Single Borehole Research Articles

Borehole Group Effect during Pile Foundation Construction in Soft Soil Areas

Existing projects indicate that a large number of pile boreholes formed via cement fly ash gravel (CFG) pile construction in soft soil could produce a borehole group effect in the absence of timely backfilling. The borehole group effect could cause ground settlement and threaten the safety of adjacent buildings, tunnels, and pipelines. However, the mechanism of the borehole group effect, causing serious deformation in surrounding soil, has seldom been studied, and there exists no simple simulation method to predict the borehole group effect. In this study, a case involving CFG pile boreholes in a soft soil area causing a significant settlement of surrounding buildings was introduced. Based on this case, a centrifuge test was designed to simulate the influence of a single borehole and multiple boreholes on the surrounding soft soil. The mechanism of the borehole group effect was investigated via centrifuge tests and the finite-element method. In the case of a single borehole, a horizontal circumferential stress arch and vertical stress transfer arch were formed in the soil, which effectively limited the shrinking deformation of the borehole wall. In the case of a large number of boreholes with a small spacing, the horizontal and vertical stress arches around the boreholes affected and weakened each other. As a result, the shrinking deformation of the borehole group was greater than that of the single borehole. In other words, the borehole group effect was found to cause serious deformation in the surrounding soil. To overcome the difficulty in the simulation of a large number of pile boreholes in engineering, the borehole-type conversion and multihole merging method was proposed. The simulation results indicated that the simplified method was reasonable.

An elastoplastic damage constitutive model for capturing dynamic enhancement effect of rock and concrete through equivalent stress history

The strength enhancement effect of rock and concrete under dynamic loading is one of the significant response characteristics differing from quasi-static loading. Typically, current dynamic constitutive models describe this phenomenon by adopting strain/stress rate dependent functions. In this study, a novel elastoplastic damage constitutive model that captures the dynamic strength increase through the equivalent stress history is developed by coupling the incubation characteristic time (ICT) criterion and the unified strength theory (UST). Specifically, the ICT criterion substitutes the stress history for the strain/stress rate to predict dynamic yield strength of rock and concrete. And the equivalent stress expressed by the UST provide an appropriate selection of the stress history in the ICT criterion under complex stress states. Furthermore, based on the non-associated plastic flow rule and the plastic-damage evolution law, combined with the equation of state, the proposed constitutive model is established and implemented numerically. The dynamic stress-strain curves of granite were reconstructed by a series of numerical tests on a single element, which verified the accuracy of the proposed constitutive model and the reliability of the numerical algorithm by comparing the experimental data. The proposed model was applied to the numerical simulation of dynamic compression-shear tests on inclined sandstone specimens loading by the split Hopkinson pressure bar (SHPB) and fragmentation experiments of single borehole granite specimens under blast loading, respectively. And the experimental dynamic crack patterns on the specimen surface captured by high-speed cameras match the simulated damage distribution well. In addition, several common concrete constitutive models and the proposed model were employed to simulate the dynamic response of reinforced concrete beams under near-field blast loading. The damage patterns and the mid-span residual displacement calculated numerically by the proposed model are more consistent with the experiments than other concrete models. As a conclusion, the proposed model, coupled the ICT criterion and the UST, provides a new perspective and paradigm for characterizing the responses of rock and concrete subjected to intense dynamic loads.

An Overview of Sandbox Experiment on Ground Heat Exchangers

As an energy-efficient and low-carbon technology, ground-source heat pumps are promising to contribute to carbon neutrality in the building sector. A crucial component of these systems is the ground heat exchanger, which has been extensively studied through sandbox experiments. These experiments play a vital role in understanding heat transfer characteristics and validating simulation results. In order to facilitate the improvement of ground heat exchangers and the development of ground-source heat-pump systems, this article provides a comprehensive summary of existing sandbox experiments. The borehole sandbox experiments are classified into the single borehole experiment, borehole group experiment, seepage experiment, and multi-layer soil experiment. It was observed that the heat transfer efficiency of a single spiral tube is only 80% compared to that of a double spiral tube. Moving on to energy-pile sandbox experiments, they are further divided into mechanical performance, thermal performance, and thermal-mechanical coupled performance tests. It was revealed that the heat transfer distance of a single U-shaped energy pile in the radial direction is three times greater than in the vertical direction. For the mentioned sandbox experiments, the sandbox design, experiment conduction, testing conditions, and result analyses are summarized. To improve the sandbox experiments, there are still some difficulties in building a similarity experiment, testing the temperatures in a small error, controlling the boundary conditions accurately, and testing the thermophysical properties of soil accurately. Furthermore, the perspectives of sandbox experiments of ground heat exchangers are also proposed. The sandbox experiments under complex environment conditions or with novel composite energy geo-structures or ground heat exchangers with new materials and new technologies would be further investigated. By addressing these aspects, this review aims to provide guidelines for the design, construction, operation, and optimization of sandbox experiments for different ground heat exchangers, ultimately promoting the wider adoption of ground-source heat pumps in achieving carbon neutrality.

Evaluation of Liquefaction Potential Index and Severity Mapping for Rangpur City Corporation

This paper aims to investigate the Liquefaction Potential Index (LPI) of different Wards and prepare a liquefaction severity map of Rangpur City Corporation. A Liquefaction severity map of a city helps the legislators and policymakers with informed land use planning as well as the development of the town. Disturbed soils from 10 Wards among 33 Wards of Rangpur were collected for different depths in a single borehole. The locations for the borehole were selected based on the seismic soil profile and engineering geology reports available for Rangpur City Corporation in CDMP-II. Standard Penetration Test (SPT) and Microtremor test were done on the field. Then, the Grain Size Distribution test and Liquefaction Potential Index (LPI) were determined for the collected samples in the laboratory. Based on the results of the tests done in the field and laboratory, the severity was classified as moderate, moderate to severe, and severe. Based on the Liquefaction Potential Index (LPI) value, four Wards of Rangpur City Corporation fall within the moderate risk class, three Wards show moderate to severe risk of liquefaction, and the remaining 3 Wards belong to the severe risk class.
 J. of Sci. and Tech. Res. 4(1): 49-58, 2022

Loess Strata Distribution Characteristics and Paleoclimate Spatial Pattern during the Last Interglacial in the Luohe River Basin

Paleoclimate studies of loess in China have focused mostly on the time series of a single borehole or profile. However, research on loess strata and regional paleoenvironmental patterns could facilitate a deeper understanding of loess as a paleoenvironmental indicator and provide new insights into interpreting loess in sedimentary records of the paleoclimate. In this study, we determined the spatial pattern and regional characteristics of the paleoclimate during the Last Interglacial period in the Luohe River Basin, Shaanxi Province, China. We selected four representative boreholes in the study area (ZK04, ZK18, ZK13, and ZK19) from different landforms and zones, distributed from the northwest to the southeast, as well as three classic profiles (JB, JD, and LC). From north to south, comparative analyses were conducted of the loess strata, magnetic susceptibility, and grain size, and we analyzed the distribution characteristics of loess and paleosols in different geomorphological regions. The results showed that both the thickness and the sedimentation rate of loess in this river basin decreased from north to south. There were few paleosol horizons in the northern Liangmao area, but numerous such horizons in the southern plateau, and the degree of paleosol development increased from north to south. The magnetic susceptibility increased, whereas the particle composition tended to become thinner from north to south. The climate fluctuations of the Last Glacial recorded by the loess and paleosols in different regions were inconsistent.

Muon tomography for detection of dynamic border tunnels

Using muon tomography to detect tunnels has been suggested and studied with promising results. This work revisits the idea in the context of border tunnels with applications to drug interdiction and military intrusions. Unlike previous studies, this work considers the detection of border tunnels as they are being built with the goal of detecting tunnels before they cross the border. An updated physics model and numerical analysis is presented and applied to the problem with the following results. For a single borehole the optimal detector depth is 40–60 m with a reach of almost 60 m for drug tunnels. A more useful configuration is an array of detectors spaced evenly along the border. This picket fence arrangement has been suggested but not studied. With detectors spaced at 130 m separation this work shows that drug tunnels could be detected 18 days before crossing the border with only 10% variation in detection time. Relaxing the uniformity requirement allows the separation to increase to 300 m. For larger, military tunnels the distance between the detectors could be increased further to 450 m.

Robust reservoir identification by multi-well cluster analysis of wireline logging data

A novel clustering method is applied to well logs for improved rock type identification in hydrocarbon formations. For grouping the objects in the multi-dimensional data space, we propose a Most Frequent Value (MFV) based clustering technique applied to natural gamma ray, bulk density, sonic, photoelectric index, and resistivity logs. The MFV method is a robust estimator, which assists in finding the cluster centers more reliably than a more noise sensitive K-means clustering approach. The result of K-means cluster analysis highly depends on the choose of the initial centroids. To reduce the risk of inappropriately chosen starting values, we apply a histogram-based selection method to give the best position of the initial cluster centers. We assure the robustness of the solution by calculating the centroid as the MFV of the cluster elements and defining the overall deviation of cluster elements from the center by a weighted Euclidean (Steiner-) distance. The proposed workflow relies on a fully automated weighting of the cluster elements, which does not require a constraint on the statistical distribution of the observed variables. The processing of synthetic data shows high noise rejection capability and efficient cluster recognition even beside considerable amount of outlying and missing data; the accuracy is measured by the difference between the estimated and the exactly known distribution of cluster numbers. The clustering tool is first applied to single borehole data, then the procedure is extended to multi-well logging datasets to reconstruct the multi-dimensional spatial distributions of clusters revealing the lithological and petrophysical characteristics of the studied formations. A large in situ dataset acquired from several boreholes traversing Hungarian gas-bearing clastic reservoirs of Miocene age is analyzed. The accuracy of the field results is confirmed by core permeability measurements, independent well log analysis and a gradient metrics characterizing the noise rejection capability of the clustering method.

Comparison of model study with field implementation of gravity blind backfilling method to control subsidence induced disaster in abandoned underground coal mines

Blind hydraulic backfilling technique is used for subsidence control in underground coal mines. A laboratory size model of underground working was developed to understand backfilling process. Observations from model were utilized for backfilling process in one of the underground mines. This study describes the results obtained in the field investigation at an old abandoned waterlogged underground coal mine of Eastern Coalfields Limited (ECL), a subsidiary of Coal India Limited and their verification with the findings obtained in the laboratory scale model study carried out on a model of underground coal mine worked by board and pillar method. The relative influence of slurry concentration and flow rates on the areas of filling from a single inlet borehole has been discussed. The relative spread of sand in different directions has also been measured using a remotely operated underground vehicle mounted camera. The empirical relationships developed under field conditions have been found to be similar to those of laboratory model.

Investigating the Influence of Groundwater Flow and Charge Cycle Duration on Deep Borehole Heat Exchangers for Heat Extraction and Borehole Thermal Energy Storage

Decarbonisation of heat is essential to meeting net zero carbon targets; however, fluctuating renewable resources, such as wind or solar, may not meet peak periods of demand. Therefore, methods of underground thermal energy storage can aid in storing heat in low demand periods to be exploited when required. Borehole thermal energy storage (BTES) is an important technology in storing surplus heat and the efficiency of such systems can be strongly influenced by groundwater flow. In this paper, the effect of groundwater flow on a single deep borehole heat exchanger (DBHEs) was modelled using OpenGeoSys (OGS) software to test the impact of varying regional Darcy velocities on the performance of heat extraction and BTES. It is anticipated that infrastructure such as ex-geothermal exploration or oil and gas development wells approaching the end of life could be repurposed. These systems may encounter fluid flow in the subsurface and the impact of this on single well deep BTES has not previously been investigated. Higher groundwater velocities can increase the performance of a DBHE operating to extract heat only for a heating season of 6 months. This is due to the reduced cooling of rocks in proximity to the DBHE as groundwater flow replenishes heat which has been removed from the rock volume around the borehole (this can also be equivalently thought of as “coolth” being transported away from the DBHE in a thermal plume). When testing varying Darcy velocities with other parameters for a DBHE of 920 m length in rock of thermal conductivity 2.55 W/(m·K), it was observed that rocks with larger Darcy velocity (1e-6 m/s) can increase the thermal output by up to 28 kW in comparison to when there is no groundwater flow. In contrast, groundwater flow inhibits single well deep BTES as it depletes the thermal store, reducing storage efficiency by up to 13% in comparison to models with no advective heat transfer in the subsurface. The highest Darcy velocity of 1e-6 m/s was shown to most influence heat extraction and BTES; however, the likelihood of this occurring regionally, and at depth of around or over 1 km is unlikely. This study also tested varying temporal resolutions of charge and cyclicity. Shorter charge periods allow a greater recovery of heat (c. 34% heat injected recovered for 1 month charge, as opposed to <17% for 6 months charge).

Infinite borehole field model—a new approach to estimate the shallow geothermal potential of urban areas applied to central Budapest, Hungary

Deploying shallow geothermal solutions is critical for meeting energy demands while supporting decarbonisation targets. In densely populated areas, drilling large numbers of boreholes may lead to thermal interactions between closely located borehole heat exchangers. This paper presents a novel method termed the infinite borehole field model to estimate the technical shallow geothermal potential, especially in urban regions. The thermal interactions between boreholes are considered using finite element models simulating the operation of a single borehole in a larger field. Mathematical optimisation is used to find the amount of thermal energy that can be annually extracted while keeping the borehole wall temperature above freezing point of water. The method considers thermogeological details of geological formations including downward-increasing ground temperature, geothermal heat flux, thermal conductivity, heat capacity, porosity, density, and advective heat transfer. Results of our case study indicate that 100 m deep thermally independent boreholes can produce 14.20 MWh/a for 50 years on average. However, boreholes in an infinite borehole field spaced 20 m apart produce 7.80 MWh/a. A further investigation including advective heat transfer indicated that high velocity groundwater flow can significantly enhance borehole yield. Our method provides a generalised approach which can be beneficial prior to detailed site investigations.

3D Discrete Fracture Network Modelling from UAV Imagery Coupled with Tracer Tests to Assess Fracture Conductivity in an Unstable Rock Slope: Implications for Rockfall Phenomena

The stability of a rock slope is strongly influenced by the pattern of groundwater flow through the fracture system, which may lead to an increase in the water pressure in partly open joints and the consequent decrease in the rock wall strength. The comprehension of the fracture pattern is a challenging but vital aspect in engineering geology since the fractures’ spatial distribution, connectivity, and aperture guide both the water movement and flow quantity within the rock volume. In the literature, the most accepted methods to hydraulically characterise fractured rocks in situ are the single borehole packer test, the high-resolution flow meters for fractures, and the artificial tracer tests performed in boreholes. However, due to the high cost a borehole requires and the general absence of wells along coastal cliffs, these methods may not be appropriate in rockfall-prone areas. In this study, an unsaturated rocky cliff, strongly affected by rockfalls, was investigated by combining kinematic analysis, Discrete Fracture Network (DFN) modelling, and artificial tracer tests. The DFN model and potential rock block failure mechanisms were derived from high-resolution 3D virtual outcrop models via the Structure from Motion (SfM) photogrammetry technique. An artificial tracer was injected using a double ring infiltrometer atop the recharge zone of the slope to determine the infiltration rate and validate the DFN results. The DFN and tracer test methods are frequently used at different spatial scales and for different disciplines. However, the integration of digital photogrammetry, DFN, and tracer tests may represent a new step in rockfall and landslide studies. This approach made possible the identification of groundwater flow patterns within the fracture system and revealed about a 10-day tracer transit time from the injection area and the monitored slope, with similar conductivity values gathered from both the DFN and tracer test. Planar and wedge failures with volumes ranging from 0.1 and 1 m3 are the most probable failure mechanisms in the areas. The results were consistent with the delay between the intense rainfall and the slope failures previously documented in the study area and with their mechanisms.

Enhancement of shallow ground heat exchanger with phase change material

Heat pumps perform better when coupled with ground as thermal source than with air. In literature, several studies and applications suggest and analyse the use of phase change materials (PCMs) coupled with single or double U-tube vertical borehole heat exchangers (BHEs). Usually, PCMs are mixed with the grouting material during the installation. An alternative solution to vertical BHEs is the use of horizontal ground heat exchangers (HGHEs). The present work investigates the possibility of coupling PCMs with a flat-panel HGHE installed inside a trench 2 m under the ground surface. The study analyses the case in which PCMs are adjacent to the HGHE, taking a cue from alternative coupling technologies which have PCMs added to the backfilling material of the trench where the HGHE is installed. The analysis has been conducted with COMSOL software tool. A simulation model of the system was developed to carry out a parametric analysis. The objective of the simulations is the investigation of the thermal behaviour of the HGHE patent pending coupled with PCMs under cycles of operation which represent how the heat pump could work in GSHP system. The results show the meaningful difference of using the PCM in direct contact with the HGHE.

Study on Gas Migration Mechanism and Multi-Borehole Spacing Optimization in Coal under Negative Pressure Extraction

In order to study the gas migration in gas-bearing coal, and reasonably arrange gas drainage boreholes to improve the efficiency of gas drainage, a gas-solid coupling model is established based on the pore-fracture dual medium porous model. The solid deformation of coal body, gas seepage and diffusion, and gas adsorption and desorption are considered in this model. The COMSOL software is used to simulate the gas change in the coal matrix and coal fracture under single borehole extraction. We analyze the effective extraction range and study the migration mechanism of gas between coal fracture and borehole, coal matrix and coal fracture, and coal matrix. The effective extraction area of multi-borehole negative pressure gas extraction varies with extraction time and borehole spacing. At 140 d, the effective extraction radius is r = 1.3 m, and the spacing of boreholes is 233 r=1.5 m, 2 r=2.6 m,4 m,5 m,and 6 m, respectively. The influence of the equilateral triangle shape of three boreholes on the gas extraction effect is studied. The simulation results show that when three boreholes are extracted for 140 days under different borehole spacing, different gas extraction effects will be affected by a superposition effect. Considering the change in gas pressure, the effect of gas extraction in the effective extraction area, and the safety and cost performance of gas extraction, it is concluded that the optimal hole spacing is 5 m around 140 d. This study aims to provide reference for underground gas drilling layout and reasonable hole spacing.

Application of the Heat Penetration Distance in the Design of the Hole Spacing of Ground-Coupled Heat Pumps

Due to issues such as heat accumulation, the site area, and project investment, the reasonable determination of the hole spacing for heat exchangers has become one of the key design points of the ground-coupled heat pump system. Based on the definition of heat penetration in heat transfer and the research method of the inverse problem, a direct algorithm of the heat penetration distance in the aquifer was proposed using the analytical solution to the mathematical model for one-dimensional heat convection–conduction problems. Taking a vertical ground-coupled heat pump project in Hefei, Anhui Province, China, as an example, a three-dimensional hydro-thermal coupling numerical simulation model was established, and the influence radius during the refrigeration and heating periods under the action of a single borehole heat exchanger was determined. Comparing the heat penetration distance with the influence radius, the results show that the relative errors of the results obtained by the two methods are less than 10%, which verifies the rationality and effectiveness of the calculated penetration distance in the aquifer. At the end of the cooling or heating period, the heat penetration distance in the aquifer is calculated to be 7.59 m. Therefore, the proposed method is straightforward and efficient, which can provide a convenient approach to determining the reasonable hole spacing of the heat pump system.

Determining the reasonable volume required to inject liquid CO2 into a single hole and displace CH4 within the coal seam in bedding boreholes: case study of SangShuPing coal mine

Previously, determining the injection volume of liquid CO2 used to displace CH4 in coal seam boreholes has lacked scientific input, and has mostly been based on engineering experience and analogy. Here, one scientific method combining theoretical analysis, physical experimentation, and underground measurements was used to determine the reasonable injection volume for a single borehole along a coal seam when injecting liquid CO2 to displace CH4 within the coal seam. Therefore, the amount of CH4, ΔXCH4 , that needs to be displaced by injecting liquid CO2 in a single borehole was obtained by theoretical calculation, and the displacement ratio, β , of CH4 displaced by injecting liquid CO2 under different pressures was analyzed. An in-situ test was designed to obtain the relationship between the injection volume and the distribution of the CO2 content in coal seams located at different distances from the injection hole and the other two key parameters in the equation, such as the influence coefficient, η, of the distribution of the CO2 content in the coal seam and the effective diffusion radius, l0. Based on the relationship between ΔXCH4 , β , η and l0, the empirical equation for the reasonable injection volume of single hole is obtained, and the correction equation is obtained after considering the influence coefficient ξ of CO2 flow non-uniformity. The results showed that to displace CH4, 6.1–7.7 m3 is a reasonable liquid CO2 injection volume range within a single borehole along the bed in 3303 working face when ΔXCH4 is 0.75 m3 t−1, β is 0.0088–0.01, η is 0.275, l0 is 16 m and ξ is 0.155. This method provides a theoretical basis and scientific guidance for applying liquid CO2 injection technology to displace CH4 in coal seams.

Comparative Analysis of the Drainage Effect Based on Improved Hydraulic Flushing Technology.

The composite structure coal seam composed of coal seams with strong and weak deformation is widely developed in the coalfields in Southwest and North China. The implementation of hydraulic flushing in soft coal stratification can effectively relieve the pressure and increase the permeability of coal seams, so it can improve the effect of underground gas drainage. Taking Xin'an Coal Mine, West Henan province, China, as an example, this paper discussed the influence of hydraulic flushing on the gas drainage effect in soft coal seam by means of field investigation, numerical simulation analysis, similar simulation, and field verification. As the same time, a fluid-solid coupling model of gas drainage based on layered hydraulic flushing technology is established, and the differences and reasons of the gas drainage effect after layered hydraulic flushing in soft and hard composite coal seams are discussed. The results show that (1) the gas concentration change of layered flushing boreholes intermittently rises rapidly, and there is no obvious law, while that of conventional flushing boreholes shows a regular attenuation trend. (2) According to the numerical simulation results, after 30 days of drainage, the effective drainage range of layered hydraulic flushing is 8.33 m3 higher than that of conventional hydraulic flushing. (3) There are not a few points where the single borehole concentration data of each row of layered flushing boreholes are in the high level. Meanwhile, the maximum concentration of all conventional flushing boreholes is less than 40%, except that one borehole can reach 70% at the beginning. (4) All of the average gas drainage concentration of single-row boreholes using layered flushing technology is greater than 20% in the vast majority of days, while that of the conventional flushing boreholes is only higher than 20% in row 5 and has only lasted for 2 days. The difference of the drainage effect is obvious.

Enhancing Methane Recovery with Cryogenic Liquid CO2 Cyclic Injection: Determination of Cyclic Injection Parameters.

Carbon dioxide (CO2) is both a primary greenhouse gas and a readily available energy source. In this study, a new underground coal permeability enhancement technique utilizing cryogenic liquid CO2 (L-CO2) cyclic injection is proposed. The key parameters that determine the feasibility of this technique are cycle period and cycle number within a fixed working period. The optimal value of these two parameters mainly depends on the pore structure evolution law of coal cores before and after cryogenic L-CO2 cyclic freeze–thaw. Accordingly, nuclear magnetic resonance (NMR) was employed to study the evolution characteristics of the fracture networks and pore structures in coal cores subjected to different freeze–thaw cyclic modes. The results demonstrated that the amplitude and width of all peaks of the T2 spectra of the three coal cores (lignite, gas coal, and 1/3 coking coal) increased with an increase in the number of injection cycles. Additionally, as the number of freeze–thaw cycles (Nc) increased, the total porosity and effective porosity of the coal cores increased linearly before stabilizing, while the residual porosity first steadily diminished and afterwards became constant. Furthermore, the variation in the total porosity and residual porosity of the coal cores continuously diminished with an increase in the level of metamorphism. The NMR permeability of the coal cores showed a similar pattern to the porosity. Accordingly, the optimal parameters for cryogenic L-CO2 cyclic injection during a complete working time were determined to be Nc = 4 and Pc = 30 min. A field test demonstrated that after L-CO2 cyclic freeze–thaw treatment, the average gas drainage concentration of a single borehole in the test region increased by 1.93 times, while the pure flow of a single gas drainage borehole increased by 2.21 times. Finally, the gas attenuation coefficient decreased from 0.036 to 0.012. We concluded that the proposed permeability enhancement technique transformed coal seams from hard-to-drain to drainable.

Failure mechanism of methane drainage borehole in soft coal seams: Insights from simulation, theoretical analysis and in-borehole imaging

Methane drainage borehole in soft coal seams is prone to instability. This could seriously compromise methane drainage efficiency, leading to risks of methane hazards at coal mines. In this paper, the failure of methane drainage boreholes in soft coal seams is studied through theoretical analysis, numerical simulation and field monitoring methods, which aims to guide the borehole protection and improve the methane drainage capacity. The functional relationship between effective radial normal stress and effective tangential shear stress at the unstable position of the borehole and the in-situ stress and pore pressure is constructed through the theoretical study. The equation to derive borehole wall collapse pressure is established, considering cohesion, internal friction angle, coal seam porosity, pore pressure, in-situ stress and other factors. The numerical modelling shows that the coal around the borehole undergoes tensile breakage towards the borehole in both single and multiple borehole conditions. The vertical displacement at the borehole top is the largest and the borehole turned into an elliptical shape under compression. Also, it shows the opening and the end of the borehole are at higher failure risks than middle sections. The horizontal and vertical displacements of multiple boreholes are larger than that of a single borehole. While the usage of multiple boreholes improves the methane drainage efficiency, it brings more challenges to the borehole stability. The creep deformation law of borehole is revealed based on new developed in-borehole imaging monitoring system, including four stages: first (transient), second (steady-state), tertiary (accelerating state) and closure. The field measurement shows that the methane drainage efficiency was relatively high in the first two stages, and then the efficiency dropped rapidly when borehole closing. The research results are critical for guiding methane drainage borehole protection, improving the methane drainage efficiency and enhancing safety in production.

Study on the Long-Distance Gas Pre-Drainage Technology in the Heading Face by Directional Long Borehole

Gas control in the heading face of a coal roadway is an important and difficult point in coal mining in China. On the basis of analyzing the disadvantages of high gas control cost and long drainage period in the existing mine heading face, a long-distance pre-drainage method of long-distance drilling is proposed to control the gas in the heading face so as to improve the tunneling speed. Applied to the engineering geological conditions of Changcun coal mine, the technology is studied in detail. First, a gas migration model considering permeability changing with time is established, and the model is put into the numerical simulation software to study the variation law of permeability and gas pressure under the conditions of single borehole and multi-borehole drainage. The results show that with the increase of drainage time, the permeability around the borehole increases gradually, the gas pressure decreases gradually, and the permeability at the borehole boundary increases the most, reaching 1.2 times the initial permeability. In the process of multi-borehole drainage, there will be mutual influence between boreholes, but with the increase of borehole spacing, the degree of this influence gradually decreases. Second, according to the results of numerical simulation, a reasonable gas drainage scheme is designed and applied in the field. The field application shows that the technology has a good gas drainage effect, the gas drainage concentration and flow are at a high level for a long time, the drilling cuttings quantity is always lower than the critical value, and the excavation length of roadway increases by more than 50 m per month. These results indicate that this technology is a promising method to realize the safe and rapid excavation of a mine coal roadway.

Heat Transfer Modeling on High-Temperature Charging and Discharging of Deep Borehole Heat Exchanger with Transient Strong Heat Flux

High-temperature charging and discharging by deep borehole heat exchanger is typical of a large heat exchange temperature difference and transient strong heat flux. Simulation of this problem is not only computationally expensive, but it is also challenging in terms of robustness and stability for numerical methods. This paper formulates a generic and efficient heat transfer model with two distinctive novelties: Firstly, it highlights unsteady- and quasi-steady-state modeling strategies for heat transfer outside and inside a borehole. Secondly, this model provides analytical solutions for the heat front propagation and heat flux density distribution for unsteady-state heat transfer in the rock zone. These analytical formulations prove to be generic and critical to relieve computational effort in the face of strong heat flux. This model is validated by a typical high-temperature heat storage case from the literature, as well as the pilot demonstration project in China. It was discovered that a large prediction error of the heat transfer model only exists in very short operation days during the initial unsteady stages of charging and discharging. Both relative errors under charging and discharging phases are within 5% during the steady-state period. A comparison of the simulation cost with OpenGeoSys software demonstrates its high efficiency. It proves that this heat transfer model achieves an acceleration ratio of 30 times relative to the fully numerical method. In general, the heat transfer model has four advantages: generic applicability, good accuracy, easy implementation, and high efficiency, but it is limited to the heat transfer of a single deep borehole heat exchanger under pure heat conduction.