Search Results (42)

The stability and safety of the vertical shaft during construction is an important problem for deep mining engineering because of the high in situ stresses. This paper conducts experimental studies on the difficulty of shaft support during the construction of No. 6 deep shaft at the Huize Mine, Yunnan Province, China. Based on the rule of similarity test, a similar material formula was developed, and standard model samples of the vertical shaft were prepared. Three different support methods were set up, including steel fiber-reinforced concrete support, drilling pressure relief support, and slot filling support. The experiments were conducted by using a true triaxial test system, and the testing process was monitored by a static stress–strain gauge and an acoustic emission system. The experimental results show that the integrity of the borehole pressure relief support shaft is optimal under the in situ stress. As the maximum principal stress increases to the instability and failure of the shaft, the peak load, cumulative number, and energy of acoustic emission events were the highest using the steel fiber concrete support method, and the peak load was the lowest using the borehole pressure relief. The borehole pressure relief transfers the stress around the shaft to the deep part. Although it ensures the integrity of the shaft, it causes internal damage to the shaft, reduces the energy storage of the shaft, and results in the lowest cumulative number and energy of acoustic emission events. After the instability and failure of the shaft, the average block size of the shaft debris is the highest under the borehole pressure relief support along the direction of the maximum principal stress. On the other hand, the mechanical properties of samples with different support methods under dynamic load conditions are studied by applying external low-frequency disturbances, and the test conclusions have been verified through numerical simulation. Field tests have verified that the steel fiber-reinforced concrete lining support can maintain the integrity of the deep shaft wall and ensure safety during mining production. Full article

To investigate the distribution of coal seam stress ahead of the working face under hard-roof conditions and analyze its impact on gas flow, this study focused on the 16,021 working face in Wu Hua No. 1 Mine. First, we established a mining model using UDEC to analyze stress distribution at different coal seam extraction distances. Second, we used COMSOL Multiphysics 6.3 to simulate the influence of stress on the permeability and gas pressure of coal seams during extraction, thereby exploring how stress distribution affects gas flow. Finally, we deployed gas extraction boreholes to validate the gas flow characteristics associated with the stress zones. The results indicate that the coal seam stress ahead of the working face forms four distinct zones, influenced by the main roof hanging: stress reduction zone I, stress concentration zone, stress reduction zone II, and original stress zone. When extraction days are equal, under high-stress conditions, the extracted coal seam exhibits low permeability and a small decrease in gas pressure, making gas extraction difficult; in contrast, under low-stress conditions, it exhibits high permeability and a large decrease in gas pressure, making gas extraction relatively easier. Field measurements show that the gas extraction flow rate initially increases and then decreases with distance from the coal wall, exhibiting a noticeable rise within the 47–62 m range before stabilizing. This trend aligns well with the characteristics of stress zoning. Full article

The process of exploiting hydrocarbon deposits is subject to many complications, some of which can make exploitation very difficult or impossible. These factors include damage to the wellbore zone by drilling fluid, which impedes the flow of reservoir fluid from the production zone to the well. This article presents the results of research conducted to develop drilling fluid compositions with the best possible ability to form a tight sealing sediment on the borehole wall. In addition to traditional carbonate blockers, modern organic agents were used as bridging agents. Research was conducted on the selection of the drilling fluid composition, the rheological parameters of which would ensure the suspension of the solid phase in the form of various types of blockers. After preparing the base drilling fluid, its composition was modified by adding different configurations of blockers. The sets of blockers added to the fluid varied in both chemical structure and particle size. Such modified fluids were then subjected to tests of technological properties, such as rheological parameters, API filtration, and pH. In the next stage, sealing tests of the filter cake formed by the tested fluids were carried out on the surface of the rock core using the PPT—Pore Pressure Transmission Test. Based on the obtained results, it can be concluded that the new type of organic blockers used allows the rapid formation of a tight filter cake on the borehole wall, and thus significantly reduces drilling fluid filtration. During PPT, the sediment formation time (t_pmax) for OB2 was 45 min; for the combination of OB1 and the carbonate inhibitor, it was 8 min; and for the carbonate inhibitor alone, it was 150 min. Full article

Aiming at the problems of borehole shrinkage and pipe sticking caused by creep in salt-gypsum rock formations during deep well drilling, multi-field coupling creep experiments on deep salt-bearing gypsum mudstone were carried out. Furthermore, a nonlinear creep constitutive model was constructed based on the Drucker–Prager criterion, and the critical value of liquid column pressure for borehole shrinkage was determined through numerical simulation. Experiments show that at 140 °C, salt-gypsum rock is mainly subjected to brittle failure with single shear fracture, while at 180 °C, multiple sets of cross-cutting shear bands form, shifting to plastic flow-dominated composite failure. The coupling effect of confining pressure and deviatoric stress is temperature-dependent; the critical deviatoric stress is independent of confining pressure at 140 °C, but decreases significantly with increasing confining pressure at 180 °C, revealing that salt-gypsum rock is more prone to plastic flow under high temperatures and confining pressure. The creep constitutive equation was further determined, and fitting parameters show that the stress exponent m = 2–5 and the time exponent n decrease linearly with the increase in deviatoric stress, and the model can accurately describe the characteristics of three-stage creep. The numerical simulation found that there is a nonlinear relationship between the drilling fluid density and borehole shrinkage; the shrinkage rate exceeds 1.47% when the density is ≤2.0 g/cm³, and the expansion amount is >1.0 mm when ≥2.4 g/cm³. The critical safe density range is 2.1–2.3 g/cm³, which is consistent with the field data in the Bayan area. The research results provide an experimental basis and quantitative method for the dynamic regulation of drilling fluid density in deep gypsum rock formations, and have engineering guiding significance for preventing borehole wall instability. Full article

The widespread application of horizontal drilling technology has significantly enhanced the development efficiency of unconventional resources, particularly shale gas, by overcoming key technical challenges in reservoir exploitation. However, wellbore instability remains a critical challenge during shale gas horizontal drilling, as borehole wall collapse often results in the accumulation of large-sized cuttings (or blocky cuttings), increasing the risk of stuck pipe incidents. In this study, a large-scale circulating loop experimental system was developed to investigate the hydrodynamic behavior of blocky cuttings transport under the influence of multiple factors, including rate of penetration (ROP), well inclination, flow rate, drilling fluid rheology, and block size. The experimental results reveal that when ROP exceeds 15 m/h, the annular solid-phase concentration increases non-linearly. At a well inclination of 60°, the axial and radial components of gravitational force reach a dynamic equilibrium, resulting in the maximum cuttings bed height. To enhance cuttings transport efficiency and mitigate deposition, a minimum flow rate of 35 L/s and a drill pipe rotation speed of 90 rpm are required to maintain sufficient turbulence in the annulus. Drilling fluid plastic viscosity (PV) in the range of 65–75 mPa·s optimizes suspension efficiency while minimizing circulating pressure loss. Additionally, increasing fluid density enhances the transport efficiency of large blocky cuttings. A drill pipe rotation speed of 80 rpm is recommended to prevent the formation of sand-wave-like cuttings beds. These findings provide valuable hydrodynamic insights and practical guidelines for optimizing hole-cleaning strategies, ensuring safer and more efficient drilling operations in shale gas horizontal wells. Full article

In the drilling process in permafrost strata, the mass and heat transfer effects may thaw the strata around the boreholes and decrease the content of pore ice, thus causing the mechanical properties of the strata to deteriorate greatly, thus influencing the stability of the borehole walls. In this work, a multiphysics coupling mathematical model was built for the stability of borehole walls in permafrost strata. Based on this model, the leading factors for the influences of the mass and heat transfer effects of drilling fluids on the stability of borehole walls were analyzed, and the influences of different drilling conditions on the stability of borehole walls were studied. The results demonstrate that the heat conduction of drilling fluids to the strata is the most important factor that influences the stability of borehole walls, and the diffusion of salt components affects the freezing temperature of pore water and the pore ice content in the frozen area. As the duration of the drilling increases, the collapsed zones of the borehole walls develop toward the radial and circumferential directions. Decreasing the temperature of the drilling fluids can improve the temperature distribution in the strata around the boreholes and is beneficial to reducing the degree of collapse. The increment in the concentration of salt components in the drilling fluids can decrease the overall temperature distribution in the strata, while the increase in the ionic concentration substantially decreases the pore ice content in permafrost and increases the borehole expansion rate. Enlarging the fluid column pressure of the drilling fluids does not intensify the mass and heat transfer effect of drilling fluids on the strata, while it greatly affects the stress distribution in the strata, shrinks the borehole collapse range, and improves the stability of the borehole walls. Full article

In hydraulic fracturing tests, the initial crack length and the compressibility of the injection system have a significant effect on the initiation and propagation of the fracture. Numerical or theoretical models that ignore the compressibility of the injection system are unable to accurately predict fracture behavior. In this paper, a 2D analytical/numerical model based on linear elastic fracture mechanics is presented for the initiation and propagation of hydraulic fracturing from two transversely symmetrical cracks in a borehole wall. It is assumed that the fracture is driven by compressible inviscid fluid in a permeable medium. To solve the problem, the governing equations are made dimensionless and the problem is solved in the compressibility–toughness-dominated propagation regime. According to the results, the initial crack length and the compressibility of the injection system have a significant effect on fracture initiation behavior. When the initial flaw length is small or compressibility effects are important, the initiation of the fracture is accompanied by instability and the occurrence of a sudden decrease in borehole pressure and a sudden increase in crack length. If the initial crack length is large or the compressibility effects are negligible, the crack propagation is stable. The leak-off coefficient has no effect on the pressure level required for crack propagation, but with an increase in leak-off, more time is required to reach the conditions for crack propagation. The results obtained in this paper provide good insights into the design of hydraulic fracturing processes. Full article

Geothermal energy, as a clean, low-carbon, widely distributed, renewable and environmentally friendly energy source, plays an important role in the transition from traditional energy sources dominated by coal and oil to clean energy. Ground source heat pump technology is a key technological tool for developing geothermal energy for widespread use. Coaxial-cased heat exchangers are the core component of the ground source heat pump system, and their heat transfer performance directly affects the heat transfer efficiency and service life of the ground source heat pump system. According to the actual working conditions of coaxial-cased heat exchangers in fractured aquifers, the coupled pressure–temperature model of the heat transfer outside the borehole was created by considering the influence of the Joule–Thompson effect. For heat transfer inside the wellbore, a multi-layer long concentric cylinder wall model was developed to obtain the fluid temperature distribution within the wellbore. Results show that the heat transfer efficiency increases with the increase of thermal conductivity, water production and effective permeability of fractures. The positive and negative values of the Joule–Thompson coefficient reflect the trend of fluid temperature changing with pressure. The larger the absolute value is, the greater the temperature change is. The increase in the initial temperature of the injected water will lead to a decrease in the theoretical heat transfer. With the increase of the water injection rate and horizontal wellbore length, the heat recovery power will also increase significantly, but the optimal value needs to be considered comprehensively. The findings of the study can not only lay a theoretical foundation for the performance evaluation and optimal design of coaxial-cased heat exchangers but also have great significance in promoting the efficient development of geothermal energy. Full article

Pipeline pullback load is a crucial basis for drill rig selection and pipeline strength design. This paper presents a new pullback load calculation model from the perspective of pipe–borehole wall contact. The pipe–borehole wall contact analysis includes the distribution of contact pressure and the relationship between the external load and compressive displacement. The friction force between the pipe and the borehole wall was calculated based on the pipe–borehole wall contact analysis and adhesion theory without depending on the empirical friction coefficient. The effects of the eccentricity were also considered when calculating the fluid drag force. Through case studies, we verified the applicability of the model and discussed the possible reasons for the errors between the theoretical and field-measured results. This study can provide a helpful tool for analyzing the pipe–borehole wall contact and pullback loads for horizontal directional drilling. Full article

Theoretical analysis and numerical simulation are used to study the influence of different parameters of variable diameter borehole pressure relief technology on the surrounding rock and support. A strain-softening model was established to analyze the intrinsic connection between the parameters of variable diameter boreholes and the evolution of surrounding rock stress, deformation law, and support strength. The results show that: (1) With the increase in shallow borehole diameter, it is easy to cause anchor de-anchoring phenomenon. (2) After the deep borehole diameter is more than 250 mm, it transfers the peak of the shallow vertical stress to the deep surrounding rock (about 16 m away from the coal wall). (3) If the position of the variable borehole aperture is set between the anchorage zone and the stress peak of the roadway, the stress transfer effect is better, and the influence and effective binding force on the surrounding rock is smaller. (4) When the spacing is 1.0 m~2.0 m, the vertical stress starts to transfer to the deep surrounding rock, the deformation of the surrounding rock is smaller, and the reduction in the effective binding force of the anchors is smaller. The result can provide a reference for similar production conditions. Full article

A foundation pit is constructed in the floodplain of Yangtze River, and a deep and thick layer of large-particle pebble gravel exists below the base slab, thus forming a connected supply channel with the adjacent Yangtze River. The large water volume, high water pressure, and strong permeability of this layer bring great risks to the foundation pit construction. In view of the fact that conventional waterproof curtain construction technologies such as the deep mixing column and high-pressure jet grouting column cannot meet the engineering requirements under these kinds of geological and environmental conditions, a new waterproof curtain construction technology that combines the trenching technology of the diaphragm wall with the TRD (Trench cutting Remixing Deep wall) technology is proposed, i.e., the trenching-and-replacing-style TRD technology, as well as the construction process of this technology, is presented. After the waterproof curtain is built using the proposed technology, the strength, integrity, uniformity, and service performance of the waterproof curtain wall are tested and evaluated by the comprehensive methods of coring, borehole television imaging, resistivity CT, and a group well pumping test. The results show that the proposed technology overcomes the adverse effects of underlying large-particle pebble gravel layer, and the waterproof curtain built by it effectively cuts off the hydraulic connection inside and outside the pit. The technical proposal can provide useful references for similar projects. Full article

In deep carbonate reservoirs, testing and production with open-hole completion can help release the maximum production capacity. However, because the reservoir is subjected to high in situ stress, if the test pressure differential is too large, the wellbore collapse and instability will occur easily, causing downhole accidents. Therefore, it is necessary to determine the state of the borehole during the open-hole test in the carbonate reservoir and analyze the ultimate test pressure differential accordingly to ensure test safety. Considering the characteristics of open-hole completion, based on the mechanical properties of the carbonate reservoir and the stress distribution around the borehole during testing, a calculation method of the elastic zone, plastic zone, and residual failure zone around the open-hole wellbore was proposed. Regarding actual engineering data, a criterion for the overall stability of the open-hole section had been established from three aspects: the volume ratio of the plastic zone; the failure zone around the wellbore; and the failure angle on the borehole wall. According to this criterion, it is possible to determine the ultimate pressure differential during the open-hole test process and provide theoretical support for designing the open-hole completion test and production parameters for deep carbonate reservoirs. Full article

Drilling with prestressed concrete (DPC) pipe pile is a nonsqueezing pile sinking technology, employing drilling, simultaneous pile sinking, a pipe pile protection wall, and pile side grouting. The unloading induced by drilling, the pipe pile supporting effect, and the dissipation of the negative excess pore-water pressure after pile sinking, all of which have significant effects on the recovery of soil pressure on the pile side, are the main concerns of this study, which aim to establish a method to reasonably evaluate the timing selection of pile side grouting. The theoretical solutions for characterizing the unloading and dissipation of the negative excess pore-water pressure are presented based on the cylindrical cavity contraction model and the separated variable method. By inverse-analyzing the measured initial pore pressure change data from borehole unloading, initial soil pressures on the pile side of each soil layer are determined using the presented theoretical solutions. Then, the presented theoretical solutions were verified through a comparative analysis with the corresponding measured results. Moreover, by introducing time-dependent coefficients α_t1 and α_t2 to characterize the pore pressure dissipation and rheology effects, the effects of the negative excess pore-water pressure dissipation on the pile-side soil pressure recovery are discussed in detail. Full article

The DIGIT experiment was launched at the Tournemire Underground Research Laboratory URL with the aim of determining the effects of temperature on the transfer of analogues of most mobile radionuclides (i.e., ³⁶Cl, ¹²⁹I and ⁷⁹Se) in the Toarcian clay rock, the properties of which are close to host rocks being considered for future deep geological disposal of high-level (HL) radioactive wastes. The experimental principle involves the monitoring of an exchange between a test water traced with stable halides and deuterium at constant concentration and the porewater of the Toarcian clay rock submitted to various temperatures. This experiment seeks to partially address questions regarding the potential spread of contaminants during the thermal phase of High Level Waste (HLW) waste packages. Specifically, the in situ experiment aims to evaluate the role of scale effects and thermodiffusion, a process that combines Fick’s law and the Soret effect, in the transfer of radionuclides. This paper presents the first steps of the study, including the scoping calculations, the experimental set-up and the first results obtained during the unheated phase. The study started with the acquisition of the initial parameters, including the rock thermal properties, the concentrations of the four tracers (chloride, bromide, iodide and deuterium) naturally present in the clay porewater and their diffusive transport parameters by using four diffusion exchange techniques (phase 0). A model coupling heat and mass transfers was then developed using Comsol Multiphysics^®, integrating data acquired so far with existing literature data. A test water with a tracer concentration around 1000 times higher than those in the pore water was proposed with a temperature imposed at the test section wall of 70 °C. A large test zone of 50 cm height and 1 m in diameter and installed in a 3 m deep vertical well located in a sound zone at the URL was then proposed. The installation of the experiments required the realization of one shaft and of nine peripheral boreholes for the monitoring of temperature, water pressure and deformation. The experiment started with phase 1, involving a traced, unheated water start-up for a period of 5 months. Then, a core sampling was conducted in the emptied well, and the same diffusion exchange techniques were applied. The results of anionic tracers were compared to simulations based on initial parameters (phase 0), revealing that tracer penetration at the end of phase 1 exceeded simulated values by approximately 2 cm. This result is interpreted as an increase in the accessible porosity to tracers, possibly due to the excavation damaged zone. Future simulations should incorporate these adjusted diffusive transport parameters. Following phase 1, the heating system was activated, applying a temperature of 70 °C to the test zone. New data will enable the comparison of tracer penetration and assess the actual impact of temperature on tracer transfer. Full article

Directional hydraulic fracturing (DHF) is popular with hydraulic fracturing operations in coal mining to create cave-hard roofs, in which radial initial notches are created around open borehole walls before injecting high-pressurized fluid. Despite extensive field application of DHF, the three-dimensional irregular hydraulic fracture (HF) geometry in DHF remains unclear, and the HF re-orientation mechanism requires comprehensive understanding. Here, we experimentally examined factors affecting HF re-orientation in DHF in transparent gelatin samples with a self-developed experimental device. We found that it is the ratio between the differential stress and gelatin elastic moduls that determines HF re-orientation rather than the absolute magnitudes of these two factors. Both shear failure and tensile failure occur during HF re-orientation. The HF tends to propagate asymmetrically, and the step-like HF geometry is likely to form in gelatin samples with low elastic moduli and under high differential stresses. HF re-orientation is not necessarily a near-borehole effect, and HFs can propagate along the notch direction for longer distances in stiffer gelatin samples under relatively low or moderate differential stresses. Finally, recommendations are provided for the effective utilization of DHF at coal mine sites. Full article