Choking cavitation number and cavitating discharge coefficient serve as core quantities to characterize venturis under choked-flow conditions, yet whether they can be consistently represented across geometrically similar venturis in a throat-Reynolds-number framework remains unclear. In this study, seven geometrically similar venturis with throat diameters ranging from 2.0 to 12.5 mm were examined in a dual-valve cavitation system under quasi-steady cavitation generated by a continuous downstream-valve sweep. Choked-flow data were obtained at multiple upstream valve openings, with three independent replicates conducted for each condition, spanning a throat Reynolds number range of 1.0 × 104 to 1.5 × 105. Critical pressures, choking cavitation numbers, and cavitating discharge coefficients were determined under a consistent experimental and data-reduction protocol, with water-quality indicators monitored and measurement uncertainties incorporated into the comparison. The results show that the throat-Reynolds-number framework has different organizing ability for the two quantities. For cavitating discharge coefficient, venturis 2–7 form a common branch, while venturi 1 remains separated, indicating substantial but incomplete cross-venturi organization. For choking cavitation number, each venturi can also be represented in the Reynolds-number domain, but the resulting curves remain distinct and do not form a common relation. Collectively, throat Reynolds number provides a useful basis for cross-venturi assessment, but it does not fully eliminate retained venturi-to-venturi dependence. The present work provides an uncertainty-informed assessment of the tested venturi set and a reproducible framework for future cross-venturi comparisons involving other geometric configurations.

Hydrodynamic cavitation is a promising technique for near-wellbore permeability enhancement. This study experimentally investigates the effects of organ-pipe geometrical configuration parameters, jet hydraulic power, pressure drop, discharge rate, standoff distance, and treatment duration on permeability enhancement in sandstone cores. The results show that increasing the jet hydraulic power and decreasing the hydraulic transmission efficiency, while keeping the geometrical configuration parameters within optimal value ranges, leads to high-intensity stimulation and maximizes permeability enhancement within a short treatment duration. Under a jet hydraulic power of 23.7 kW, a discharge rate of 72.9 L/min, and a pressure drop of 32 MPa, Nozzle 1 achieved maximum, average and minimum permeability enhancement rates of 102.5

This study establishes a numerical simulation method for modeling the dynamic response of porous rocks under high strain-rate impacts and verifies its robustness, accuracy, and reproducibility. A systematic investigation was conducted to evaluate the effects of four pore structure parameters—porosity, pore size, aspect ratio, and orientation angle—on the dynamic response of rocks. The results indicate that increasing porosity significantly reduces normalized strength (σnorm), dynamic Young’s modulus (Ed), and energy dissipation density (Uv), with the degree of weakening influenced by pore geometry. A greater aspect ratio difference leads to higher Uv, suggesting that more flattened pores promote complex fracture development and energy dissipation. Furthermore, the study reveals a coupled mechanism between pore structure and impact loading characteristics: Porosity exerts the most significant control on the rock’s impact response under different stress-wave loading conditions, whereas the effects of pore shape and size are comparatively secondary. For strength weakening, non-optimal loading periods (T = 200 or 800 μs) are recommended to avoid the strengthening effect observed at T = 400 μs. To improve energy efficiency, the optimal loading period should be selected based on porosity. When porosity is less than 3%, applying an impact stress wave with an amplitude 12 times the rock matrix strength achieves an optimal balance between strength reduction and energy efficiency; when porosity exceeds 5%, an amplitude of 6 times the matrix strength yields superior energy utilization. The findings propose a porosity-driven impact loading design strategy, providing theoretical and quantitative guidance for field-scale engineering applications.

Fracability evaluation is a crucial basis for fracturing and production enhancement in tight reservoirs. Due to the complexity of the geological environment, factors influencing reservoir fracability exhibit significant uncertainty. Ignoring the uncertainty of relevant parameters and conducting fracability evaluation based on deterministic parameters may lead to deviations from actual fracability results. To address this issue, this paper proposes a comprehensive evaluation method for the fracability of unconventional oil and gas reservoirs, considering reservoir description uncertainty. Based on fundamental evaluation methods, a reservoir fracability evaluation model is constructed, incorporating the Monte Carlo stochastic simulation method to determine the comprehensive probability distribution of the reservoir fracability evaluation index. This approach enables a more scientific and reliable evaluation of reservoir fracability. The research results indicate that the assumed distribution of input parameters has a certain impact on fracability evaluation results, with normal distribution demonstrating significant disturbance resistance. Additionally, brittleness index is found to be the most sensitive factor affecting fracability evaluation. The proposed evaluation method and insights can provide theoretical references for the fracability assessment of highly heterogeneous tight reservoirs. (c) 2026 Southwest Petroleum University. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).

Hydraulic fracturing is a promising approach for the efficient exploitation of flammable ice reservoirs, for which Mode-I fracture toughness (KIC) is a key indicator of fracturability. Given the similar physical–mechanical properties of flammable ice and ordinary ice, this study employs synthetic ice-bearing loose sediments as flammable ice analogs to investigate Mode-I fracture behaviors. Artificial specimens with six controlled ice saturation levels (S = 0–90

Particle impact drilling (PID) technology is a breakthrough for achieving low mechanical drilling speeds in hard and abrasive formations. A design method for a new type of PID bit, which is a key component of the PID system, was proposed. The newly designed PID bit can continuously excavate rock formations using only the impingement of the particle discharge streams. The rock-breaking experiments show that the designed PID bit transforms the hydraulic power to the momentum of the particles, and the emergence angles of multiple-particle water jets can be continuously adjusted to cover the entire borehole bottom to produce an integral wellbore. The design method is suitable for designation PID bits of different sizes. Single-nozzle and multi-nozzle combination rock-breaking experiments with granite and conglomerate sand rock representing hard and abrasive formations, respectively, were conducted under confining pressure. The experiments showed that the optimum dimensionless standoff distance was 7.5, and the optimum range of emergence angle was 9–20°. A strong relationship exists between the borehole bottom profile produced by multiple tilted particle water jets and rock strength. The borehole bottom profile is more sensitive to the emergence angle of particle water jets with greater rock strength. The optimized emergence angles of the designed triple-nozzle PID bit were 18.8, 10.6, and 11.3° while breaking granite. Meanwhile, the borehole bottom profile produced, while breaking conglomerate sand rock was insensitive to the emergence angles of the nozzles. The borehole bottom profile and rock-breaking efficiency were unaffected by the PID bit rotation rate. The emergence angle optimized to break hard formations is also suitable for rock-breaking abrasive formations. The maximum penetration rate obtained for breaking granite with the designed PID bit was 4.0 m/h without any mechanical auxiliaries, while the maximum penetration rate for sand conglomerate rock exceeded 4.0 m/h. A comprehensive rock-breaking experiment shows that the designed PID bit produced an integral borehole using different forward speeds without contact with the rock and drilling pressure. The produced borehole profile was smooth and had a borehole enlargement rate of 3.3

The sensitivities of the cavitation inception conditions and the cavitation discharge coefficient, together with the path independence and uniqueness of inception, underpin reliable use of cavitating Venturi tube under ambient conditions. With a downstream sweep rate of 1/6 °/s verified in preliminary tests, forward and backward quasi-dynamic cavitation evolution processes were sampled to construct pressure- and flow-rate-control paths for the Venturi tube used in this study. Analysis of the control paths shows that, as the critical inlet pressure approaches infinity, the critical pressure ratio and the cavitation discharge coefficient approach characteristic values of 0.6364 and 0.5573. With ±3% practical equivalence margins, they become insensitive to the critical inlet pressure above thresholds of 2.4496 and 1.7167 MPa, respectively. The inception conditions are unique; hysteresis in the critical pressure ratio emerges for critical inlet pressures of 0.40–0.45 MPa, and path independence no longer holds for critical inlet pressure exceeding 0.45 MPa. The findings indicate that detecting and controlling Venturi cavitation under ambient conditions must account for the sensitivities and path-dependence of cavitation characteristics, and they provide a useful baseline reference.

The structure parameters of the organ-pipe nozzle served as the cavitation generator to improve near-well reservoir permeability are optimized by simulation, and a feasibility test is carried out to verify the improvement of the permeability of sandstone and shale with the structure-optimized organ-pipe nozzle. With the gas content as the evaluation standard, numerical simulation results show that the outlet shrinkage ratio ( Φ_D /Φ_d )^2 = 4–9, length diameter ratio of diffusion section S/Φ_D = 3.3–4.6, and diffusion angle α = 60°–80° are the optimal parameter ranges of organ-pipe nozzle flow passage. The feasibility tests show that the influences of cavitation effects produced by the nozzles whose flow channel parameters are within the optimal ranges on the permeability only have little difference. However, after the treatment of the sandstone cores and shale cores by the nozzle whose flow channel parameters are not within the optimal ranges, permeability improving rates decrease significantly compared to those treated by nozzles whose flow channel parameters are within the optimal ranges. The feasibility tests verified the rationality of numerical calculation, and show that the fluid cavitation effect is suitable for the permeability improvement of low-permeability sandstone but not for the ultra-low permeability shale. This study reveals that the fluid flow cavitation effect has the ability to improve the near-wellbore sandstone reservoir permeability and the structure-optimized organ-pipe nozzle could be used as the cavitation generator.

Pore pressure (Pp) is crucial for oil and gas exploration and development, and accurate prediction ensures safe and efficient drilling. In this paper, a novel method for real-time Pp prediction is proposed based on the NARX neural network method by considering variation patterns of surface logging data. This approach uses upper formation data to predict the lower one. We collected surface logging data from two wells at South China Sea. The surface logging data of one well was used for modeling, while the other serves for an additional test to evaluate the generalization performance of the method. Six surface logging parameters, including hook load (WOH), weight on bit (WOB), torque (TOR), flow rate (FLW), rate of penetration (ROP), and stand-pipe pressure (SPP), were selected as inputs for the model, with Pp gradient as the output variable. Similar to the time delay windowin theNARX neural network model, this paper introduces a sequential length to include proper information of upper formations. The optimal sequential length is 10 m, resulting in the best-fitting model with a determination coefficient (R2) of 0.9918, a root mean square error (RMSE) of 0.00459 g/cm3, and a computational time of 18.34 s. Furthermore, this selected model is successfully applied to neighboring wells within the same block, achieving notably high prediction accuracy.

Gas kick is a common and high-risk drilling trouble in geo-energy engineering, especially for deep-water drilling and ultra-deep well drilling. The most favorable way of early detection of gas kick is to move the detection from the wellhead or near surface to the downhole. In this paper, a new conception of downhole detection of gas kick using low-frequency elastic wave was put forward. The framework of coupling the multiple physic processes of gas-liquid two-phase annulus flow, bubble migration and low-frequency elastic wave propagation during gas kick was developed. The characteristic responses of low frequency elastic wave to gas kick were quantitatively analyzed based on multiphysics modeling under the ideal case and different levels of background gas (BGG), respectively. The modeling results indicated that: (i) The velocity decreases cliff-like and the attenuation coefficient increases significantly at the gas-kick bubble front. The change magnitudes of the velocity and the attenuation coefficient increase with the gas intrusion rate, and decrease with the pump displacement. The change magnitude of attenuation coefficient decreases with mud density, while the increase of velocity is not sensitive to the mud density. (ii) For the ideal case of the vertical well, the effective detection within the depth section of (2014–2568) meter could identify gas kick about (14.3–29.2) minutes earlier than the conventional pit-gain method. The gas kick could be identified earlier when the deviation angle is smaller and/or the monitoring point goes deeper. (iii) The changing magnitudes and rates of the velocity and the attenuation coefficient decrease with the BGG. The downhole detection of gas kick using low-frequency elastic wave should be reliable when the BGG is lower than 5%. The effects of rock cuttings, phase transition of the gas, and drill-string vibrations should be further investigated in future study.

Particle impact drilling (PID) is a promising technology to enhance the rate of penetration in hard and abrasive formations. The changes to physical and mechanical properties near the crater of a full-size tight sandstone sample after successive impacts of a steel-particle water jet are investigated by different scales of rock physics measurements to estimate the range of the damage zone. Similar measurements are also carried out on a sample from the same formation without steel-particle water jet as a reference. The results indicate that: (i) there is a damage zone around the crater with induced cracks and pore collapse caused by the impact stress wave produced by the steel-particle water jet. The dimensionless depth of the damage zone from the crater’s surface is about 0.69 times the jet diameter (d0). Within the damage zone, the dense compaction zone is just near the crater surface within a dimensionless distance of about (0.02–0.06) d0, where the porosity and the permeability slightly decrease, and show significant strength decrease. (ii) The changes of the porosity and the P- and S-wave velocities after steel-particle water jet at core scale are neglectable. Compared to the reference sample, the permeability at the lateral of the crater is (4.52–30.90) times higher, while the permeability beneath the bottom of the crater is 79

Numerical models are established based on the coupled SPH-FEM algorithm to explore the damage mechanism of rock containing cracks by impacts of steel-particle water jet. The results indicate that the collapse zone appears above the crack because of the blocking of the stress wave propagation by the crack, and the damage inside the collapse zone is accumulated in a stepwise way. Moreover, the volume of the broken hole increases nonlinearly with the crack length while the depth is basically unchanged. The crack inclination affects the impact direction of the steel-particle water jet, which changes the shape of the broken hole. There exists interference between cracks when the dimensionless crack spacing is less than 2.0. For the upper surface center of the lower crack, there is a one-step surge in damage value before completely broken, and the corresponding damage value at the turning point increases with the crack spacing.

Gas kick monitoring is of great significance for prevention of blow-out accidents, especially in deep drilling and deep-water drilling. In this study, a machine learning (ML) model for early-monitoring of gas kick is developed using the ensemble learning algorithms based on 7363 lines of drilling logging data at South China Sea. The selected input parameters based on mechanism analysis of gas kick are six fast engineering parameters, including hook load (WHO), weight on bit (WOB), torque (TOR), flow rate (FLW), rate of penetration (ROP) and stand-pipe pressure (SPP), and two slow mud property parameters, i.e. electrical conductivity (CON) and mud outlet density (DEN). The model is constructed using RUSboosted, Subspace-KNN and Bagged Trees algorithms, and is compared with the neural network algorithm. We propose a comprehensive error to quantitatively evaluate the performance of the gas kick monitoring models. The models for early-monitoring of gas kick are applied for a single well and multiple wells, respectively. The results indicate that: (i) The optimal combination of input parameters is made up of six fast engineering parameters and two slow mud parameters. When there is a higher requirement on timeliness, only use of the six fast engineering parameters is also acceptable. (ii) The ensemble learning models work well when the input data expand from single well to multiple wells in the same block. For most cases, the prediction error of the optimal model is below 10%. The RUSboosted algorithm performed best in most data sets. (iii) Gas kick identification from lots of drilling logging records is mathematically a small-sample problem. The output labelling of a potential gas kick should be based on the field practical requirement. The recommended positive length of continuous-point labelling method is 5 m for the studied area, which can effectively reduce the average error from 8.02% to 5.48%.

The process of drilling generally produces sound noise as by-products and different types of drilling materials generate acoustic signals with various frequencies and amplitudes. Thus, how to use these signals as an important diagnostic tool for drilling materials identification is highly desirable. In this paper, different types of rock specimens (limestone, shale and cement) and steel which is analogue to wellbore casing were subjected to rotary drilling with roller cone bit at different conditions while their sound pressure signals were simultaneously recorded during drilling materials disintegration process. The empirical mode decomposition (EMD) was used to decompose the vibration signal signals, and time and frequency domain analysis is performed to extract acoustic signal features. Then, a back-propagation -artificial neural network (BP-ANN) model and principal component analysis (PCA) approach were established to recognize the drilling materials with noise produced in drilling. Results indicate increased drilling pressure can generate more evident differences during drilling process, but shale and sandstone cannot be directly recognized because of overlapping signals in FFT graph. With the help of artificial intelligence technology, the drilling materials can be effectively recognized by acoustic signals at the same drilling pressure. In addition, the combination of PCA and BP-ANN give the best results comparing to conventional BP-ANN model and was proved to be a powerful integrated method to recognize signals from different drilling materials with different sound characteristics.

The formation of gas hydrate is a serious threat to the safe and effective completion of deepwater drilling and transportation operations, although it is considered as a potential energy resource. The inorganic salts are generally used as thermodynamic inhibitors; CaCl2 as a common additive in drilling fluids exhibits unique properties. In this study, we explored the dissociation mechanism of CH4 hydrate in CaCl2 solutions at the macroscopic and microscopic scale using experiment and molecular dynamics (MD) simulation. The experimental results showed that CaCl2 accelerated the dissociation rate of CH4 hydrate. The dissociation rate of CH4 hydrate increased with the increase of CaCl2 concentration at large depressurization pressure and was mainly affected by pressure when the depressurization pressure was lower. MD simulations were used to give an atomic scale interpretation of the macroscopic results obtained from the experiment. The results showed that the addition of CaCl2 destroyed the resistance liquid film formed during CH4 hydrate dissociation, thus accelerating the dissociation process, in good agreement with experimental results. HIGHLIGHTS: • The amount of CaCl2 affects CH4 hydrate dissociation at large depressurization pressure. • The dissociation of CH4 hydrate at low depressurization pressure is dependent on pressure. • Ca2+ destroys effectively the resistance liquid film produced during hydrate dissociation. • MD simulation results are in agreement with those of the experiment.

A numerical modelling method for the low-frequency (similar to 10(2) Hz) acoustic wave propagation in a bubbly fluid with a low gas fraction is developed in this paper. Based on the digital image analysis of the bubbly fluid, the geometrical model of polydisperse bubble populations is created. The developed method is first verified by the Wood's (1955) equation and the Commander & Prosperetti (1989) model. Then the influence of gas fraction, excitation frequency and bubble radius on the acoustic properties of the bubbly fluid is systematically examined. The obtained results confirmed that the velocities and attenuations for different bubble radius distributions are close to those obtained for the monodisperse models. The gas fraction is the key controlling factor of the acoustic velocity, while the influence of the bubble size at low frequencies can be neglected. The velocity dispersion with frequency is about 5% even in Wood's regime and the attenuation increases with a higher gas fraction and excitation frequency, while it decreases with a smaller bubble radius. By estimation of bubble expansion during the migration of gas kick, variation trends of acoustic velocity and attenuation along the wellbore are discussed. This implies that variations of the velocity and attenuation of drilling fluid at low-frequencies during the gas migration can be used for an accurate and earlier gas kick detection than currently used methods.

There are still two deficiencies in the study of crack propagation in plane steel. One is that there are few studies on the theory of crack cracking and the cracking criterion from the strain field at the crack tip; the other is that there are few studies using the block discrete element method to explore the crack propagation. In view of the above two points, this paper proposes a strain strength criterion to explain the extension of tensile cracks and shear cracks in steel. This strain strength criterion assumes: (1) Tensile cracks grow along the direction of the maximum principal strain. When the principal strain γ reaches a critical value, the tensile cracks begin to grow; (2) Shear cracks grow in the direction of the most dangerous stress state. When the Mohr circle exceeds the Mohr Coulomb failure line, the shear crack starts to expand. In addition, this paper applies the strain strength criterion to the block discrete element method to simulate the macro-mechanical response characteristics of steel under load and the propagation and evolution process of cracks. It is shown (1) the stress-strain curve appears elastic, stagnant, fluctuating, and falling back, etc. The basic law, and (2) the characteristics of stable and unstable growth of cracks during the whole loading process.

In this work, we present molecular dynamics (MD) simulations of single poly (N-vinyl caprolactam) (PN-VCL) in the water at different temperatures, from 275 to 335 K with a gradient of 10 K. The force fields of OPLS-AA and SPC model were used for PNVCL and water molecules, respectively. Simulations with durations of 200 ns were performed at each temperature for a 20-mer PNVCL chain in an isothermal-isobaric (NPT) and canonical (NVT) ensembles. The results showed that the phase transition of PNVCL chain during heating is driven by H-bonding transformation of amide groups, followed by the hydrophobic dehydration of C-H group in the caprolactam ring. PNVCL exhibited a coil-to-globule transition at 305 K, corresponding to its low critical solution temperature (LCST). Below LCST, H-bonding predominated while hydrophobic interaction was dominant above LCST. PNVCL collapsed at elevated temperatures due to the weakening of the attractive forces between its hydrophobic/hydrophilic group and water molecules. The analysis of the H-bonds formation showed that PNVCL is hydrophilic and only carboxyl oxygen (C = O) in the amide group formed H-bonds with water molecules. In the collapsed state, PNVCL formed no intra H-bonds between its residues and the globule conformation was mainly due to the hydrophobic interactions between PNVCL residues. The H-bond formation played an important role in the coil-to-globule transition of PNVCL and is believed to be the key factor of its solubility. (C) 2020 Elsevier B.V. All rights reserved.

Particle Impact Drilling (PID) is a new technology to effectively improve the rate of penetration (ROP) for oil and gas drilling in hard and strongly abrasive formations. In this paper, numerical simulation method is used to analyse the motion characteristics and the modulation method of particle swarm in high-pressure tank for the particle injection system based on differential pressure ejection in PID. The numerical simulation results show that: when there is no modulation elements, the motion of particle swarm in the high-pressure tank follows an asymmetric funnel flow with pulsating state, which could be divided into vertical flow domain, fast flow domain, slow flow domain and stagnation domain. The unstable dynamic arching effect of the funnel flow, the viscous effect of the liquid bridge force and the collapsing effect of the particle swarm could probably lead to the blockage of the discharge port of the high-pressure tank. When the semiapex angles of the high-pressure tank decreases, the volume flow rate of particles increases and the stagnation domain becomes smaller, but it becomes easier to form arching and blockage. The modelling results indicate that the pulsation of the funnel flow is minimum when the semiapex angle is 45° without the mutilation element, which means the funnel flow of the particle swarm is relatively stable. By introducing a conical modulating element above the discharge port, the unstable funnel flow of the particle swarm could be transformed to an overall uniform flow. The modelling results indicate that the installation height of the modulation element has the greatest influence on the pulsation degree. The optimized parameters for the conical modulation element based on numerical modelling tests are 70° for the vertex angle, 35 mm for the length of the flank and 70 mm for the installation heigh.