Journal of Natural Gas Science and Engineering最新文献

The quantitative characterization of underground transport phenomena remains challenging due to the complex behavior of the gas movement in soil. Conversely, this inhibits the accurate prediction of the risk arising from the underground transport of hazardous materials. This work proposed and qualitatively evaluated a computational model that spans a wide range of underground gas flow regimes, ranging from gas migration, to ground uplift, and crater formation, depending on the release characteristics. The model followed the multiphase Eulerian approach and adopted the standard k-ω turbulence model and the kinetic theory of granular flow for the ground description with the Syamlal-O’Brien granular viscosity expression. The model's optimum configuration was checked against experimental data using a new mechanistic approach to link the qualitative observations with quantitative model outputs. The effect of pipeline pressure, burial depth, and release orientation on the regime was studied and the outcomes were utilized to enhance a literature nomograph for the flow regime identification. Emphasis was given to fill in the literature gaps and improve the delineation of the boundaries between the regimes rather than deriving specific quantities. The resulted nomograph is a cost-effective screening tool to identify the regime and select among the available strategies of risk assessment.

Water retention curves play a critical role in numerical simulations for predicting fluid production and sediment deformation behaviors in gas hydrate-bearing sediments (GHBSs). This study uses a new testing assembly that combines gas drainage and low-field nuclear magnetic resonance (NMR) tests to determine the water retention curves of artificially synthesized clay silty specimens. The effect of hydrate on the pore size distributions and water retention curves is analyzed via NMR transverse relaxation time curve distributions, and the mechanism of changes in the water retention curve parameters is further discussed. The results show that hydrate formation decreases the proportion of pores with sizes greater than 15 μm and increases the proportion of pores with sizes less than 3.5 μm in clay silty sediments. Hydrate formation increases capillary pressure and prevents available water migration. The presence of hydrate exponentially increases the normalized capillary pressure but exponentially decreases the normalized curve shape factor, yielding narrower curve distributions. The gas entry pressure and curve shape factor exhibit linear correlations with the pore size distribution parameters. The results imply that the changes in the water retention curves are strongly related to the initial pore size distributions. This study offers a deep understanding of capillary effects-related water retention characteristics and their underlying links with the pore size distributions, and demonstrates that low-field NMR has great potential for characterizing water retention curves of GHBSs.

With the continuous development of unconventional oil and gas in recent years, researchers have set higher requirements on crosslinker performance. Therefore, research on the crosslinking agent that is suitable for the high-temperature/high-pressure (HT/HP) fracturing fluid system is particularly important. This work reviews the development status of a crosslinker for the HT/HP fracturing fluid system. Our results show that the cost of a boron crosslinking agent is low; however, it is not suitable under very high temperatures. Metal crosslinking agents are suitable for a wide range of pH but have poor shear resistance. Meanwhile, the nano crosslinking agent has a high applicable temperature and a strong comprehensive performance but is relatively expensive. How to further improve the adaptability of crosslinking agents to high temperature and pressure and reduce the additive dosage are the focus of future research.

Methane extraction in deep coal seams is a multi-field coupling process affected by multiple factors, and a systematic multi-field coupling theory is of guiding significance for ensuring the safety and efficiency of such extraction. In this study, the theoretical framework was constructed for the multi-field coupling theory of methane extraction in coal mines. First, the key scientific issues were identified. Subsequently, based on the stress distribution and structure of the coal seam, the longwall panel was divided into three zones, i.e., the virgin zone, the excavation disturbed zone and the goaf. Furthermore, the research progress of multi-field coupling during methane extraction were analyzed for the three zones, respectively. Finally, the deficiencies of current researches on multi-field coupling theory for methane extraction were revealed, and the future research directions were pointed out. The following research findings were obtained: Deep coal mining is accompanied by a notable multi-field coupling phenomenon, and the occurrence of compound disasters promote the difficulty in disaster control. However, we can adopt appropriate measures to decouple compound disasters after fully grasping the triggering mechanism and development process of the fields. In this way, the disasters can be controlled separately. In addition, from the perspective of the whole longwall panel, the main development directions are to reveal the gas flow in cross-scale structures and to develop a unified model for gas migration in the multi-scale structure.

The Directional Long Borehole (DLB) technique, which has the benefits of low construction costs and high drainage efficiency, will be extensively used in future mining operations for gas drainage and utilization. Choosing the right borehole parameter is critical for improving the drilling stability and drainage efficiency of DLB. This paper begins with research on the gas migration and enrichment law in the mining-induced fissure, and proposes a gas migration channel zoning model. Furthermore, the dynamic process of gas migration in fissure fields is analyzed, as well as the key borehole parameters are identified. The optimal DLB parameters are eventually addressed and presented for the 2205 working face via combined UDEC and COMSOL. Research indicates that: (1) The mining-induced fissure serves as a channel for gas flow, and presents different pore morphology in space, forming a gas migration channel zoning model. The model's permeability increases initially and then decreases from the gob upward, increasing in an O-ring outward diffusion from the gob's center, it can be represented as a rectangular ladder platform with 15 zones. (2) Gas goes through a dynamic process of state change during drainage that involves adsorption/desorption, diffusion, and seepage. The main factors that affect this process are negative pressure, borehole length, borehole diameter, and the location of the borehole in the fissure zone. (3) The optimal parameters of DLB in the 2205 working face, are 2 drill sites, each with 5 sets of 500 m long boreholes, with a diameter of 133 mm, and a pumping negative pressure of 13 kPa, and placed in a layer height of 22 m. DLB provides benefits over the high-level suction highway in terms of construction cost, drainage effectiveness, and timeliness. The findings can be used to guide the design of DLB, enhance gas energy utilization, and prevent gas disasters.

Bypass pigging is an emerging strategy in effectively overcoming inherent issues of high pig speed and overflow of pigging-induced slug volume concomitantly caused by traditional pigging for natural gas pipelines, which is attracting wide attention for flow assurance in oil and gas industry. In view of most bypass pigs with simple bypass structures, the risk of pig stalling or pipeline blockage accidents can be encountered when suffering from increased resistance forces. Accordingly, a novel bypass pig prototype with a self-regulated module is proposed in this study as an improved solution to strengthening pigging safety, efficiency and flow assurance. A self-regulated, easily assembled bypass pig prototype was fabricated for experimental studies in a horizontal transparent gas pipeline system. The pressure mitigation and pig velocity characteristics were fully evaluated under varying bypass fractions. Specifically, when the bypass fraction increases from 0% to 3%, the average pig velocity can be reduced by 64.3–81.5% and pressure fluctuations are more stable. Besides, as an important structural parameter in affecting pig velocity and accuracy of dynamic pigging simulation, the pressure drop coefficient of gas through the bypass pig structure with an internal regulating valve was numerically studied. Compared to the common bypass pig with only a simple bypass port, the addition of the bypass regulating valve will notably increase the pressure drop coefficient. In particular, when the bypass fraction is increased to 7%, pressure drop coefficients for the bypass pig without or with an internal valve are 1.03 and 1.97, respectively, with the deviation of 90.9%. Finally, an optimal design scheme for bypass pigging operations was newly proposed to optimize bypass fractions in accordance with practical scenarios. This study can provide an effective pathway towards the implementation of self-regulated bypass pigging technology in substantially improving flow assurance of natural gas pipeline systems.

For shale gas reservoir, fracture network fracturing in horizontal well is the key technology to guarantee its commercial exploitation, and the load recovery is a critical parameter which determines the post-fracturing performance. It has been reported that there is a huge difference in load recovery but the control factors are not well understood. It seriously affects the stimulation effect of fracture network fracturing in shale gas wells. Therefore, it is important to analyze the main control factors affecting the load recovery to optimize the design of fracture network fracturing. Further, the load recovery is affected by many factors such as geological, engineering, and production. However, traditional methods are blind to the accurate analysis of the impact on the load recovery. Notably, machine learning (ML) technology has achieved remarkable success in solving the problems of multi-factor nonlinear fitting and black box prediction. Therefore, the genetic expression programming (GEP) is adopted to express the nonlinear relationship in a clear and precise manner in this paper. The data of 189 wells were collected in southern Sichuan, including geological and engineering factors. A feature comprehensive index calculation method was established, and the relative importance of these features analyzed, and then screened out 18 reconstructed features based on geological and engineering factors that affect flow back. The mutual influence between the features was eliminated through principal component analysis of the reconstructed features. Thus the load recovery calculation model was developed and the influence of main control features (variables) on the flow back was analyzed by using partial dependence plot. Statistical parameters showed that satisfactory performance can be obtained through GEP model (training set R = 0.835, test set R = 0.815). The research results show that the GEP calculation model can quickly and accurately calculate the load recovery, obtain the influence law of main controlling factors of geological engineering on shale gas flow back and improve the control of load recovery. Therefore, the method based on GEP can effectively study the main control factors affecting the flow back of shale gas, and hence it can be used as a fast reliable tool to effectively evaluate the load recovery.

–Despite numerous experimental data on gas hydrate equilibrium conditions in the presence of glycols, alkanolamines, and nitrogenated additives that are frequently utilized in the gas refinery, the apparent lack of a precise predictive thermodynamic model is still perceived. This study presents an unprecedented thermodynamic framework benefitting from the modified van der Waals-Platteeuw (vdW-P) model for the hydrate phase, the Peng-Robinson equation of state (PR EoS) for the vapor/gas phase, and combinations of free-volume Flory Huggins (FVFH) and Pitzer-Debye-Hückel (PDH) equations for the water activity in the aqueous phase, in which the FVFH activity model is utilized for the additives with molecular interactions solely, while the PDH model is employed when the ionic interactions also exist. When the model assessed a databank of 1075 data points, 0.29% (0.80 K) and 9.67% (0.49 MPa) deviations were observed in the temperature and pressure calculations, respectively. In particular, for 877 data points (glycols, urea, acetamide, and formamide), employing FVFH solely resulted in 0.32% (0.88 K) and 10.54% (0.50 MPa) temperature and pressure deviations, respectively, whereas the combination of FVFH + PDH yielded 0.17% (0.48 K) and 5.81% (0.47 MPa) errors in temperature and pressure estimations, respectively in 198 data points of the systems comprised of amines, hydrazine, and piperazine. The maximum deviation of temperature prediction did not exceed 6.80 K (2.39%). The results reveal the effective performance of the proposed calculation approach.

Acid gases such as hydrogen sulphide (H₂S) and carbon dioxide (CO₂) are abundant in natural gas, which affect the economics of plant operations and the environment. Chemical absorption is one of the most established technologies for acid gas removal. However, it suffers with a major drawback, i.e. high energy consumption. In this work, an integrated simulation-optimisation approach was employed to minimise energy consumption and hence operating cost in an acid gas removal (AGR) system for natural gas processing. The integrated approach made use of commercial simulation software Aspen HYSYS and optimisation software LINGO to establish a surrogate model that has the best operating conditions while satisfying sales gas requirements. Operational parameters such as alkanolamine flowrates, absorber pressure, and alkanolamine temperature were taken into account. Moreover, Pareto analysis is carried out for multi-objective optimisation in maximising profit and minimising CO₂ content. The integrated approach was demonstrated on a case study involving an AGR system in a natural gas processing plant. Results showed that with the optimal operating conditions, profit of the plant is predicted to increase by 9.15% for the same CO₂ basis (i.e. 0.77 mol%); the profit is expected to increase by 23.3% at higher CO₂ content (i.e. 1 mol%). It was observed that the maximum profit and minimum CO₂ content is achieved at amine recirculation rate of 1914.49 m³/h, pressure of 54 kg/cm², and temperature of 49.54 °C. Furthermore, sensitivity analysis illustrated that profit is proportional to the sweet gas price whereas electricity cost is the most vital parameter in reducing the overall profitability.

In a natural coal reservoir environment, the coal seam is constrained by in-situ stress and gas pressure. Damage on the coal microstructure due to microwave irradiation (MI) differs significantly different from that under no-load conditions. In this study, the effect of MI on the pore-fracture structure and seepage characteristics of load-constrained coal is investigated using a custom-developed microwave fracturing experimental device, nuclear magnetic resonance test device, and permeability test device. Based on the relationship between microwave and pore-fracture structure parameters and the permeability of loaded coal, the pore-fracture structure evolution and permeability growth law of loaded coal under MI are determined. The results show that the number of micropores in coal decreases and the T₂ curve of micropores is shifted to the right under the combined effect of MI and external stress load. The numbers of mesopores, macro-pores, and micro-fractures increase, and the T₂ curve exhibits a broader peak span. The pore-fracture structure evolution effect of loaded coal increases with the microwave power and MI time. Under high-power MI, the pore-fracture structure evolution of the loaded coal shows a “decrease - increase – decrease” trend as the stress load increases, whereas a “decrease – increase” trend is shown under low-power MI. Under the same microwave parameters, the permeability of unloaded and loaded coal increases by a maximum of 15.7 and 364.7 times, respectively. In particular, the permeability increases by 3.1–11.4 and 17.8–49.7 times under external stress loads of 4 and 2 MPa, respectively. The combination of high MI power and short MI duration under the same microwave energy facilitates the development of the pore-fracture structure and increases the permeability of loaded coal. Microwaves have a differential thermal effect on mineral in coal, which reduce the physical properties of the coal. Pore-fracture structure and permeability are further enhanced by the stress load.