Search Results (17)

Vertical cross-layer propagation of hydraulic fractures is critical for the efficient stimulation of interbedded sandstone reservoirs in the Lianggaoshan Formation. To investigate the vertical fracture propagation mechanisms and controlling factors in this complex lithological setting, a three-dimensional (3D) numerical model was established using the continuum–discontinuum element method (CDEM) based on the typical “mudstone–sandstone–mudstone” geological structure of the Lianggaoshan Formation. The effects of geological parameters, including interlayer stress contrast, vertical stress contrast, elastic modulus ratio, and reservoir thickness, as well as engineering parameters, including fluid viscosity and injection rate, were systematically evaluated. The results show that interlayer stress contrast is the primary factor restricting vertical fracture growth. As the interlayer stress contrast increases from 2 MPa to 8 MPa, the fracture morphology gradually changes from effective cross-layer propagation to complete containment within the sandstone layer, while the injection pressure at 300 s increases from 55.91 MPa to 58.90 MPa and the fracture width increases from 4.58 mm to 5.05 mm. In contrast, vertical stress contrast has a limited influence under the horizontal-stratification conditions investigated. Increasing fluid viscosity and injection rate can enhance intra-fracture net pressure and promote interface breakthrough. When the fluid viscosity increases from 5 mPa·s to 50 mPa·s, the breakdown pressure increases from 61.05 MPa to 69.59 MPa and the fracture width increases from 4.79 mm to 6.37 mm. When the injection rate increases from 0.6 m³/min to 3.6 m³/min, the breakdown pressure increases from 58.57 MPa to 63.35 MPa and the fracture width increases from 4.28 mm to 5.17 mm. Based on the Effective Vertical Propagation Index (EVI), three vertical propagation modes were identified: restricted vertical propagation, partially effective cross-layer propagation, and effective vertical propagation. Gray Relational Analysis (GRA) revealed the following sensitivity ranking: interlayer stress contrast > injection rate > fluid viscosity > elastic modulus ratio > reservoir thickness > vertical stress contrast. For reservoirs with a typical 4 MPa stress barrier, effective vertical breakthrough can be achieved when the fluid viscosity exceeds 25 mPa·s or the injection rate reaches 3.6 m³/min. These findings provide quantitative guidance for optimizing fracturing parameters in interbedded sandstone reservoirs. Full article

Hydraulic fracture height growth in thin sandstone–mudstone interbeds is often limited by bedding interface failure and multi-cluster stress interference. In this study, a coupled fracture–matrix interface finite element model was developed for the He-8 sandstone–mudstone interbeds in the Sulige Gas Field and validated against previously published true triaxial hydraulic fracturing experiments. The simulations indicate that vertical–horizontal stress difference (VSD; the difference between overburden stress and minimum horizontal stress within a layer) promotes fracture-height growth, whereas interlayer stress difference (ISD; the minimum horizontal stress contrast between adjacent layers) acts as a stress barrier that promotes bedding interface shear failure and arrests vertical growth. For the investigated reservoir configuration, each 4 MPa increase in VSD increased fracture height by approximately 1.5 m in the three-cluster case and 1.8 m in the four-cluster case, whereas each 2 MPa increase in ISD reduced the average fracture height by approximately 4.0 m in the three-cluster case and 3.5 m in the four-cluster case. Under moderate ISD, increasing the fluid viscosity was more effective than increasing the injection rate alone, although the benefit depended on cluster number and interface failure state. These results clarify how stress contrast, interface strength, and multi-cluster stress shadows jointly control cross-layer fracture propagation in thin interbedded reservoirs. Full article

The bottom water of the Shizhouji Formation tight sandstone reservoir in the Tazhong Shun 9 well area is developed. General fracturing faces the problem of excessive extension of hydraulic fractures and easy communication with water layers. A true triaxial fracturing physical simulation experiment was conducted on the sandstone and mudstone outcrops of the same layer to explore the expansion laws of hydraulic fractures in the tight sandstone reservoir and consider the influence of mudstone interlayers, horizontal stress difference, fracturing fluid flow rate, and viscosity. The mechanism of multi-cluster fractures/artificial fractures penetrating through the layers was revealed. The research results show that the existence of mudstone interlayers greatly increases the complexity of fractures, from 1.88 to 2.96, an increase of 57%. When there is a mudstone interlayer in the rock, the fracturing process is prone to open weak planes, hindering the expansion of hydraulic fractures. The hydraulic fractures of Sample No. 4 were cut off four times and penetrated through the layers once. The larger the flow rate, the greater the complexity of hydraulic fractures, and the easier the fractures penetrate through the layers. The fractures with a large flow rate (200 mL/min) were cut off three times, and the stress difference was larger, the hydraulic fractures tended to be simple, and the penetration through the layers was zero times at a high-level stress difference (18 MPa); the greater the viscosity, the greater the fracture pressure, and the complexity of fractures first increased and then decreased; the greater the viscosity, the more easily the hydraulic fractures penetrate through the layers, with low viscosity cutting off three times, medium viscosity cutting off four times, and high viscosity cutting off five times. Therefore, considering the limitation requirements of the on-site fracturing on the extension of fracture height, it is recommended that the on-site fracturing construction flow rate be 6 m³/min, and the fracturing fluid viscosity be 10 mPa·s. Full article

The strength of Xigeda strata decreases significantly upon contact with water, and the shear strength between sandstone and mudstone layers is lower than that within the individual layers. Therefore, the interlayer dip angle plays an important role in determining the failure mode of rainfall-induced landslides. To investigate the effect of interlayer dip angle on the mechanical response of Xigeda sandstone–mudstone slopes under rainfall conditions, a total of five model slope tests were conducted. Different ratios of model materials were selected for the sandstone and mudstone, and artificial rainfall with intensities representative of the Panxi region was simulated using a calibrated rainfall device. A combination of photography and instrument measurements was employed to study the seepage field, deformation field, and slope failure characteristics at five interlayer dip angles. It is shown that when the interlayer dip angle is smaller than the slope angle, an increase in the interlayer dip angle accelerates the movement of the wetting front along the weak interlayer plane. At the same time, this increase shortens the time to the occurrence of abrupt displacement and increases the corresponding displacement magnitude, which makes slope failure prediction more challenging. The shoulders of all slopes experienced displacement earliest and exhibited the largest displacement amplitude. The slope failure mode transitioned from shallow surface sliding to interlayer sliding. When the interlayer dip angle surpassed the slope angle, the weak interlayer plane was no longer the dominant control surface. Slope stability was thereby moderately enhanced, with the failure mode shifting to through-layer sliding. Full article

To address stability issues induced by dynamic fracture of weak interlayers in sandstone–mudstone interbedded slopes during blasting excavation, this study investigates the Qingnian Hub diversion channel project of the Ping-Lu Canal through an integrated methodology combining field blasting tests, laboratory dynamic rock experiments, and numerical simulation validation. Field monitoring captured slope dynamic responses, while ultrasonic testing and Split Hopkinson Pressure Bar (SHPB) dynamic splitting tests determined rock mass mechanical parameters. A high-fidelity 3D numerical model developed in ANSYS/LS-DYNA was validated against experimental data, demonstrating reliability with relative errors in peak particle velocity (PPV) below 20% at most monitoring points. Results reveal that increasing interlayer dip angle reduces fracture length along the lower interface while causing internal oblique cracks to initially lengthen and then shorten, with optimal oblique crack development observed at 10–15°. Conversely, greater interlayer spacing first decreases and then stabilizes lower-interface fracture length, whereas oblique crack length peaks at 4.8 m for a 4 m spacing. Based on 25 parametric simulations, a safety criterion using crack-initiation vibration velocity was established, yielding a predictive model dependent on dip angle and spacing. The derived criterion defines a critical vibration velocity range of 5.6–10.0 cm/s for the studied slope configurations. Compared to existing empirical guidelines that rely solely on peak particle velocity, the proposed criterion innovatively incorporates the controlling influence of geological stratigraphic geometry. This study provides theoretical and practical guidance for optimizing blasting parameters and ensuring slope stability in similar engineering contexts. Full article

Carbon Capture and Storage (CCS) is a key technology for achieving carbon neutrality goals. Relevant foreign research began in the 1970s, but overall it remains in the exploration and demonstration stage. Clarifying the geological parameters and characteristics of reservoir–caprock systems in CCS projects is of great significance to the effectiveness and safety of long-term storage. By reviewing 15 typical global CCS projects, this paper identifies that ideal reservoirs are gently structured sandstones with few faults (characterized by high porosity, high permeability, and large scale, which are conducive to CO₂ diffusion) or basalts (which can react with CO₂ for mineralization, enabling permanent storage). Caprocks are mainly composed of thick mudstone and shale; composite caprocks consisting of multi-layer low-permeability formations and tight interlayers within reservoirs have stronger sealing performance. Additionally, they should be far from faults, and sufficient caprock thickness is required to reduce leakage risks. Meanwhile, this paper points out the challenges faced by CCS technology, such as complex site selection, limitations in long-term monitoring, difficulties in designing injection parameters, and challenges in large-scale deployment. It proposes suggestions including establishing a quantitative site selection system, building a comprehensive monitoring network, and strengthening collaborative optimization of parameters, so as to provide a basis for safe site selection and assessment. Full article

With the development of subway transportation, how to excavate large-section tunnels and find more convenient and reliable support methods has become an issue that cannot be ignored. This paper addresses issues such as low construction efficiency of core rock columns during the construction of large-section subway tunnels in sandstone–mudstone interbedded geological conditions. It proposes an optimized support scheme that replaces traditional core rock columns with temporary steel supports (steel columns). Finite element analysis was used to compare the deformation of the surrounding rock when retaining the core rock columns, using temporary steel columns to replace the core rock columns, and not providing additional support. Five interlayer positions and four interlayer angles were analyzed to identify the most dangerous geological conditions. Based on this analysis, the reasonable spacing of the temporary steel columns was investigated. The results indicate that temporary steel columns and core rock columns can effectively reduce vertical deformation of the surrounding rock, with steel columns showing slightly better results. Replacing core rock columns with steel columns is feasible. To control tunnel rock mass deformation, this project should ensure that the spacing between temporary steel columns is maintained between 21.88 m and 56.80 m. However, in construction sections with good rock mass conditions, the spacing can be extended as long as safety is ensured. Full article

The Zigui Basin, located in the Three Gorges Reservoir Area, has developed numerous landslides due to its interlayering of sandstone and mudstone, geological structure, and reservoir operations. This study identifies a fourth type of landslide failure mode: an oblique-dip slope wedge (OdSW) landslide, based on the Wanshuitian landslide. Following four heavy rainfall events from 3 to 13 July 2024, this landslide exhibited significant deformation on the 17th and was completely destroyed within 40 min. The dimensions of the landslide were 350 m in length, 160 m in width, and 20 m in thickness, with a volume estimated at 8.0 × 10⁵ m³. The characteristics of landslide deformation and the changes in moisture content within the shallow slide body were ascertained using unmanned aerial vehicles, moisture meters, and mobile phone photography. The landslide was identified to have occurred within the weathered residual layer of mudstone, situated between two sandstone layers, with the eastern boundary defined by an inclined rock layer. Upon transitioning into the accelerated deformation stage, the landslide initially exhibited uniform overall sliding deformation, culminating in accelerated deformation destruction. The dip structure created terrain disparities, resulting in a step-like terrain on the left bank and gentler slopes on the right bank, with interbedded soil and rock in a shallow layer, because the interlayered soft and hard geological conditions caused varied weathering and erosion patterns on the riverbank slopes. The interbedded weak–hard stratum layer fostered the development of the oblique-dip slope wedge landslide. Based on the improved Green–Ampt model, we developed a stability prediction methodology for an oblique-dip slope wedge landslide and determined the rainfall infiltration depth threshold of the Wanshuitian landslide (9.8 m). This study aimed not merely to sharpen the evolution mechanism and stability prediction of the Wanshuitian landslide but also to formulate more effective landslide-monitoring strategies and emergency management measures. Full article

The characteristics of interlayer oxidation zones constrain sandstone-type uranium mineralization. This study conducted a quantitative characterization of the interlayer oxidation zones in the uranium-bearing reservoir of the Saihan Formation in the central Wulanchabu Subbasin of the Erlian Basin through sand dispersion system mapping, the analysis of sedimentary debris components, environmentally sensitive parameters, and elemental geochemical characteristics. The formation mechanisms and controlling factors of interlayer oxidation zones were investigated, along with uranium mineralization patterns. Research findings reveal that the sandbodies in the study area primarily consist of red sandstone, yellow sandstone, gray ore-bearing sandstone, and primary gray sandstone, representing strong oxidation zones, weak oxidation zones, transitional zones, and reduction zones, respectively. Although the mineral debris content shows minimal variation among different zones, feldspar dissolution is more prevalent in oxidized zones. During interlayer oxidation, environmentally sensitive parameters exhibit an ascending trend from strong oxidation zones through weak oxidation zones and reduction zones to mineralized transitional zones. Four transition metal elements (Co, Ni, Zn, and Mo) demonstrate enrichment in mineralized transitional zones. The development of interlayer oxidation zones is directly controlled by reservoir heterogeneity and sedimentary environments. Oxidation subzones primarily occur in sandbodies with moderate thickness (40–80 m), sand content ratios of 40%–80%, and 2–10 or 10–18 mudstone barriers (approximately 20 m thick), mainly in braided river channels and channel margin deposits. Reduction zones develop in thicker sandbodies (~100 m) with higher sand contents (~80%), fewer mudstone barriers (2–8 layers), greater thickness (40–80 m), and predominantly channel margin deposits. Transitional zones mainly occur in braided distributary channels and floodplain deposits. When oxygen-bearing uranium fluids infiltrate reservoirs, oxygen reacts with reductants like organic matter, whereFe²⁺ oxidizes to Fe³⁺, S²⁻ reacts with oxygen, and U⁴⁺ oxidizes to U⁶⁺, migrating as uranyl complexes. As oxygen depletes, Fe³⁺ reduces to Fe²⁺, combining with S²⁻ to form pyrite between mineral grains. Uranyl complexes reduce to precipitate as pitchblende, while some U⁴⁺ reacts with SiO₄⁴⁻, forming coffinite, occurring as colloids around quartz debris or pyrite. The concurrent enrichment of certain transition metal elements occurs during this process. Full article

Ultra-deep tight sandy conglomerate reservoirs in the Junggar Basin are characterized by vertically alternating lithologies that include mudstone, sandy conglomerate, and sandstone. High in situ stresses and formation temperatures contribute to a brittle–ductile transition process in the reservoir rocks. However, the brittle behavior and ductile hydraulic fracture propagation mechanisms under in situ conditions remain inadequately understood. In this study, ultra-deep core samples were subjected to triaxial compression tests under varying confining pressures and temperatures to simulate different burial depths and evaluate their brittleness. A three-dimensional hydraulic fracture propagation model was developed in ABAQUS 2023 finite element software, incorporating a cohesive zone ductile constitutive model. Numerical simulations were conducted, considering interlayer horizontal stress differences, injection rate, and fracturing fluid viscosity, to systematically analyze the influence of geological and engineering factors on ductile fracture propagation. A fracture length–height competition diagram was constructed to illustrate the propagation mechanisms. The results reveal that high temperatures significantly accelerate the brittle–ductile transition, which occurs at confining pressures between 55 and 65 MPa. Following this transition, failure modes shift from single-shear failure to a multi-localized fracture with bulging deformation. Interlayer horizontal stress differences were found to strongly influence fracture penetration, with larger stress differences hindering vertical growth. Increasing injection rates promoted the uniform distribution of lateral fractures and fracture tip development, while medium- to high-viscosity fracturing fluids enhanced fracture width and vertical stimulation uniformity. These findings provide important insights for optimizing fracturing strategies and expanding the effective stimulation volume in the ultra-deep tight sandy conglomerate reservoirs of the Junggar Basin. Full article

A numerical analysis model for sand-mudstone interbedded fracturing based on field application in South China is presented in this paper. The proposed model can analyze the influence laws of different longitudinal lithology changes, stress difference changes, different interlayer positions, and fracturing fluid construction parameters on fracture characteristics. Based on the study of fracture characteristics of low-modulus mudstone, a set of layered stress loading experimental devices was independently designed and developed. Experimental analysis shows that the stress difference has a limited limiting effect on the interlayer propagation of hydraulic fracturing fractures in the Weizhou Formation, and the fracture height is prone to interlayer propagation. The injection of high-rate and high-viscosity fracturing fluid has a significant impact on the hydraulic fracture surface penetration. Numerical simulation analysis shows that the smaller the elastic modulus of the mudstone interlayer and the lower the minimum horizontal principal stress compared to the sandstone layer, the more favorable it is for fracture propagation. Field application showed that the highest injection rate of the fracturing pump in well A was 7 m³/min for south-west Weizhou oilfield shale oil. The interpretation results of the acoustic logging after fracturing showed obvious response characteristics of the formation fractures, and the farthest detection fracture response well distance was 12 m, indicating a good fracturing transformation effect and providing technical support for subsequent offshore shale oil fracturing construction. Full article

This study presents a precise and efficient methodology for evaluating the water-richness of the aquifer overlying the No. 3 coal seam in Hancheng Mine. A comprehensive assessment model was developed by integrating subjective and objective weighting through the sequential relationship analysis–entropy value method. This model facilitated the delineation of water-richness zones within the sandstone aquifer of the Shanxi Group associated with the No. 3 coal seam. Five key evaluation indices were selected based on the aquifer’s water-richness index: core recovery rate, thickness of water-rich sandstone, number of sand–mudstone interlayers, sandstone lithology coefficient, and the thickness ratio of brittle to plastic rock. Furthermore, an advanced evaluation model combining set pair analysis and variable fuzzy sets was established to assess the water-richness risk levels across the entire Hancheng mining area. The results reveal distinct spatial patterns in water-richness: the northeastern region exhibits strong water-richness, while the southwestern area is characterized by medium to weak water-richness over a broad expanse. Overall, the No. 3 coal seam in the Hancheng mining area is classified as having a medium risk level of water-richness. Full article

The Weixinan Sag in the Beibuwan Basin is rich in shale oil resources. However, the reservoirs exhibit rapid phase changes, strong compartmentalization, thin individual layers, and high-frequency vertical variations in the thin interbedded sandstone and mudstone. These factors can restrict the height of hydraulic fracture propagation. Additionally, the low-porosity and low-permeability shale oil reservoirs face challenges such as low production rates and rapid decline. To address these issues, the Plannar3D full 3D fracturing model was used to simulate hydraulic fracture propagation and to study the main controlling factors for fracture propagation in the second member of the Liushagang Formation. Based on the concept of geological–engineering integration, a sweet spot evaluation was conducted to identify reservoirs with relatively better brittleness, reservoir properties, and oil content as the fracturing targets for horizontal wells. The UFM model was then applied to optimize fracturing parameters. This study indicates that the matrix-type oil shale has a high clay mineral content, resulting in a low Young’s modulus and poor brittleness. This makes hydraulic fracture propagation difficult and leads to less effective reservoir stimulation. In contrast, hydraulic fractures propagate more easily in high-brittleness interlayer-type oil shale. Therefore, it is recommended to prioritize the extraction of shale oil from interlayer-type oil shale reservoirs. The difference in interlayer stress is identified as the primary controlling factor for cross-layer fracture propagation in the study area. Based on the concept of geological–engineering integration, a sweet spot evaluation standard was established for the second member of the Liushagang Formation, considering both reservoir quality and engineering quality. Four sweet spot zones of interlayer-type oil shale reservoirs were identified according to this evaluation standard. To achieve uniform fracture initiation, a differentiated segment and cluster design was implemented for certain high-angle sections of well WZ11-6-5d. Interlayer-type oil shale was selected as the fracturing target, and the UFM was used for hydraulic fracture propagation simulation. Fracturing parameters were optimized with a focus on hydraulic fracture characteristics and the estimated ultimate recovery (EUR). The optimization results were as follows: a single-stage length of 50 m, cluster spacing of 15 m, pump injection rate of 10 m³/min, fluid intensity of 25 m³/m, and proppant intensity of 3.5 t/m. The application of these optimized fracturing parameters in field operations resulted in successful fracturing and the achievement of industrial oil flow. Full article

The H8 and S1 reservoirs in the lower Shihezi Formation and Shanxi Formation of the central block in the Sulige Gas Field are typical fluvial tight sandstone reservoirs. Due to frequent river channel migrations during deposition, the reservoirs exhibit complex spatial structures with developed intra-sand mudstone interlayers. As the field has entered the middle and late stages of development, the distribution of remaining gas is intricately controlled by these interlayers, necessitating research on their distribution to understand the remaining gas patterns and types for effective extraction enhancement. However, the thinness of interlayers presents a challenge for precise prediction. Addressing this, this study delineates different interlayer types and their origins, applies reservoir architecture theory, and utilizes bounding surfaces characterization, planar and sectional distribution studies, unit scale analysis, horizontal well data, and quantitative characterization methods to investigate the internal reservoir architecture bounding surfaces. The study finely portrays the interlayer distribution, analyzes the control of reservoir architecture bounding surfaces on remaining gas, and establishes a multi-tiered reservoir architecture model in the study area. Numerical simulation of the gas reservoir clarifies the types of remaining gas enrichment. This study also identifies and quantitatively characterizes the 5–3 level architecture bounding surfaces within the sandbody, categorizing the remaining gas into bounding surfaces-controlled, well-network uncontrolled, and single-layer unperforated types, proposing targeted enhancement measures for each type. Based on the findings, four vertical wells and three horizontal wells were deployed, improving the well network density to three wells per square kilometer. The first completed horizontal well encountered an effective drilling rate of 61.7%, marking significant implications for the exploitation and recovery enhancement of similar tight sandstone gas reservoirs. Full article

Organic-rich lacustrine shales in the Upper Triassic Yanchang Formation with thermal maturity mainly in the oil window are the main shale oil and shale gas system in the lacustrine strata of the Ordos Basin, China. Pore systems are important for the storage and transfer of shale oil and gas. The main objectives of this study are to identify the pore types and pore structures and investigate the controlling factors for pore types, pore structures, and total porosities of the lacustrine Yanchang Shale. In this study, organic-rich mudstones, mudstones with siltstone interlayers, siltstone, and sandstones were selected from 15 wells in the southern Ordos Basin. X-ray diffraction, pyrolysis, scanning electron microscopy (SEM), low-pressure nitrogen adsorption analysis, and helium porosimetry were conducted to investigate the mineral compositions, pore types, pore structures, porosities, and controlling factors. Siltstone and sandstone interlayers heterogeneously developed in the Yanchang Shale. The petrology, mineral composition, geochemistry, pore type, pore structure, and porosity of siltstone interlayers are different from those of mudstones. The siltstone and sandstone interlayers usually have more quartz and feldspars, greater detrital grain sizes, and relatively better grain sorting but are lower in clay minerals, total organic carbon (TOC), amount of free liquid hydrocarbons values (S₁), and total residual hydrocarbons values (S₂), compared to mudstones. Interparticle (interP), intraparticle (intraP) pores, and organic pores (OPs) were developed in both siltstones and mudstones. OPs were observed in samples with lower thermal maturity (e.g., 0.5–0.85%). The inorganic pore size is greater than that of OPs. Additionally, the inorganic pore diameters in siltstone interlayers are also greater than those in mudstones. Organic-rich mudstones generally have higher pore volumes (PVs) of pores with sizes less than 10 nm, pore volumes of pores with sizes between 10 and 50 nm (PV, 10–50 nm), and specific surface area (SSA), but they have lower PVs of pores with sizes greater than 50 nm, total PV, and porosity when compared to siltstone and sandstone interlayers. The dominant pore type in mudstones is OPs and TOC (first order), sources and OM types (second order), and thermal maturity (third order), while the abundances of rigid grains with greater sizes and grain sorting are the main controlling factors of pore structures, SSA and PV. Both inorganic pores and organic pores are abundant in the siltstone interlayers. The pore size distribution (PSD), PV, and porosity of siltstone interlayers are related to the abundance of rigid grains (first order), grain sorting (second order), grain size (third order), and carbonate cement content. The total PV and porosity of Yanchang Shale reservoirs may have increased with the increased abundance of siltstone and sandstone interlayers. Full article