Studying Deformation Behaviors in Austenitic Stainless Steels within a Temperature Range of 143 K < T < 420 K

This work deals with studying staging and macroscopic strain localization in austenitic stainless steel 12Kh18N9T within a temperature range of 143 K < T < 420 K. The visualization and evolution of macroscopic localized plastic deformation bands at different stages of work hardening were carried out by the method of the double-exposure speckle photography (DESP), which allows registering displacement fields with a high accuracy by tracing changes on the surface of the material under study and then comparing the specklograms recorded during uniaxial tension. The shape of the tensile curves σ(ε) undergoes a significant change with a decreasing temperature due to the γ-α'-phase transformation induced by plastic deformation. The processing of the deformation curves of the steel samples made it possible to distinguish the following stages of strain hardening, i.e. the stage of linear hardening and jerky flow stage. A comparative analysis of the design diagrams (with the introduction of additional parameters of the Ludwigson equation) and experimental diagrams of tension of steel 12Kh18N9T for different temperatures is carried out. The analysis of local strains distributions showed that at the stage of linear work hardening, a mobile system of plastic strain localization centers is observed. The temperature dependence of the parameters of plastic deformation localization at the stages of linear work hardening has been established. Unlike the linear hardening, the jerky flow possesses the propagation of single plastic strain fronts that occur one after another through the sample due to the γ-α' phase transition and the Portevin-Le Chatelier effect. It was found that at the jerky flow stage, which is the final stage before the destruction of the sample, the centers of deformation localization do not merge, leading to the neck formation.

The Pressure Buildup Well Test Analysis considering Stress Sensitivity Effect for Deepwater Composite Gas Reservoir with High Temperature and Pressure

Some deepwater gas reservoirs with high temperature and pressure have obvious stress sensitivity effect resulting in difficulty in well test interpretations. The influence of stress sensitivity effect on the pressure drawdown well test is discussed in many papers. However, the influence on the pressure buildup well test is barely discussed. For practices in oilfields, the quality of pressure data from the drawdown stage of well test is poor due to the influence of production fluctuation. Thus, the pressure data from the buildup stage is used for well test interpretations in most cases. In order to analyze the influence of stress sensitivity effect on the pressure buildup well test, this paper establishes a composite gas reservoir pressure buildup well test model considering the stress sensitivity effect and the hysteresis effect. Numerical solutions to both pressure drawdown and buildup well test models are obtained by the numerical differentiation method. The numerical solutions are verified by comparing with analytical solutions and the homogeneous gas reservoir well test solution. Then, the differences between pressure drawdown and buildup well test curves considering the stress sensitivity effect are compared. The parameter sensitivity analysis is conducted. Compared with the conventional well test curve, the pressure derivative curve of pressure drawdown well test considering the stress sensitivity effect deviates upward from the 0.5 horizontal line at the inner zone radial flow stage, while it deviates upward from the M/2 (mobility ratio/2) horizontal line at the outer zone radial flow stage. However, for the pressure buildup well test curve considering the stress sensitivity effect, the pressure derivative curve gradually descends to the 0.5 horizontal line at the inner zone radial flow stage, while it descends to the M/2 (mobility ratio/2) horizontal line at the outer zone radial flow stage. The pressure derivative curve of pressure buildup well test considering the hysteresis effect is higher than the curve without considering the hysteresis effect, because the permeability cannot be recovered to its original value in the buildup stage after considering the hysteresis effect. Meanwhile, skin factor and mobility ratio have different effects on pressure drawdown and buildup well test curves. Based on the model, a well test interpretation case from a deepwater gas reservoir with high temperature and pressure is studied. The result indicates that the accuracy of the interpretation is improved after considering the stress sensitivity effect, and the skin factor will be exaggerated without considering the stress sensitivity effect.

Semi-analytical model for the transient analysis of the pressure in vertically fractured wells in reservoirs considering the influence of natural fractures

In addition to artificial fractures generated by hydraulic fracturing technology, natural fractures distributed in reservoirs will also affect the fluid flow process. To study the transient behavior of the pressure in fluid flows in reservoirs containing natural fractures, a semi-analytical model for vertically fractured wells with complex natural fracture networks was established. This model was based on the linear source function theory and the fracture discretization and coupling methods. It was solved by the Stehfest numerical inversion and the matrix transformation. The results of the study on the fluid flow stages in a reservoir with natural fractures indicated that the presence of natural fractures increased natural fracture flows. These flows were dominated by natural fractures and fracture interference stages and were different from the fluid flows observed in vertically fractured wells with a single main fracture. The sensitivity analysis on the influences of the fluid flow factors in the reservoirs with three types of natural fractures could provide a more detailed reference for the identification of the reservoir parameters and the transient characteristics of the flow stage. The different characteristic curves of the fluid flow in the reservoirs with different scale natural fractures could also provide a theoretical basis for determining the distribution of natural fractures in reservoirs.

Numerical Simulation of Transient Characteristics of Start-Up Transition Process of Large Vertical Siphon Axial Flow Pump Station

In order to explore the transient characteristics of the large-scale vertical siphon axial flow pumping station during the start-up and exhaust process, numerical simulations were carried out on the start-up process of the axial flow pumping station under the two starting modes of pre-opening the vacuum breaking valve and keeping the vacuum breaking valve closed. The calculation results show that during the start-up phase of the unit, the flow separation phenomenon of the impeller channel of the pump device with the vacuum breaking valve closed is serious, the large-scale vortex in the guide vane blocks the flow channel, and the instantaneous impact on the blade surface is strong. The flow field of the pump device with pre-open vacuum failure valve is obviously less affected by the instantaneous impact characteristics during the start-up of the pump. The range of high entropy production area in the impeller channel is reduced, the duration of high entropy production area is significantly shortened, and the instantaneous impact on the blade surface is weak. Under the two starting modes, the internal flow field of the pump device is similar in the evolutionary law. The unstable flow phenomenon of the pump device is most prominent in the weir flow stage. The maximum instantaneous impact on the blade surface also mainly occurs in the weir flow stage. A very small part of the remaining gas in the siphon formation stage is difficult to discharge and takes a long time. After the pump device is exhausted and enters a stable operation state, the external characteristic parameters are in good agreement with the test results. Compared with the starting method in which the vacuum breaking valve is kept closed, the method of pre-opening the vacuum breaking valve reduces the maximum starting head by 20% and the exhaust time by 43%. The pre-open vacuum breaking valve effectively avoids the system instability caused by the start-up and exhaust of the pump device.

Study on Viscosity Retention of Polymer System and Its Application in a Bohai Oilfield

Due to the high viscosity of crude oil and high water-oil mobility ratio, water channeling is serious in a Bohai oilfield. Polymer flooding has been carried out in the oilfield, and good results have been achieved. When polymer flooding is implemented in the oilfield, only the wellhead viscosity of polymer is known, but the viscosity of polymer system in the formation is not accurately known. The viscosity of polymer system in the formation is an important parameter for polymer flooding effect and later polymer injection parameter optimization. Due to the lack of data and the difficulty of operation in offshore oilfield, it is urgent to study the viscosity retention of polymer after being sheared from the borehole. The flow of polymer solution is divided into two stages. The first flow stage is that the flow of polymer solution in the wellbore is equivalent to the flow of an equal diameter circular pipe. Assuming that the solution system is incompressible and is one-dimensional stable flow, the mathematical model of apparent viscosity is established by momentum theory and the constitutive equation of pseudo-plastic fluid. Finally, the apparent viscosity and shear rate of the solution system are calculated by the mathematical model, which keep unchanged along the flow direction in the equal diameter circular pipe. The second flow stage is that the flow process of polymer solution through the borehole is equivalent to the flow process of shrinkage and expansion in a variable cross-section pipeline. The viscosity mathematical model of the solution system after the borehole shearing is established. The viscosity retention is calculated by the mathematical model, and the influence of perforation radius and other indexes on formation working viscosity is analyzed. The results show that the viscosity retention of the polymer system is 34.1%∼36.9% by using the new model. Through the analysis of the influencing factors, it is concluded that the consistency coefficient and perforation radius have the greatest influence on the viscosity retention. By applying the calculated viscosity retention obtained by the new model to the numerical simulation, the water cut history fitting of single well is improved. Due to the same concentration injected in the whole oilfield, the effect of polymer flooding in some areas is not obvious. The viscosity of polymer in the formation is calculated by the new model. After concentration optimization and adjustment, the concentration of polymer injection in three wells increases from 800mg/L, 1000mg/L and 1200mg/L to 1500mg/L respectively, and the oil production of the surrounding production wells increases significantly, and the daily oil production increases 105m3. The new technology has been widely used in five wells of other two oilfields in Bohai Oilfield. On the basis of calculating the viscosity retention rate, good results have been achieved by adjusting the injection concentration, and the total oil increase has reached 5×104m3. There are some assumptions in the calculation of this technology. In the future, the fluid flow will be further studied under the condition of removing the assumptions.

Simulation of the Flowability of Fresh Concrete by Discrete Element Method

The discrete element method (DEM) was used to establish the slump model and J-Ring model of concrete to describe the flow behavior in the slump test and J-Ring test. Then, the contact parameters of particle-particle and particle-geometry for the concrete DEM model, including restitution coefficient, rolling friction coefficient, static friction coefficient, and surface energy, were measured. In order to avoid the influence of the shape and size of the aggregate, this paper used high-precision glass spheres as the aggregate of the concrete for meso-calibration test, slump test, and J-Ring test. Comparing the simulation results of DEM model with slump test result, a very high agreement between the initial stage, the rapid flow stage, and the slow flow stage of the slump flow–time curve can be found as well as the final slump and slump flow. Moreover, similar to the slump DEM model, the DEM models of J-Ring test, V-funnel test, and U-channel test were established to study the passing ability and filling ability of concrete with outstanding accuracy. Therefore, the concrete DEM model with contact parameters and JKR model can be adopted to study the flow behavior of the fresh concrete.

Determination and application of a reasonable drainage rate of coalbed methane in vertical wells in the single-phase flow stage

The determination of a reasonable drainage rate of coalbed methane (CBM) in vertical wells in the single-phase flow stage can provide maximise the transmission of water pressure over distance. Based on the principle of effective stress and Darcy’s law, a mathematical model for dynamic changes of the permeability in the single-phase flow stage was established; on this basis, the relationship between permeability and threshold pressure gradient was experimentally attained; according to the linkage of changes of the transmission distance of water pressure, permeability, and pressure drop in the wellbore in the drainage process, a mathematical model for a reasonable reduction rate of the working fluid level in the single-phase flow stage taking the change of the permeability into account was established. The accuracy of the mathematical model was verified according to practical drainage data from CBM wells in Daning Block in Qinshui Basin, Shanxi Province, China. The results show that the rate of pressure drop decreases in a negative exponential manner with the increase of the drainage time. Different rates of pressure drop were required in coal reservoirs with different permeabilities; when keeping other conditions constant, the larger the permeability of coal reservoirs, the lower the threshold pressure gradient and the lower the rate of daily pressure drop. The research results provide a theoretical basis and reference for the reasonable drainage system in the single-phase flow stage.

Autowave Mechanics of Plastic Flow

The notions of plastic flow localization are reviewed here. It have been shown that each type of localized plasticity pattern corresponds to a given stage of deformation hardening. In the course of plastic flow development a changeover in the types of localization patterns occurs. The types of localization patterns are limited to a total of four pattern types. A correspondence has been set up between the emergent localization pattern and the respective flow stage. It is found that the localization patterns are manifestations of the autowave nature of plastic flow localization process, with each pattern type corresponding to a definite type of autowave. Propagation velocity, dispersion and grain size dependence of wavelength have been determined experimentally for the phase autowave. An elastic-plastic strain invariant has also been introduced to relate the elastic and plastic properties of the deforming medium. It is found that the autowave’s characteristics follow directly from the latter invariant. A hypothetic quasi-particle has been introduced which correlates with the localized plasticity autowave; the probable properties of the quasi-particle have been estimated. Taking the quasi-particle approach, the characteristics of the plastic flow localization process are considered herein.