| In oil-gas injection production, the environment in three-dimensional borehole is complex,which is influenced by several factors, such as hole curvature, torsion variation, hightemperature and pressure, CO2/H2S corrosion and multiphase-fluid flow. This results in athree-dimensional borehole mechanical system, which is composed of pipe string temperatureand corrosion stress, borehole contact and fraction, fluid viscosity, tubing string vibration andthe coupling effect between fluid and string. In this system, the dynamics of multiphase-fluidand pipe string structure is the key problem.During gas injection in thermal recovery for a vertical well, two-phase flow of water andvapor and gravitational differentiation could take place. This reduces the steam absorptioneffect and producing ratio. Moreover, influenced by the coupling effect of hole curvature andcomplex loading, when it is running in the hole, the perforation string with a super-thin rodstructure is prone to endure engineering accidents, such as buckling failure, blocked detentionand fracture. In consider of the theoretical and engineering problems occurred in heavy-oilthermal recovery and perforating string operation, which have been mentioned above, theborehole water-vapor mixing mechanism and string structure safety assessment are studied inthis paper, based on the dynamics theory and methods of both multiphase fluid and stringstructure.In this paper, the main works are as follows:1. Gravitational differentiation and Mixing Mechanism of Water-Vapor Two-Phaseflow in the borehole of heavy oil.(1) The parameter analysis model of borehole pressure and dryness for vertical borehole isdeveloped. This model is further solved by using iteration method. Afterwards, water-vaporflow condition was analyzed based on Beggers-Brill flow pattern discriminance. The resultsshow that, when the steam injection matches steam absorption, a water-vapor slug flow withunsteady intermittence forms. In this condition, the liquid plug with low gas content and thebubble with high gas content appear alternately. The water and vapor are irregularly distributed, with a poor uniformity. There exists a huge difference in the density betweenwater and steam, thus an oil-water interface forms as the gas stays on the top while water sinkat the bottom, especially for a thicker reservoir.(2) With considering the steam injection technology in thermal recovery for vertical well,both ball-filling and vaned water-steam mixers were designed, and the corresponding mixingscheme was proposed.(3) By using the fluid mechanics calculation software COMSOL Multiphysics, thenumerical simulation about the mixing process of steam and water in a vaned mixer wascarried out. The results show that, when the fluid flows through the spiral blade in the mixer,strong rotational and radial flows were produced to promote the material exchange betweenthe fluids near the pipe wall and that close to the axis, and thus the mixing achieved. Inaddition, the influence of four factors, namely, the number of elements, the output pattern, theratio of length and radial and the injection velocity, on the fluid characteristics in the vanedmixer was investigated.(4) Based on the hydromechanics affinity theory, a ground performance test system ofthe wellbore water-steam two-phase mixer was designed and set up. By using air and water asthe test media, the mixing effects of both ball-filling and avaned water/steam mixers wereinvestigated, and the comparative result show that the vaned water-steam mixer is moresuitable to be applied into practice.2. Dynamic characteristics of Pipe String in complex borehole environment(1) According to the downhole operation state of the pipe string, the string geometricconfiguration in the actual wellbore trajectory was described. With consideration of bothgeometric nonlinear and contact nonlinear characteristics of the slender strings, the stringmovement process can be regarded as the mechanical problem of an elastic rod withlongitudinal vibration. The dynamic model was further presented based on theinfinitesimal-element analytical method.(2) The solution method of the string dynamics was analyzed systematically, and twotypical engineering problems, namely, the dynamic problem of the perforation stringcombined with bridge plug while pumping down the well and the impact dynamic problem ofTCP strings, were studied here. For these two problems, the theoretical analyses andnumerical simulations were carried out to investigate the strings’ mechanical responses andthe influence rules of the key engineering factors. Based on this, the correspondingapplication software system was developed by using Visual Basic6.0, which can be used toprovide guidance for field operations... |
