Enhanced Geothermal Systems (EGS), engineered deep rock heat exchangers, are often touted for their massive untapped renewable energy potential. However, numerous technical and financial challenges ...must be overcome before the technology is widely deployed. To be viable, EGS must engineer a heat sweep by circulating fluid through a large volume of rock while minimizing fast pathways that create thermal short circuits. Here, we propose a new horizontal EGS well design with partially bridging multi-stage hydraulic fractures (Fig. 1b) to improve heat extraction from hot dry rock (HDR). In order to increase fluid circulation through the stimulated reservoir volume (SRV) between fractures, the hydraulic fractures are created in an alternating pattern between injection and production wells, with each stopping short of connecting the second well. To test this design, we developed a thermal-hydro-mechanical (THM) coupling model and investigated heat extraction performance and reservoir stress evolution during operation. Fractures were modelled as opening contact surfaces with system stresses evolving according to thermal, poroelastic, and fracture opening effects. Based on the model, the temperature performance of proposed EGS design model is compared with the commonly used fully bridging EGS design model (Fig. 1a). An investigation of thermal performance showed that the proposed design obtains higher production temperatures and delays thermal breakthrough by several years compared to a fully-bridging fracture design. It also results in a greater degree of secondary stimulation of the SRV as cold fluids are forced further into the rock matrix. These results indicate that the partially-bridging fracture design is a promising candidate for practical EGS implementation.
•A novel partially bridging horizontal EGS well design model is proposed.•Multistage fractures are modelled as opening contact surfaces.•A 3D THM coupling model with system stress evolving according to thermal, poroelastic, fracture opening effects is presented.•Heat extraction performance, reservoir stress evolution of the proposed design are compared with fully bridging design model.•The separate role of thermoelastic, poroelastic, fracture opening effects in EGS reservoir stress evolution are discussed.
Thermal cycling induced micro-cracks can change the physical and mechanical properties of geothermal energy reservoir, which may influence the heat and mass transfer performance as well as the ...stability of the reservoir. In this study, thermal cycling treatment was carried out on granite over the temperature range of 20–300 °C with 1–20 thermal cycles. The results show that thermal cycling promotes the initiation and propagation of intergranular and intragranular cracks, which are evenly distributed in all directions. With increase in the number of thermal cycles, the crack density (Pl) increases, resulting in increased permeability (K). The path of seepage passage is mainly between the mineral particles. Moreover, the critical crack propagation radius (rc) of rock decreases with increasing cracking degree, which leads to the decrease in rock fracture resistance. In particular, the fracture toughness (Keff) of granite decreases most when it is subjected to 1–5 thermal cycles. Water-cooling thermal cycling and cooling rate can significantly affect the micro-crack evolution, permeability and ability of granite to resist fracture. The changes in mechanical and physical properties observed in this work can provide basic theoretical reference for the rational selection of geothermal energy mining methods and process parameters, as well as the study of reservoir stability evaluation.
•We introduce a simulation framework to assess the techno-economic feasibility of EGS district heating using a partially-bridging multi-fracture design.•Stimulated permeability, reservoir ...temperature, and fracture number are essential for maximising constant stable thermal power output.•Applied to the Qiabuqia EGS field, China, we show that a 14-fracture EGS is economically viable, heating greater than 28 000 dwellings over 20 years with emissions reduction of 1.5 Mt CO2.
Here, we investigate the techno-economic feasibility of an enhanced geothermal system (EGS) for direct heating, using a new horizontal-well with partially-bridging fractures design. To do this, we developed numerical models to evaluate reservoir dynamics and output, economic performance and environmental benefits. This model was used to determine optimal thermal power output over a 20-year operation for given parameters. Then, we ranked the effects of key parameters on commercial and environmental outcomes. Stimulated reservoir permeability and reservoir temperature are the primary determinants of economic viability, with lesser effects due to reservoir depth and fracture number. A case study was developed for the Qiabuqia EGS, China, capturing uncertainty through sampling of a surrogate model. We show that a 14-fracture design was economically preferred, providing up to 2.8 km2 of indoor heating over 20 years with considerable emissions offset (∼1.5 Mt of CO2) and economic benefit (24–185 M$).
Hydraulic fracturing aimed at building a network of fractures in hot dry rock (HDR) often induces strong seismic risks. However, few studies have focused on seismic risks, and methods for increasing ...the stimulated reservoir volume (SRV) while reducing seismic risks have not been adequately studied. Therefore, based on the full hydro-mechanical coupling, a seismic-fracturing model with a discrete fracture network (DFN) was developed, and the methods for increasing the SRV while reducing the seismic risk of horizontal wells at the FORGE site in Utah were discussed. The results showed that the fluid channeling effect between each segment is the main reason for the uneven fracture growth in the horizontal well. Additionally, the spatiotemporal evolution of fracturing-induced seismicity shows that the two ends (L1 and L3) of the horizontal well tend to form high stress concentrations and induce larger seismic events than the middle (L2). To solve these problems, a novel hydraulic fracturing method with a differentiated-stage, variable-injection volume suitable for the multi-fracturing of horizontal wells in HDR was proposed. With the application of the method, the SRV of the horizontal well was improved effectively, and the average magnitude induced by hydraulic fracturing was also decreased by 15.4%.
The EGS Collab project acquired continuous active-source seismic monitoring (CASSM) data before, during, and after hydraulic stimulations at the first testbed at the depth of 4850 ft (1478 m) at the ...Sanford Underground Research Facility in Lead, South Dakota, for monitoring fracture creation and evolution. CASSM acquisition was conducted using 24 hydrophones, 18 accelerometers, and 17 piezoelectric sources within four fracture-parallel wells and two orthogonal wells. 3D anisotropic traveltime tomography and anisotropic elastic-waveform inversion of the campaign cross-borehole seismic data show that the rock within the stimulation region is a heterogeneous horizontal transverse isotropic medium. We use these inversion results as the initial models and apply 3D anisotropic first-arrival traveltime tomography and 3D anisotropic elastic-waveform inversion to the CASSM data acquired after each stimulation in May, 2018 and December, 2018. We observe the spatiotemporal evolution of seismic velocities and anisotropic parameters caused by hydraulic fracture stimulations, showing the regions of rock alternation caused by hydraulic fracture stimulation.
This article reviews laboratory experimental studies on hydraulic fracturing under triaxial and true triaxial stress conditions in crystalline rock for geothermal purposes, and places particular ...focus on the stimulation of Enhanced Geothermal Systems. First, parameters that influence hydraulic fracture initiation and propagation and breakdown pressure are reviewed and discussed. The parameters including micro-structure, fluid viscosity, injection rate, and fluid infiltration, and stress conditions are identified as the key controlling factors in hydraulic fracture growth in hard rock. Second, innovative injection schemes, such as cyclic and fatigue hydraulic fracturing, are reviewed because they show advantages both in fracture network creation in granite and in mitigating and controlling induced seismicity via fluid injection. Third, this review includes fracture-inspection techniques, non-destructive methods of acoustic emission (AE) monitoring and X-ray computed tomography (CT), and microscopic observations used for quantifying the efficiency of injection protocols. In addition to AE parameters, such as AE event rate and source location, we emphasize the importance of in-depth AE analysis on the failure mode and radiated seismic energy. X-ray CT and microscopic observation enable fractures in the rock volume to be quantified, and thereby lead to a better understanding the mechanism behind hydraulic fracturing. Combined measurements of AE and CT yield insights into the complex process of hydraulic fracture and permeability enhancement. The discussion section is enriched with diagrams that connect the injection rate and the resulting fluid infiltration zone and fracture process zone, granite-specific hydraulic fracturing behavior, and practical upscaling elements for potential field applications in geothermal fields.
To improve the microearthquake detection capability during the 2012 Newberry enhanced geothermal system (EGS) demonstration, we applied two advanced earthquake detection algorithms to the continuous ...seismic data. These techniques have the power to detect microearthquakes within the seismic noise that would otherwise be missed using traditional techniques. We compare the efficacy of a matched field processing algorithm with a multi-channel waveform correlation algorithm. Both methods identified between 60%–80% more events, with each slightly outperforming the other in different seismic noise contexts. Incorporating either template-matching technique into a seismic processing routine could therefore effectively increase the sensitivity of the seismic network without changes to the existing hardware. Additionally, lowering the magnitude of completeness would significantly add to the number of events that could subsequently inform future decision making at the reservoir level.
An estimation of the Enhanced Geothermal System's theoretical technical potential for the Iberian Peninsula is presented in this work. As a first step, the temperature at different depths (from ...3500 m to 9500 m, in 1000 m steps) has been estimated from existing heat flow, temperature at 1000 m and temperature at 2000 m depth data. From the obtained temperature-at-depth data, an evaluation of the available heat stored for each 1 km thick layer between 3 and 10 km depth, under some limiting hypotheses, has been made. Results are presented as the net electrical power that could be installed, considering that the available thermal energy stored is extracted during a 30 year project life. The results are presented globally for the Iberian Peninsula and separately for Portugal (continental Portugal), Spain (continental Spain plus the Balearic Islands) and for each one of the administrative regions included in the study. Nearly 6% of the surface of the Iberian Peninsula, at a depth of 3500 m has a temperature higher than 150 °C. This surface increases to more than 50% at 5500 m depth, and more than 90% at 7500 m depth. The Enhanced Geothermal System's theoretical technical potential in the Iberian Peninsula, up to a 10 km depth (3 km–10 km) and for temperatures above 150 °C, expressed as potential installed electrical power, is as high as 700 GWe, which is more than 5 times today's total electricity capacity installed in the Iberian Peninsula (renewable, conventional thermal and nuclear).
•The temperatures at 3500 m–9500 m depth for the Iberian Peninsula are calculated.•An estimation of the EGS theoretical technical potential for the Iberian Peninsula is presented.•The EGS theoretical technical potential for the Iberian Peninsula is as high as 700 GWe.
•THM modeling of deformation of rock matrix and joints with nonlinear stiffness.•Contraction of rock decreases flow impedance of fracture, leading to flow channeling and reduction of energy ...production.•Rapid pore pressure diffusion in rock matrix causes the deformation of reservoir at early-stages.•Thermal effects are more pronounced for higher permeability rocks.•Deformation of ground surface is an important effect resulting from geothermal energy production.
Hot water extraction and cold water injection into an underground geothermal reservoir cause mechanical deformation of rock matrix and rock joints/fractures. That leads to alteration of hydraulic transmissivity. To study the evolution of reservoir transmissivity we performed coupled Thermo-Hydro-Mechanical (THM) simulations using a robust code called Finite Element for Heat and Mass Transfer (FEHM) for a 3-D domain with a single fracture connecting the injection and production wells. Rock fracture was modeled as a thin equivalent porous medium. We established dynamic relations between the properties of the equivalent porous medium and fracture aperture. In this paper we discuss the alteration of fracture aperture due to heat extraction. The channeling of flow between injection and production wells by THM effects causes faster temperature drawdown and reduces energy production. The model also predicted fracture opening near injection well and closure at far field locations. We also simulated the aperture alteration for different joint stiffness, thermal expansion coefficients and rock matrix permeabilities. Increase in rock matrix permeability not only causes the leakage of injected water but also increases matrix contraction due to cooling and therefore the aperture growth. Additionally we reported the effect of thermo-poro-elastic deformation on the expansion and contraction of the formation for different reservoir properties. We established that in the early-stages the compaction/expansion of the formation was controlled by pore pressure change but in the late-stage it was controlled by thermal contraction.
Tau protein has important physiological functions at both presynaptic and postsynaptic terminals. Pathological tau species are also associated with synaptic dysfunctions in several neurodegenerative ...disorders, especially Alzheimer's disease. To understand tau distribution inside synaptic compartments, super-resolution imaging is required. Here, we describe a facile protocol to immobilize and image brain synaptosomes without aggregation artefacts, by substituting the standard fixative paraformaldehyde with ethylene glycol bis(succinimidyl succinate) (EGS). Super-resolution imaging of tau proteins is achieved through three-color direct stochastic optical reconstruction microscopy (dSTORM). Tau protein is found to colocalize with synaptic vesicles as well as postsynaptic densities.
