A coupled thermal-hydro-mechanical (THM) model based on the combined finite-discrete element method (FDEM) is presented for simulating rock cracking driven by multi-physics. The THM model contains ...three parts: a fracture-pore mixed seepage model, a heat transfer model, and a fracture mechanics calculation model. By combining any two of the above three models, a coupled thermal-mechanical (TM) model, a coupled hydrothermal (TH) model, and a coupled hydromechanical (HM) model are constructed. Then, the TM model, TH model, and HM model are combined to build the THM model, which is implemented in a GPU parallel multiphysics finite-discrete element software, namely MultiFracs. Finally, we use this THM model to study the hydraulic fracturing process of hot dry rock. The simulation results indicate that in addition to the primary fracture perpendicular to the direction of the minimum in situ stress, branching fractures along the direction of the minimum in situ stress are also produced during the hydraulic fracturing process. The proposed THM model can simulate heat and fluid transfer in fractured reservoirs, crack initiation, propagation, and intersection.
The MEET Special Issue aims at showing the gains in geothermal energy that can be achieved using a variety of techniques, depending on the geological setting of the underground. Among the list of ...exploitation concepts, enhanced geothermal systems (EGS) are particularly interesting, as their application is much less dependent of the underground setting, allowing, in turn, a large geographical deployment and market penetration in Europe. The challenges of EGS are multiple in terms of investment costs, the testing of novel reservoir exploitation approaches with an inherent risk of induced seismicity, and the presence of aggressive geothermal brines, damaging infrastructures. The conversion of oil wells or coproduction of heat or electricity together with oil is also addressed. This Special Issue summarizes the output of the H2020 MEET project based on laboratory experiments, geological field works on high-quality analogues, advanced reservoir modeling, the development of a decision-maker tool for investors and specific demonstration activities, such as chemical stimulation or the innovative monitoring of deep geothermal wells, and the production of electrical power via small-scale binary technology tested in various geological contexts in Europe.
In this study, novel analytical solutions are presented for predicting and optimizing energy (heat and power) extraction from an idealized single hot-dry rock (HDR) or hot-wet rock (HWR) through a ...multistage hydraulically fractured horizontal well doublet (MHFWD). The solutions are based on a two-dimensional analytical model that accounts for heat transfer mechanisms through an MHFWD completed in a homogeneous nearly impermeable matrix of the HDR/HWR system. The method of Laplace transformation is used to obtain the solutions. The solutions are inverted numerically and analytically to compute the fracture water outlet temperature and the temperature inside the matrix in the real-time domain. The analytical solutions presented in the study have not been presented elsewhere. The fluid outlet and matrix temperature distributions computed from the analytical model were validated against the results of a commercial simulator. The derived analytical solutions were integrated to convert thermal energy to electric power, and a sensitivity study was performed to investigate the effect of certain parameters, such as fracture height, number, half spacing, injection rate, and injection temperature. In addition, the study provides insights and guidelines to predict, design, and optimize heat recovery and electricity production from an EGS system stimulated by an MHFWD.
•Novel analytical solutions are given for predicting fracture outlet temperatures.•The solutions are for a multi-fractured horizontal-well doublet in hot-dry rocks.•The solutions account for geothermal gradient between the wells.•The solutions can be usef for predicting heat extraction and power generation.•A sensitivity study using the solutions shows the effect of the parameters.
Sarcopenia (a decline of skeletal muscle mass) has been identified as a predictor of poor postoperative outcomes. The impact of sarcopenia in emergency general surgery (EGS) remains undetermined. The ...aim of this study was to evaluate the association between sarcopenia and outcomes after EGS.
A 3-y (2012-15) review of all EGS patients aged ≥45 y was presented to our institution. Patients who underwent computer tomography–abdomen were included. Sarcopenia was defined as the lowest sex-specific quartile of total psoas index (computer tomography–measured psoas area normalized for body surface area). Patients were divided into sarcopenic (SA) and nonsarcopenic. Primary outcome measures were in-hospital complications, hospital-length of stay h-LOS, intensive care unit-length of stay, adverse discharge disposition, and in-hospital mortality. Our secondary outcome measures were 30-d complications, readmissions, and mortality.
Four hundred fifty-two patients undergoing EGS were included. Mean age was 58 ± 8.7 y, and 60% were males. Hundred thirteen patients were categorized as SA. Compared to nonsarcopenic, SA patients had higher rates of minor complications (28% versus 17%, P = 0.01), longer hospital-length of stay (7d versus 5d, P = 0.02), and were more likely to be discharged to skilled nursing facility/Rehab (35% versus 17%, P = 0.01). There was no difference between the two groups regarding major complications, intensive care unit-length of stay, mortality, and 30-d outcomes. On regression analysis, sarcopenia was an independent predictor of minor complications (OR 1.8 1.6-3.7) and discharge to rehab/SNIF (OR: 1.9 1.5-3.2). However, there was no association with major complications, mortality, 30-d complications, readmissions, and mortality.
Sarcopenia is an independent predictor of minor postoperative complications, prolonged hospital-length of stay, and an adverse discharge disposition in patients undergoing EGS. Identifying SA EGS patients will improve both resource allocation and discussion about the patient's prognosis between physicians, patients, and their families.
•A fully coupled thermo-poroelastic finite element model with DFN is developed.•The model is used to simulate and analyze reservoir stimulation in the Newberry EGS Demonstration (Phase ...2.2).•Injection profile, permeability evolution and induced micro-seismic events are simulated.•The results agree with field observations.•Injection induced stress state changes around the injection well are observed.
Heat production from an enhanced geothermal system (EGS) requires a successful stimulation. Cold water is injected into hot rocks to enhance the heat reservoir permeability. During injection, pore pressure, temperature and the stress field in the reservoir change significantly. Most hot dry rock masses are to some extent naturally fractured at various scales. Fractures could dilate and slip in shear possibly and propagate as a result of stress changes, increasing reservoir permeability. The objective of this work is to analyze the response of a naturally fractured heat reservoir to water injection. A fully coupled thermo-poroelastic finite element model is developed and used to describe the interaction between fluid flow, rock deformation, and heat transfer within the fractured rock. A fracture network model is generated based on field data and implemented into the coupled model. The model is used to simulate and analyze reservoir stimulation in the Newberry EGS Demonstration. In particular, the Phase 2.2 stimulation is modeled using field data on the fracture network, in-situ stress and laboratory data on rock and fracture properties. The simulated injection profile, the evolution of permeability and induced micro-seismic events agree with field observations. Simulation results also show that injection induced stress state changes occur around the injection well explaining the noted differences between the stress regimes obtained prior and subsequent to stimulation.
CO2 may be a better heat transmission fluid than water for Enhanced Geothermal Systems (EGS). The advantages and disadvantages of these two kinds of EGS are the focus of this study. The water and ...CO2-EGS system models including simple subsurface heat transfer and flow models and a surface energy conversion system model were designed based on the reservoir grade and the ambient temperatures. The results indicate that the operating parameters including the injection pressure, turbine outlet pressure and reservoir stimulated area should be optimized to match the actual CO2-EGS conditions. CO2-EGS produce more power than water-EGS for reservoirs with low recoverable thermal energies due to less irreversible losses compared to ORC or flash cycles for water-EGS. However, high resistance losses caused by high mass flow rates degrade the CO2-EGS performance; thus, the water-EGS has better performance than CO2-EGS for larger energy content reservoirs.
•Comparing the performance of CO2-EGS and water-EGS for various conditions.•Presenting the scope of applications for these two kinds of EGS systems.•Cooling after compression before the CO2 is injected improves CO2-EGS performance.•There is an optimum recoverable thermal energy content for CO2-EGS.
Chemico-microstructural and mineralogically-induced time-dependent mechanical alterations of the reservoir rock found in the peripheral zone of a CO2-based EGS is experimentally investigated. A ...series of uniaxial compressive and triaxial (under 30 MPa constant confining pressure with different temperatures; 25-300 °C) strength tests on granite specimens reacted in ScCO2+water medium for different time periods was performed by coupling acoustic emission (AE) technique. The results were compared with those for intact and water-reacted rock specimens in order to better understand this weakening phenomenon. Scanning electron microscopy (SEM) was also incorporated to evaluate the microstructural and mineralogical alterations.
The results indicated that the mineralogical structure of Harcourt granite rock specimens was mainly altered by the dissolution of silicate minerals and plagioclase phase feldspar minerals. In addition, enhanced precipitation of secondary clay minerals including kaolinite, smectite, and illite was observed in the ScCO2+water-reacted rock specimens. The average compressive strength of ScCO2/water-reacted granite specimens found to be reduced with the increase of reaction period. Furthermore, 3.81%, 5.08%, 5.66% and 0.52% reductions in failure strength resulted under 30 MPa confining pressure in 6 months ScCO2/water-reacted granite specimens at 25 °C, 100 °C, 200 °C and 300 °C temperatures, respectively, compared to intact rock values at these temperatures. The post-failure analysis also revealed that the ScCO2/water reaction significantly alters the post-failure behaviour due to micro-mineralogically-induced weakening inside the rock pore structure. It was concluded that enhanced mineral precipitation and dissolution influence in time-dependent degradation of the surrounding rock mass which influence in long-term operation of the geothermal system.
•Microstructural and mineralogically induced mechanical alterations in CO2-based EGSs.•Time dependant mineralogical and microstructural alterations in rock specimens were observed.•Changes in mechanical properties were evaluated.•Tri-axial tests were performed under high temperature and pressure conditions.•Post failure analysis of rock specimens was conducted.
Accurate and efficient simulation of fluid and heat flow in fractures has long been a topic of interest for fractured reservoirs, e.g., enhanced geothermal systems (EGS) and unconventional oil/gas ...formations. In this paper, we propose a flexible and effective modeling approach, the extended embedded discrete fracture model (XEDFM), to simulate fluid and heat flow in fractured reservoirs with 3-D non-planar fracture networks. Compared with the conventional embedded discrete fracture model (EDFM), the XEDFM possesses two major merits: (1) separation of fracture discretization and matrix gridding provides maximum flexibility in handling fractures with complex geometry/topology, regardless of the resolution of the matrix grid; and (2) the combination of connection-list strategy and the concept of non-neighboring connection facilitates the construction of fluid-heat flux between the fracture and the matrix/fracture. With systematically validated XEDFM, the impacts of fracture roughness and heat extraction strategy on hydrothermal behaviors and heat mining efficiency are investigated. Another example introduces a workflow for design and modeling of 3-D non-planar fracture networks, with which the performance of horizontal and vertical wells in tapping heat energy from EGS are explored. This work presents a flexible and effective approach for modeling fluid/heat transfer accurately in 3-D non-planar fractures, and provides a set of framework and efficient algorithms for non-planar fractures design, discretization and simulation, establishing the foundation to build and simulate models with complicated fractures.
•Proposed an extended Embedded Discrete Fracture Model to model nonplanar fractures.•The fluid-heat flow in fracture is verified against analytical/numerical solutions.•Introduced an efficient workflow for designing/modeling 3-D non-planar fractures.•Impacts of fracture roughness in enhanced geothermal systems (EGS) are investigated.•Performance of numerous heat extraction strategies associated with EGS is evaluated.
Enhanced geothermal system (EGS) is often envisioned to consist of at least two wells spaced sufficiently apart and connected by hydraulic fractures that serve as flow paths. All the flow paths must ...be utilized efficiently to ensure the system is operated at its highest potential. However, building an efficient and sustainable EGS is a complicated process as the fluid always chooses the path of least resistance, which can lead to uneven flow distribution. This study focuses on several critical parameters related to well designs, which can potentially allow for optimized flow distribution. An analytical model (written in Python) is developed based on Kirchhoff's law to calculate the flow distribution in any doublet EGS. Wellbore perforations, the completed wellbores and the fractures are simulated as resistance while the fluid is simulated as a current analog. The model solves the pressure at each node, analogous to voltage, using pipe flow equations and Darcy's law. Three different doublets EGS designs (parallel, anti-parallel and non-parallel) were simulated using the model, and a detailed sensitivity study was performed. Anti-parallel doublet systems perform the best in terms of better fluid distribution and at a lower frictional loss. It was also observed that the flow distribution in a doublet system can be affected by fracture permeability, perforation size and flow rate. Higher permeability fracture leads to poor fluid distribution. Smaller perforation size improves the fluid distribution, but it leads to huge frictional losses. Low flow rates also help with optimized fluid distribution but would eventually lead to low heat output.
•Reservoir properties and working fluid types for EGS are summarized based on related global projects.•The dissolution or precipitation process of common minerals and its influencing factors are ...described in details.•Salt precipitation in saline aquifers and its effect on reservoir permeability are illustrated.•The impacts of working fluid-rock interactions on reservoir permeability, mechanical properties and heat extraction capacity are summarized.•The prospects of CO2-EGS and some limitations of current studies have been discussed.
Geothermal energy from Hot Dry Rock (HDR) with poor permeability is recognized as a potential future energy source, and it can be exploited by Enhanced Geothermal System (EGS) technology. In this paper, geochemical mechanisms of typical working fluids interacting with the primary rock types of EGS reservoirs, and their impacts on reservoir properties are summarized. While traditional EGS primarily utilizes granite reservoirs, the technology has been gradually extended to include sandstone and carbonate rock formations. Regarding the working fluid, water is currently the only one that has been put into practice. Mineral dissolution and precipitation and salt precipitation are the two dominant geochemical mechanisms during the heat extraction process using water or CO2. These mechanisms are influenced by fluid pH, reservoir temperature, pressure, and flow rate. The dissolution of feldspar minerals plays an important role in increasing permeability and the availability of cations for the precipitation of secondary minerals. Carbonate minerals are often the quickest to respond to changes in fluid chemistry induced by CO2 injection, and the re-precipitation of carbonates is triggered by increasing temperature and pH. It is difficult to predict variations in permeability due to the involvement of many factors, including particle sizes and salt precipitation. The mechanical properties of rocks are significantly weakened following interactions with working fluids. The weakening effect on rock mechanical properties is more pronounced when CO2 is injected into water-saturated formations. Although water-rock interactions can change the thermal conductivity of the rock, the characteristics of fracture networks appear to be of greater importance because working fluids mainly flow through fractures, and maximizing heat extraction from EGS depends strongly on effective circulation. Further economic evaluations for CO2-EGS are necessary to confirm the economic viability of using CO2 as a working fluid. Additionally, the THMCB (Thermo-Hydro-Mechanical-Chemical-Biological) coupling effects during water/CO2 interactions should be comprehensively studied based on both long-term field tests and truly integrated numerical models.
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