Geothermal Wellbore Research Articles

A Novel Geothermal Wellbore Model Based on the Drift-Flux Approach

Geothermal energy, being a clean energy source, has immense potential, and accurate wellbore modeling is crucial for optimizing the drilling process and ensuring safety. This paper presents a novel geothermal wellbore model based on the drift-flux approach, tested under three different temperature and pressure well conditions. The proposed model integrates the conservation equations of mass, momentum, and energy, incorporating the gas–liquid two-phase flow drift-flux model and heat transfer model. The key features include handling the heat transfer between the formation and the wellbore, addressing the slip relationship between the gas and liquid phases, and accounting for wellbore friction. The nonlinear equations are discretized using the finite difference method, and the highly nonlinear system is solved using the Newton–Raphson method. The numerical simulation, validation, and comparison with existing models demonstrate the enhanced accuracy of this model. In our tests, the model achieved a high accuracy in calculating the bottom-hole pressure and temperature, with mean relative errors (MREs) significantly lower than those of other models. For example, the MREs for the bottom-hole pressure and temperature of the Rongxi area well in Xiongan, calculated by this model, are 1.491% and 1.323%, respectively. These results offer valuable insights for optimizing drilling parameters and ensuring drilling safety. Comparisons indicate that this approach significantly outperforms others in capturing the complex dynamics of geothermal wellbores, making it a superior tool for geothermal energy development.

Study on the integrity assessment of geothermal wellbore under thermal and non-uniform in-situ stresses coupling effects

During the development of geothermal resources, thermal and non-uniform in-situ stresses can lead to wellbore integrity failures in geothermal wells. To evaluate the integrity of casing-cement-formation combination system under coupled thermal and non-uniform in-situ stress, an integrity evaluation mathematical model is established based on thermodynamics, elastodynamics, and the equivalent stress criterion. The performance of cement sheath mechanics and combination system sealing under different temperature conditions is investigated based on the designed experimental apparatus. By integrating numerical simulation and experimental approaches, a method to evaluate the integrity of the wellbore combination system in geothermal wells has been developed and the risk of temperature variations and non-uniform in-situ stresses on the casing, cement sheath, Cementing I and II interfaces has been analysed. Experimental results show significantly reduced mechanical and sealing performance under high temperature conditions. Numerical simulations indicate that the Cementing I and II interfaces, as well as the cement sheath, serve as weak points for potential integrity failure in geothermal wells, but the potential failure risk of Cementing I interface is higher. The developed method can effectively evaluate the wellbore combination system integrity under the coupling effect of thermal stress and non-uniform in-situ stress, which is crucial for ensuring the wellbore integrity and safe development of geothermal energy.

An elastoplastic semi-analytical solution for enhanced geothermal wellbore stability considering temperature-sensitive failure criterion

Temperature alters the rocks’ mechanical behavior and influences the stability and operation of enhanced geothermal wells. The stability of the well is a concern during construction, just after the construction, at the time of production, and injection. Most available analytical and semi-analytical solutions for wellbore stability do not account for rock's temperature-dependent yield behavior. In the present study, we proposed an elastoplastic constitutive model based on the temperature-sensitive yield criterion and a semi-analytical solution for wellbore stability. The applicability of the constitutive model is demonstrated by comparing the experimental results of Comiso limestone with the model output. The model elastic perfectly plastic assumption reasonability captures the stress-strain behavior at elevated temperatures. Temperature-sensitive yield criterion effectively predicts the yield point of Comiso limestone, even the substantial decay in yield stress after the temperature of 400 οC. The semi-analytical solution of wellbore stability analysis is validated using FE (Finite element) analysis. An implementation of the semi-analytical solution is showcased for a vertical wellbore, considering changes in in-situ stresses and temperature. A parametric analysis establishes that external factors, like vertical stress increase, do not consistently decrease the size of the plastic zone. Instead, a specific threshold of vertical stress exists at which the plastic zone radius reaches its minimum value. In the presented case, the least radius of the plastic zone is observed for in-situ vertical stress of 80 MPa. Furthermore, the parametric study examines the impact of horizontal stress, support pressure, temperature, and dilation parameter, which followed the generally reported trends.

Enhancing Geothermal Wellbore Heat Exchanger Performance Through Rectangular Protrusion Integration in Repurposed Abandoned Oil Wells

Enhancing Geothermal Wellbore Heat Exchanger Performance Through Rectangular Protrusion Integration in Repurposed Abandoned Oil Wells

A sustainable clean energy source for mitigating CO2 emissions: numerical simulation of Hamit granitoid, Central Anatolian Massif

Türkiye relies on coal-fired power plants for approximately 18 GW of annual electricity generation, with significantly higher CO2 emissions compared to geothermal power plants. On the other hand, geothermal energy resources, such as Enhanced Geothermal Systems (EGS) and hydrothermal systems, offer low CO2 emissions and baseload power, making them attractive clean energy sources. Radiogenic granitoid, with high heat generation capacity, is a potential and cleaner energy source using EGS. The Anatolian plateau hosts numerous tectonic zones with plutonic rocks containing high concentrations of radioactive elements, such as the Central Anatolian Massif. This study evaluates the power generation capacity of the Hamit granitoid (HG) and presents a thermo-hydraulic-mechanical (THM) model for a closed-loop geothermal well for harnessing heat from this granitoid. A sensitivity analysis based on fluid injection rates and wellbore length emphasizes the importance of fluid resident time for effective heat extraction. Closed-loop systems pose fewer geomechanical risks than fractured systems and can be developed through site selection, system design, and monitoring. Geothermal wellbore casing material must withstand high temperatures, corrosive environments, and should have low thermal conductivity. The HG exhibits the highest heat generation capacity among Anatolian granitoid intrusions and offers potential for sustainable energy development through EGS, thereby reducing CO2 emissions.

Investigating heat transmission in a wellbore for Low-Temperature, Open-Loop geothermal systems

Heat transfer processes in geothermal wellbores are an essential component to overall system performance, but can be overlooked in subsurface modelling studies that tend to focus on reservoir response. This study addresses heat transfer in the wellbore through a comprehensive modelling study of 10 different parameters on OpenGeoSys software designed to evaluate heat losses during open-loop production conditions for typical low-temperature (<100 °C) single phase geothermal systems. Models were set-up to focus on wellbore effects only, using a single production well with constant fluid inlet temperature boundary condition. Results indicate that under base case conditions for a 2-km deep well surrounded by rock formations with a thermal conductivity of 2.5 W/(m.K) and bottom-hole temperature of 60 °C, the difference in inlet (reservoir) and production (wellhead) temperature at the end of a 40-year production period is 2.06 °C. This corresponds to minimum heat losses into the surrounding formations of −63.3 W/m, which is ∼ 7 % of the thermal power recorded at the wellhead (assuming a rejection temperature of 30 °C) and a 3.4 % difference to bottom-hole temperature. These losses in heat can be significant, particularly when in combination with surface losses through the heat exchanger or in the reservoir through thermal breakthrough. Wellbore insulation can reduce losses, but it would appear this only impacts the short-term. Wellbore performance can also be improved, with heat losses minimized, when developing wells above the reservoir interval in low thermal conductivity rock with high geothermal gradients.

Designs of ultra-HPHT Electrical Component Packages for Downhole and Geothermal Wellbore Logging Tool Integrations

Conventional dielectric sealing materials (PEEK, glass and glass-ceramic) used for sealing electrical feedthroughs, connectors, interconnectors and bulkheads in electronic packages can provide reliable downhole wellbore long-term logging tool service up to 150C or short-term logging service for greater than/equal 200C, due primarily to low glass transition temperature of the used sealing materials. This paper will demonstrate that the maximum allowable operating pressure and temperature could be from 20KSI to 100KSI and from 200C to 400C with Borosilicate and Bismuth-Boron-Silica (XTS) sealing glasses. Borosilicate glass sealed Inconel-Kovar/Kovar electronic housing could maintain reliability up to the maximum pressures of ~35KSI at 200C or 25KSI at 400C. Similarly, a Bismuth-Boron-Silica XTS glass sealed stainless steel electronic package housing could have a maximum operating pressure of ~100KSI at 150C or ~50KSI at 300C temperature. It has been found that high glass transition temperatures of (440-560)C for Borosilicate glass and (44010)C for Bismuth-Boron-Silicate (XTS) glass are key for making highly reliable ultra-HPHT electronic component packages. It is shown that the tetrahedral diamond-like microstructures of these sealing glasses are keys to ensure water-repelling properties to maintain sufficient electrical insulation resistance regardless of their use in water or moisture-rich wellbores. Moreover, compressive pin stress is found to be additional key for mitigate frequently observed glass seal cracks during logging tool field services.

A new method for calculating the minimum sand-carrying velocity in geothermal well production

Formation sand production is an important engineering and technical problem that restricts the safe and efficient development of geothermal wells. Accurate prediction of the minimum sand carrying velocity of sand production is the key to effectively controlling the formation sand production of geothermal wells. Compared with conventional vertical well sand production, geothermal wellbore sand carrying has the characteristics of high concentration, high fluid temperature, and easy silting, which makes it more difficult to calculate the minimum sand carrying velocity. In order to derive the formula of sand carrying law suitable for complex geothermal conditions, this paper considers the temperature and concentration conditions, based on the basic sedimentation principle and indoor sand carrying experiment, abandons the single correction coefficient correction of the conventional calculation formula, integrates the influence of temperature and high sand content on debris migration and deposition, and adopts the double correction coefficient of temperature-viscosity and sand content-mixed density, the actual minimum sand carrying velocity calculation formula of geothermal well under different sand content and temperature is obtained, and a new theoretical calculation method of minimum sand carrying velocity of the geothermal wellbore is formed. With the help of indoor experiments, a reasonable experimental group was designed to carry sand law experimental research. The experimental results show that the calculated value is in good agreement with the measured data, and can be used to calculate the minimum sand-carrying velocity of unconsolidated sandstone in the geothermal wellbore, which provides a basis for sand-carrying failure and sand sinking prevention in geothermal wellbore site.

Radial impact fracture characteristics and crack initiation criterion of concentric perforated granite after high temperature-water cycle

In this study, the radial impact fracture characteristics and crack initiation criterion of concentric perforated granite after high temperature-water cycle were evaluated based on the engineering environment such as high temperature, water encounter and dynamic disturbance experienced by the wellbore surrounding rock in the energy storage area during deep geothermal energy mining. And the combination of mechanical test, numerical simulation and theoretical analysis was used. The results revealed that the concentric perforated granite treated with high temperature and water soaking under radial impact load successively underwent three stages: transient elastic deformation, stable and unstable expansion of internal cracks. Finally, the tensile failure was produced and the granite was split into four pieces of approximately symmetrical. The main fracture direction of the rock sample was the impact direction revealed by the strain, fracture history and lateral strain clouds of the inner bore wall of the sample. Meanwhile, the tensile cracks firstly sprouted from the inner bore wall of the sample in the impact direction and expanded to the outer wall, and then from the outer wall in the vertical direction to the inner bore wall. Then, based on the test results, the damage variables were defined according to the proportion of damage degree caused by single factor and the law of crack germination inside the rock sample. It was calculated that the damage accumulation rate of concentric perforated granite under radial impact load after high temperature and water immersion experienced three typical stages: “slow-fast-slow”. Finally, the energy structure relationship of the micro-element of concentric perforated granite was constructed. The crack initiation criterion of the rock sample with concentric perforated structure under radial impact load was established by considering the initial damage caused by heating and water immersion and using the parameters such as the energy storage limit of the rock sample and the energy required for crack initiation. In addition, the rationality was also verified. The research can provide a theoretical reference for the formation of heat transfer channel caused by fracture of stored rock mass and the stability control of surrounding rock of geothermal wellbore, which have great engineering practice and scientific significance.

A transient geothermal wellbore simulator

A new transient geothermal wellbore simulator is presented. It is based on a mathematical model consisting of three conservation equations (mass, momentum and energy) and a slip model describing how the liquid and vapour phases move differently. The three conservation equations are discretised using the finite volume method with thermodynamic variables such as pressure and temperature defined at cell centres and flow variables such as volume flux of liquid and vapour velocity defined at cell boundaries. Implicit differencing in time is used and upstream weighting in space is essential.The three discrete conservation equations are solved together with the slip equation, giving a system of 4N nonlinear algebraic equations to be solved for 4N unknowns for a numerical model containing N blocks. This system of equations is solved using the Newton-Raphson method.For solving difficult transient problems e.g., starting and stopping production in a high temperature well, some subtle modifications of the standard upwinding approach were introduced. The simulator was then able to solve all the test cases we tried, including completion tests, opening and closing of wells (with some high enthalpy cases) and well stimulation by air-lifting and air-compression. The well stimulation simulations used a version of the simulator modified to handle a mixture of water and a noncondensible gas (NCG) by including two mass conservation equations, one for water and one for the NCG.Our simulator appears to be very robust and can solve a wide range of interesting transient flows in geothermal wells.

Simulation of complex transient flows in geothermal wells

In a recent paper we discussed the development of a new transient geothermal wellbore simulator. In the present paper we discuss the application of this simulator to the modelling of challenging and interesting flows that occur in a geothermal wellbore. The first group of problems involve the opening and closing of hot geothermal wells with rapid transient flows. In some cases, this involves counter-flow with steam flowing upwards and liquid water flowing downwards. The final test case in this category is the simulation of a completion test which involves four stages: cold water injection, heating up of a shut-in well, discharge of the well to the atmosphere, connection of the well to a separator.Simulation of well stimulation by air-lifting and air-compression demonstrate the capability of the simulator for modelling flows of air-water mixtures with a moving interface between the phases.Finally, we demonstrate the ability of the simulator to model injection of water-carbon dioxide mixtures, with an increasing proportion of CO2. This is of particular importance as operators look to carbon sequestration as a way of mitigating green-house gas emissions from geothermal power plants.These are a selection of the more interesting problems of the many test cases that were used during the development of our simulator to ensure that it is robust and can simulate the full range of complex transient flows that can occur in a geothermal well.

Study on creep characteristics and fractional creep damage constitutive model of limestone under thermal-mechanical action

To study influence of thermal-mechanical action on creep for the geothermal wellbore, Dalian limestone was taken as the research object, and triaxial creep tests were carried out on samples under different temperatures and the effects of different temperatures on the creep deformation, creep rate and long-term strength of limestone are discussed. The test results show that the creep process of limestone under thermal-mechanical action shows three distinct phases. As the temperature increases, the creep time and long-term strength gradually decrease. The macroscopic failure modes of limestone under different temperatures are shear failure and the fracture surface is more obvious. Meanwhile, based on the principle of statistical damage and fractional calculus, the damage body under high temperature is proposed, and a fractional creep damage model that can characterize the triaxial creep mechanical behavior of limestone under different temperatures is constructed. The reasonableness of the model is proved by comparing. The parameters analysis shows that the fractional order δ mainly affects the creep rate, which is expressed in the stable and accelerated phases. The aging damage parameter λ is related to the time coordinate scale. The research results in this paper can provide a certain guiding significance for the long-term stability evaluation of the surrounding rock of the geothermal wellbore.

A review of mathematical models for geothermal wellbore simulation

A wellbore simulator is an important tool for understanding flow in geothermal wells, however, the literature on the topic is confusing because previous wellbore simulators have been based on a variety of versions of the equations for conservation of mass, momentum and energy. This lack of consistency in describing the behaviour of flow in a geothermal well makes it unclear as to whether some models correctly conserve key quantities and presents difficulties when comparing models. This study aims to give clarity by providing a comprehensive review of conservation equations used in modelling both transient and steady multiphase flow in a geothermal wellbore.We present as a standard for comparison a complete, physically accurate three-equation model for transient flow in a geothermal wellbore. Two of the four previous transient geothermal wellbore simulators use this three-equation model, while the other two simulators are based on approximate equations that do not fully conserve all quantities.We also review previous steady-state geothermal wellbore simulators and find that some are based on mathematical models that do not correctly conserve key quantities. Some models approximate the momentum flux term using average variables, while others incorrectly account for the effect of changes in cross-sectional area on momentum flux. Our analysis shows that both assumptions can have a significant impact on simulation results under certain conditions, however, under normal operating conditions, these differences are relatively small.

Computational study of two-phase flashing flow in a calcite scaled geothermal wellbore

This work presents a computational study to understand different heat transfer and phase transition processes of two-phase flow in a geothermal wellbore. The simulation uses a six-equation model with mass, momentum, and energy balance equations for the liquid and vapour phase, which is available in Ansys Fluent. The model considers the effect of phase transition due to flashing, caused by thermal non-equilibrium at the liquid-vapour interface. Two wellbore scenarios, one with a constant inner diameter and the other a variable inner diameter along the flow direction due to calcite scaling, were simulated using the model. Comparison of the simulation results with measured pressure log data shows good agreement. Simulation studies for the fluid flow in a wellbore with variable diameter occurring due to calcite scaling shows higher mass transfer rate and wall normal bubble flux in the region of scaling, resulting in higher chances of mineral precipitation and deposition in the same region.

A two-dimensional temperature field simulation of the La Primavera geothermal area, México

A reasonable estimation of the geothermal potential of any geothermal system depends on the reliable knowledge of its geological, geophysical and geochemical parameters. The heat flow is an important geophysical criterion used to describe the subsurface temperature profiles in prospecting areas. These profiles are helpful tools to locate high temperatures and thermal disturbances (heating or cooling) that could indicate permeable or fractured zones in the subsurface. A good area to apply this approach is the La Primavera caldera (LP), México, with an estimated geothermal potential high enough to install a 75 MW power plant. A conductive model of La Primavera would provide vital information to understand the heat flow during future work on and eventual exploitation of the geothermal field. We developed a 2D conceptualized numerical model of La Primavera based on the heat transfer equations. Then, we applied the Tri-Diagonal Matrix Algorithm (TDMA) method to discretize the equations. The resulting model described the temperature distribution as a function of depth in 10 geothermal wellbores drilled within an area of 4 km2 inside the caldera. The lithology of these exploration wellbores (subsurface stratigraphy) was used to correlate the proposed 2D geological profiles and to construct the model geometry and thermal conductivity at depth. The temperature profiles were used as boundary conditions to calibrate and validate the numerical model. The results indicate that estimated temperatures (450-500 °C) occur at depths of 3 km hosted within the granitic basement beneath La Primavera. These high temperatures may be linked to remnant intrusive bodies located in depth. These results suggest that La Primavera is a promising target for future geothermal exploitation.

Study on Thermo-Hydro-Mechanical Coupling and the Stability of a Geothermal Wellbore Structure

For processes such as water injection in deep geothermal production, heat transfer and fluid flow are coupled and affect one another, which leads to numerous challenges in wellbore structure safety. Due to complicated wellbore structures, consisting of casing, cement sheaths, and formations under high temperature, pressure, and in situ stress, the effects of thermo-hydro-mechanical (THM) coupling are crucial for the instability control of geothermal wellbores. A THM-coupled model was developed to describe the thermal, fluid, and mechanical behavior of the casing, cement sheath, and geological environment around the geothermal wellbore. The results show that a significant disturbance of effective stress occurred mainly due to the excess pore pressure and temperature changes during cold water injection. The effective stress gradually propagated to the far-field and disrupted the integrity of the wellbore structure. A serious thermal stress concentration occurred at the junction of the cased-hole and open-hole section. When the temperature difference between the injected water and the formation was up to 160 °C, the maximum hoop tensile stress in the granite formation reached up to 43.7 MPa, as high as twice the tensile strength, which may increase the risk of collapse or rupture of the wellbore structure. The tensile radial stress, with a maximum of 31.9 MPa concentrated at the interface between the casing and cement sheath, can cause the debonding of the cementing sheath. This study provides a reference for both the prediction of THM responses and the design of drilling fluid density in geothermal development.

Alkali-activated aluminosilicate sealing system for deep high-temperature well applications

This study proposes potential replacement for the Portland-based cement, which is a common sealant used in deep high-temperature geothermal wellbore completions. The experimental laboratory studies were focussed on alkali-activated aluminosilicates. It was proven, that the so-called, geopolymer-based sealing systems exhibit high compressive and flexural strength at elevated temperatures, good resistance towards thermal cyclic loading, high ductility, acid insensitivity, and improved water permeability. Additionally, alkali-activated aluminosilicate sealing systems are economically feasible and their CO2 emission, during the manufacturing process, is significantly lower in comparison with the conventional Portland-based cement types, making them an environmentally friendly option for deep wellbore completions in unconventional high-temperature geothermal systems.

Structural and chemical changes from CO2 exposure to self-healing polymer cement composites for geothermal wellbores

Wellbore cement is subjected to a number of mechanical, thermal and chemical stress regimes over its lifetime. Therefore, next-generation wellbore cement formulations need to be evaluated in conditions relevant to these environments. In this work, we investigate the mechanism of the alteration of a novel self-healing polymer-cement composite after exposure to a CO2-rich environment by using synchrotron-based X-ray Fluorescence (XRF), X-ray absorption near edge structure (XANES), and scanning electron microscopy coupled with energy dispersive spectroscopy. Results showed that a chemical alteration of the polymer-cement follows the rim carbonation mechanism, similar to conventional cement, although carbonation takes place to a lesser extent in polymer-cements despite the higher porosity. Along with detailed mechanistic insights on carbonation in polymer-cement composite, the performance of these in CO2-rich environment is further studied using standard compressive strength analysis.

Seismic Imaging of Convective Geothermal Flow Systems to Increase Well Productivity

A core feature of convective geothermal resource production is wellbore energy flow Q ~ ρC x T x V. E.g., for wellbore fluid of volume heat capacity ρC ~ 4.3MJ/m3∙oC, temperature T ~ 230oC, and volumetric flow V ~ 50L/s, wellbore heat energy production is Q~ 50MWth ~ 5MWe. Wellbore fluid flow V =2πr0φv0ℓ for open wellbore length ℓ is given in turn by the spatially variable product crustal porosity times crustal fluid velocity v ≡ φv0 at the wellbore radius r0. For a geothermal wellbore to be productive (nominal Q ~ 5MWe), locally variable bulk inflow rates v = φv0 across crustal volumes of dimension ℓ must be adequate to sustain high wellbore flows (nominal V ~ 50L/s). Wide-ranging crustal well productivity statistics show that few crustal wells flow at these rates. This is not surprising as local bulk flow ~ 10-2 m/s needed for production wellbores is decades greater than ambient bulk fluid flow ~ 10-8-10-7m/s characteristic of natural convective geothermal systems. Such rare high flow locales must be found. While existing crustal surveys generally fix resource temperatures T with nominal 500m spatial resolution, as yet no survey technique provides data to locate production-grade flow rates at nominal ℓ ~ 50m spatial resolution. In consequence, many costly unproductive wells are drilled before sites of productive local geothermal fluid bulk flow v = φv0 ~ 10-2 m/s are located. A seismic survey methodology sensitive to crustal flow v = φv0 at ℓ ~25m resolution has evolved from multi-channel seismic reflection technology applied to the production of shale hydrocarbons. The field-proven seismic flow-imaging methodology can be adapted from sedimentary terrains to volcanic terrains through appropriate seismic refraction means for generating effective seismic velocity models for convective geothermal flow volumes. Numerical simulation for characteristic ambient crustal heterogeneous flow distributions v = φv0 at ℓ ~50m resolution shows that travel-time data for seismic source energy refracted from deep geothermal wellbores to surface seismic sensor arrays can replicate the multi-channel seismic flow-imaging capability demonstrated for shale formations. Multi-channel seismic reservoir flow monitor detection and mapping of flow-induced seismic emissions in shale formations can be adapted to achieve similar mapping of convective geothermal flow system structures with v = φv0 ~ 10-2m/s at nominal ℓ ~ 50m resolution. Such seismic emission flow structure maps can greatly reduce the uncertainty and hence the cost of drilling effective production wells at brownfield sites and assessment/development wells at greenfield sites

A rigorous approach to scale formation and deposition modelling in geothermal wellbores

Lots of studies have been done to model the flow behavior and probable processes in producing geothermal wells. As the deposition of particles can cause a decline in the expected flow rate, and it is not desirable, so the scale has to been predicted, treated, and removed. In this study, a geothermal wellbore simulator program is presented that predicts scale precipitation and deposition during drilling in addition to modeling production conditions. The novelty of this study is wellbore simulating and scale modeling in one package, but the most important limitation of the model is its applicability just in cased parts.