Single Borehole Research Articles

A 3D space-marching analytical model for geothermal borehole systems with multiple heat exchangers

We present a novel, three-dimensional (3D), space-marching analytical framework that accurately predicts and evaluates the thermal performance of a geothermal borehole system with single or multiple N-by-N heat exchangers. The procedure for this model which is associated with the key novelties of this work is threefold. First, a radial temperature profile in a single borehole arrangement is solved through an exact solution using the Green’s function. A time-dependent convective boundary is prescribed around the borehole wall. Second, an axial temperature distribution in this single borehole is calculated by a space-marching algorithm, which updates the convective boundary at every depth by obtaining the heat transfer fluid (HTF) temperature via an energy balance. Third, the single borehole in 2D is extended to a N-by-N arrangement in 3D by the thermal superposition with algebraic equation based on each borehole’s location. The developed 3D model is verified with numerical results using the finite element method and validated against a field-scaled experimental data in the literature in regards to the HTF temperature. Further, the influences of borehole distance, ground thermal conductivity and mass flow rate of the HTF are studied. It can be concluded that this 3D space-marching analytical framework is capable of predicting transient temperature profile of any N-by-N geothermal boreholes subjected to a time-dependent boundary in an accurate and computationally efficient manner, which in turn facilitates the thermal design and implementation of geothermal systems for energy extraction.

Development and validation of a computationally efficient hybrid model for temporal high‐resolution simulations of geothermal bore fields

AbstractThis paper presents a hybrid simulation approach for 1U‐, 2U‐ and coaxial single, and field of, borehole heat exchangers. We implement a novel combination of existing solutions for the simulation of heat transfer processes within the borehole and the surrounding ground. The heat transfer in the ground is modelled with a combination of analytically determined g‐functions, the borehole models utilize thermal resistance capacity models and the finite volume method. Critically, we improve the computational efficiency of long‐term simulations by sub‐dividing the time scale into multiple periods, where the influence of past periods on future periods is calculated using the fast Fourier transform (FFT). The accuracy and efficiency of the proposed method is validated against single borehole numerical models and a 40 bore field case study. The method achieves computational times reductions of over 90% in some cases, with greater improvement as the simulation time increases.

Investigation of the Heat Transfer Performance of Multi-Borehole Double-Pipe Heat Exchangers in Medium-Shallow Strata

Just as the double-pipe heat exchanger is being utilized in an increasing number of applications, its research content is also deepening. For this paper, based on the air-conditioning cold and heat source project of a building in Handan, Hebei Province, a 300-meter medium-shallow well double-pipe heat exchanger was used for heating and cooling, and a corresponding heat transfer model was established. The changes of parameters such as the inlet and outlet temperature, heat exchange (with and without a temperature gradient), and borehole wall temperature distribution between a single borehole, double boreholes, and four boreholes over one year in medium-shallow wells were simulated and analyzed. By comparing the obtained experimental data and the simulation data, the accuracy of the heat transfer model was verified. This provides a theoretical basis for the further advancement of the project and lays the foundation for an in-depth study of multi-borehole double-pipe heat exchangers.

Multi-Factor Evaluation of Deep Karst Dolomite Reservoir Based on Paleogeomorphological Reconstruction, a Case Study From the 4th Member of the Dengying Formation in the Central Sichuan Basin, China

The evaluation of reservoirs is of great importance in exploration practices, and reservoir distribution is dependent on multiple geological factors. The carbonate platform of the fourth member (Z2dn4) of the Dengying Formation (Z2dn) in the Gaoshiti-Moxi area (GMA) was uplifted above sea level during episode II of the Tongwan movement and underwent prolonged karstification by meteoric freshwater in the terminal Sinian, leading to a strong heterogeneity in reservoirs. Paleogeomorphology reflects hydraulic gradients that can affect the intensities of surface karstification and may be approximated by terrain slope gradients. Karst products (especially solution vugs and caverns) exist prevailingly in the Z2dn4; their development mirrors the extents of internal dissolution. Based on paleogeomorphological reconstruction using the elevation method, the slope angles of single boreholes were calculated through the digital elevation model (DEM). In accordance with single-borewell Fullbore Formation MicroImager (FMI) logging data, four electrofacies (host rock, vug, fracture, and cave) were identified in boreholes possessing FMI data. The identified electrofacies were matched with corresponding conventional logging curves to obtain the training data. A model generalized for other boreholes without FMI data was established with the random forest algorithm and the thicknesses of the four electrofacies in each borehole were determined to characterize the extent of karstification. Slope gradients and electrofacies thicknesses, together with multiple physical parameters of reservoirs in boreholes, were utilized for factor analysis. Four factors were obtained and fused into one comprehensive score based on their ratios of variance proportions. The favorability of reservoirs was described by the comprehensive scores of each borehole. These analyses showed that the most advantageous reservoirs are distributed in the vicinity of the platform margin in the west of the GMA, where they enjoy several benefits. Deposition of porous mound-shoal complexes occurs predominantly in the platform margin where large-scale caves are more developed in the mixing karst zone of seawater and freshwater. Meanwhile, the transmeridional lateral migration of hydrocarbons from the regional hydrocarbon-generating center abutting the west of the GMA contributes to earlier entrapment and accumulation in the mound-shoal complexes in the platform margin.

Dynamics of coalbed methane emission from coal cores under various stress paths and its application in gas extraction in mining-disturbed coal seam

Uncontrolled and excessive gas emission may affect normal production and even result in serious disasters such as gas explosion and coal and gas outburst. To find out the essence of abnormal gas emission under mining induced stress, in this work, we conducted experiments on gas emitting from coal cores under mining-induced stress paths, and systematically discussed the controlling mechanism of stress change on gas emission. The results show that at the initial period, gas emission rate decreases regardless of the stress path. In this period, stress has little effect on gas emission, and it is mainly controlled by the attenuation of gas pressure gradient. In the middle period, the gas emission rate fluctuates with time. The tipping points of the gas emission rate in this stage is mainly caused by expansion of pores, opening of primary fractures and newly exposed surface induced by shear slip of the primary isolated fractures. With the further increase of the stress difference, failure of the coal core may occur, which creates a large amount of new exposure surfaces, and significantly improves the gas emission rate. Comparing with the minimum value before failure, the gas emission rate of PDS coal after failure increases by 82.35%–990.91%, and that of the JZS coal increases by 75.0%–175.0%. To verify the laboratory test results, in a recently completed research project, we implemented the technique of stress-relieved gas re-extraction in Shoushan coal mine in China. The results show that mining activity can affect the gas extraction in coal seams within 50 m from the coal wall of the working face. The average gas concentration of the boreholes drilled in the mining-disturbed zone is above 40%, and the average net gas flowrate of single borehole reaches 30 L/min. The gas extraction amount in the mining-disturbed zone, with borehole number less than 1% of the total, accounts for 55.1% of the total gas extraction amount in the roadway. This practice could be used as a valuable example for other coal mines with similar geological conditions.

Optimal design of a hybrid ground source heat pump for an official building with thermal load imbalance and limited space for the ground heat exchanger

This work presents the optimal design of a hybrid ground source heat pump (GSHP), taking into account thermal imbalance and space limitation for the ground heat exchanger field (GHE), applied to an official building. Once the building loads are calculated and devices selected, experiments carried out from a single vertical borehole obtain the ground thermal characteristics, including a local short-term period function (STGF). From them, the Finite Line-Source (FLS) model simulates the GHE behavior, from decomposing the ground thermal loads in hourly linear steps for 50 years. A set of input variables, such as geometric configuration data of boreholes field, and additional terms associated with this hybrid operation, must be provided to the model. For optimization purposes, a design of experiments (DoE) considers the thermal ground characteristics and input factors, providing both energy savings and the internal rate of return as outputs (objective functions). Pareto's optimal solutions method provides the selected case, considering a compromise between economic and environmental benefits. It has been established for 18 boreholes (rectangular disposition) of 120 m deep, providing a 33.12% energy saving and an internal rate of return of 3.9%, also showing 89% of the total building load supported by the GHE.

Use of the Alternating Directions Method in Cylindrical Coordinates, Applied to the Two-Dimension Unsteady Conduction Heat Transfer

Abstract Finding analytical solutions for the unsteady conduction heat transfer equation is generally very difficult due to the intrinsic complexity of the problem. In most cases, the equation has no analytical solutions. A numerical approach that surmounts these difficulties consists of the finite difference technique. The alternate directions finite-difference technique especially suits to the two-dimension heat conduction. The paper considers a hollow cylinder consisting of a conduction heat transfer medium, and the two-direction unsteady heat propagation within this medium is studied by means of the alternate directions method. The approach simulates the heat conduction process during a charge-discharge cycle of a single borehole heat storage.

Power ultrasound assisted coalbed methane enhancement recovery: Field application and performance evaluation in underground coal mine

Power ultrasonic assisted reservoir modification is a promising technique for enhanced coalbed methane recovery. The current studies confirm the coal structural alteration and the improvement of gas deliverability in coal pores with ultrasound stimulation. However, the in-site performance of ultrasound-assisted CBM production has not yet been revealed. In the current study, the in-situ antireflection test was conducted with high power ultrasound ∼18 kW in underground coal seam, and the antireflection performance was evaluated by measuring the borehole drainage gas data in the field test zone. The result shows, within 40 days’ drainage after ultrasonic antireflection in coal seam, the average gas concentration of single borehole in the experimental group increased by 81.4% ∼ 227.3% than that in control group, the average borehole gas flowrate has a 20% ∼106% improvement over the control group, and the pure methane production in single borehole increased by about 3.83 times. The ultrasonic anti-reflection influence radius exceeds 8 m in the coalseam with water injection in this study, and the increment of methane production in single borehole decreases with the distance increase from the ultrasound source, which is due to the energy attenuation of ultrasound propagation in the porous coalseam. This study preliminarily demonstrates the on-site implementability and effectiveness of power ultrasound in enhancing coalbed methane recovery.

Performance evaluation of U-tube borehole heat exchangers employing nanofluids as the heat carrier fluid

The present study aims to investigate comprehensively the performance of various nanofluids in single U-tube borehole heat exchangers (BHEs). Seven common nanoparticles with the volume fraction ranging from 0.1% to 2.0% are selected to be evaluated as the heat carrier fluid. Firstly, a comparative techno-economic analysis is performed in order to highlight the merits and drawbacks of each nanofluid. Then, a sensitivity analysis is performed to optimise the decrement percentage of BHE thermal resistance. Finally, by means of the linear regression of numerical results obtained for different nanofluids, simple equations are proposed allowing evaluation of the outlet fluid temperature for nanofluids. The obtained results indicate that Ag- and Cu-based nanofluids are characterised by the highest heat transfer enhancement, although this improvement is at penalty of a higher pressure drop and up to 31% higher required pumping power. On the other hand, SiO2- and TiO2-water nanofluids are the worst cases in terms of thermal performance, but at the same time, they are characterised by the lowest pressure drop. The optimum decrement percentage of thermal resistance yielded in presence of Cu-water nanofluid is equal to 4.31%. Furtheremore, it is shown that employing nanofluids for the purpose of BHE length reduction is not a promising choice. Economic analysis revealed that the cost of electrical energy for nanofluids due to the higher energy consumption of pump is negligible in comparison with the capital cost of nanoparticles. The SiO2 nanoparticles with a capital cost ranging from 5.8 to 17.5 €/m is the cheapest nanoparticle to employ, unlike the Ag nanoparticles.

Numerical study on heat transfer efficiency for borehole heat exchangers in Linqu County, Shandong Province, China

Developing shallow geothermal energy with borehole heat exchangers helps energy supply and CO2 emission reduction. This paper comprehensively investigates the influencing factors on heat transfer efficiency of a single borehole heat exchanger based on the field investigations in Linqu County, Shandong Province, China. We built a three-dimensional numerical model validated by a thermal response test. We systemically investigated the influence of groundwater seepage, circulating water flow rate, pipes spacing and length, the operation interval, and the annual operation mode on the heat transfer efficiency. The results show that the heat transfer efficiency decreases with long-term operation due to surrounding soil’s cold/heat accumulation. The groundwater seepage increases the heat transfer efficiency. The heat transfer efficiency increases with the pipe length, inlet and outlet pipes spacing, and operation interval. It decreases with the circulating water flow rate. The annual operation mode is of greatest importance in the heat transfer efficiency among the studied influencing factors. Compared to working only in winter, the average heat transfer efficiency coefficient increased by 0.152 when working in winter and summer. The results provide a reference for the practical installation and optimization of the borehole heat exchanger for sustainable utilization of shallow geothermal energy.

Experimental study on crushing of concrete columns by high voltage pulse discharge

High voltage pulse discharge (HVPD) crushing tests were carried out on 30 concrete columns with section sizes of 400 mm × 400 mm, 500 mm × 500 mm, 600 mm × 600 mm and 700 mm × 700 mm respectively. Based on the comparative analysis of the electrohydraulic effect and the copper wire explosion on the fracture test of a concrete beam, the latter is determined as the optimal method. The parameters in the test of crushing concrete columns by HVPD include strength grade of concrete, column borehole aperture, hole spacing, discharge voltage and number of copper wires. Columns with sectional dimensions of 400 mm × 400 mm and 500 mm × 500 mm are arranged in single row boreholes, and columns with sectional dimensions of 600 mm × 600 mm and 700 mm × 700 mm are arranged in double rows of boreholes. The results show that the crushing effect of concrete columns increases obviously with the increase of discharging voltage and the number of copper wires, and slightly increases with the decrease of concrete strength. When other parameters are the same, the crushing effect of the concrete columns with hole diameter of 40 mm is better than that of the ones with hole diameter of 50 mm. The crack connectivity of concrete columns with hole spacing of 300 mm is similar to that of the ones with hole spacing of 400 mm. According to the test results, the mathematical expressions of the damage degree of concrete columns with single and double rows of holes (i.e., the width of concrete cracks around boreholes) and the output voltage, the number of copper wires and the strength grade of concrete are established respectively. In order to meet the needs of the project, the suggestion of further crushing construction combined with mechanical method was put forward

Investigation of the Ultrasonic Stimulation Performance on Permeability Enhancement in Coal Seam: Field Tests and In Situ Permeability Evaluation

Power ultrasonic-assisted reservoir modification is a promising technique for enhanced coalbed methane recovery. However, the in-site performance of ultrasound-assisted CBM production has not yet been revealed. In the current study, the in situ antireflection test was conducted with high-power ultrasound ~18 kW in underground coal seam, and the antireflection performance was investigated by measuring the borehole drainage gas data in the field test zone, and then, the in situ permeability change of the target coal seam was evaluated numerically. The result shows that, within 40 days’ drainage after ultrasonic antireflection in coal seam, the average gas concentration of single borehole in the experimental group increased by 81.4 %~227.3% than that in control group, the average borehole gas flowrate has a 20%~106% improvement over the control group, and the pure methane production in single borehole increased by about 3.83 times. The permeability inversion indicates that the in situ coal seam permeability has increased by at least 2.36 times after the ultrasound stimulation within the range of 8 m from the ultrasound source.

Suitability of Screened Monitoring Wells for Temperature Measurements Regarding Large-Scale Geothermal Collector Systems

Groundwater temperature (GWT) is usually measured using screened monitoring wells (MWs). The aim of this study was to investigate whether MWs are suitable for monitoring the effects of large-scale geothermal collector systems (LSCs) on GWT, focusing on possible vertical flows within the MWs due to both natural and forced convection. Comparative temperature depth profiles were therefore recorded over a period of nine months in both shallow MWs and in small-diameter, non-screened temperature monitoring stations (TMSs), each of which was installed in a single borehole. Particularly high temperature deviations were measured in MWs in the upper part of the water column where the GWT reached up to 1.8 K warmer than in the surrounding subsurface. These deviations correlate unambiguously with the prevailing positive thermal gradients and are caused by thermal convection. Where forced convection occurred, the GWT was measured to be up to 0.8 K colder. Potential temperature deviations must be considered when monitoring very shallow GWT as thermal gradients can be particularly high in these zones. For monitoring concepts of LSCs, a combination of MW and TMS is proposed for GWT measurements decoupled by the effects of convection and in order to enable further investigations such as pumping tests.

Seasonal changes in thermal process based on thermal response test of borehole heat exchanger

The ground-source heat pump (GSHP) system is one of the most energy-efficient cooling and heating systems popularly used worldwide. The subsurface thermal properties, especially the apparent thermal conductivity, are applied in designing an optimal system. A thermal response test (TRT) is usually conducted once to estimate the apparent thermal conductivity. However, seasonal changes in the apparent thermal conductivity obtained from the TRT have not been sufficiently discussed. Subsurface conditions such as groundwater flow and temperature distribution can vary seasonally because of natural or artificial causes. Therefore, to optimally design a GSHP system, an evaluation of the variation of apparent thermal conductivity caused by seasonal changes in subsurface conditions such as groundwater flow and temperature distribution is imperative. We performed TRT with temperature monitoring three times in January, May, and October, and analyzed the results using numerical simulation. The apparent thermal conductivity in January (winter season) was estimated to be approximately 10%–15% greater than that in May and October owing to the rapid groundwater flow in the partial aquifer, assumed to be because of pumping well operation for snow melting. To evaluate the rate of increase of the apparent thermal conductivity caused by the partial groundwater flow, numerical simulations were performed using a three-dimensional single borehole heat exchanger model. It was found that the parameter of dispersivity plays an important role in the numerical simulation of the thermal process with groundwater flow, and the rate of increase of thermal conductivity has a linear relationship with the mass flow rate, regardless of the partial aquifer thickness and dispersivity. This finding may aid in the estimation of apparent thermal conductivity when considering seasonal changes in groundwater.

Gas extraction technology and application of near horizontal high directional drilling

In order to solve the problem of gas accumulation in the upper corner caused by large gas emission due to high mining intensity in the fully mechanized top coal caving face of Wangjialing coal mine, we used numerical simulation method combined with the key stratum layer control theory to study the roof caving trend and development law of inclined mining fissure. Then we analyzed the evolution process of goaf fissure with the advance of the working face and the development height of roof fissure. Finally, we got the best hole layout position and drilling structure of near horizontal high-level directional long drilling. The practice indicates that we achieved nice gas extraction effect by setting near horizontal high-level directional long drilling borehole according to the above research results. The maximum net extraction volume of a single borehole is 2.10 m3/min, the maximum extraction volume fraction is 31.39%, and the total net amount of gas extracted from the four boreholes under construction is 289900 m3. The gas concentration in the upper corner of the working face decreased to 0.46% during extraction, and the problem of gas overrun in the upper corner was effectively solved.

3D Numerical Modeling to Evaluate the Thermal Performance of Single and Double U-tube Ground-coupled Heat Pump

The heat transfer rate and borehole design represent great challenges to the thermal equipment designer of the ground-coupled heat pump. The present model represents a mathematical and numerical technique implemented to tackle such a problem. A thermal assessment was established to estimate the total energy dissipated to the ground zone for a heat pump utilized for cooling purposes in the summer season. Comsol Multiphysics 5.4 software was used to build a 3-dimensional model to assess the thermal performance of single and double U-tube boreholes that circulate water as a thermal transfer medium. The (Heat Transfer) module has been implemented for this investigation under the (Stationary) study option. The model couples both heat conduction in solids, including tube metal, grout, and soil regions, and thermal medium fluid flow inside the U-tubes. The numerical solutions were compared for both heat exchangers at fixed borehole geometry, diameter, and depth and constant operating conditions in a steady-state mode. The double U-tube heat exchanger was tested in the parallel circuiting orientation of the U-tubes. The total mean resistance of the single U-tube borehole was higher than the half-loading double U-tube heat exchangers by 14.6%. The results also revealed that the heat transfer rate enhancement for the double U-tube was in the range of 10–14% when operating at the same fluid mass flow rate and inlet temperature for a given borehole design. Doi: 10.28991/HIJ-2022-03-02-01 Full Text: PDF

Thermal disturbance analysis in rock-soil induced by heat extraction from the abandoned well

Medium-deep geothermal energy has obvious advantages in building heating in cold regions. The abandoned oil well can be retrofitted into a closed-loop coaxial borehole heat exchanger to save costs. However, continuous heat extraction and recovery in long-term operation can cause potential deformation, and the thermal disturbance scope in the surrounding formation should be determined. In this study, the thermal process and disturbance induced by heat extraction are analyzed. The effects of heat load, flow rate, rock-soil thermal conductivity, outer pipe thermal conductivity, rock-soil density and depth for a single borehole are discussed. And the effects of distance for two boreholes on the thermal disturbance were evaluated. The results demonstrate that heat conduction between the circulating fluid and the formation is opposite at the shallow and deep depth, and the thermal process is enhanced at the bottom borehole. The thermal disturbance scope at the depth of 2000 m can exceed 40 m following 20 years of operation. The superimposed thermal process of the two boreholes induces more thermal disturbance. The thermal disturbance scope of each borehole was able to 75 m for the layout of the two boreholes. The results can provide a basis for layout of the coaxial borehole heat exchanger.

Effect of fault activation on the hydraulic connectivity of faults in mudstone

An injection test and repeated packer tests were conducted for a fault in siliceous mudstone in order to activate the fault and investigate changes in hydraulic connectivity of the fault before and after fault activation. The injection test successfully induced a significant dilational shear failure within the fault. Pressure changes measured by the repeated packer tests were analyzed before and after the failure, where log-log plots of the pressure derivatives changed after the failure from an upward trend indicating a limited extent of fluid flow paths to a horizontal trend suggesting well-connected fluid flow paths. After the borehole had been open for six weeks, the pressure derivatives were restored to an upward trend. This reversible change in pressure derivatives means that the hydraulic connectivity of the fault increased temporarily during and just after the injection test, but fault activation did not irreversibly affect the initially low hydraulic connectivity of the fault. This transition in the hydraulic connectivity of the fault is also consistent with the variation in fluid pressure monitored at a neighboring observation hole. We propose that analyzing the pressure derivatives obtained by repeated packer tests before and after an injection test in a single borehole is effective for assessing the sensitivity of the hydraulic disconnectivity of faults to fault activation, which is key information for risk assessment of radioactive waste disposal.

An Analytical Model for the Thermal Assessment of a Vertical Double U-Tube Ground-Coupled Heat Pump System in Steady-State Conditions

An analytical model was built to predict the thermal resistance of a vertical double U-tube ground-coupled heat pump that operates under steady-state conditions. It included a geometry obstruction factor for heat transfer throughout the backfill medium due to the presence of the second loop. The verification of the model was achieved by the implementation of five different borehole configurations and a comparison with other correlations in the available literature. The model considered a U-tube spacing range between (2) and (4) times the U-tube outside diameter producing a geometry configuration factor range of (0.29–0.6). The results of the model were utilized for the assessment of the DX ground heat exchanger coupled heat pump system. For similar geometrical configurations, the borehole thermal resistance experienced a decrease as the geometry factor increased. The single U-tube borehole thermal resistance was higher than that of the double U-tube heat exchanger by (10–27)% for the examined geometry configurations. The borehole thermal resistance at tube spacing of twice the tube diameter was higher than the predicted value at the triple diameter and fell in the range of (18–34)%.

A Novel Semi-Analytical Solution for Thermal Response Analysis of Single U-Tube Borehole Heat Exchangers (Bhes)

A Novel Semi-Analytical Solution for Thermal Response Analysis of Single U-Tube Borehole Heat Exchangers (Bhes)