oil & gas and civil industries. He has been involved in wide variety of technical and managerial efforts; including basic R&D associated with the development of high­ temperature drilling tools (e.g., Diagnostics-While-Drilling); the planning, development and supervision of  grassroots drilling exploration programs; in-situ stress measurements and well testing in deep boreholes; coordination and development of underground drilling programs; design and installation of instrumentation systems for underground and surface excavations, and numerical analyses of drilled and mined excavations in geologic materials. Mr. Blankenship was a member of the National Laboratory response team supporting the Secretary of Energy during the Deepwater Horizon event. He has a B.S. in Civil Engineering and a M.S. in Geological Engineering from the University of California at Berkeley.

ABSTRACT

The geomechanics challenges of geothermal reservoir development are exemplified by issues associated with engineered/enhance geothermal system (EGS). The issues are many and not separate from those facing any endeavor into the subsurface. However, there exist some challenges that are exasperated by the application space, not the least of which are the elevated temperatures and flow rates needed for an economically viable geothermal system. The talk will focus on developing a conversation on the common geomechanics challenges, as well those of somewhat unique to EGS development. 

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Hydrothermal and Sedimentary Reservoirs: Classification and Challenges

William D. Gosnold        
Professor of Geophysics
Harold Hamm School of Geology and Geological Engineering
University of North Dakota
Interim Chair of the Department of Petroleum Engineering

Dr. Gosnold earned a baccalaureate degree in Physics from the State University of West Georgia in 1971 and the Doctor of Philosophy degree in Geophysics from Southern Methodist University in 1977. Dr. Gosnold is Custodian of the Global Heat Flow Data Base of the International Heat Flow Commission (IHFC). He is currently Director of the Petroleum Research Education and Entrepreneurship Center (PREEC) at the University of North Dakota. He has conducted research on heat flow and geothermal resources since 1979. He is a member of Sigma Xi, the American Geophysical Union, the European Geosciences Union, the Geological Society of America and The American Association of Petroleum Geologists. In 2006 he received the highest award of the University of North Dakota: Chester Fritz Distinguished Professor.

ABSTRACT

The key components of a geothermal reservoir are temperature, fluid availability and permeability. The first-order control of temperature is tectonic setting, and geologically young regions with near-surface magma bodies and active tectonics have greatest potential for geothermal power. Fluid circulation through a reservoir is critical for access to the thermal energy and a two-variable matrix of temperature and fluid availability provides a useful map of geothermal potential. Fluid availability depends on rock permeability and porosity and on the hydrologic setting of the reservoir. Three types of hydrothermal system capable of power generation: vapor-dominated, hot water and moderate-temperature water, range in temperature from greater than 200 °C to 100 °C. These systems occur in the western states in several different settings including near magma bodies and associated with young fault systems. Intermediate to low temperature geothermal reservoirs occur in sedimentary basins east of the Rockies and present potential for development with oil and gas settings.

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Hydraulic Fracturing and Stimulation of EGS Reservoirs

Andrew Bunger
Assistant Professor University of Pittsburgh

Professor Bunger is an Assistant Professor in the University of Pittsburgh's Department of Civil and Environmental Engineering. He recently joined the University of Pittsburgh after spending 10 years in Melbourne, Australia working in the Geomechanics Group within the Commonwealth Scientific and Industrial Research Organization (CSIRO). His research interests include the mechanics of hydraulic fractures, coupled fluid-shale interaction, and the emplacement dynamics of magma-driven dykes and sills. He holds a Ph.D. in Geological Engineering from the University of Minnesota.

ABSTRACT

The reservoir stimulation concept for EGS is intended to generate a uniform stimulation of a volume of reservoir in the vicinity of a long, open-hole section of well. The EGS experience is that stimulation tends to be far more localized than is intended. This presentation will critically review the approach of low pressure injection of low viscosity fluid into a long section of open-hole completion in light of complimentary work in both the petroleum industry and the modeling of magma-driven dyke swarms in Nature.

Discrete Element Modeling of Fractured Reservoirs

Branko Damjanac           
Principal Engineer
Itasca Consulting Group, Inc.

Dr. Damjanac has undergraduate degree in civil engineering from Belgrade University, Serbia and Ph.D. in Civil Engineering (with specialization in geotechnical engineering) from University of Minnesota. He is a principal engineer at Itasca Consulting Group. Inc. who has almost 25 years of experience in the design and analysis of surface and underground excavations in soils and rocks related to civil (e.g., dams and bridges), mining (underground and surface mines) and nuclear waste projects. His expertise is in development and application of numerical models to problems of deformation and stability of geomaterials under static and dynamic loading conditions and simulation of complex transport processes (fluid flow and heat transport) in geomaterials coupled with deformation. In recent years he is involved in analysis of rock mass treatment by fluid injection with application to oil and gas, mining and geothermal industries. He worked on the development of numerical code 3DEC and the implementation of the lattice approach in numerical codes for slope stability analysis and simulation of hydraulic fracturing.

ABSTRACT

The Distinct Element Method (DEM) represents a rock mass as an assembly of blocks (polygonal or polyhedral). Contacts between blocks correspond to discontinuities (i.e., fractures or joints) that can exhibit non-linear mechanical behavior, including slip and opening. If flow in rock fractures is approximated using lubrication equation, the coupled hydromechanical DEM models can be used for simulation of fractured rock mass treatment by fluid injection. However, this approach has limited capability to simulate fracture propagation. The synthetic rock mass (SRM) concept overcomes this limitation. In SRM, the bonded particle model (BPM), which is an assembly of circular or spherical particles bonded to each other, represents deformation and damage of intact rock. If pre-existing discontinuities are represented in the BPM, the resulting model, referred to as SRM, has the capability of simulating of hydraulic fracturing in naturally fractured reservoirs in which general hydraulic fracture trajectory is the model solution resulting from fracturing of the intact rock and sliding and opening of pre-existing joints. Examples of application of DEM models to simulation of fractured rock mass treatment by fluid injection (in oil and gas and geothermal industries) will be presented.

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Three-Dimensional Simulation of Geothermal Drilling

Masami Nakagawa
Associate Professor
Colorado School of Mines

Dr. Nakagawa (Ph.D. Theoretical Mechanics, Cornell University} is associate professor at Colorado School of Mines (CSM), has been working on various aspects of particle mechanics including the Moon Dust project with NASA for their space exploration directory. Most               recently,              he           leads      the effort            to           create   geothermal educational/research programs at CSM and in the state of Colorado. He was awarded funds from the Governor's Energy Office to develop geothermal energy program at. CSM in collaboration with the School for Renewable Energy Science in Iceland. In May of 2009, he created the Geothermal Academy with its main goal of becoming the focal institution to help increase the chance of a successful development of geothermal energy through EGS, energy conservation through direct use and applications of: ground source heat pumps. In December of 2009, the U.S. Department of Energy (USDOE) awarded a fund for the Geothermal Academy. Dr. Nakagawa is involved in several USDOE funded geothermal projects at CSM. He also held a joint appointment with the geothermal program at the National Renewable Energy Laboratory (NREL). For the last three years, he has been giving short courses, lectures and workshops on sustainable natural resources development (including geothermal) for the Peruvian, Bolivian, Indonesian and Japanese government.

ABSTRACT

The future geothermal resources development requires better understanding of technical challenges associated with the drilling process into hard, dry rock at higher pressure and temperature.  In simulating drilling process, we use a discrete element approach because it is well suited to model rock fragmentation processes. The Bonded Particle Model (BPM) developed by Itasca can provide the basic features for modeling rock samples and drilling mechanisms such as crushing, chipping and spalling. The BPM uses  a  cement-like  bond that  connects  element  particles  to  model  macroscopic properties of a rock sample  of interest and its fragmentation  processes. We have created a model rock that represents mechanical properties of typical granite. Our preliminary three-dimensional drilling simulation results show a unique potential of this approach to model the drilled tracks, fracture patters and the failure modes.

Microseismic Monitoring: Practical Aspects.  Field Instrumentation, Data Acquisition and Interpretation.  Case Study from Newber Volcano EGS Demonstration.

Trenton Cladouhos
Senior Vice President R&D
AltaRock Energy, Inc.

Dr. Cladouhos holds a B.Sc. degree in Geology from Stanford University and a Ph.D. in Geological Sciences from Cornell University. His research specialty, both in academia and industry, has been the mechanics and fluid flow in fractured and faulted rock. At AltaRock Energy, he is responsible for developing, improving and testing AltaRock's, technologies for improving production in both EGS and conventional geothermal reservoirs. He manages the geologic and geophysical aspects of AltaRock EGS and stimulation projects. Trenton was the primary author on the Newberry Induced Seismicity Mitigation Plan, and has managed the design, procurement, installation and operation of AltaRock's microseismic arrays for monitoring EGS reservoir growth at various projects.

ABSTRACT

Hydraulic stimulation of an existing deep, hot well on the west flank of Newberry Volcano was performed in the fall of 2012 as part of the Newberry Volcano EGS Demonstration. Prior to the stimulation, a microseismic array (MSA) was designed and installed to map the growth of the EGS reservoir and as a key component of an Induced Seismicity Mitigation Plan. The real-time data collected by the MSA was used to inform ' the stimulation operations. Advanced  processing  after  the  stimulation  resulted  in moment  tensors  to  study  source  mechanisms,  improved  hypocenter  locations  by relative   relocation   methods,   an   improved   shear-velocity   model   using  seismic Interferometry, and a more complete catalog using matched field processing.

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Microseismicity and Geomechanical Modeling for Fracture Network Engineering

William Pettitt  
General Manager and Principal Geophysicist
Itasca Consulting Group, Inc.

Dr. Pettitt is Vice President, General Manager and Principal Geophysicist at Itasca Consulting Group, Inc.  (ICG) in Minneapolis, USA.  Dr.  Pettitt  specializes  in the application of passive seismology techniques to induced seismicity focused on the microseismic and acoustic emission imaging of rock fractures and the integration of these  techniques  with  numerical  models,  principally  for  petroleum,  geothermal, mining and radioactive waste and laboratory testing markets. Will has managed field operations, equipment installations, real-time data acquisition, post-processing and interpretation studies for oil & gas, geothermal and nuclear waste acquisitions. He is  the  creator  of  InSite,  a  leading  commercial  microseismic  software  system  with  licensees that include companies and universities expert in the field deployment and processing for  microseismic  monitoring, and  has developed  unique  high-frequency data acquisition equipment for fracture monitoring across a range of applications. Dr. Pettitt is based in Minneapolis and leads the business and technological development of ICG's products, consultancy and support services, and supervises an internationally respected team of engineers, software developers and supporting staff.

ABSTRACT

Geomechanical computer modeling is being used as an interpretation tool for induced seismicity and microseismic imaging within the resource engineering community. The reason for this interest is due to the capabilities that geomechanical models offer for accurately determining the causality of the microseismicity in the field. Induced seismicity has become a popular tool for describing fracturing within a rock volume as it can delineate the abundance, position, magnitude and failure mechanism of the rock disturbance that creates the emission. Microseismic information of any form, whether it relates to injection induced seismicity in petroleum, C02 or geothermal industries, or whether it relates to a mechanical response in mining or civil engineering, therefore can tell us accurately what has happened within the monitored volume. However, understanding the causes of the activity requires an interpretation of the data that often depends on complex rock mechanical behavior and properties.

In order for geomechanical models to provide this interpretative tool for microseismic observations, they need to reflect accurately the geological parameters, stress conditions and physical behavior of the rock deformation processes. Geomechanical models designed for rock engineering purposes are becoming ever more sophisticated due to the combined increases in computational efficiency and our developing understanding of rock mechanical processes. The computer models now can model reservoir scale (or mine scale) in three dimensions with thousands of joints in complex patterns and include thermal-hydromechanical coupling and chemical processes. Joints can open and slide and the rock can fracture and fragment. An integrated approach with geomechanical models unlocks microseismics to be a useful tool for optimizing engineering design (e.g., using a Fracture Network Engineering approach for hydraulic fracturing) and provides the capability to research the causes of induced seismicity (e.g., from waste-water injection). This paper provides an introduction to Fracture Network Engineering and a discussion of some of the challenges associated with its implementation.

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Three-Dimensional Discrete Fracture (DFN) Analysis of Heat and Mass Flow for Enhanced Geothermal (EGS) Systems

William Dershowitz        
Technical Director
FracMan Technology Group Golder Associates Inc.

Dr. William Dershowitz is the Technical Director of Golder Associates' FracMan Technology Group in Redmond, WA. He has been developing and applying the Discrete Fracture Network (DFN) approach for civil, geothermal, mining and oil projects since 1976 when he joined Prof. Einstein's Rock Mechanics group at MIT. Dr. Dershowitz has analyzed geomechanics, flow and transport for geothermal developments in New Zealand, California, Nevada and Texas.  He is currently carrying out EGS feasibility research together with Golder colleagues Dr. Thomas Doe, Dr. Aleta Finilla and Dr. Rob Mclaren and sponsored by the U.S. Department of Energy.

ABSTRACT

This paper explores the importance of Discrete Fracture Network intensity, connectivity and spatial structure for development of enhances geothermal (EGS) systems. This paper includes geological and geomechanical simulation, geometric fracture connectivity analyses, dual porosity DFN heat and mass flow and transport modeling, and  hydraulic fracture simulation.

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