Geothermal Energy Site Research Articles

3D modeling of deep borehole electromagnetic measurements with energized casing source for fracture mapping at the Utah Frontier Observatory for Research in Geothermal Energy

AbstractWe present a 3D numerical modelling analysis evaluating the deployment of a borehole electromagnetic measurement tool to detect and image a stimulated zone at the Utah Frontier Observatory for Research in Geothermal Energy geothermal site. As the depth to the geothermal reservoir is several kilometres and the size of the stimulated zone is limited to several 100 m, surface‐based controlled‐source electromagnetic measurements lack the sensitivity for detecting changes in electrical resistivity caused by the stimulation. To overcome the limitation, the study evaluates the feasibility of using a three‐component borehole magnetic receiver system at the Frontier Observatory for Research in Geothermal Energy site. To provide sufficient currents inside and around the enhanced geothermal reservoir, we use an injection well as an energized casing source. To efficiently simulate energizing the injection well in a realistic 3D resistivity model, we introduce a novel modelling workflow that leverages the strengths of both 3D cylindrical‐mesh‐based electromagnetic modelling code and 3D tetrahedral‐mesh‐based electromagnetic modelling code. The former is particularly well‐suited for modelling hollow cylindrical objects like casings, whereas the latter excels at representing more complex 3D geological structures. In this workflow, our initial step involves computing current densities along a vertical steel‐cased well using a 3D cylindrical electromagnetic modelling code. Subsequently, we distribute a series of equivalent current sources along the well's trajectory within a complex 3D resistivity model. We then discretize this model using a tetrahedral mesh and simulate the borehole electromagnetic responses excited by the casing source using a 3D finite‐element electromagnetic code. This multi‐step approach enables us to simulate 3D casing source electromagnetic responses within a complex 3D resistivity model, without the need for explicit discretization of the well using an excessive number of fine cells. We discuss the applicability and limitations of this proposed workflow within an electromagnetic modelling scenario where an energized well is deviated, such as at the Frontier Observatory for Research in Geothermal Energy site. Using the workflow, we demonstrate that the combined use of the energized casing source and the borehole electromagnetic receiver system offer measurable magnetic field amplitudes and sensitivity to the deep localized stimulated zone. The measurements can also distinguish between parallel‐fracture anisotropic reservoirs and isotropic cases, providing valuable insights into the fracture system of the stimulated zone. Besides the magnetic field measurements, vertical electric field measurements in the open well sections are also highly sensitive to the stimulated zone and can be used as additional data for detecting and imaging the target. We can also acquire additional multiple‐source data by grounding the surface electrode at various locations and repeating borehole electromagnetic measurements. This approach can increase the number of monitoring data by several factors, providing a more comprehensive dataset for analysing the deep‐localized stimulated zone. The numerical analysis indicates that it is feasible to use the combination of the energized casing and downhole electromagnetic measurements in monitoring localized stimulated zone at large depths.

Reservoir and Fracture Characterization for Enhanced Geothermal Systems: A Case Study Using Multifractured Wells at the Utah Frontier Observatory for Research in Geothermal Energy Site

Summary For enhanced geothermal systems (EGS), multistage hydraulic fracturing along a deviated or horizontal well is a key technology used to create a high-conductivity fracture network between injection and production wells in deep, low-permeability geothermal reservoirs. The purpose of the created fracture network is to allow for the efficient transfer of fluid, heated by the geothermal reservoir, from the injection to the production well; therefore, well spacing (between injection and production wells) and hydraulic fracturing must be designed not only to promote connectivity between well pairs but also to mitigate thermal short-circuiting and thermal breakthrough. Analysis of post-fracture pressure decay (PFPD) data measured after each stage of a hydraulic fracturing treatment can be used to provide critical reservoir and fracture parameters required for well and hydraulic fracturing design optimization; this method provides a low-cost alternative and complementary approach to in-situ observation techniques, such as core-through experiments, fiber optics, or image logs in offset wells. Until now, PFPD has primarily been applied to multifractured horizontal wells (MFHWs) completed in low-permeability hydrocarbon reservoirs. The goal of this study is therefore to develop a methodology to estimate fracture and reservoir parameters using stage-by-stage PFPD data associated with EGS projects. An analytical model is proposed herein to estimate fracturing fluid efficiency, fracture length, average fracture aperture, average fracture conductivity, and reservoir permeability for different possible fracture geometries in EGS reservoirs. PFPD data collected for three hydraulic fracture stages in the injection well at the Utah Frontier Observatory for Research in Geothermal Energy (FORGE) site were analyzed to demonstrate the practical application of the proposed method. The results of this study indicate that, due to the presence of natural fractures in the target (granitic) reservoir, the hydraulic fracturing treatment (using slickwater) in the openhole section resulted in low fracturing fluid efficiency and small hydraulic fractures. In contrast, hydraulic fracturing treatments conducted in the perforated casedhole wellbore resulted in higher fracturing fluid efficiency and created larger hydraulic fractures even with smaller injected volumes. The results of the PFPD analysis were confirmed using a Formation MicroScanner image log and microseismic data collected during each stage of hydraulic fracturing.

Automatic classification with an autoencoder of seismic signals on a distributed acoustic sensing cable

This study probes the association between fluid injection in enhanced geothermal systems and certain kinds of seismicity that may result from hydraulic fracturing occurring at depth using unsupervised machine learning. In April and May 2019, a distributed acoustic sensing borehole array at the Frontier Observatory for Research in Geothermal Energy site near Milford, Utah recorded seismic data during hydraulic injection stimulation of a nearby well. Using an autoencoder, a type of deep neural network, we reduce the dimensionality of spectrograms of the detected signals to a lower-dimensional latent feature space with just nine dimensions. Next, Gaussian mixture model clustering is performed on this latent feature space, assigning each detected signal to one of 7 classes. For each signal class, we examine spatiotemporal distributions of the clustered results and find that total detections exhibit a bimodal distribution with respect to channel depth. The shallow mode occurs between 250 and 500 m, and the deep mode is centered around 750 m. In the temporal distribution, clustering results show the two best-clustered signal classes exhibit weak or no correlation with injection-related activities. More generally, we demonstrate the ability to discern not just when and where signals are detected, but also what kind, thus enabling rapid and targeted data exploration and providing constraints on source mechanisms.

Flowback Test Analyses at the Utah Frontier Observatory for Research in Geothermal Energy (FORGE) Site

In 2017 and 2019, injection testing was carried out in three zones in a vertical well in granite at the Frontier Observatory for Research in Geothermal Energy site near Milford, Utah, USA. In several injection cycles, flowback was implemented rather than shut-in. The goal was to explore an alternative to prolonged shut-in periods for inferring closure stress, formation compressibility, and formation permeability (permeability thickness product). The flowback procedures involved a cyclic flowback/shut-in, while pressure decreased. The flowback data are presented, and analyses are shown. The inferred closure stress(es) from flowback analyses are lower than for equivalent injection cycles that were strictly shut-in. Relatively high formation compressibility obtained from the flowback analyses indicates an extensive, fractured system. This study also includes numerical simulation of the flowback events. The numerical model shows that the rebound pressure is not necessarily the lower bound of the minimum principal stress. The signature of stiffness change can be identified as the process when the depletion mainly transitions from hydraulic fracture to natural fractures from numerical analysis. Overall, flowback potentially has advantages over shut-in because of the reduced time to closure.

Electromagnetic methods for exploration and monitoring of enhanced geothermal systems – A virtual experiment

We present the concept of virtual electromagnetic experiments, which aims at setting up and optimizing the monitoring design for a potential stimulated geothermal energy reservoir. The concept builds on existing 3D geologic models, which are processed, meshed, assigned realistic electrical parameters and used for our finite element simulations on unstructured tetrahedral grids. The available simulation codes are able to cope with the high degree of geometric complexity in close-to-reality scenarios.To demonstrate the procedure and the capabilities of our method, we show the model preparation and the simulation results for the transient electromagnetic method in the context of a prospective enhanced deep geothermal energy site at the fault ‘Roter Kamm’ near Schneeberg in Saxony/eastern Germany. We show the changes in the measuring signal caused by the stimulated fracture system in more than 5000m depth for a range of electrical resistivity contrasts, which represent different porosity, pore connectivity or fluid resistivity variations. When a borehole receiver is advantageously placed at depth, percentage changes in the measuring signal of 25% can be expected. Our study demonstrates, that the optimal positioning of source and receiver is crucial for a successful monitoring and may only be achieved with advanced simulation techniques.

Characteristics of microseismic events induced during hydraulic fracturing experiments at the Hijiori hot dry rock geothermal energy site, Yamagata, Japan

Microseismicity accompanying hydraulic injection experiments at the Hijiori hot dry rock site was monitored by a network of ten borehole seismic stations deployed at an average distance of 2 km from the injection well. While expanding hydraulic fractures are almost aseismic, they can induce microseismic events. These events are probably caused by shear failures induced by high pore fluid pressures occurring on planes of weakness in the rock surrounding the main hydraulic fracture. Thus we can use these induced events to locate the hydraulic fracture and follow its growth. Microseismic events induced during the 1988 hydraulic fracturing experiment with a high injection pressure were located near the injection point in the early stage of the experiment and clearly migrated towards the east and distributed along a vertical plane. The strike of seismicity is nearly parallel to the direction of the maximum principal stress. The vertical orientation and east–west strike of the seismic events are essentially coplanar with the caldera ring-fault structure in the southern portion of the Hijiori Caldera. This indicates that a preexisting fracture zone was being re-opened and developed in the direction of the maximum principal stress, although microseismic events were caused by shear failures. The space–time distribution of the microseismic events shows that the events migrated at a rate proportional to time to the power 2/3. Assuming that migration of events is attributed to fracture propagation, the propagation of the hydraulically stimulated fracture can be explained by one of two petroleum industry models tested. Seismicity accompanying the 1989 circulation test with a low injection pressure was diffuse and showed a seismic cloud. The permeability was estimated from the hypocenter migration as 10 −16 m 2, which is intermediate between the permeability of core samples of granodiorite taken from the production well and the permeability of fractured rocks obtained by an injection test between the injection well and the production well. It is therefore concluded that the seismic cloud accompanying the circulation test is due to the permeation of water into joints which slip when the effective stress is reduced by the increased pore fluid pressure accompanying the hydraulic injection. Microseismic events occur and migrate along those joints.

Induced microseismicity and reservoir growth at the Fjällbacka hot dry rocks project, Sweden

Although still relatively uncommon, microseismic detection is accepted as a valuable technique in monitoring large-scale hydraulic injection or extraction from rock at depth. Even through microseismicity may potentially provide information on rock mass behaviour that is unavailable from other remote techniques, the interpretation of microsiesmic activity is often restricted to the analysis of their locations. This restricted usage is primarily due to our current limited understanding of fundamental mechanisms of microseismic energy release and their relations to the mechanics of rock deformation. In this paper, microseismic data will be used to investigate the mechanism of reservoir growth during viscous fluid injection into a jointed granitic rock mass at the Fjällbacka Hot Dry Rock geothermal energy site in Sweden. It will be demonstrated that using simple seismological and rock mechanics interpretation techniques it is possible to obtain a valuable insight into the behaviour of a jointed rock mass during fluid injection. It will be shown through the combined interpretation of microseismic fault plane solutions and in situ stress data, using the simple rock mechanics concept of peak shear strength, that reservoir growth took place through the shear failure of shallowly dipping natural joints due to sufficiently elevated pore fluid pressures.

Stress control of seismicity patterns observed during hydraulic fracturing experiments at the Fenton Hill hot dry rock geothermal energy site, New Mexico : Fehler, M C Int J Rock Mech Min SciV26, N3/4, July 1989, P211–219

Seismicity accompanying hydraulic injections into granitic rock is often diffuse rather than falling along a single plane. This diffuse zone of seismicity cannot be attributed to systematic errors in locations of the events. It has often been asserted that seismicity occurs along preexisting joints in the rock that are favorably aligned with the stress field so that slip can occur along them when effective stress is reduced by increasing pore fluid pressure. A new scheme for determining orientations and locations of planes along which the microearthquakes occurred was recently developed. The basic assumption of the method, called the three point method, is that many of the events fall along well defined planes; these planes are often difficult to identify visually in the data because planes of many orientations are present. The method has been applied to four hydraulic fracturing experiments conducted at Fenton Hill as part of a hot dry rock geothermal energy project. While multiple planes are found for each experiment; one plane is common to all experiments. The ratio of shear to normal stress along planes of all orientations is calculated using a best estimate of the current stress state at Fenton Hill. The plane common to all experiments has the highest ratio of shear to normal stress acting along it, so it is the plane most likely to slip. The other planes found by the three point method all have orientations with respect to current principal stresses that are favorable for slip to occur along preexisting planes of weakness. These results are consistent with the assertion that the rock contains pre-existing joints which slip when the effective stress is reduced by the increased pore fluid pressure accompanying the hydraulic injection. Microearthquakes occur along those planes that are favorably aligned with respect to the current stress field.

Stress control of seismicity patterns observed during hydraulic fracturing experiments at the Fenton Hill hot dry rock geothermal energy site, New Mexico

Abstract Seismicity accompanying hydraulic injections into granitic rock is often diffuse rather than falling along a single plane. This diffuse zone of seismicity cannot be attributed to systematic errors in locations of the events. A new scheme for determining orientations and locations of planes along which the microearthquakes occurred was recently developed. The basic assumption of the method, called the three-point method, is that many of the events fall along well-defined planes; these planes are often difficult to identify visually in the data because planes of many orientations are present. The method has been applied to four hydraulic fracturing experiments conducted at Fenton Hill as part of a hot dry rock geothermal energy project. While multiple planes are found for each experiment, one plane is common to all experiments. The ratio of shear to normal stress along planes of all orientations is calculated using a best estimate of the current stress state at Fenton Hill. The plane common to all experiments has the highest ratio of shear to normal stress acting along it, so it is the plane most likely to slip. The other planes found by the three-point method all have orientations with respect to current principal stresses that are favourable for slip to occur along pre-existing planes of weakness. These results are consistent with the assertion that the rock contains pre-existing joints which slip when the effective stress is reduced by the increased pore fluid pressure accompanying the hydraulic injection. Microearthquakes occur along those planes that are favourably aligned with respect to the current stress field.

Theory of thermal recovery from a spherically stimulated hot dry rock reservoir

A conceptual model is presented to describe thermal recovery from a semi‐infinite hot dry rock (HDR) geothermal reservoir containing an equidimensional permeable zone. Transient behavior may be represented uniquely by five dimensionless parameters. Variation in production temperature TD with time tD is influenced by reservoir throughput QD thermal porosity ϕD and depth ratio a/z. Of these, only throughput QD exercises significant control on transient performance, the parameter being directly proportional to reservoir circulation rate and inversely proportional to the effective radius of the stimulated zone. Steady production temperature TD is indexed to throughput QD and depth ratio a/z, only. Steady production temperatures are always highest for a host medium bounded by a proximal constant temperature surface and lowest for an insulated boundary. Boundary effects are insignificant for reservoir burial depths up to an order of magnitude greater than the reservoir radius. A threshold behavior in time (tDQD) is evident for very large reservoir throughput (QD). This bounding behavior describes, in dimensionless time, the maximum rate at which thermal depletion may occur. This state is evident for large dimensionless throughput magnitudes (QD) corresponding directly with high circulation rates within the reservoir. Predictions compare favorably with results from a 300‐day circulation test at the Fenton Hill Geothermal Energy Site, New Mexico.