Search Results (3,505)

In this study, deep tight sandstone was selected as an example to propose a complete method for predicting reservoir pore structure by capillary pressure curves and conventional well log data. This method pioneers the integration of grey relational analysis, principal component analysis, ensemble clustering, and deep neural networks to establish a systematic predictive framework for transitioning from conventional logging data to pore structure types. A total of 186 core data from three wells were used in this study. First, sensitive pore structure parameters from mercury injection capillary pressure data were extracted using grey correlation analysis and principal component analysis. Then, unsupervised clustering analysis was applied to classify the reservoir pore structures in the study area, dividing it into three categories. These category labels were combined with conventional well logs to create learning samples for a deep neural network (DNN) model developed to predict reservoir pore structure categories. The accuracy of the training set of the model reached 88.2%, while the accuracy of the testing set was 80.43%. Finally, the method was applied to field well log data. The results showed significant differences in pore structure classifications among gas layers, water–gas layers, and dry layers. This method is versatile, with its core components transferable to other deep sandstone reservoir studies, and can accurately predict the pore structure of tight sandstone reservoirs, which is critical for advancing the characterization of deep and complex oil and gas reservoirs. Full article

During the drilling process of 10,000 m deep wells, loss zones face complex environments with ultra-high temperatures and pressures. Traditional bridging plugging materials exhibit insufficient temperature resistance and tend to carbonize under downhole high-temperature conditions, leading to recurrent loss. To address the technical challenges of drilling fluid loss in ultra-high-temperature formations of 10,000 m deep wells, experimental research was conducted to evaluate the properties of plugging materials resistant to 240 °C. Rigid particles, elastic particles, flaky materials, and fiber materials resistant to 240 °C were optimized. An experimental evaluation method for ultra-high-temperature dense pressure-bearing loss prevention and plugging formulations was established. The ultra-high-temperature while-drilling leak prevention formulation was optimized through sand disk plugging experiments. Millimeter-scale fracture plugging simulation experiments optimized ultra-high-temperature stop-drilling plugging formulations for different fracture apertures, achieving a bearing capacity of 15 MPa within 1–5 mm fracture apertures. Through the synergistic effects of various loss prevention materials, a reinforced force chain network structure forming a dense pressure-bearing plugging layer was achieved under 240 °C high-temperature conditions. This research provides material and system support for the solving drilling fluid loss challenges in high-temperature formations of 10,000 m ultra-deep wells. Full article

Hydraulic fracturing is crucial for the effective development of unconventional oil and gas reservoirs. This paper systematically reviews the damage issues caused by conventional fracturing fluids in tight unconventional reservoirs, highlighting problems such as significant formation damage and high risks of scale deposition and plugging. To address these shortcomings, a phase-change self-propping fracturing fluid is proposed and compared with a guar gum fracturing fluid and slickwater fracturing fluid. The self-propping fluid offers advantages of low damage and low fluid loss. It can undergo a phase transition to form solid particles that effectively prop the fractures, thereby significantly reducing damage such as reservoir pore structure blockage. This study demonstrates that the phase-change self-propping fracturing fluid is well-suited for tight, low-permeability reservoirs due to its ability to minimize formation damage. Furthermore, the reservoir damage evaluation methodology established in this work provides an effective means for analyzing damage mechanisms and assessing effectiveness during the fracturing process. Full article

Accurate downhole depth measurement is essential for oil and gas well operations, directly influencing reservoir contact, production efficiency, and operational safety. Collar correlation using a casing collar locator (CCL) is fundamental for precise depth calibration. While neural network has achieved significant progress in collar recognition, preprocessing methods for such applications remain underdeveloped. Moreover, the limited availability of real well data poses substantial challenges for training neural network models that require extensive datasets. This paper presents a system integrated into a downhole toolstring for CCL log acquisition to facilitate dataset construction. Comprehensive preprocessing methods for data augmentation are proposed, and their effectiveness is evaluated using baseline neural network models. Through systematic experimentation across diverse configurations, the contribution of each augmentation method is analyzed. Results demonstrate that standardization, label distribution smoothing (LDS), and random cropping are fundamental prerequisites for model training, while label smoothing regularization (LSR), time scaling, and multiple sampling significantly enhance model generalization capabilities. Incorporating the proposed augmentation methods into the two baseline models results in maximum F1 score improvements of 0.027 and 0.024 for the TAN and MAN models, respectively. Furthermore, applying these techniques yields F1 score gains of up to 0.045 for the TAN model and 0.057 for the MAN model compared to prior studies. Performance evaluation on real CCL waveforms confirms the effectiveness and practical applicability of our approach. This work addresses the existing gaps in data augmentation methodologies for training casing collar recognition models under CCL data-limited conditions, and provides a technical foundation for the future automation of downhole operations. Full article

A comprehensive study on the distribution characteristics and exploitation strategies of remaining oil was carried out in the Ordovician ultra-deep fault-controlled fractured–vuggy carbonate reservoir within the Shunbei No. 1 strike–slip fault zone. This research addresses challenges such as severe watered-out and gas channeling encountered during multi-stage development, marking a shift toward a development phase focused on residual oil recovery. By integrating seismic attributes, drilling, logging, and production performance data—and building upon previous methodologies of “hierarchical constraint and genetic modeling”—a three-dimensional geological model was constructed with a five-tiered architecture: strike–slip fault affected zone, fault-controlled unit, cave-like structure, cluster fillings, and fracture zone. Numerical simulations were subsequently performed based on this model. The results demonstrate that the distribution of remaining oil is dominantly controlled by the coupling between key geological factors—including fault kinematics, reservoir architecture formed by karst evolution, and fracture–vug connectivity—and the injection–production well pattern. Three major categories with five sub-types of residual oil distribution patterns were identified: (1) local low permeability, weak hydrodynamics; (2) shielded connectivity pathways; and (3) Well Pattern-Dependent. Accordingly, two types of potential-tapping measures are proposed: improve well control through optimized well placement and sidetrack drilling and reservoir flow field modification via adjusted injection–production parameters and sealing of high-permeability channels. Techniques such as gas (nitrogen) huff-and-puff, gravity-assisted segregation, and injection–production pattern restructuring are recommended to improve reserve control and sweep efficiency, thereby increasing ultimate recovery. This study provides valuable guidance for the efficient development of similar ultra-deep fractured–vuggy carbonate reservoirs. Full article

Unique fractal characteristics are significantly controlled by shale lithofacies, mineralogical characteristics, and OM features, which in turn determine reservoir properties and gas-bearing capacity. However, a comprehensive understanding of fractal features has remained insufficient. This study presents a systematic investigation into the full-scale pore size distribution for the Wufeng–Longmaxi shales in Sichuan Basin which employed low-pressure CO₂ adsorption (CO₂GA), N₂ adsorption (N₂GA), and mercury injection capillary pressure (MICP), as well as field emission scanning electron microscope (FE-SEM) techniques. The fractal dimensions of pores across different pressure ranges were revealed by different fractal models. The results demonstrate that the shale pores are dominated by micro- to mesopores and partial extremely larger pores, contributed primarily by organic matter (OM) pores and microcracks, respectively. Fractal dimensions follow a consistent increasing order: D_C < D_N1 < D_N2 < D_M or D_C < D_N1 < D_M < D_N2, suggesting that larger pores with diameters lager than 5 nm are more heterogeneous and complex compared to the pores less than 5 nm (smaller pores). This is because smaller pores are predominantly composed of OM pores, while larger pores comprise a mixture of OM pores, mineral-related pores, and microcracks. Different fractal dimensions, in turn, are influenced by distinct factors. The D_C value exhibits a positive correlation with micropore volume. D_N1 and D_N2 values are positively correlated with the content of brittle minerals and TOC, while they show negative correlations with the content of clay minerals. Notably, D_M values do not demonstrate a significant correlation with shale compositions, primarily owing to the development of microcracks. Fractal dimensions, particularly D_N1 and D_N2, are significantly controlled by the lithofacies of shale. The highest D_N1 and D_N2 values occur in the siliceous shale lithofacies, and the mixed shale lithofacies exhibit moderate D_N1 and D_N2 values, whereas the lowest D_N1 and D_N2 values primarily occur in clay-rich shale lithofacies. Different fractal dimensions show various correlations with shale gas content. The Langmuir volume as well as total gas content exhibit significant correlations with D_N1 and D_N2 values, while they exhibit no obvious correlations with D_C and D_M values. This implies that pores with diameters of 1.8–55 nm serve as primary storage sites for both adsorbed and free gas. The findings can significantly improve the cognition of adsorbed gas and free gas behavior in shale reservoirs. Full article

In the current context of the energy transition, the reuse of offshore oil and gas (O&G) structures that have reached the end of their operational life presents new engineering challenges. Many projects aim to adapt existing facilities for a range of alternative uses. This paper outlines guidelines for identifying the most suitable conversion options aligned with the goals of the ongoing energy transition, focusing on the Italian offshore area. The study promotes the reuse—instead of partial or full removal—of existing offshore platforms originally built for the exploitation of hydrocarbon reservoirs. From an engineering perspective, the project describes the development of guidelines based on an innovative methodology to identify new uses for both offshore oil and gas platforms and the depleted reservoirs, with a focus on safety and environmental impact. The guidelines identify the most suitable and effective conversion option for the platform–reservoir system under consideration. To ensure a realistic approach, the developed methodology allows one to identify the preferable conversion option even when some piece of information is missing or incomplete, as often happens in the early stages of a feasibility study. The screening process provides an associated level of uncertainty related to the degree of data incompleteness. The outcome is a complete evaluation procedure divided into five phases: definition of criteria; assignment of an importance scale to determine how critical each criterion is; connection of indices and weights to each criterion; and analysis of the relationships between them. The guidelines are implemented in a software tool that supports and simplifies the decision-making process. The results are very promising. The developed methodology and the related guidelines applied to a case study have proven to be an effective decision-support for analysts. The study shows that it is possible to identify the most suitable conversion option from a technical, engineering, and operational point of view while also considering its environmental impact and safety implications. Full article

The homogeneity of methane hydrates in marine sediments plays a significant role in determining the efficiency of gas production during exploitation processes. Revealing their distribution mechanisms is crucial for optimizing the development of gas hydrates. This work systematically investigates the evolution patterns of effective thermal conductivity (ETC) during the formation and dissociation of methane hydrate in marine sediments, focusing on their major mineral components, such as quartz sand, illite, and montmorillonite. The results reveal the influence of thermal conductivity (TC) characteristics in porous media on hydrate phase transition behavior and spatial distribution. Key findings demonstrate that the TC characteristics of porous media are one of the dominant factors controlling hydrate formation rates. High-conductivity porous media significantly accelerate hydrate formation through efficient heat transfer. The swelling characteristics of montmorillonite and its coupling effects with salt ions impair heat transfer pathways, thereby inhibiting hydrate formation. Further analysis reveals that the spatial heterogeneity in reservoir TC is the primary intrinsic mechanism responsible for the macroscopic heterogeneous distribution of hydrates. Additionally, the hydrate dissociation process disrupts solid-state thermal bridging and generates gaseous thermal barriers, causing irreversible attenuation of reservoir TC. This phenomenon exacerbates the non-uniformity of the front during dissociation and increases the risk of secondary formation during exploitation. From a novel perspective of reservoir TC heterogeneity, this study establishes mechanistic links between the thermophysical properties of porous media and the spatial distribution patterns of hydrates. This provides significant theoretical guidance for resource exploration and the safe, efficient exploitation of marine gas hydrate reservoirs. Full article

Low gas recovery in the Sebei-2 gas field is linked to residual gas trapping under water encroachment. This study investigates the pore-scale trapping behaviour of residual gas in three types of layer: conventional, low-resistivity, and low-acoustic high-resistivity. High-fidelity pore structures were reconstructed by integrating mercury intrusion porosimetry with thin-section data and microfluidic models were designed using the Quartet Structure Generation Set method and fabricated by wet etching. Visualized displacement experiments were performed under different wettability conditions and water invasion rates, and image analysis was used to quantify the distribution of trapped gas. Results show that the low-resistivity gas layer exhibits the highest residual gas saturation (30.57%), followed by the low-acoustic high-resistivity gas layer (20.20%), while the conventional gas layer has the lowest (15.29%). These values correspond to apparent pore-scale gas recoveries of about 48.95%, 65.01%, and 72.14% for the low-resistivity, low-acoustic high-resistivity and conventional gas layers, respectively. In hydrophilic systems, wetting-film thickening and flow diversion are the main trapping processes, whereas in hydrophobic systems, flow diversion dominates and residual gas decreases markedly. Increasing the water invasion rate reduces trapped gas in the conventional and low-resistivity layers, whereas in the strongly heterogeneous low-acoustic high-resistivity layer, higher invasion intensity strengthens preferential channelling/viscous fingering, leading to a non-monotonic residual gas response. These findings clarify the differentiated pore-scale trapping mechanisms of gas under water encroachment and highlight that mitigating water film-controlled trapping in low-resistivity layers and flow diversion trapping in low-acoustic high-resistivity layers is essential for mobilizing trapped gas, improving dynamic reserves, and ultimately enhancing the economic recovery of water-bearing gas reservoirs similar to the Sebei-2 gas field. Full article

Horizontal wellbore temporary plugging and diversion fracturing serves as a critical technical approach for the economical and efficient development of unconventional oil and gas reservoirs. A degradable knot temporary plugging agent (TPA) offers distinct advantages for perforation plugging in horizontal wellbore; however, existing research remains limited, and the influence of knot TPA parameters on perforation temporary plugging mechanisms has not been clearly elucidated. This study employs a CFD-DBCM coupled model to conduct numerical simulations of temporary plugging with a knot TPA. The simulation is validated through visualized temporary plugging experiments, followed by an optimization analysis focusing on the flank length and structural configurations of the knot TPA. Research indicates that, when the flank is less than 1.6 times the central diameter, its plugging capacity is significantly compromised. Once the flank exceeds 1.6 times the central diameter, the total plugging performance of the knot TPA improves to a certain extent, and the temporary plugging capacity for the upper perforations increases particularly significantly. When flank lengths are identical, a knot TPA with uniformly distributed four flanks exhibits superior plugging performance compared to configurations featuring only single or double flanks. Given formation heterogeneity, a temporary plugging simulation analysis of the combined knot TPA was conducted. The results indicate that employing a combined knot TPA achieves a higher valid plugging rate compared to using only one type of knot TPA, with valid plugging accounting for the majority of cases. Field application of knot TPA was conducted in the fracturing stage of an oil well in Zhejiang, and the changes in on-site data verified the effectiveness of the temporary plugging technique of knot TPA. Full article

The South Sakhalin mud volcano (Sakhalin Island, Russia) emits HCO₃-Cl/Na-Mg water, emanates CO₂ prevailing over CH₄ in the gas phase, and extrudes mud bearing five carbonate mineral species. The study focuses on the distribution, diversity, and origin of the carbonate minerals from the mud volcano (MV) ejecta, in terms of carbon cycle processes. The data presented include a synthesis of field observations, compositions of MV gases and waters, chemistry of carbonate minerals, as well as stable isotope geochemistry of MV waters (δ¹³C, δD, and δ¹⁸O) and carbonates (δ¹³C and δ¹⁸O). The sampled MV waters are isotopically heavy, with δ¹⁸O = +5.7‰ to +7.5‰ VSMOW, δD = −18.0‰ to −11.0‰ VSMOW, and ¹³C (δ¹³C_DIC = +6.9‰ to +8.1‰ VPDB). This composition may be due to the dilution of basinal water with dehydration water released during the diagenetic illitization of smectite. Carbonates in the sampled mud masses belong to three genetically different groups. Mg-rich siderite, (Fe_0.54–0.81Mg_0.04–0.30Ca_0.05–0.23Mn_0.00–0.08)CO₃, disseminated in abundance throughout the mud masses, coexists with common calcite and sporadic ankerite. The trace-element chemistry of Mg-siderite, as well as the oxygen (δ¹⁸O = +34.4‰ to +36.8‰ VSMOW) and carbon (δ¹³C = −1.3‰ to +0.6‰ VPDB) isotopic signatures, confirms its authigenic origin. Siderite formed during early diagenesis of the Upper Cretaceous sandy and clayey marine sediments mobilized by mud volcanism in the area. Another assemblage, composed of dawsonite, siderite, and vein calcite (±kaolinite), represents altered arkose sandstones found as few fragments in the mud. This assemblage may be a marker of later CO₂ flooding into the sandstone aquifer in the geological past. The trace-element chemistry, particular morphology, and heavy C (δ¹³C = +5.5‰ to +7.0‰ VPDB) and O (δ¹⁸O = +39.1‰ to +39.5‰ VSMOW) isotope compositions indicate that aragonite is the only carbonate species that is related to the current MV activity. It crystallized in a shallow reservoir and was maintained by CO₂ released from rapidly ascending liquefied mud and HCO₃-Cl/Na-Mg-type of MV waters. Full article

With the intensive development of oil and gas fields, multi-well layouts with reduced inter-well spacing are increasingly adopted to improve production efficiency. Such configurations, however, may significantly enhance seismic interaction among adjacent wells. In this study, a nonlinear three-dimensional finite element model incorporating soil–structure interaction is developed using GTS NX to investigate the seismic dynamic response of closely spaced oil well casings. A representative dual-well system is analyzed under horizontal earthquake ground motion. The influence of inter-well spacing on displacement response characteristics is systematically examined. Numerical simulations are conducted for three center-to-center spacing distances (5 m, 7.5 m, and 10 m). The spatial distribution of displacement responses in both the casings and the surrounding soil is analyzed at different depths and monitoring sections. The results indicate that reduced well spacing significantly amplifies dynamic coupling effects, leading to increased displacement responses in the casing–soil system. These findings provide quantitative insight into spacing-dependent seismic interaction mechanisms and offer theoretical support for seismic design and spatial optimization of multi-well systems. Full article

Deep fractured-porous carbonate reservoirs used for underground gas storage (UGS) experience simultaneous changes in temperature and effective stress during cyclic injection and withdrawal, so predicting permeability evolution is essential for evaluating long-term injectivity and deliverability. Using the Xiangguosi UGS as the engineering background, we measured steady-state gas permeability of three fractured-porous carbonate cores under representative conditions (20–80 °C; 15–35 MPa). Permeability decreases nonlinearly under coupled loading: changing temperature or effective stress alone typically reduces permeability by 30–70%, while the maximum reduction under concurrent increases in both variables exceeds 80% relative to the reference condition. An exponential model was fitted to quantify the decay parameter of permeability with effective stress (0.038–0.046 MPa⁻¹) and with temperature (0.016–0.020 °C⁻¹). In addition, the temperature-related exponential decay parameter decreases with increasing effective stress, because compliant fractures and larger pores are progressively pre-closed, weakening the permeability response to temperature. Finally, we propose a parsimonious separable exponential model that reproduces the measurements with a mean relative error below 12%, providing a practical constitutive relation for multiphysics simulations of UGS in fractured-porous carbonates. Full article

Fly ash (FA), a major by-product of coal combustion, has long been regarded as a challenging industrial solid waste. Its inherent abundance of alkaline-earth oxides positioned it as a promising candidate for CO₂ sequestration through mineral carbonation. This study systematically investigated the effects of key operational parameters, including time, stirring rate, ultrasonic treatment, and solid-to-liquid ratio, on the leaching efficiency of calcium ions and subsequent CO₂ fixation. Ultrasonic treatment, a solid-to-liquid ratio of 1:7, a stirring speed of 600 rpm, and 7% monoethanolamine (MEA) collectively enhanced the calcium leaching efficiency (χ_e) to 16.7%, thereby supplying a substantial reservoir of calcium ions for CO₂ fixation. Additionally, the CO₂ injection into fly ash slurry and the slurry spraying into CO₂ gas were investigated to optimize reactor configurations. The latter method demonstrated superior performance, attaining a CO₂ fixation efficiency of 7.23%. This corresponds to a carbonation conversion efficiency (ηc) of approximately 44.5%, indicating that nearly half of the leached calcium ions were successfully converted into stable carbonates. Advanced characterization techniques (SEM-EDS, XRD, FTIR) confirmed the formation of stable carbonates and highlighted the role of additives in enhancing reactivity. The environmental benefit of this approach is addressing fly ash wastes and transforming fly ash into a CO₂ fixation material. These findings provided critical insights for calcium leaching and CO₂ fixation of fly ash. Full article

The “bauxite gas reservoir” in the Ordos Basin represents a novel exploration domain, yet the mechanisms governing its widespread aluminous fracture fillings remain unclear. This study integrates core observation, thin-section analysis, geochemical simulation, and rock physics to investigate the formation and impact of these fracture systems. Results identify a characteristic filling evolutionary sequence of “wall-lining film → oolitic/globular → plug-like → vermicular.” Geochemical simulations confirm that increasing pH and decreasing Eh driven by water–rock interactions are the key drivers for aluminous mineral precipitation. A distinct well log response model characterized by high GR, DEN, and CNL values coupled with low AC and RT is established for effective identification. Seepage experiments reveal that while Al–Si colloidal fracture fillings reduce permeability, they act as natural proppants to preserve effective flow channels, acting as a crucial high-permeability network for gas migration despite the mineral occlusion. These findings refine the accumulation theory for bauxite series reservoirs and provide geological evidence for deep tight gas exploration. Full article