Geofluids最新文献

China’s shale gas has undergone nearly 20 years of exploration; unconventional oil and gas geological evaluation theories and research methods have been greatly enriched, but how to quickly, conveniently, and accurately identify the sweet spots of shale gas is still puzzling many researchers. This study focuses on the black shale of the Wufeng–Longmaxi Formation in the southeastern edge of the Sichuan Basin; lithofacies classification, the relationship between lithofacies and depositional environments, and the correlation between lithofacies and shale gas–bearing capacity are discussed. At last, we have established the lithofacies classification criteria; the Wufeng–Longmaxi Formation deposited eight types of lithofacies, which the paleoenvironment during deposition evolved gradually from anaerobic environment to oxygen-poor and oxygen-rich environment. The black high-carbon and high-silicon shale lithofacies and the black carbon-rich and silicon-rich shale lithofacies are rich in organic matter, and they were deposited in high primary productivity, low terrigenous detritus input, and euxinic environment. The black medium-carbon medium-silica shale lithofacies and the black medium-carbon and high-silica shale lithofacies contain organic matter, which are deposited in medium primary productivity, middle terrigenous detritus input, and oxygen-poor and low hydrodynamic environment. The gray–black low-carbon low-silicon clay-rich shale lithofacies, the gray low-carbon and high-silicon shale lithofacies, and the gray–white low-carbon and silicon-rich shale lithofacies are poor in organic matter, which are deposited in a transitional environment of low primary productivity and oxygen poor–oxygen rich. In the analysis of the relationship between organic matter–rich black shale facies and sedimentary environment, it is shown that the enrichment of organic matter is positively correlated with the oxidation–reduction discrimination indicators Ni/Co, U/Th ratio of ancient oceans, and the evaluation indicators Ba_bio and Ba/Al ratios of primary productivity. Only under the favorable sedimentary geochemical conditions and good preservation conditions can deposit lithofacies sections (zones). Based on the optimization of shale gas dessert section and the drilling of horizontal wells, the optimization of favorable black shale lithofacies types and the classification of shale gas dessert section are the key to shale gas exploration. The shale gas–bearing capacity is closely related to lithofacies. Black carbon-rich silicon-rich shale lithofacies and black high-carbon high-silicon shale lithofacies have the best gas-bearing capacity and are favorable lithofacies.

To thoroughly investigate the mechanisms behind coal and gas outbursts in folded structural areas, we conducted similarity simulation experiments using a custom-built apparatus designed to replicate these structures. The objective was to analyze the stress distribution characteristics of coal rock masses under horizontal structural stress within folded zones. The experimental outcomes reveal that, under horizontal loading, shear cracks progressively develop along layer directions within the anticline wing, anticline axis, and syncline axis, evolving continuously along the interlayer direction. In these folded structures, horizontal stress consistently remains compressive, with the highest compressive stress concentrations observed at the anticline axis, followed by the wings and turning points of the anticline, and the lowest in the syncline axis area. The stress coefficient (k) in the anticline axis reached values as high as 3.18, while the syncline axis exhibited much lower stress concentrations, with k values of 0.66. Vertically, the anticline axis and its wings primarily experience tensile stress, whereas the syncline and its wings mainly undergo vertical compressive stress. The anticline axis region, subjected to horizontal structural stress, tends to develop tension cracks, which adversely affect gas retention. The combination of horizontal tension and vertical tensile stress in this region reduces the risk of coal and gas outbursts. Conversely, the syncline axis area, experiencing triaxial compressive stress, exhibits a higher degree of stress concentration and superior gas sealing capacity, rendering it more vulnerable to coal and gas outbursts. These findings provide essential insights for refining coal mining methodologies in fold structures, particularly for addressing the safety challenges posed by coal and gas outbursts.

Rock masses characterized by X-type joints are prevalent in cold region rock engineering projects. A precise understanding of the mechanical mechanisms governing the fracture initiation strength of these jointed rock masses after experiencing freeze–thaw damage is paramount for ensuring the safety and stability of associated engineering structures. Leveraging the mutual constraint relationship between the displacements at the tips of intersecting joints under compressive shear conditions, a computational approach has been developed to determine the stress intensity factor at the tip of the main joint, taking into account the interference effects arising from both main and subjoints. Furthermore, the fine-grained defects within the rock mass are abstracted as elliptical microcracks, and deterioration equations for rock cohesion and fracture toughness under freeze–thaw cycling are derived using frost heave theory. Taking into account the mutual interference effects between main and subjoints, as well as the degradation of rock mechanical properties caused by freeze–thaw cycles, a computational approach for determining the initiation strength of X-type jointed rock masses has been developed. The validity of this method has been confirmed through rigorous model testing. The findings reveal that the wing cracks in X-type jointed rock masses predominantly propagate along the tips of the main joints, while the extension of subjoints is constrained. When the X-joints have the same inclination, the initiation strength of the subjoint exceeds that of the single-joint rock mass when its inclination is less than the main joint’s but is lower when the subjoint’s inclination exceeds that of the main joint. The interference effect between oppositely inclined intersecting joints enhances the initiation strength of the rock mass, with the maximum occurring when the subjoint is at an inclination of 120°. When the freezing time is less than 18 h and the temperature is below −16°C, variations in both time and temperature are more sensitive in affecting the initiation strength of the X-jointed rock mass. Rocks with a high elastic modulus and low tensile strength experience a greater rate of freeze–thaw damage, and brittle rocks are more susceptible to frost heaving failure.

Although various fractal flow models have recently been developed to investigate pressure responses of fractured vertical wells, almost all of the existing fractal models ignore the quadratic gradient term (QGT), which makes them violate mass conservation. In this paper, fractal theory is introduced to develop a nonlinear flow model of fractured vertical wells with stimulated reservoir volume (SRV). The QGT is reserved so that the present model fully obeys material balance. Function transforms are used to linearize the nonlinear flow model, and then, Laplace transform and Laplace numerical inversion algorithm are employed to derive the pressure solution. Type curves are provided to analyze the flow characteristic and identify the flow regimes. The effects of some parameters on the pressure responses are discussed in detail. It is found that the existence of the QGT leads to the decrease of the pressure drop, especially at a large nonlinear coefficient and a large time scale. Fractal parameters and SRV radius not only affect type curves but also affect the relative error caused by neglecting the QGT.

Taking the carbonate of the Majiagou Formation in the Ordos Basin as an example, this paper introduces a method for predicting the S-wave velocity of carbonate based on rock physics modeling. By analyzing the samples in the study area, we can find that the carbonate reservoirs in the study area have the following characteristics: (1) The lithology of the Majiagou Formation in the Ordos Basin is relatively complex, mainly composed of dolomite, lime dolomite, dolomitic limestone, gypsum, and gypsum-bearing dolomite. The pore types include intergranular pores formed by dolomitization, intergranular dissolution pores formed by dissolution, and fractures. (2) Due to the diverse types and complex distribution of rock-forming minerals, there are always some rock samples whose matrix modulus is beyond the upper or lower limits. Those were calculated using the Voigt–Reuss–Hill (VRH) average method. (3) The pore structure of carbonate is very complex due to diagenesis. Based on the influence of pore shape characteristics on rock elastic parameters, pore shapes are divided into three types using the pore aspect ratio. Among them, the aspect ratio of intergranular pores is the largest, while that of the fracture pores is the smallest, and the aspect ratio of intergranular dissolved pores falls between the two. Therefore, the accuracy of predicting S-wave velocity in this area based on traditional rock physics modeling methods is low. In this paper, we will introduce a new model that is aimed at improving the traditional rock physics model. The first improvement is based on a variable matrix modulus, which can be used for matrix modeling to mitigate the influence of uneven mineral distribution. The second enhancement involves quantitatively characterizing the impact of different pore aspect ratios on the S-wave velocity of carbonate rocks, using a porous differential equivalent medium (DEM) model.

The optimization of coal dust management in fluidized mining environments is of paramount importance, yet it is currently impeded by a gap in understanding chemical dust suppression mechanisms. This study combines indoor experiments with molecular simulation to investigate the mechanisms by which three anionic surfactants with different hydrophilic and hydrophobic groups (SDBS, SDS, and SLS) influence coal wettability. Using hydrophobic bituminous coal as the experimental subject, basic physical and chemical properties are analyzed through proximate analysis, XRD, and FTIR. The effect of different surfactants on coal wettability is characterized based on sedimentation experiments, while the coal–surfactant–water three-phase model examines the equilibrium adsorption configuration, water molecule diffusion coefficient, and interaction energy in different adsorption systems. The surface free energy of coal dust and its components is measured before and after surfactant adsorption, verifying the adsorption-wetting mechanism of surfactants at the coal–water interface. Results show that anionic surfactants enhance wettability through a bidirectional adsorption mechanism at the coal–water interface: the hydrophobic tail adheres to the coal surface via van der Waals forces, while the hydrophilic head faces the water phase, driven by electrostatic and hydrogen bonding interactions. This coordinated adsorption process alters water diffusion and the surface free energy of coal, thereby improving wettability. SDBS, due to its benzene ring, significantly amplifies the bidirectional adsorption effect, achieving the most substantial improvement in coal dust wettability. The findings provide a robust theoretical framework for developing dust control strategies in fluidized mining operations, advancing the field toward more efficient and sustainable mining practices.

Using changes in ground temperature to reflect the flow status of groundwater is one of the methods for predicting mine water inrush. In this study, in order to make this method suitable for different geological conditions, an improved method for predicting mine water inrush is established based on the theories of heat transfer and nonlinear water flow in fractal porous media. A water inrush judging criterion based on the critical pressure gradient of nonlinear flow is first established. Then, an internal structural model of the crushed rocks and a mathematical model of nonlinear flow in crushed rocks are derived based on the fractal theory. Finally, a thermal, hydraulic, and mechanical (THM) coupling model is established to study the nonlinear water inrush process and temperature changes. The improved method is established based on the numerical simulation results of the THM coupling model. Results show that the water inrush judging criterion can simultaneously consider the water-resisting capacity of intact and crushed rocks and quantitatively calculate the water-resisting capacity of crushed rocks compared with the traditional method. The improved method is suitable for different cases with different water-resisting capacities, ground temperature change ranges and gradients, and aquifer water pressures, which can improve the applicability of using ground temperature to predict mine water inrush.

Composite aquiclude rock is an important part of the water barrier layer of coal seam floor, and its mechanical properties and failure characteristics have an important impact on the safety of coal seam mining. In order to explore the uniaxial compression failure characteristics and mechanical properties of composite aquiclude rocks, the mineral composition and microstructure of the composite cement rock were measured by XRD and SEM. Using a WAW-1000D electrohydraulic servo universal test system, uniaxial compression tests were carried out on samples of four structural types, namely, mudstone/siltstone, mudstone/fine sandstone, fine sandstone/siltstone, and siltstone/mudstone/fine sandstone composites soaked in saline water with various immersion periods and pH values. The experimental results show that the failure of composite rock samples was mainly concentrated in the mudstone part. With the increased proportion of soft rock in the structure, the rock samples exhibited changes in tensile, X-shaped conjugate inclined plane shear, and single inclined plane shear failure modes. The overall compressive strength of composite rock samples decreased with immersion time, and their failure mode shifted from shear to tensile with increasing immersion time. The strength-weakening effect of high-salinity water with different acidity and alkalinity on composite rock samples was significant. The increased pH promoted the tensile failure-shear failure-tensile failure evolution. The results of this study can provide important experimental data and theoretical basis for the stability control of composite aquiclude rock in coal mining.

The bearing capacity of traditional prestressed high-strength concrete (PHC) pipe pile is hampered by the poor mechanical properties of surrounding soil in soft soil areas, and the PHC nodular pile can improve the behavior of pile foundation in soft soils. The PHC nodular pile installation process will induce larger disturbance to the surrounding soil compared to the PHC pipe pile, and there is little research on the installation effect of the PHC nodular pile. In this paper, the coupled Eulerian-Lagrangian (CEL) finite element method was adopted to simulate the penetration process of PHC nodular piles and pipe piles in soft soil. The radial stress and displacement in soil induced by the PHC nodular pile and pipe pile and the soil resistance at different parts of the PHC nodular pile were analyzed. The simulation results showed that the penetration resistance of the PHC nodular pile was larger than that of the PHC pipe pile. The penetration resistance of PHC nodular piles was mainly provided by the pile shaft resistance. The uplift height of soil surface caused by the PHC nodular pile and pipe pile penetration was approximately the same. The influence range of compaction effect for PHC nodular pile and pipe pile was both concentrated on 10R (R is the pile diameter).

As one of the comprehensive exploration techniques for underground geological resources, surface geochemical methods could play an important role in geothermal exploration, which requires detailed and systematic investigations. In this study, we take the Shiba geothermal basin in Huizhou, China, as the research object and apply surface geochemical testing methods to analyze the intrinsic relationship between the geothermal system and the surface soil. The contents of soil gases and elements are mainly determined, among which hydrogen (H₂) and radon (Rn) show three obvious negative anomalies, corresponding to three positive anomalies of soil elements (Fe, U, Cr, V, Cu, and As) that are easy to migrate. The largest negative and positive anomalies correspond to the surface above the fault, which is related to the dominant channel from underground to the surface and is caused by the gas loss effect and the dissolution and migration of inorganic ions. However, the effects of the surface environment and organisms cannot be ignored. Only when the geothermal system has a significant impact on a certain geochemical parameter could the anomaly be manifested in the surface soil. Otherwise, most of the geothermal information, including thermal conduction, will be covered by surface factors. After surface geochemical anomalies related to the geothermal system have been identified, anomaly patterns (especially the top anomaly pattern) can be preliminarily established, which can be used for geothermal exploration. Furthermore, based on the empirical regional geothermal formula, the temperature and depth of the Shiba geothermal system are evaluated at 170°C and 4500 m, respectively, indicating that surface geochemical methods have a good practical prospect in the prediction of geothermal resources.