Doublet System Research Articles

Cooling-induced reactivation of distant faults during long-term geothermal energy production in hot sedimentary aquifers

Deep geothermal energy (DGE) represents an opportunity for a sustainable and carbon-free energy supply. One of the main concerns of DGE is induced seismicity that may produce damaging earthquakes, challenging its widespread exploitation. It is widely believed that the seismicity risk can be controlled by using doublet systems circulating water to minimize the injection-induced pressure changes. However, cold water reinjection may also give rise to thermal stresses within and beyond the cooled region, whose potential impacts on fault reactivation are less well understood. Here, we investigate by coupled thermo-hydro-mechanical modeling the processes that may lead to fault reactivation in a hot sedimentary aquifer (HSA) in which water is circulated through a doublet. We show that thermal stresses are transmitted much ahead of the cooled region and are likely to destabilize faults located far away from the doublet. Meanwhile, the fault permeability mainly controls the fault reactivation timing, which entails the importance of employing appropriate characterization methods. This investigation is crucial for understanding the mechanisms controlling induced seismicity associated with DGE in a HSA and allows the success of future DGE projects.

Lab-Scale Permeability Enhancement by Chemical Treatment in Fractured Granite (Cornubian Batholith) for the United Downs Deep Geothermal Power Project, Cornwall (UK)

A hydrothermal doublet system was drilled in a fault-related granitic reservoir in Cornwall. It targets the Porthtowan Fault Zone (PTF), which transects the Carnmenellis granite, one of the onshore plutons of the Cornubian Batholith in SW England. At 5058 m depth (TVD, 5275 m MD) up to 190 °C were reached in the dedicated production well. The injection well is aligned vertically above the production well and reaches a depth of 2393 m MD. As part of the design process for potential chemical stimulation of the open-hole sections of the hydrothermal doublet, lab-scale acidification experiments were performed on outcrop analogue samples from the Cornubian Batholith, which include mineralised veins. The experimental setup comprised autoclave experiments on sample powder and plugs, and core flooding tests on sample plugs to investigate to what degree the permeability of natural and artificial (saw-cut) fractures can be enhanced. All samples were petrologically and petrophysically analysed before and after the acidification experiments to track all changes resulting from the acidification. Based on the comparison of the mineralogical composition of the OAS samples with the drill cuttings from the production well, the results can be transferred to the hydrothermally altered zones around the faults and fractures of the PTF. Core Flooding Tests and Autoclave Experiments result in permeability enhancement factors of 4 to >20 and 0.1 to 40, respectively. Mineral reprecipitation can be avoided in the stimulated samples by sufficient post-flushing.

Comparison of Thermodynamic Performances in Three Geothermal Power Plants Using Flash Steam

Abstract Three geothermal systems, including single-flash, double-flash, and double-flash connected turbine flash geothermal power plants, are compared in terms of electrical power production and exergy efficiency. In the double-flash connected turbine (double-T) geothermal electrical power production systems, the outlet stream from the first steam turbine is recovered in the mixing chamber and combined with the vapor product of the second separator. The thermodynamic model for the single-flash, double-flash, and double-T geothermal systems is developed using energy and exergy balances for each component of the systems. From the thermodynamic model, the optimum flash chambers pressures, at which the electrical power production is a maximum, can be determined. It is found that, for an input geothermal source temperature of 230 °C and an input geothermal water mass flowrate of 230 kg/s, the optimum pressures for the first flash chamber are 300 kPa, 350 kPa, and 350 kPa for the single-flash, double-flash, and double-T geothermal systems, respectively. The electrical power produced in these systems at their corresponding optimum flashing pressures, respectively, are 16,000 kW, 19,500 kW, and 20,600 kW. Also, for the single-flash, double-flash, and double-T geothermal systems, the exergy efficiency at the optimum flash chamber pressures are found to be 44.2%, 47.1%, and 48.5%, respectively.

Performance analysis of pinnate horizontal well in enhanced geothermal system

• Feasibility of enhanced geothermal system with pinnate horizontal wells is studied. • Total heat production is improved by 21.4% with pinnate horizontal wells. • Effect of wells structure on heat extraction performance is evaluated. • Reliable layout of pinnate horizontal well is presented. The well layout is crucial for improving the utilization of hot dry rock geothermal resources. In present work, a three-dimensional enhanced geothermal system (EGS) with pinnate horizontal well is proposed. Furthermore, the effect of pinnate horizontal well structure on the heat extraction performance of present EGS is numerically investigated. For scCO 2 -based EGS and water-based EGS, the stable heat extraction state and a competitive heat production can be obtained by present EGS. Compared with the conventional EGS with doublet system, the production temperature and the total heat production of EGS with pinnate horizontal well increase by 21.54 K and 21.4 %, respectively. The heat extraction performance of EGS with pinnate horizontal well is positively correlated with the main well bore number and length. In comparison, the branch well structure and number have less effect on the heat extraction performance. Consequently, main wellbore number with 3 or main wellbore length with 300 m can be taken as criterion for the design of the main wellbore. Besides, the branch well with staggered pattern, inclination angel of 60° and the number of 4, is proved to be the optimum arrangement.

Prospects for development of geothermal energy in Lviv region

The data of geological exploration works carried out in Lviv region allow us to speak about the significant energy potential of geothermal waters in this region. At present, six bore-wells have been discovered in which the temperature of geothermal waters reaches 120oC at a depth of more than 3000 m. However, today the energy potential of geothermal waters of Lviv region is not used for both technical and economic reasons. This article, analyze the most common schemes for the use of geothermal resources based on the experience of countries with developed geothermal energy For geothermal systems with parameters typical of Lviv region, the greatest effect can be achieved by using doublet systems, which consist of two wells: suction and injection, and the resulting heat energy can be used to heat a wide group of consumers.

Simulation Study of Novel Methods for Water Reinjection Efficiency Improvement of a Doublet System in Guantao Sandstone Geothermal Reservoir

Simulation Study of Novel Methods for Water Reinjection Efficiency Improvement of a Doublet System in Guantao Sandstone Geothermal Reservoir

Techno-Economic Performance Optimization of Hydrothermal Doublet Systems: Application to the Al Wajh Basin, Western Saudi Arabia

Techno-Economic Performance Optimization of Hydrothermal Doublet Systems: Application to the Al Wajh Basin, Western Saudi Arabia

Coupled thermo-hydro-mechanical modelling for geothermal doublet system with 3D fractal fracture

• 3D fractal fracture existed in a geothermal doublet system. • Coupled thermo-hydro-mechanical interaction for bedrock and fracture. • Normal and shear deformations of fracture based on thin elastic layer assumption. • Evaluation of thermal breakthrough due to preferential fracture flow. The existence of preferential flow paths, such as fractures and/or fault play an vital role on the thermal breakthrough of geothermal doublet system. The interaction between the preferential flow path and bedrock is often uneven and may have typical fractal characteristic. This study proposes a thermo-hydro-mechanical coupling model considering the deformation of fractal fractures. The fractal fracture is regarded as a thin elastic layer existed in the bedrock, whoes deformation depends to the bedrock and its own mechanical properties. Subsequently, the geothermal doublet system with a three-dimensional fractal fracture is modelled and the parameters affecting thermal breakthrough are investigated numerically. Research results indicate that the thin elastic layer assumption is remarkably robust for modelling fracture opening and closing under coupling conditions. Owing to complex fracture geometry, the fracture permeability evolution presents certain heterogeneity, which is related to fractal dimension, in-situ stress, and geothermal wells layout. The cool water in fracture with larger fractal dimension can interact with the bedrock more fully and further affect the thermal breakthrough. This further suggests that the site selection of geothermal wells should consider the specific geometry of preferential flow paths to avoid premature thermal breakthrough inducing low system efficiency.

Injection related issues of a doublet system in a sandstone aquifer - A generalized concept to understand and avoid problem sources in geothermal systems

This study proposes a concept and presents a workflow to examine potential reasons for low injectivity of sandstone aquifers. Injection related problems are a major challenge for the sustainable utilization of geothermal waters. In order to completely understand and avoid the geothermal reinjection problems, potential problem sources acting on different scales should be taken into consideration. Thus, in the workflow, possible problem sources are considered on regional, reservoir and local scale and categorized into 1) effect of regional hydraulics (potential presence of overpressure and upward flow) 2) inadequate reservoir performance (limited extent, low permeability and performance) and 3) local clogging processes (particle migration, mineral precipitation, microbial activity). Hydraulic conditions are characterized by defining the pressure regime and the direction of vertical driving forces. The reservoir properties are given by determining the grain size and the size of the reservoir layers, as well as the permeability and the transmissivity of the reservoir and the capacity of the injector. Physical, chemical, and biological clogging processes are investigated by specifying the rock properties and determining particle content of the fluid; by analysing the type, probability and amount of the scaling and estimating the potential for corrosion; and by evaluating the possibility of biofilm formation. The concept and the workflow were first tested on a geothermal site (Mezőberény, SE Hungary, installed in 2012) that had to stop operation because of unsuccessful reinjection. The low injectivity of the well is a consequence of several separate problems and their interaction: Reservoir properties are insufficient due to low permeability and transmissivity of the reservoir and the limited vertical and horizontal extension of the sandstone bodies. Precipitation of carbonates, iron and manganese minerals is predicted in hydrogeochemical models and observed in solid phase analysis. Microbial material is produced from the particularly high organic content of the produced thermal water. Injection problems due to hydraulic effects are not expected since the regional pressure regime is slightly subhydrostatic. In summary, reservoir properties determine a low injectivity, which is further decreased to a critical level by the clogging processes. The proposed generalized concept guides a detailed reservoir and geothermal system analysis which is essential for a sustainable geothermal operation.

Utilizing geothermal energy from enhanced geothermal systems as a heat source for oil sands separation: A numerical evaluation

Burning natural gas to heat water for oil sands separation is and energy and carbon-intensive process. This study presents a COMSOL Multiphysics simulation model that comprehensively investigates the feasibility of utilizing geothermal energy from enhanced geothermal systems (EGSs) as a heat source for oil sands separation. The simulation model was built with discrete fractures and was verified with analytical results. The results demonstrate that a doublet EGS performs better than a triplet EGS for heat production, because a doublet system has more uniform flow distribution and longer flow paths. The heat production performance of an EGS can be improved by decreasing the injection temperature or creating more fractures and wider fracture apertures to increase the injection flow rate in a deeper geothermal reservoir. By adjusting the parameters of water, fractures, and the geothermal reservoir, the theoretical model shows that a doublet EGS can provide hot water (≥60 °C) for 30 years, thus saving 957.35 × 106 m3 of natural gas and reducing 1.96 × 106 tonnes of greenhouse gas emissions. This study provides a rational approach for evaluating potential EGS projects in future to provide hot water for oil sands separation.

Geothermal Sustainability or Heat Mining?

Heat mining” is, in fact a complete deceptive misnomer. When a mineral deposit (e.g. copper) is mined and the ore has been taken out, it will be gone forever. Not so with geothermal resources: The heat and the fluid are coming back! Namely, the heat and fluid extraction create heat sinks and hydraulic minima; around these, strong temperature and pressure gradients develop. Along the gradients, natural inflow of heat and fluid arises to replenish the deficits. The inflow from the surroundings can be strong: around borehole heat exchangers, heat flow densities of several W/m2 result, whereas terrestrial heat flow amounts only to about 50 – 100 mW/m2.
 The regeneration of geothermal resources after production, in other words, extraction of fluid and/or heat) is a process that runs over different timescales, depending on the kind and size of the utilization system, the production rate, and the resource characteristics. The resource renewal depends directly on the heat/fluid backflow rate. Heat, respectively fluid production from geothermal resources can be accomplished with different withdrawal rates. Although forced production is more attractive financially (with quick payback), it can nevertheless degrade the resource permanently. The longevity of the resource (and thus the sustainability of production) can be ensured by moderate production rates. The sustainable geothermal production level depends on the utilization technology as well as on the local geologic conditions. The stipulation of the sustainable production level requires specific clarifications, especially by numerical modelling, based on long-term production strategies. In general, resource regeneration proceeds asymptotically: strong at the beginning and slowing down subsequently, reaching the original conditions only after infinite time. However, regeneration to 95 % can be achieved much earlier, e.g. within the lifetime of the extraction/production system. In other words, geothermal resources may under certain circumstances may be considered as having potential regrowth, like biomass. Concerning the requirements for such sustainable production, it is convenient to consider four resource types and utilization schemes. These may be treated by numerical model simulations that consider heat extraction by geothermal heat pumps, hydrothermal aquifer, used by a doublet system for space heating, high enthalpy two-phase reservoir, tapped to generate electricity, and enhanced Geothermal Systems (EGS).

Well spacing optimization of hydrothermal doublet system

It is of vital importance to study the influence of well spacing on geothermal system. A three-dimensional hydrothermal coupling model of doublet system is established with a geothermal system in Shandong province as background. Under different well spacing, temperature field distribution values are calculated by finite element software, low-temperature area developments are analysed and values of heat extraction ratio are compared. The results indicate that with a smaller well spacing, low-temperature area will develop faster, thermal breakthrough will happen earlier, and service life will be shorter. Moreover, the larger the well spacing is, the faster the growth rate of the heat extraction ratio is, and the higher final heat extraction ratio is. However, the final heat extraction ratio value become more and more close while the well spacing increase.

The Potential of Depleted Oil Reservoirs for High-Temperature Storage Systems

HT-ATES (high-temperature aquifer thermal energy storage) systems are a future option to shift large amounts of high-temperature excess heat from summer to winter using the deep underground. Among others, water-bearing reservoirs in former hydrocarbon formations show favorable storage conditions for HT-ATES locations. This study characterizes these reservoirs in the Upper Rhine Graben (URG) and quantifies their heat storage potential numerically. Assuming a doublet system with seasonal injection and production cycles, injection at 140 °C in a typical 70 °C reservoir leads to an annual storage capacity of up to 12 GWh and significant recovery efficiencies increasing up to 82% after ten years of operation. Our numerical modeling-based sensitivity analysis of operational conditions identifies the specific underground conditions as well as drilling configuration (horizontal/vertical) as the most influencing parameters. With about 90% of the investigated reservoirs in the URG transferable into HT-ATES, our analyses reveal a large storage potential of these well-explored oil fields. In summary, it points to a total storage capacity in depleted oil reservoirs of approximately 10 TWh a−1, which is a considerable portion of the thermal energy needs in this area.

Pulse Testing for Monitoring the Thermal Front in Aquifer Thermal Energy Storage

Seasonal storage of heat in shallow aquifers for increasing the efficiency of geothermal energy systems requires a proper monitoring strategy. We expanded our earlier work on harmonic pulse testing (HPT) to incorporate the effect of a temperature front moving into the reservoir due to injection of hot (or cold) water. Our analytical solutions were applied to monitor the thermal front evolution in a doublet system. Thermal front position and average temperature around the injector could indeed be characterized through the application of the proposed HPT interpretation. Additional analyses were carried out adding noise to evaluate the robustness of the interpretation methodology.

Analysis of Magnetoacoustic Quadrupole Resonance and Application to Probe Quadrupole Degrees of Freedom in Quantum Magnets

Motivated by the recent progress of high-frequency ultrasonic measurements, we propose a theory of magnetoacoustic resonance as a microscopic probe for quadrupole degrees of freedom hidden in magnetic materials. A local strain driven by an acoustic wave couples to electronic states of a magnetic ion through various quadrupole-strain couplings, and this provides a periodically time-dependent oscillating field. As a typical two-level system with the quadrupole, we consider a non-Kramers doublet and investigate single- and multiphonon-mediated transition processes on the basis of the Floquet theory. An analytic form of the transition probability is derived within the weak coupling theory, which helps us analyze the magnetoacoustic quadrupole resonance. We apply the theory to realistic non-Kramers doublet systems for the f2 configuration in Oh and D4h symmetries, and discuss how to identify the relevant quadrupole by controlling the quadrupole-strain coupling with an applied magnetic field in ultrasonic measurements.

Field-Orientation Effect on Ferro-Quadrupole Order in PrTi2Al20

Ferro-quadrupole (FQ) order in the non-Kramers $\Gamma_3$ doublet system PrTi$_2$Al$_{20}$ has been investigated via angle-resolved measurements of the specific heat, rotational magnetocaloric effect, and entropy, under a rotating magnetic field within the $(1\bar{1}0)$ plane. The FQ transition occurring at 2 K is robust when the magnetic field $B$ is applied precisely along the $[111]$ direction. By contrast, the magnetic field of larger than 1 T tilted away from the $[111]$ direction sensitively changes the FQ transition to a crossover. The energy gap between the ground and first-excited states in the FQ order increases remarkably with the magnetic field in $B \parallel [001]$, but hardly depends on the magnetic-field strength, at least up to 5 T, in the field orientation between the $[111]$ and $[110]$ axes. These features can be reproduced by using a phenomenological model for FQ order assuming an anisotropic field-dependent interaction between quadrupoles, which has been recently proposed to explain the field-induced first-order phase transition in PrTi$_2$Al$_{20}$. The present study demonstrates the great potential of the field-angle-resolved measurements for evaluating possible scenarios for multipole orders.

Evaluation of geothermal development considering proppant embedment in hydraulic fractures

Conductivity in hydraulic fractures embedded with proppants has a great impact on the efficiency of geothermal development in an enhanced geothermal system. This study explores the effects of proppants and hydraulic fracture arrangement on the efficiency of geothermal development by developing a thermo-hydro-mechanical coupled model. The evolution of the fracture aperture is captured based on the Hertz contact theory and the Kozeny-Carman equation. The finite element method (FEM) is used to analyse the mechanical behaviour of the fractured system. The thermo-hydro circulated process is investigated using the unified pipe-network method (UPM). In the simulation of a doublet system embedded with propped hydraulic fractures, the increase of proppant distribution density enhances the net heat-extraction rate and reduces the thermal breakthrough time and accumulated thermal energy. The influence of reservoir properties on the heat production is further investigated. In the designing of the hydraulic stimulation, both the increase of the propped hydraulic fracture number (at least two hydraulic fractures) and the spacing promote the efficiency of geothermal development, however, the cost of the field design should be considered for optimization.

Interdependencies between physical, design and operational parameters for direct use geothermal heat in faulted hydrothermal reservoirs

Interest in direct use geothermal systems is increasing due to their ability to supply renewable, environmentally friendly heat. Such systems are mostly developed in conduction dominated geological settings where faults are often encountered. Interdependencies between physical, design and operational parameters make it difficult to assess the performance of such systems. Interaction with faults could potentially have adverse effects on system lifetime, generated Net Present Value (NPV) and produced energy. In this work a single doublet system in the enthalpy range of 140 kJ/kg to 350 kJ/kg is analysed using COMSOL Multiphysics. A choice of design (well spacing and placement), physical (layered reservoir, fault flow properties, fault throw) and operational (injection and production flow rates) parameters are considered in a full factorial design that includes 2430 3D reservoir simulations. Results show that fault flow properties characterization is more significant than fault throw structural characterization. For the considered reservoir properties, increasing the flow rate four times results in an NPV increase of a factor seven, despite the shorter system lifetime. A sealing fault renders the system lifetime less sensitive to the doublet positioning. Synthetic model results shown can serve as guidelines to reducing full scale field models. Importance and relevance of these results remains very high for horizontally homogeneous, layered reservoirs. The analysis expands the understanding of interdependencies for direct use geothermal systems and informs on their further development.

Oil saturation distribution in an aquifer with a doublet of extraction/injection wells

A new physico-mathematical approach is developed for modeling oil saturation distribution within a plane polluted aquifer subjected to a flow field governed by a doublet system. This system consists of extraction and injection (E/I) wells, located in a contaminated aquifer, which are used for groundwater remediation applications. The development of the model is done by considering a two-dimensional, nonlinear diffusion-convection transport equation, applying a self-similar scheme. A closed-form analytical solution is obtained for the steady oil saturation distribution in the aquifer for the above-mentioned flow field. The mathematical solution developed here is designed to simulate hydraulic interactions of E/I wells with the oil polluted aquifer. As such, the model was calibrated and its applicability has been shown.

Introduction to the geothermal play and reservoir geology of the Netherlands

Abstract The Netherlands has ample geothermal resources. During the last decade, development of these resources has picked up fast. In 2007 one geothermal system had been realised; to date (1 January 2019), 24 have been. Total geothermal heat production in 2018 was 3.7 PJ from 18 geothermal systems. The geothermal sources are located in the same reservoirs/aquifers in which the oil and gas accumulations are hosted: Cenozoic, Upper Jurassic – Lower Cretaceous, Triassic and Rotliegend reservoirs. Additionally, the yet unproven hydrocarbon play in the Lower Carboniferous (Dinantian) Limestones delivered geothermal heat in two geothermal systems. This is in contrast to the Upper Cretaceous and Upper Carboniferous with no producing geothermal systems but producing hydrocarbon fields. Similar to hydrocarbon development, developing the geothermal source relies on fluid flow through the reservoir. For geothermal application a transmissivity of 10 Dm is presently thought to be a minimum value for a standard doublet system. Regional mapping of the geothermal plays, with subsequent resource mapping, by TNO discloses the areas with favourable transmissivity within play areas for geothermal development. The website www.ThermoGis.nl provides the tool to evaluate the geothermal plays on a sub-regional scale. The Dutch geothermal source and resource portfolio can be classified using geothermal play classification of, for example, Moeck (2014). An appropriate adjective for play classification for the Dutch situation would be the predominant permeability type: matrix, karst, fracture or fault permeability. The Dutch geothermal play is a matrix-permeability dominated ‘Hot Sedimentary Aquifer’, ‘Hydrothermal’ or ‘Intra-cratonic Conductive’ play. The Dutch ‘Hot Sedimentary Aquifer’ play is subdivided according to the lithostratigraphical annotation of the reservoir. The main geothermal plays are the Delft Sandstone and Slochteren Sandstone plays.