Geofluids最新文献

The prediction of hydrocarbon expulsion by overpressure rupture (HEOR) is still a difficult problem in the study of primary oil and gas migration. Through thin section observation, pressure analysis, formation breakdown test, and basin numerical simulation, the overpressure fracturing of Cenozoic source rocks in Bodong Depression is studied, and its oil–gas geological significance is discussed. The research result shows that (1) overpressure is generally developed in Cenozoic source rocks in Bodong Depression, the source rocks are fractured into network fractures under the action of overpressure induced by hydrocarbon generation and undercompaction, the overpressure fractures are filled with asphalt, the rupture pressure of the source rock is controlled by the vertical principal stress, and the deep rupture pressure gradient is larger than that of the shallow layer, which makes the deep fracture more difficult; (2) the numerical simulation shows that there are two stages of rapid pressurization and one stage of slow pressurization in Bodong Sag, namely, the second thermal subsidence, the overpressure distribution in Bodong Depression has the characteristics of “strong in south and weak in north, early in south and late in north”; (3) the pressure of source rocks in the Southern Bodong Sag reached the formation rupture pressure in the rapid pressurization period since the neotectonic movement, and the hydrocarbon was quickly and efficiently expulsed. The starting times of the HEOR in Sha3 Member is 3.5 Ma~present, that in Sha1–Sha2 Member is 4~2 Ma, and that in Dong3 Member is 5.1 Ma~present. The convergent flow plus HEOR is most conducive to hydrocarbon migration and accumulation. At present, the discovered A12 and A20 oil fields developed in the path of HEOR plus convergence flow, and this study also guides the discovery of A22 oil field developed in the path of convergence flow plus HEOR.

The Ediacaran Doushantuo Formation in South China has been increasingly recognized as a significant potential source rock for hydrocarbon reservoirs, particularly in relation to the overlying Dengying Formation. This study conducts a comprehensive geochemical analysis of the organic-rich shale from the Ebian-Xianfeng section within the Sichuan Basin, aimed at elucidating the mechanisms of organic matter enrichment. The shale in Member II is characterized by high total organic carbon (TOC) content, ranging from 1.03 to 4.06 wt%. Geochemical proxies, including the chemical index of alteration (CIA) and redox-sensitive trace elements (Mo, U, and V), exhibit an upward increase, indicative of a paleoenvironmental transition towards more humid and anoxic conditions. The positive correlation between enrichment factors (Mo-EF, U-EF, and V-EF) and TOC content suggests that redox conditions were a primary control on organic matter preservation. Minimal evidence of hydrothermal influence indicates that organic matter remained largely unaltered postdeposition. The original findings suggest that the period of organic-rich shale deposition was likely a relatively warm and humid time, during which enhanced weathering input more nutrients, thereby increasing productivity and leading to the generation and sedimentation of more organic matter into the shale. Moreover, the cooling period following the warm phase may have intensified the oxidation of dissolved organic carbon (DOC) in the water column and porewater. This process consumed oxygen and sulfate, thereby creating anoxic conditions that were favorable for the preservation of organic matter. The geochemical signatures further suggest that the organic-rich shale deposited in the intrashelf basin may not be as rich in organic matter as those in deepwater basins and slopes, but it still holds certain potential for oil and gas. This study emphasizes the significance of climatic fluctuations, redox dynamics, and hydrothermal stability in the formation of organic-rich shale within the Doushantuo Formation, thereby contributing to the hydrocarbon potential of the Sichuan Basin.

The remaining coal pillars and roof form an integral coal pillar–roof system (CPRS) that plays an important role in the safety of the room mining goaf. In this research, two different sets of parallel dual coal pillar–roof combinations (PDCRCs) were developed to model the CPRS. One set of PDCRC is formed by two-component combinations featuring identical mechanical properties, whereas another set is constituted by two-component combinations exhibiting distinct mechanical properties. Building upon this foundation, a sequence of uniaxial compression tests was carried out on PDCRC. These tests integrated laboratory experimentation and numerical simulation with the particle flow code (PFC). From both macroscopic and microscopic perspectives, the load-bearing capacities, acoustic emission (AE) features, crack development processes, force chain evolution laws, and deformation features of the PDCRC were recorded. The results indicate that the initial failure of a specific coal can trigger and dominate the instability of its corresponding combination, thereby leading to a chain instability in the other combination and the entire system. For PDCRC composed of two combinations with identical mechanical properties, the two combinations share the external load equally and fail in coordination. Once any component combination loses its ability to withstand the external load, the other component combination and the entire system will immediately and synchronously lose their load-bearing capacity. For PDCRC composed of two-component combinations with distinct mechanical properties, the component combination with low strength first fails and loses its load-bearing capacity, resulting in the synchronous transfer of the originally external load to the high-strength component combination. Once the high-strength component combination loses its load-bearing capacity, the entire system becomes unable to sustain the external load simultaneously. The overall load-bearing capacity of PDCRC with identical mechanical properties is approximately equal to the sum of the two-component combinations, while that of PDCRC with distinct mechanical properties is less than the combined total. In summary, the premature instability of certain coal pillars serves as the primary initiating factor for the instability of the CPRS. When conducting stability assessments of room mining goafs, it is essential to adopt a holistic perspective to comprehensively evaluate the load-bearing capacity of the CPRS as an integrated whole.

Optimizing multicluster fracturing designs in heterogeneous conglomerate reservoirs is critical due to their complex characteristics. This study employs the continuous–discontinuous element method (CDEM) to conduct engineering-scale 3D simulations using a mathematical model incorporating rock strength heterogeneity. A predictive model relating stimulated reservoir volume (SRV) to in situ stress, stage, cluster parameters, and well azimuth was developed, with SRV maximization as the objective. Results demonstrate that larger stage lengths combined with increased cluster counts enhance SRV as stress difference increases. However, a significant bottleneck in SRV growth occurs once the stress difference exceeds 20 MPa, rendering further stage/cluster adjustments ineffective. Crucially, the optimal stage/cluster combination depends strongly on well azimuth. For conglomerate reservoirs with high stress differences, strategically adjusting the well azimuth can increase fracture complexity, effectively overcoming the SRV bottleneck and enabling sustained high SRV even under elevated stress differences.

The Hoek–Brown (H-B) criterion is widely recognized as a standard in geotechnical engineering for assessing rock mass strength across various rock mass qualities. However, challenges arise in explicitly defining the Mohr failure envelope, particularly when the strength parameter “a” deviates from the conventional value of 0.5. This study investigates the compressive strength of rock masses in the Himalayas, particularly in the context of deep tunneling and slope stability, using the H-B and Mohr–Coulomb (MC) criteria. Initially, the MC and H-B criteria were combined while varying the angle of internal friction, revealing an inconsistent trend in friction angles regarding rock mass compressive strength. The relationship between tunnel depth, slope height, and rock mass compressive strength was then examined by combining equations involving RMR, RQD, and modified H-B criteria. The combination of H-B and MC resulted in lower rock mass compressive strength values, while noncombined equations yielded higher values. Incorporating the geological strength index (GSI) provided higher and more suitable compressive strength values. For the Himalayas, the suggested H-B equations with GSI are recommended for both surface and subsurface excavations.

The abundant geothermal resources of the Haihu New District of Xining City are significantly constrained in their development and utilization, as the geothermal water is characterized by high salinity. This study aimed to investigate the hydrogeochemical characteristics and genesis of geothermal water in the Haihu New District by analyzing water chemistry data from five geothermal wells using traditional hydrogeochemical methods, statistical analysis, isotope analysis, and geochemical simulations. The findings revealed the chemical characteristics of the geothermal water and identified its recharge sources, elevation, circulation depth, and reservoir temperature. Representative reaction pathways were selected to simulate water–rock interactions along the geothermal water flow path based on the genetic model of geothermal water and the analysis of rock mineral compositions. This study examined the dissolution and precipitation characteristics of rock minerals, elucidating the chemical genesis of geothermal water in the district. The results showed that (1) the analyzed geothermal waters reveal a significant salinity level and a mildly alkaline nature, with Na⁺ as the dominant cation and SO₄²⁻ and Cl⁻ as the dominant anions. For this reason, these waters can be classified as Na-SO₄•Cl waters. (2) The geothermal system in the study area was classified as medium to low temperature. Atmospheric precipitation infiltrates from the Laji and Laoye mountains, with recharge elevations ranging from 2910 to 2980 m. The reservoir temperature is estimated to range from 52.40°C to 70.45°C, with circulation depths between 1000 and 1600 m. (3) The primary sources of Na⁺ and Cl⁻ in the geothermal water are halite dissolution and cation exchange, while SO₄²⁻ primarily originates from gypsum dissolution, with additional influence from H₂S oxidation and the common ion effect. This study provides a comprehensive understanding of the hydrogeochemical characteristics and genesis mechanisms of the geothermal system in the Haihu New District, offering critical insights for the effective and sustainable development of geothermal resources in the region.

Using a tessellation method in Neper, the simplified 3DEC-GBM (three-dimensional discrete element grain–based model) is proposed for generating joint models with different roughnesses. The model is extended by implementing the generalized effective stress law to mimic the shear behavior of unsaturated sandstone. The grain-scale mechanical parameters of the model were calibrated to correspond to the mechanical behavior of sandstone samples measured in the laboratories. The simulations accurately explain complex macroscopic shear behavior in terms of the mesoscale interaction of grains. The modeling results show that water has a deteriorating effect that weakens the strength of the joint, and the degree of reduction in joint strength rises with increasing saturation. Increasing normal stress and roughness can all improve the shear strength of the joint. We conclude that the proposed model is a convenient approach to analyze the shear response of unsaturated sandstone at varying roughness and normal stress.

The Trans-Amazon Drilling Project (TADP) drilled a sequence of claystones, siltstones, and sandstones in the Acre sedimentary basin, reaching a final depth of 923 m. This study characterizes the occurrence and compositional variation of light gaseous hydrocarbons detected using the online gas analysis (OLGA) monitoring system deployed during drilling, along with methane (CH₄) and carbon dioxide (CO₂) concentrations measured in discrete gas samples extracted from cores during drilling operations. The gaseous hydrocarbons detected by the OLGA system are predominantly CH₄ but with the regular presence of ethane (C₂H₆), propane (C₃H₈), isobutane (i-C₄H₁₀), and n-butane (n-C₄H₁₀). Zones with higher CH₄, C₂H₆, and C₃H₈ concentrations were observed at depth intervals of 250–380 and 420–588 m. These higher concentrations of CH₄, C₂H₆, and C₃H₈ occur in siltstone or sandstone layers capped by claystones, suggesting that these lithological associations act as stratigraphic gas traps. The Bernard parameter (CH₄/C₂H₆ + C₃H₈) varied from a low value of 2 at 466 m depth to a maximum value of 1904 at 621 m depth. Stable carbon isotope ratios of CH₄ show δ¹³C values between −35‰ and −25‰, suggesting the nearly ubiquitous presence of thermogenic gas. The discrete gas samples from cores exhibited CO₂ concentrations between 230 and 1400 ppm in claystones, 850 and 950 ppm in siltstones, and 240–820 ppm in sandstones, indicating higher concentrations in fine-grained sediments. The CH₄ concentration ranges from 2 to 6 ppm in sandstone layers and from 2 to 4 ppm in siltstone and claystone layers. There is no significant correlation between CH₄ and CO₂ concentrations. These results provide evidence of light hydrocarbon migration from deeper thermally mature source rocks, with entrapment in sandstone layers capped by fine-grained sedimentary rock layers. The high concentration of CO₂ relative to CH₄ in fine-grained rock layers points to restricted conditions for microbial gas generation in the drilled sediments, possibly due to a combination of low organic carbon content and oxidizing conditions. This is in accordance with the abundance of reddish fine-grained paleosols in the drilled sedimentary units. The combination of online gas monitoring and discrete sampling methods allowed the comparison between gas collected during drilling and in situ gas, contributing to a better understanding of the processes of the subsurface carbon cycle.

The study focuses on a key component of the Yangtze–Huaihe River Diversion Project—the Qiliqiao to Xincheng water supply pipeline section, which includes a megascale inverted siphon structure. A one-dimensional mathematical model is developed to simulate water hammer phenomena in transmission pipelines containing the large siphon. A comparative analysis is conducted to evaluate the protective effects of linear and nonlinear valve closure strategies. The optimal valve closure scheme is explored by parameterizing the valve closing duration and buffering time. The impact of air valves installed near the inverted siphon during hydraulic transition processes is examined, and the transient variations of key hydraulic parameters during the entire valve closure operation are systematically characterized. Under long-term operational conditions with Manning’s coefficient degradation, the designed pipeline maintains a hydraulic head surplus of 7.34 m, fully meeting long-distance water supply requirements. Air valves effectively reduce the peak pressure magnitude by 80%. Under linear valve closure conditions, only the high-elevation air valves exhibit significant exhaust behavior. In contrast, nonlinear closure strategies reduce the minimum internal pressure of the pipeline and suppress vaporization, thereby reducing the air valve discharge volume. Finally, the study identifies an optimal time coordination scheme by adjusting the closure timing of individual pumps and the interval between adjacent pump shutdowns.

Coal spontaneous combustion is a major threat to coal mine safety. The accumulation of heat from broken coal in goaf is prone to coal spontaneous combustion. A deep study of the evolution mechanism and theoretical model of porosity and permeability in crushed coal under compression is crucial for understanding the process of coal spontaneous combustion. This paper utilizes an experimental apparatus for gas seepage in crushed coal under compression developed by our research team. It systematically explores the changes in strain, porosity, and permeability of single particle and mixed particle coal during stress infiltration. The experimental results show that the strain of coal with different grain sizes has a negative exponential relationship with stress, and porosity and permeability vary inversely and linearly with strain, respectively. Theoretical analysis indicates that the sliding and filling effect of particles is the primary cause of the changes in strain, porosity, and permeability. There is a one-to-one mapping relationship between the total strain, residual porosity, and residual permeability of coal with different grain sizes after compression and their respective change paths. Based on these findings, this paper establishes theoretical relationship models of stress–strain, strain–porosity, and strain–permeability, with total strain, residual porosity, and residual permeability as parameters. These research results will provide theoretical support for a deeper understanding of the occurrence and development processes of coal spontaneous combustion.