Search Results (407)

Understanding the thermal evolution of foreland basins is essential for reconstructing hydrocarbon generation, migration, and accumulation, and for improving petroleum-system models in thrust-influenced foreland basins worldwide, including the Pre-Uralian Foredeep. However, the thermal regime of Paleozoic formations along the eastern margin of the Pre-Caspian Basin remains poorly constrained, particularly with respect to the relative roles of burial heating and tectonically driven hydrothermal circulation. This study investigates the thermal history and petroleum implications of calcite-hosted fluid inclusions in fractures and veins within Upper Silurian to Lower Permian sedimentary rocks of the Sakmara accretionary zone (Mugodzhar, Southern Urals), the Pre-Uralian Foredeep, and the eastern Pre-Caspian Basin, western Kazakhstan. Aqueous fluid inclusions yield homogenization temperatures ranging from ~50 to 280 °C and define several fluid populations. The dominant population (Th ≈ 87–146 °C) is interpreted as burial-related, whereas low-temperature inclusions (50–78 °C) reflect early diagenetic and/or late-stage fluid circulation. Higher-temperature inclusions (>180 °C) record transient hydrothermal events associated with fault-controlled fluid flow. These results indicate that the basin’s thermal evolution was governed by the combined effects of burial heating and episodic hydrothermal activity. Comparison with available Rock–Eval, vitrinite reflectance, molecular maturity, and burial-history data suggests that the sampled Paleozoic formations were generally immature to marginally mature, whereas hydrocarbons were likely generated in deeper or adjacent, more mature zones (>4 km) and subsequently migrated into the studied units. The results provide a regional framework for understanding fluid-flow and thermal processes in the Uralian foreland system and demonstrate the broader value of integrating fluid-inclusion microthermometry with structural and burial-history analyses to reconstruct basin evolution and hydrocarbon migration in foreland basins worldwide. Full article

Accurate identification of acid-producing layers is key to controlling acid mine drainage (AMD) in abandoned coal mines. This study collected 337 core samples from 34 boreholes in the Modigou mining area, Shanxi, China, and established a combined static–mineralogical–kinetic approach to evaluate the acid-generating and neutralization potentials of sulfur-bearing rocks. Three-stage net acid generation (NAG) tests identified the pyrite-bearing layer of the Benxi Formation and the No. 10 coal seam of the Taiyuan Formation as the main acid producers, with NAG values of 360.41 and 97.87 kg H₂SO₄/t, respectively, while the Taiyuan limestone showed a high neutralization capacity (ANC = 490 kg H₂SO₄/t). NAG pH was strongly negatively correlated with sulfur content (Pearson r = −0.75, p < 0.01). Sulfide oxidation acid production showed staged attenuation, with average decreases of 64.81% and 47.65% in the second and third stages. Humidity cell experiments demonstrated continuous acid production over 63 days under dry–wet cycles, with increased acid generation rates at higher flow velocities (Darcy flux: 3.54 × 10⁻³ cm/s for accelerated vs. 8.84 × 10⁻⁴ cm/s for standard conditions). Multi-dimensional flow-through simulations confirmed the AMD formation mechanism of “acid supply, buffer, and fracture conduction”. The identified acid-producing layers matched well with field discharge points. This multi-method coupling system provides a theoretical basis for source control of AMD in abandoned high-sulfur coal mines in the Yellow River Basin. This study did not account for microbial catalysis, which is a key limitation of the static chemical oxidation method used. Full article

The pronounced heterogeneity of tight sandstone reservoirs in the Sulige Gas Field poses significant challenges to the uniform propagation of multi-cluster hydraulic fractures during horizontal well staged fracturing, often leading to uneven stimulation and compromised productivity. To address this issue, a coupled fluid–solid fracture propagation model based on the displacement discontinuity method (DDM) was developed, incorporating dynamic fluid distribution, rock deformation, and temporary plugging mechanisms. The model was validated against microseismic monitoring data from the Sulige field and subsequently employed to investigate the effects of reservoir heterogeneity—including porosity, permeability, and in situ stress—on multi-cluster fracture growth. Results indicate that permeability and stress heterogeneity exert the most significant influence on fracture non-uniformity, as reflected by increased coefficients of variation in fracture length. Engineering measures such as the use of high-viscosity guar gum fracturing fluids, variable perforation strategies (e.g., 6, 10, and 16 holes per cluster), and optimized temporary plugging parameters (timing of 0.5 with 12 balls) were shown to effectively mitigate these effects and promote more balanced fracture propagation. This study provides a quantitative framework for optimizing fracturing design in heterogeneous tight gas reservoirs and offers practical guidance for enhancing stimulation uniformity and gas recovery efficiency in the Sulige Gas Field. Full article

Combined Vertical–Horizontal Well Patterns (CVHWPs) have been increasingly applied in mature and complex reservoirs, such as the C5 Block. Their application is attractive because they provide extensive reservoir coverage and high development efficiency. However, close well spacing and the three-dimensional configuration of vertical and horizontal wells can induce strong stress-shadow interference. This interference makes fracture propagation difficult to control and may reduce stimulation effectiveness. To address this problem, a multi-well, multi-fracture induced-stress model for CVHWP stimulation was developed in this study. The model was validated using laboratory three-stage fracturing experiments, including two horizontal-well stages and one vertical-well stage, together with field observations. Across three stages, the calculated stress intensity factors at breakdown are closely matched, validating the induced-stress model. When the vertical well was fractured first, the horizontal principal-stress difference at the adjacent horizontal stage increased by 2.01 MPa, which was unfavorable for branched fracture development. In contrast, when the horizontal stage was fractured first, the stress difference decreased by 3.25 MPa at the subsequent horizontal stage and by 3.89 MPa at the vertical-well stage. This sequence is preferable because fractures generated from the vertical well impose a stronger stress perturbation on adjacent horizontal-well fractures than fractures generated from the horizontal well impose on the subsequent vertical-well fracture. Under the tested CVHWP conditions, the horizontal-well fractures tended to form nearly symmetric bi-wing planar fractures, whereas branched fractures were more likely to develop in the vertical well. Therefore, for CVHWP reservoirs with close vertical–horizontal well spacing and significant stress interference, fracturing the horizontal well before the vertical well is recommended to control fracture propagation and promote multiple-fracture formation. Field application of this sequence showed notable production improvement, indicating that the proposed method can provide practical guidance for unconventional well-pattern fracturing design. Full article

The Late Triassic Bolila Formation in the central Qiangtang Basin is a typical carbonate buildup deposited during a regional transgression in the eastern Tethyan realm. Understanding its sedimentary evolution and reservoir-forming mechanisms is crucial for hydrocarbon exploration. This study integrates petrology, detrital zircon U-Pb geochronology, carbon-oxygen isotopes, and reservoir property analysis of the Quemudongda section. The results show: (1) detrital zircon dating provides a maximum depositional age of 225.7–235.7 Ma (Carnian–Norian), correcting the previous Jurassic misassignment on the 1:250,000 geological map. Carbon-oxygen isotopes (average δ¹³C = +3.2‰, δ¹⁸O = −11.1‰) are consistent with the global Carnian–Norian positive δ¹³C excursion. (2) The section reveals a platform-margin reef (hexactinellid and calcareous sponges) and slump breccia (seven layers) association, representing a steep-rimmed carbonate platform margin. The sedimentary evolution comprises three stages: reef initiation, reef flourishing with frequent slumping, and reef decline with dolomitization. (3) Reservoirs are mainly breccia and reef dolostones, with intergranular, intercrystalline, and fracture-related pores. Porosity averages 2.8% (0.8%–7.2%), permeability averages 0.35 mD (0.001–8.5 mD), defining a low-porosity, ultra-low-permeability fracture-pore reservoir. Breccia dolostone has better properties (porosity 3.71%, permeability 2.412 mD). (4) Reservoir formation is controlled by sedimentation (platform-margin facies), diagenesis (dolomitization generates pores, but high-temperature recrystallization causes densification), and tectonics (microfractures enhance permeability). High-quality reservoirs occur where breccia dolostone and fractures overlap. (5) The Bolila reef-shoal complex and the overlying Bagong Formation source rocks form a “lower reservoir—upper source” assemblage, representing a new exploration target in the Tuonamu area. The breccia dolostone–fracture overlap zone is the core “sweet spot”. Full article

Rock pillars in deep underground mines are subjected to complex stress environments. The combined effects of in situ stress and cyclic disturbances from mining activities lead to a redistribution of the surrounding rock mass stress field, which readily triggers instability and failure, posing severe threats to mining engineering safety. To investigate the damage mechanism of cyclic loading on rock and its weakening effect on the bearing capacity of mine pillars, this study takes limestone as the research object. A series of uniaxial compression tests were conducted on limestone specimens subjected to triaxial cyclic pre-damage, complemented by numerical simulations to further characterize the energy and deformation evolution of the damaged limestone under cyclic loading conditions. The findings are as follows: (i) Triaxial cyclic tests on limestone show that both the input energy and dissipated energy follow similar trends, decreasing rapidly in the initial stage before stabilizing. The elastic strain energy remains largely constant, with most of the input energy being stored as elastic strain energy. Under constant stress levels and cycle numbers, increases in confining pressure and frequency reduce the rock’s input energy, elastic strain energy, and dissipated energy. (ii) The peak stress of damaged limestone exhibits a positive correlation with the pre-damage confining pressure and cyclic frequency, while it decreases with an increasing number of cycles. Higher confining pressure and frequency raise the input energy, elastic potential energy, and dissipated energy at the peak stress point. (iii) Deformation and failure in damaged limestone originate from the development and propagation of localized deformation zones. Increased lateral displacement within these zones promotes the formation of macroscopic fractures. Due to significant structural heterogeneity inside the localized areas, the evolution of deformation energy is influenced by regional characteristics. (iv) Simulation results indicate that the uniaxial compressive failure of limestone involves the accumulation and propagation of micro-scale tensile cracks, which ultimately coalesce into macro-scale shear fracture surfaces. During uniaxial loading of pre-damaged limestone, newly generated cracks predominantly initiate around pre-existing cracks, with only a limited number distributed randomly. Their peak intensity shows a positive correlation with the pre-damage confining pressure. Full article

Shale oil is a vital unconventional resource. Large-scale hydraulic fracturing serves as the core technology for the efficient development of shale oil reservoirs. Well testing can be applied to characterize the reservoir parameters of fractured shale formations. Nevertheless, conventional well testing approaches fail to account for numerous discrete fractures and complex formation geometries. Based on the embedded discrete fracture model (EDFM)—an effective tool for simulating flow in discrete fractures—this work proposes a numerical well testing approach for horizontal wells in hydraulically fractured shale reservoirs. The effects of fracture permeability, number of fracture clusters, matrix permeability, and water saturation on well testing curves are also investigated. The results showed that the parameters such as the main fracture permeability, the number of fracture clusters, and the matrix permeability all have significant effects on the well test curves. When the permeability of main fractures exceeds 20D, radial flow characteristics appear in Stage V. For the distance between fracturing intervals and pressure monitoring points within 0 m to 200 m, it imposes the most significant impact on Stage I and Stage II. The half-length of main fractures, the SRV extent in the Y-direction, and boundary conditions mainly affect Stage VI and Stage VII. Full article

Borate-crosslinked hydroxypropyl guar (HPG) gels are widely used as water-based fracturing fluids in oilfield stimulation. During hydraulic fracturing, their effectiveness depends on the rapid formation of a low-permeability filter cake on fracture walls, which helps reduce fluid invasion, maintain fracture pressure, and support fracture propagation. In high- and ultra-high-permeability reservoirs, however, rapid matrix invasion may occur faster than effective filter-cake formation, causing severe pre-cake spurt loss or even uncontrolled leakoff. Conventional filter-paper tests tend to emphasize stabilized wall-building behavior and may therefore fail to represent the early-time spurt loss in porous reservoir media. In this study, the leakoff behavior of borate-crosslinked HPG fracturing fluids was investigated using a modified static fluid-loss apparatus. Experiments were conducted at differential pressures of 0.5–6.0 MPa through filter paper and artificial sandstone disks with permeabilities from 0.120 to more than 4.0 μm². The filter-paper tests showed typical wall-building behavior, with limited spurt loss and stable late-time leakoff. In contrast, the sandstone-disk tests revealed a transition from cake-controlled leakoff to early-time spurt-loss-dominated leakoff as permeability and differential pressure increased. When permeability exceeded approximately 1.55–2.42 μm², spurt loss (V_sp) became the main contributor to total leakoff, whereas the late-time wall-building coefficient (C_w) was much less sensitive to permeability. This indicates that permeability mainly controls the pre-cake invasion stage rather than the stabilized leakoff stage. Based on these results, an empirical spurt-loss model considering permeability and pressure differential was developed, and spurt-loss zoning maps were constructed for engineering evaluation. Limited ultra-high-permeability tests further showed that quartz particles promoted early bridging and reduced leakoff under moderate pressure differentials, but the particle-assisted barrier lost effectiveness under higher pressure differentials. These findings demonstrate that filter-paper-based criteria are insufficient for evaluating HPG gel performance in extreme-permeability formations and that a spurt-loss-based framework is needed for fluid-loss-control design and fracturing-fluid selection in high-permeability reservoirs. Full article

To address the industry challenge that the formation breathing effect in fractured formations narrows the safe mud weight window and significantly increases well control difficulty, this study employs two approaches—a self-designed experimental apparatus for the formation breathing effect and a combined finite-discrete element method (FDEM) numerical model—to systematically reveal the characteristic behavior and underlying mechanism of this effect, and to establish a prediction method for near-wellbore lost-circulation pressure that accounts for the breathing effect. The numerical simulation achieves high quantitative accuracy, with errors of less than 2.1% during the loss stage and less than 4.1% during the flowback stage. The results show that the typical signature of the breathing effect in fractured formations is a sustained loss of drilling fluid followed by rapid flowback, resulting in a pronounced reversible volume change in the wellbore. The intrinsic mechanism lies in the switching between fracture opening and closure triggered by the shift in the pressure differential between the wellbore and the formation. Parametric sensitivity analysis indicates that increasing wellbore pressure intensifies the breathing effect; formations with low fracture opening pressure, high porosity, and high permeability are more prone to severe breathing effects. Increasing the plastic viscosity and yield point of the drilling fluid can suppress the breathing effect, but careful management of the resulting increase in circulating friction and equivalent circulating density (ECD), which raises bottomhole pressure, is required. Field case calculations for a well in the Cameroon block show that, after improving the lost-circulation pressure calculation method to incorporate the breathing effect, the safe mud weight window can narrow by up to 0.03 g/cm³. This study advances the understanding of breathing effects in fractured formations and provides theoretical support for safe drilling within the narrow mud weight windows commonly encountered in such formations. Full article

To address delayed roof fracture, severe stress concentration, and strong strata pressure under thick–hard roof conditions, this study investigated the 1014 mining face of Yushuquan Coal Mine. A staged thick-plate model incorporating boundary-condition degradation was established based on Mindlin–Reissner thick-plate theory to analyze the deformation and stress redistribution characteristics of the thick–hard roof during mining. The evolution mechanism of the stress-concentration shell was systematically studied through theoretical analysis, physical simulation, numerical simulation, and field application. The results show that, with mining advancement, the boundary constraints of the thick–hard roof gradually evolve from four-sided clamped support to four-sided simply supported conditions. Meanwhile, the high-stress zone migrates from the goaf boundary toward the central suspended roof region. The stress-concentration shell undergoes a dynamic process of formation, expansion, failure, and reconstruction, and its instability is the main driving mechanism of large-scale roof caving. The plastic zone expands upward in an inverted funnel shape, while acoustic emission signals increase significantly before roof instability and exhibit strong precursor characteristics. Based on the evolution characteristics of the stress-concentration shell, a three-stage coordinated blasting technology was proposed to regulate the overburden load-bearing structure. Field application shows that this method effectively reduces suspended roof distance, caving block size, surrounding rock deformation, and hydraulic support pressure, thereby improving roof stability and mining safety. The results provide theoretical and engineering references for stability control of thick–hard roofs under similar mining conditions. Full article

Multi-cluster perforation staged fracturing in horizontal wells has become an important means of completion stimulation for unconventional oil and gas reservoirs. However, the non-uniform propagation of multi-cluster hydraulic fractures remains one of the key challenges restricting efficient reservoir stimulation. In this study, based on the finite element method and considering factors such as frictional pressure drop along the wellbore for power-law fluid, perforation friction, and stress interference, a fracture propagation model with dynamic multi-stage flow distribution coupling formation, perforation, and wellbore flow was constructed. The effects of non-uniform perforation schemes, total number of perforations, and perforation non-uniformity coefficient on multi-cluster fracture propagation behavior were systematically investigated, and the characteristics of dynamic flow distribution were clarified. The results show that the order of fluid intake uniformity among different perforation schemes is as follows: spindle-shaped perforation, uniform perforation, and Tapered perforation. Reducing the number of perforations and decreasing the perforation non-uniformity coefficient can improve the uniformity of fracture propagation to a certain extent. The findings of this study can provide a theoretical basis and practical reference for efficient fracturing stimulation of shale oil in the Cangdong Sag. Full article

Deep-to-ultra-deep marine carbonate reservoirs represent an important frontier for hydrocarbon exploration in the Tarim Basin, yet fluid sources and accumulation processes in the Ediacaran (Sinian) succession remain poorly constrained due to extreme burial depth and complex tectono-thermal evolution. Here, we investigate fracture–vug reservoirs of the Sinian Qigebulake Formation in Well LT3 (Tabei Uplift) using an integrated dataset including petrography and cathodoluminescence, fluid-inclusion microthermometry, fluorescence and Raman spectroscopy, in situ major/trace element analysis and C–O–Sr isotope geochemistry, and LA-ICP-MS carbonate U–Pb dating of authigenic minerals. The paragenetic sequence comprises early dolomite (Dol-I), later dolomite (Dol-II), co-precipitated calcite (Cal-I) and quartz (Qtz-I), and late solid bitumen (Bit). Dolomite veins show PAAS-normalized REE patterns and 87Sr/86Sr ratios (0.70918–0.70984; average 0.70942) comparable to the surrounding Sinian marine wall rocks, indicating precipitation from diagenetic fluids dominated by closed-system water–rock interaction. In contrast, Cal-I displays LREE enrichment, pronounced positive Eu anomalies (δEu = 4.91–7.21), radiogenic ⁸⁷Sr/⁸⁶Sr ratios (0.71161–0.71417; average 0.71256), and negative δ¹⁸O_VPDB values (down to −9.439‰), suggesting a large-scale influx of deep-seated, high-temperature, Sr-rich hydrothermal fluids likely linked to fault-assisted fluid circulation. Fluid inclusions record four hydrocarbon charging episodes, evolving from lower- to higher-maturity oils and ultimately to dry gas. Dol-II hosts pale-yellow to pale-blue oil inclusions, whereas Cal-I and Qtz-I predominantly contain deep-blue oil inclusions and methane-rich gas inclusions (Raman peak near 2917 cm⁻¹). Carbonate U–Pb ages constrain dolomite precipitation to the Middle Ordovician (~468–463 Ma) and hydrothermal-related carbonate filling to the Early Triassic (~247–244 Ma). Collectively, these results support a time-resolved evolution in which early diagenetic fluid circulation in a marine carbonate system was overprinted by a later hydrothermal pulse that modified pore structures and thermal conditions, followed by late-stage deep burial leading to cracking of retained liquids, widespread bitumen formation, and methane charging. This framework provides new information on the constraints for fluid–rock interaction and hydrocarbon evolution in deep marine carbonate successions. Full article

The Shizhushan super-large wollastonite deposit, situated in the western segment of the Pingle Depression within the Qin–Hang metallogenic belt, represents the largest known wollastonite deposit in the world. The deposit formed mainly through contact metamorphism, followed by contact metasomatism. The ore bodies are hosted in the carbonate rocks of the Middle Permian Maokou Formation. The deposit consists of 12 ore bodies, which are controlled by the strata and occur as layered and lenticular bodies. The mineralization process is divided into an early contact metamorphism period (wollastonite–calcite and wollastonite–calcite–quartz stages) and later hydrothermal metasomatism period (wollastonite–garnet stage). This study examined the fluid inclusions in the contact metamorphic wollastonite ore bodies in this deposit. The results show that the types of inclusions are mainly liquid-rich two-phase inclusions, pure liquid-phase inclusions, and pure vapor-phase inclusions, with a small number of CO₂-H₂O three-phase inclusions. The homogenization temperature ranges of the different stages are 409.50~260.10 °C for the wollastonite–calcite stage and 283.60~132.40 °C for the wollastonite–calcite–quartz stage. The temperature and salinity of ore-forming fluids show an evolution from medium–high temperature and medium–low salinity to low temperature and low salinity. The minimum ore-forming depth of the deposit is 0.79~4.74 km, indicating a shallow, low-pressure environment. Based on the macrogeological characteristics, the tectonic fractures formed during the contact metamorphic period triggered decompression boiling of the ore-forming fluids, which led to the precipitation of mineralization materials and the subsequent mineralization. Based on a comprehensive analysis of the metallogenic geological setting, material sources, metallogenic process, and structural control factors, a trinity metallogenic model of the “carbonate rock + pluton + semi-open structural system” is established for the Shizhushan wollastonite deposit. Full article

Tight oil reservoirs are characterized by poor petrophysical properties. After hydraulic fracturing, the low flowback rate of fracturing fluid readily leads to the formation of a water invasion zone in the near-wellbore region, which severely restricts the performance of Carbon dioxide (CO₂) huff-n-puff. To clarify the damage mechanism of the water invasion zone on CO₂ huff-n-puff in tight oil reservoirs and determine the key regulatory parameters, tight cores with a relative water invasion zone length Δδ = 0.3 were adopted as the research subject. Five groups of injection–soaking–production time combinations were designed, and single-factor analysis was implemented using the control variable method. Integrated with numerical simulation and nuclear magnetic resonance (NMR) testing, the influence of the water invasion zone, pore crude oil mobilization characteristics, and parameter regulation effects were systematically explored. The results demonstrate that the water invasion zone occupies effective pore throats to form a continuous water-phase barrier, hindering CO₂ seepage and mass transfer. After four huff-n-puff cycles, the cumulative recovery factor of the water-invaded model is 4.13 percentage points lower than that of the water-free model. After four huff-n-puff cycles, the cumulative recovery factor of the water-invaded model is 4.13 percentage points lower than that of the water-free model. The NMR T₂ spectra of cores with and without water invasion exhibit remarkable discrepancies: the water-free core presents a unimodal structure, while the water-invaded core features a distinctive bimodal structure, with obvious staged characteristics in crude oil mobilization. The recovery factor declines nonlinearly and sharply with the increase of Δδ, verifying that the water invasion zone length is the dominant controlling factor. The regulation effects of injection, soaking, and production time differ significantly: injection time serves as the pivotal parameter for enhancing oil recovery. Prolonging injection time can strengthen displacement intensity and dismantle the water-phase barrier, thereby elevating the recovery factor, whereas soaking time and production time have no significant improvement effect. The results can provide valuable references for the parameter optimization of CO₂ huff-n-puff in water-invaded tight oil reservoirs. Full article

In the Ordos Basin, one of the most important oil- and gas-bearing basins, the Triassic Yanchang Formation has formed important source rocks, but is also a typical representative of continental deep-water sedimentation. In the Huacheng area of the Upper Triassic Yanchang Formation, the deep-water gravity flow sedimentation characteristics of the lake-bottom fan are complex, and the spatial distribution pattern and stacking style of the sand bodies are of great significance for oil and gas resource exploration. Based on core observation, by combining well logging and analysis of signs of sedimentary facies, including petrologic features and primary sedimentary structures, the thick massive sand bodies of the Chang 6 Member belong to deep-water gravity flow deposits, and they develop in a semi-deep to deep lacustrine environment in the Huachi area, Ordos Basin. The primary sedimentary structures of deep-water gravity flows include massive bedding, graded bedding, sliding fractures, slumping deformation structures, turbidite sequences, and synsedimentary offsets. Two kinds of deep-water gravity flows of the channel system, namely sandy debris flows and turbidity currents, were identified in the sublacustrine fan. The sublacustrine provided accommodation space for the rapid unloading and accumulation of gravity flows. Deposited sandy debris flows are the most widely distributed in the sublacustrine fan. Three types of stacked sand bodies developed in the Chang 6 Member of the Huachi area, including multi-stacked thick-layered, sandstone–mudstone interbedded, and sand-thin and mud-thick types. The multi-stacked thick-layered sand bodies consist of multi-period massive sandstones, which are interpreted as sandy debris flow deposits. Sandstone–mudstone interbedded types exhibit diverse lithologies, including massive sandstone and deformed structural sandstone. In addition, the turbidity current is the primary factor controlling the stacked sand bodies. Sand-thin and mud-thick sand bodies consist primarily of laminated mudstone, massive mudstone, and flaser-bedded sandstone, and these deposits were formed by waning-stage turbidity currents and the rigid heads of sandy debris flows. Full article