Showing papers on "Overpressure published in 2022"

Geological Characteristics and Controlling Factors of Deep Shale Gas Enrichment of the Wufeng-Longmaxi Formation in the Southern Sichuan Basin, China

Abstract: Abstract To identify the factors controlling high-quality deep shale gas reservoirs and the exploration and development potential of the Lower Paleozoic marine shale in the Sichuan Basin, the sedimentary environment of deep shale was comprehensively analysed using core thin sections, scanning electron microscopy, gamma ray spectrometry logging, and elemental logging data. In addition, the geological conditions of deep shale gas accumulation and the effect of tectonic processes on the preservation conditions are discussed based on the experimental data of mineral composition analysis, geochemical features, and reservoir spatial characteristics. (1) The sedimentary environment changes from an anoxic water environment to an oxygen-rich oxidizing environment from bottom to top in the Wufeng-Longmaxi Formation in southern Sichuan. The deep shale gas reservoir shows overpressure and rich gas characteristics, namely, high formation pressure (2.0~2.2), high porosity (20%~55%), and high gas content (4.0~5.0 m3/t). (2) The favourable sedimentary environment has a higher hydrocarbon generation potential and deposits of rich organic matter and siliceous particles. During the hydrocarbon generation process, the rich organic matter generates a large number of organic pores and a large specific surface area, which provides the main reservoir and adsorption space for free and adsorbed shale gas. A large number of biogenic siliceous particles provide a solid rock support framework for the shale reservoir, thereby maintaining excellent reservoir physical properties. (3) Late and small stratigraphic uplifts result in a short shale gas escape time and favourable preservation conditions. Additionally, the small-scale faults and a high-angle intersection between the fracture strike and the geostress direction are conducive to the preservation of shale gas. (4) A high formation pressure coefficient, a sedimentary environment rich in organic siliceous deep-water continental shelf microfacies, and a relatively stable tectonic structure are conducive to the accumulation of deep shale gas.

Analysis of obstacle shape on gas explosion characteristics

Abstract: The safe design and operation of clean fuels like hydrogen require a detailed understanding of their explosion characteristics. An experimental study of the flame propagation behavior influenced by quadrangular, cylindrical, and triangular obstacles was executed in a 530 mm × 82 mm × 82 mm pipe. The results confirm the understanding that obstacles with a tip promote the generation of flow instability and produce a more intense burning behavior of the flame. The shear layers shed fewer larger vortices after the quadrangular obstacle; however, these vortices can be dislodged to form smaller and more vortices after passing through the triangular obstacles. The shear layer has weak shedding properties behind obstacles with curved edges. In the flame propagation process, the quadrangular obstacles have a more obvious promoting effect of the initial explosion, but the degree is weaker than with the triangular obstacles. The effect of the quadrangular obstacles on flame velocity is mainly influenced by gas flow at the flame front. In triangular obstacles, the shear layer became prominent in the later stage of the explosion process and this contributes to enhancing the flame velocity and overpressure.

Forecast of Airblast Vibrations Induced by Blasting Using Support Vector Regression Optimized by the Grasshopper Optimization (SVR-GO) Technique

Abstract: Air overpressure (AOp) is an undesirable environmental effect of blasting. To date, a variety of empirical equations have been developed to forecast this phenomenon and prevent its negative impacts with accuracy. However, the accuracy of these methods is not sufficient. In addition, they are resource-consuming. This study employed support vector regression (SVR) optimized with the grasshopper optimizer (GO) algorithm to forecast AOp resulting from blasting. Additionally, a novel input selection technique, the Boruta algorithm (BFS), was applied. A new algorithm, the SVR-GA-BFS7, was developed by combining the models mentioned above. The findings showed that the SVR-GO-BFS7 model was the best technique (R2 = 0.983, RMSE = 1.332). The superiority of this model means that using the seven most important inputs was enough to forecast the AOp in the present investigation. Furthermore, the performance of SVR-GO-BFS7 was compared with various machine learning techniques, and the model outperformed the base models. The GO was compared with some other optimization techniques, and the superiority of this algorithm over the others was confirmed. Therefore, the suggested method presents a framework for accurate AOp prediction that supports the resource-saving forecasting methods.

Effects of the length and pressure relief conditions on propagation characteristics of natural gas explosion in utility tunnels

Abstract: With the development of urbanization, the construction of the utility tunnel facilitates people's lives, but also brings some new potential hazards. For example, gas explosion occurring in the natural gas compartment can pose great threats to the safe operation of utility tunnels. In this paper, an experimental apparatus for modeling the process of the natural gas explosion in utility tunnels is established to investigate the effects of the gas compartment length and pressure relief conditions on the flame behaviors, and several parameters related to the consequence severity of gas explosion are studied comprehensively. The results indicate that ancillary facilities in the natural gas compartment (seen as obstacles) that cannot generate the large-scale recirculation zone but cause the small size of turbulence. The maximum overpressure firstly increases and then decays with the increase of the length of the gas compartment. Besides, it is also found that different pressure relief locations and strengths have a great influence on explosion characteristics in utility tunnels. This study can provide technical supports for safe management of utility tunnels. Meanwhile, some suggestions are put forward for the explosion suppression techniques and the design of pressure relief ports in the utility tunnel.

Numerical simulation of overpressure loads generated by gas explosions in utility tunnels

Abstract: Studies of overpressure loads are essential to mitigate the potential hazards of public safety induced by gas explosion accidents. Based on the computational fluid dynamics (CFD) code FLACS, numerical models of gas explosions in large-scale tunnels were developed and verified by comparing with testing data. Numerical simulations of gas explosions in confined and side-vented utility tunnels were carried out. The effects of the venting condition, gas cloud volume, and ignition position were analyzed and discussed. It is found that gas explosions in utility tunnels can be divided into three stages according to the pressure and flame development, i.e., the combustion-induced rise stage, oscillation rise stage, and oscillation decline stage. The pressure oscillation is significant, and a side vent near the tunnel end can reduce the peak pressure by 42~78% and the oscillation peak by 35~87%. The peak pressure, oscillation peak, and duration of oscillation rise stage varied significantly with the gas cloud volume. When the premixed gas cloud is ignited at the 1/4 L, the peak pressure and oscillation peak reach the maximum of 1242.5 kPa and 490.4 kPa for confined cases, which are one time higher than those of side-vented cases. What is more, based on the equivalent single degree of freedom (SDOF) approach in the elastic range, the dynamic effects of gas explosions in utility tunnels are figured out, and suggestions for engineering practice are proposed.

Dissociation of gas hydrates by hydrocarbon migration and accumulation-derived slope failures: An example from the South China Sea

Abstract: • Submarine slope failure by overpressure caused by gas migration/accumulation. • Buoyancy extrusion and seepage-derived deformation can generate slope failure. • Slope sediment permeability controls gas buoyancy and slope failures. • Slope failure results in the complete dissociation of the gas hydrate accumulation. The mechanism of slope failure associated with overpressure that is caused by hydrocarbon migration and accumulation remains unclear. High-resolution seismic data and gas hydrate drilling data collected from the Shenhu gas hydrate field (site SH5) offer a valuable opportunity to study the relations between submarine slope failure and hydrocarbon accumulation and flow that is associated with a ∼2 km-diameter gas chimney developed beneath site SH5 where none gas hydrates had been recovered by drilling and sampling despite the presence of distinct bottom simulating reflectors (BSRs) and favorable gas hydrate indication. The mechanism of submarine slope failure resulted from buoyancy extrusion and seepage-derived deformation which were caused by overpressure from a ∼1100 m-high gas column in a gas chimney was studied via numerical simulation. The ∼9.55 MPa overpressure caused by hydrocarbons that migrated through the gas chimney and then accumulated beneath subsurface gas hydrate-bearing impermeable sediments. This may have resulted in a submarine slope failure, which disequilibrated the gas hydrate-bearing zone and completely decomposed the gas hydrate once precipitated at site SH5. Before the gas hydrate decomposition, the largely impermeable sediments overlying the gas chimney may have undergone a major upward deformation due to the buoyancy extrusion of the overpressure in the gas chimney, and slope failure was initiated from plastic strain of the sediments and reduced internal strength. Slope failure subsequently resulted in partial gas hydrate decomposition and sediment permeability increase. The pressurized gas in the gas chimney may have diffused into the overlying sediments controlled by seepage-derived deformation, causing an effective stress reduction at the base of the sediments and significant plastic deformation. This may have formed a new cycle of submarine slope failure and finally the total gas hydrate dissociation. The modeling results of buoyancy extrusion and seepage-derived deformation of the overpressure in the gas chimney would provide new understanding in the development of submarine slope failure and the link between slope failure and gas hydrate accumulation and dissociation.

Submarine slope failure due to overpressure fluid associated with gas hydrate dissociation

Abstract: The dissociation of gas hydrates can increase pore pressures greatly, thereby causing the shallow layers of submarine slopes to fail. Given the high failure risk of shallow subsea soils, it is important to understand the stratum response mechanisms after hydrate dissociation. In this paper, submarine slope failure triggered by overpressure fluid associated with gas hydrate dissociation is investigated in laboratory experiments. A two-layer geological model is built based on actual geological data, and pressurised air is injected into the model to simulate the overpressure fluid. The pore pressures, surface displacements and internal deformations of soils are measured and compared under different conditions, and their evolution processes are analysed for various parameter values. The results show that the accumulation of pore pressure increases with the thickness of the soil layer and leads to layered fractures. The failure pattern can be generalised into two types: (a) disc-shaped failure and (b) penetration failure. In disc-shaped failure, a major failure occurs when the shear stress reaches the shear strength, whereas tensile fracturing is a major effect in penetration failure. This achievement is very important for a deep understanding of submarine landslides induced by overpressure fluid, as well as for risk assessments of ocean engineering sites.

CFD analysis of large-scale hydrogen detonation and blast wave overpressure in partially confined spaces

Abstract: The effect of confinement on the magnitude of overpressure due to a gaseous detonation of a hydrogen-air mixture was studied with the aid of numerical simulations. A simplified combustion reaction kinetic model applied along with computational fluid dynamics (CFD) allowed for the numerical simulation of large-scale detonations of hydrogen-air mixtures. The model was validated against experimental data reported in the literature for the case of a large-scale (300 m 3 ) surface (unconfined space) detonation of a hydrogen-air mixture and the results of a hydrogen-air (confined space) detonation test performed in a 263 m 3 tunnel facility. The predicted overpressure and detonation velocity was in agreement with the measurements in both unconfined and confined detonation cases. To verify the effect of confinement on the magnitude of the blast wave overpressure due to detonation of a combustible gas cloud, a series of CFD simulations of hydrogen detonations followed by propagation of a non-reactive blast wave in various geometries were carried out. The results were compared with the correlation applicable for unconfined detonations, which was also found to be applicable for partially confined detonations after the transformation of the distance from explosion centre to spherical blast wave equivalent radius.

Experimental study on pressure and flow characteristics of self-ignition hydrogen flowing into the unconfined space

Abstract: Accidental release of high-pressure hydrogen can result in self-ignition, non-premixed jet flame and high overpressure, which will create potential security risks to people, buildings and equipment. In this study, pressure dynamics and flame induced by the self-combustible hydrogen flowing into the unconfined space were experimentally studied. The entire process was characterized by pressure sensors and cameras. Results show that the velocity of the hydrogen jet increases first and then decreases after it flows out of the tube. Its overpressure decays rapidly and stabilizes quickly. The dramatic changes for the flow field parameters in the near-field region can cause the self-ignition region to extinguish first and then reignite. And the overpressure in the near-field region, caused by the self-ignition jet flowing out of the tube, is lower in the unconfined space than that in the semi-confined space. In addition, axial and radial variations for the jet flame are characterized by phased development, which can be divided into three stages based on features of flame morphology and temperature distribution. Among them, a typical tadpole-like flame is formed. Its head with large size shows asymmetry, develops downward deflection and eventually separates from the flame body which are all affected by asymmetric large-scale vortices. Besides, the re-ignition of the jet can be induced during the nitrogen purge process, which is much more dangerous in real accident scenarios.