Geofluids

Retaining wall is essential for stopes mining in two steps, for it can prevent the instability and collapse of backfill. In this study, taking the retaining wall of backfilled stope as the research object, a stability analysis method of retaining wall based on the close coupling of catastrophe theory and numerical analysis was proposed. First, by extracting the unit failure rate of the retaining wall from the numerical simulation results and fitting it with the mining depth, the functional expression between them was established. Second, the function relation was transformed into the normal form according to catastrophe theory, and the instability criterion of retaining wall was deduced. Furthermore, an effort was made to analyze the changing law of the state of retaining wall and calculate the critical span of stope, under different thickness conditions. On this basis, the application test of retaining wall was carried out by using this method. The results show that with the thickness decreasing, the values of splitting variables and show a reverse trend, which leads to the discriminant of instability criterion decreasing and turning from positive to negative, resulting in the collapse. Meanwhile, in order to ensure the stability, the wider the span of the stope, the thicker the retaining wall is required, and conversely, the thicker the retaining wall, the higher the adaptability to the span of stope. In addition, it can be found from the application test that instability was bound to occur with a thickness of 3 m, but the retaining wall with a thickness of 4 m maintained stable, which tended to be consistent with the analysis. Therefore, the stability analysis method proposed in this study provides a way to accurately evaluate the stability of the retaining wall and calculate the critical thickness of that, and its application value is expected to be further explored.

After the rupture of pressurized water supply pipes in urban underground areas, seepage-induced ground subsidence becomes a severe geological hazard. Understanding the permeation and diffusion patterns of water in soil is crucial for deciphering the mechanisms underlying soil settlement and damage. Notably, the pressure within water supply pipes significantly influences the settlement and damage of the soil. Therefore, this study simulated experiments on soil settlement and damage caused by water seepage from a preexisting damaged pipeline under various internal pipe pressure conditions using an indoor model apparatus. The results indicate that the internal pressure of the pipe significantly influences the settlement of the soil. High-pressure seepage causes noticeable erosion in the soil, forming cavities within it. In contrast, low-pressure seepage results in water diffusing in an ellipsoidal pattern, leading to the formation of circular surface cracks. The degree of surface settlement increases with higher pipe pressure. The onset of subsidence at a specific point on the ground is not directly related to whether the moistening front within the soil has reached that point horizontally. Instead, it is associated with the moisture content below the subsidence point within the soil. The research results further illustrate the water diffusion and moisture content increase processes after water seepage from pipes with different pressures, revealing the influence of pipe pressure on the degree and form of soil settlement damage and clarifying the relationship between water diffusion and settlement in the soil.

This study presents a novel perspective for improving the understanding of permeable structures at geothermal prospects by jointly diagnosing the responses of conventional pressure transient and tracer testing. The pressure and tracer responses individually yield apparent porosity–thickness products. The difference between them implies the existence of unknown dead-end features involved in a reservoir model. Laboratory experiments and numerical simulations validate this concept. Potential application to hypothetical exploration demonstrates that the logarithmic ratio of the porosity–thickness products, determined based on pressure and tracer responses, indicates the accuracy of the reservoir model to be successively updated with the progress of the exploration. The reservoir model successfully reproduced the synthetic observations regardless of the accuracy of permeable structure if different porosity–thickness products were allowed to be assumed to individually reproduce pressure and tracer responses. These porosity–thickness products coincided only if the reservoir model correctly captured the permeable structure. This novel perspective will provide strategic guides for successful exploration and development at the prospects of geothermal and, potentially, general geofluid resources.

In the process of long-term water flooding in the Gaoqian Southern Area with an average porosity of 30% and an average permeability of  μm², the fluid-solid interaction among oil, water, and rock has a great influence on the pore structure. It has resulted in changes in reservoir parameters with the extension of time. This paper used electron microscopy scanning, mercury injection, X-ray diffraction, physical properties, and oil-water relative permeability curves to study the variation of clay mineral content, pore throat structure, porosity, permeability, and relative permeability curves of high-permeability sandstone after high-pressure water flooding. The results showed that clay minerals such as montmorillonite and kaolinite were dissolved, hydrated, and migrated after long-term water flooding, which resulted in the decrease of clay mineral content in fine sandstone and medium sandstone, the increase of pore throat radius, and the decrease of displacement pressure, median pressure, and separation coefficient. The saturation of the isotonic point of the oil-water relative permeability curve was obviously shifted to the right, the hydrophilicity was significantly enhanced, and the porosity and permeability were effectively improved, but there was a blockage of the throat less than 2 μm in the fine sandstone. In addition, this paper established the equations of water injection, permeability, irreducible water saturation, residual oil saturation, and oil-water relative permeability curve coefficient and establishes the initial permeability model with the well data before water flooding. The logging interpretation results of development wells in the process of water flooding as verification data were used, and the relative error of permeability far lower than the general requirement of permeability error within an order of magnitude was less than 30%, which verified the rationality of the method.

The relationship between biodegraded oil and its source has long been a complex and contentious topic. The Western Halaalate area is located in the Piedmont area on the northwest margin of the Junggar Basin. Source rocks of the Fengcheng Formation exist both locally and in the nearby Mahu Depression. In order to determine the source of biodegraded crude oil in this area, the molecular marker characteristics of biodegraded crude oil were analyzed by gas chromatography-mass spectrometry. The results showed that the vitrinite reflectance (Ro) of source rocks of the Permian Fengcheng Formation in the nearby Mahu Depression is greater than 1.3% and has entered a high mature stage of condensate oil and moisture gas; the source rock of Permian Fengcheng Formation in Western Halaalate area is in a mature stage with Ro of 0.79%~1.13%. The ascending configuration of tricyclic terpenes C₂₀-C₂₁-C₂₃ for the crude oil samples found in the Carboniferous strata of the Western Halaalate Area is consistent with the characteristics of the Fengcheng Formation source rocks, which are present in both the Western Halaalate Area and the nearby Mahu Depression. Chromatography spectrometry examination shows that crude oils have undergone a varying degree of biodegradation. The Carboniferous oil was originated from the in situ Fengcheng Formation source rocks based on the application of molecular markers resistant to biodegradation, such as maturity parameters, salinity parameters, the new gammacerane index, and aromatic hydrocarbon parameters, combined with the analysis of hydrocarbon migration pathway. In addition, the oil biodegradation alteration rules in the Western Halaalate area were clarified, which advances regional knowledge of the relationship between biodegraded oil and source rocks.

Microbially induced carbonate precipitation (MICP) has been utilized as a new method to improve loess soil strength. In this study, we investigated the influence of the main parameters on the shear strength of MICP-treated loess specimens. Initially, culture media with different formulas and pH values were examined to identify the most efficient medium for loess soil. To explore the shear behavior of MICP-treated loess under general stress levels, unconfined compressive strength (UCS) tests and triaxial tests relevant to the compression strength and vertical loads were performed on MICP-treated loess with different calcium sources, cementation concentrations, and curing periods. Subsequently, calcium chloride was selected as the optimal calcium source based on the ultimate strength of the MICP-treated loess. The effective cementation concentration in the loess soil was between 1.0 and 1.25 M. The ultimate strength of the MICP-treated loess was 3.6 times of the untreated loess. The stress-strain curves indicate that a higher cementing effect can be expected with an increase in the curing period. The formation process of calcium carbonate and the micromorphology of the MICP-treated loess samples were examined using scanning electron microscopy. In this study, we present an environmentally friendly technique for improving loess soil strength.