Geofluids最新文献

Coal mining in China is increasingly moving towards deeper resources. In certain Carboniferous–Permian coal fields in North China, there is a typical problem of repeated mining over short distances and multiple coal seams, and the distance between the main coal seam and the mined shallow coal seam is relatively small. This leads to repetitive disturbance damage to the surrounding rock during closed-coal seam mining. This was followed by more serious threats from water disasters. Therefore, it is particularly important to systematically study the disturbance and failure characteristics of rock masses under repeated mining conditions in multiple coal seams and to investigate repeated mining damage law and characteristics of the overlying rock of a close coal seam. This study focuses on six mining areas in the Jiyang coal mine. This study also predicts the damage height of the top and bottom slabs of mined No. 7 coal and main mined No. 10 coal based on an empirical formula. An on-site investigation was conducted using downhole drilling and segmental water injection to determine the damage depths of the two seams. A numerical simulation was then conducted to study the height, displacement, and stress of the overlying rock damage caused by mining close to the coal seams. Changes in height, displacement, and stress of the overburden rock mining damage were also studied through numerical simulations. Results indicated the following: (1) The height of the plastic zone of the overburden rock increased by 6.25%, (2) maximum settlement displacement increased by 5.05%, and maximum horizontal displacement increased by 9%. It is important to note that these findings are objective and based solely on presented data. Repetitive disturbances with larger amplitudes caused a 27% change in horizontal displacement. (3) Maximum principal stress in the stress field decreased with the overall value of the vertical stress. The influence range increased slightly and the two sides of the hollow area at the open cutting eye and stopping line were also affected. These findings were obtained through numerical simulations. Stress concentration was more pronounced in the open-cut eye and near-the-stop line.

The traditional specific storage coefficient (S_s) was defined under two assumptions. One is that aquifer rock deforms only in the vertical direction, and the other is that the average rock stress remains unchanged. Consequently, S_s is irrelevant to the shear modulus of rock (G). In this paper, the Biot theory is used to derive a new specific storage coefficient ($$) with the natural deformation of rock. $$ appears to be relevant to G. Compressed glass beads and Berea sandstone are used for illustration. At frequencies lower than 10 kHz, the equation of groundwater flow with $$ yields the same phase velocity and quality factor as the Biot theory, and therefore, it is capable of accurately predicting fluid pressure diffusion in the low-frequency regime. The results also show that S_s is 16%–17% higher than $$. In conclusion, the latter one is superior to the former in its consistency with the Biot theory and unconstraint by the aforementioned two assumptions.

The Ordos Basin is the main tight gas-producing basin in China. Under the background of strong heterogeneous physical property, there is still high porosity and permeability “sweet spot” in tight reservoirs. Feldspar dissolution exerts a significant effect on porosity/permeability, while its genetic mechanism is unclear, which restricts the prediction of effective reservoir. In this paper, genesis of feldspar dissolution and its effect on reservoir heterogeneity were studies. Carboniferous-Permian Formation, which is the main gas-producing strata in northeast margin of Ordos Basin, is taken as the target. Based on the methods of thin section observation, physical property test, inclusion, isotope and productivity analysis, mechanism of feldspar dissolution, and its positive modification to tight sandstone reservoir are studied. The results show that the target sandstone is dominantly made up of litharenite and feldspathic litharenite. About 98.6% of the samples possess permeability less than 1 mD, while 89.7% of the samples possess porosity less than 10%. The high porosity/permeability space within tight reservoirs is dominated by dissolved pores, accounting for more than 80% of the total pore space. The dissolution minerals are mainly associated with feldspar, and the amount of feldspar dissolution is positively correlated with porosity and permeability. According to the fluid chemical information of quartz overgrowth and ankerite (related to feldspar dissolution), feldspar dissolved pores are caused by organic acid derived from the thermal maturity of organic matter. According to production data, no daily production can be achieved in single well, when the porosity and permeability are less than 7% and 0.1 mD, respectively. However, once it exceeds this porosity/permeability threshold value, the production capacity is exponentially improved. Furthermore, when surface porosity of feldspar dissolution exceeds 7%, most values of the porosity and permeability exceed the threshold values of the allowable productivity, indicating that extensive feldspar dissolution is favorable factor for effective reservoir space development in tight reservoirs.

This study is aimed at solving the issue of mining under the boundary coal pillar of the close-distance coal seam that causes roof falling. This study established a new key bearing structure model for analyzing the structural instability mechanism when mining under the coal pillar at the working face by taking Shaping Coal Mine as an example. The purpose of this study is to analyze the formation process, load transfer mechanism, and two failure types of the key bearing structure using theoretical analysis and numerical simulation. Additionally, this study discussed the timing and method of roof control for different key bearing structure failure types. Research shows that the instability of the key bearing structure composed of the coal pillar, interlayer rock, and lower coal body is the important reason behind the roof falling. The instability types of key bearing structures include the coal pillar instability type and the cantilever beam instability type. The stability width of the interlayer rock cantilever beam and the coal pillar jointly ascertain the failure type of the key bearing structure. In the 9204 working face, the key bearing structure was destroyed when the coal pillar was 14 m wide, resulting in the roof stress being as high as 31.81 MPa. The stress drop phenomenon can be used as a boundary to divide the failure process of the key bearing structure into three stages. The pressure relief of the coal pillar and interlayer rock cantilever beam is an effective way to deal with this problem, and the coal pillar instability type needs to be pressure relieved earlier than the cantilever beam instability type. The research findings offer new insights into the roof stability control of mining under the coal pillar.

This study examines energy dissipation patterns and failure mechanisms in coal under cyclic impact, crucial for preventing dynamic disasters like rock bursts and coal and gas outbursts. Using a 75-mm split Hopkinson pressure bar (SHPB) experimental system, the dynamic mechanical characteristics and fragment size distribution patterns of coal samples were analysed under a confining pressure of 10 MPa, axial pressure of 12 MPa, and impact pressures of 0.25, 0.30, 0.35, 0.40, and 0.45 MPa for 1, 2, and 3 cycles. The experimental data indicate that as the number of impacts increases, the energy reflected by the coal samples gradually increases, while the transmitted energy correspondingly decreases. The energy absorbed per unit volume of the coal samples under the first, second, and third dynamic loading cycles and confining pressure is 0.56, 0.61, and 0.66 J/cm³, respectively, with energy absorption rates ranging from 16.2% to 33.8%. Under different impact pressures, the fractal dimension of coal fragmentation shows a linear change, and as the impact pressure increases, the degree of fragmentation intensifies, and the mass of the fragmented coal decreases. The strength reduction in the energy dissipation patterns of coal samples under dynamic loading provides important theoretical support for the prevention of rock bursts during coal mining.

Abundant evidence exists for deep crustal penetration of meteoric fluids along faults, including emergence of hot, dilute, and isotopically light geothermal fluids in extensional settings; however, the nature of the fluid conduits supporting this rapid circulation from surface to the brittle-ductile transition and back remains mysterious. Metamorphic core complexes (MCCs) are the sites of rapid exhumation of rocks from that depth, and their associated detachment faults are known loci of fluid migration. This study utilizes spot infrared (IR) spectroscopy of drill core and outcrop to unravel the fluid history of the late-Neogene Silver Peak-Lone Mountain MCC and detachment fault (SPLMDF) in SW Nevada. That history begins with Mesozoic regional burial metamorphism of Paleozoic sediments, minor late Mesozoic contact metamorphism by silicic intrusives, followed by upwelling of hot metamorphic fluids after detachment initiation (11 MYA), later circulation of moderate-temperature meteoric-geothermal fluids, and young (< 5 MYA) hot epithermal fluids upwelling along detachment-cutting normal faults. Each of these stages is characterized by distinct changes in sheet silicate mineral crystallinity and hydration. These are conveniently summarized by maturity indicators based on IR absorption peak ratios, for example, illite spectral maturity (ISM). Burial metamorphism up to greenschist facies is indicated by steadily increasing ISM versus depth in core from a detachment-penetrating geothermal exploration borehole. A sharp decrease in ISM characterizes the detachment damage zone, accompanied by reappearance of smectite, zeolite, and abundant iron oxides, indicating much cooler alteration by meteoric-origin fluids. Low-ISM zones are concentrated in the damage zone ± 10 m from the fault, resulting from an accumulation of very narrow alteration bands (10–50 cm wide). About 1/3 of the SPLMDF fault trace exhibits this low-temperature circulation. Another third of the trace is overprinted by postdetachment epithermal alteration with extreme ISM, often in zones extending along the detachment near cross-cutting normal faults.

The development of urban underground space is a crucial aspect of current urban development. However, various factors pose challenges during the development process. In particular, in the valley cities of western China, groundwater has a significant impact on the utilization of underground space. This study focuses on understanding the influence of hydrogeological characteristics on the development of underground space in Ledu District, Haidong. Firstly, the hydrogeological characteristics of the aquifer in the study area are analyzed to determine the recharge and discharge characteristics. The dynamic changes in the groundwater table over time are then examined. Subsequently, the hydrochemical characteristics of groundwater are investigated to expound on the influence mechanism of groundwater on the development of underground space resources in Ledu District. The study reveals that the hydrodynamic effect of groundwater in the study area is substantial and has a significant physical scouring effect on underground structures. The fluctuation of groundwater levels affects the stability of buildings by increasing groundwater buoyancy. Additionally, this fluctuation also leads to engineering and environmental problems such as differential settlement and land salinization. The chemical characteristics of groundwater primarily impact the stability of building foundations through the corrosive effects of groundwater composition. This research clarifies the influence mechanism of groundwater on the development of underground space in typical valley cities of western China. The findings have practical significance for the evaluation, planning, development, and utilization of urban underground space resources in Haidong.

Fluid inclusions in mineralized fracture infillings (i.e., veins) could preserve information about subsurface fluids like temperature and salinity. The isotopic composition of water in these fluid inclusions could provide direct evidence of the provenance of these mineral-forming fluids. So far, the isotope compositions of fluid inclusions have been mainly derived from carbonate veins and other precipitates, like speleothems. The aim of this study is to analyse the δ¹⁸O and δ²H isotopic compositions of aqueous fluid inclusions of quartz veins using a cavity ring-down spectroscopy (CRDS) analyser in combination with a moisturized nitrogen background and mechanical crusher. For this study, we analysed δ¹⁸O and δ²H values of fluid inclusions in quartz veins from three north-western European locations formed during the Variscan orogeny. Prior to crushing, the fluid-rich quartz fraction was separated from the pure quartz fraction, from other mineral phases and host rock by using conventional heavy liquids and magnet separation. Raman spectrometry detected some rare occurrences of hydrocarbon, methane, and nitrogen in the fluid inclusions. The samples were sequentially crushed to elucidate the potential impact of different fluid inclusion assemblages (FIA) on the δ¹⁸O and δ²H values. The results from single and sequential mechanical crushing, together with interlaboratory comparisons, exhibit reliable and consistent isotopic patterns across locations with high precision (for δ¹⁸O: 1σ SD < 0.8‰; for δ²H: 1σ SD < 1.5‰). The obtained data occur in three different clusters for three study zones, providing evidence for the presence of meteoric-derived fluids in the fold-and-thrust belts of the Variscan orogeny. These findings demonstrate that the CRDS approach can be successfully applied to quartz minerals, investigating fluid pathways within the upper crust and the formation of these secondary minerals.

Surface hydraulic fracturing is an important measure for increasing reservoir permeability, which has advantages such as engineering safety and a large impact range and can be implemented ahead of the mine’s underground engineering. However, its underground outburst reduction range and effect are rarely reported, and there is a lack of connection with underground fracturing wells. Taking coalbed methane wells in the Lu’an Mining Area as an example, underground observation, microseismic monitoring, and numerical simulation methods were used to study the fracturing range and outburst control effect of surface wells. The fracturing zone of coalbed methane wells is approximately elliptical in shape, with the main fractures extending along the direction of the maximum horizontal principal stress. It can be divided into five zones: sand laying zone (radius of 140~150 m), fracture propagation zone (radius of about 180 m), fracturing fluid permeability zone (width of about 1 m), gas surge zone (width of 2~3 m), and final desorption zone (width of about 2 m). The stress around the fracturing zone increases along the direction of maximum and minimum principal stresses, while the stress value within the zone decreases, with a range of approximately 5~20% of the original geostress. The outburst reduction index Δh₂ in the fracturing zone significantly decreased after fracturing. The percent of Δh₂ (< 150 Pa) increased from 38.3% to 100% after fracturing. A model for evaluating the effectiveness of surface fracturing and outburst prevention was proposed, and the model was used in Tunliu Mine. The results showed that the standard-reaching rate of extraction was high, and the danger of outburst could be completely eliminated. The research results can provide a reference for the arrangement of the coalbed methane wells and can also provide effective guidance for outburst prevention and control work during mining and excavation on a more macro scale. It provides a new idea and method for making up for the shortcomings such as the small impact range and safety hazards of underground fracturing.

The lower the volatile value of coal, the higher the metamorphic degree of coal. As a vector, the spatial variation of the volatilization value of the coal seam be used to explore the location of concealed pluton and to characterize the maximum influence intensity of the magma thermal field. The eastern side of the Huyanshan intrusive pluton has amassed a large amount of C-P coal measurement and coal quality exploration data. The zonation distribution of coal seam volatile fraction data within the exploration area serves as an efficacious record of the intrusion process of the rock mass and a valuable resource for the exploration of the location of concealed plutons. Based on the data from 470 boreholes in the exploration area, contour maps of coal seams No. 02, No. 2, No. 6, No. 8, and No. 9; the true thickness contour maps of each coal layer; and the volatilizing profile map were created. The results indicate the following: (1) In the central region of the exploration area, from No. 9 to No. 8 to No. 6 to No. 2 to No. 02, the contours of volatiles in each coal seam exhibit a ring pattern, with the size of the ring and the size of the volatiles change regularly. This pattern strongly suggests that the concealed pluton is developed in the underlying layer of the coal seam. (2) According to the linear fitting of the relevant data, it is predicted that the concealed pluton is located at a depth of 65.4 m below the No. 9 coal seam. At a vertical distance of 159.1 m, the thermal field influence of the concealed pluton is lost, which aligns with the spatial distribution characteristics of volatiles in the coal seam as observed through drilling. (3) The occurrence of a significant interlayer slip between coal seam No. 6 and coal seam No. 2 in the overlying concealed pluton is indicated by the results of the hotspot migration analysis. The results are helpful to the search for magmatic-hydrothermal deposits and the exploration and development of coal and coalbed methane.