Study On Annular Multiphase Flow Pattern Transition Mechanism Considering Gas Hydrate Phase Transition

Study on annular multiphase flow pattern transition considering gas hydrate phase transition is one of the key research subjects on studying wellbore hydrodynamics during deep water drilling. The present multiphase flow mechanisms can not meet the engineering requirement due to the gas hydrate phase transition. The new breakthrough of this theory is going to be of great significancy in deep water oil/gas and gas hydrate exploitation of China.Through experiment and theory analysis, phase equilibrium condition, formation rate and decomposition rate of gas hydrate are gotten in the paper. Through simulation of gas hydrate phase transition in deep water wellbore, the result shows: the gas hydrate formation rate is much higher than decomposition rate. Once gas entering the hydrae formation region, it will turn into gas hydrate soon. And temperature in wellbore has a great influence in hydrae phase transition."Annular multiphase flow pattern transition considering gas hydrate phase transition simulation system" designing is finished with pressure and large diameter. The conclusions are as follows.Bubble flow, cap-bubble flow, cap-churn flow and churn flow are identified with increase of gas injection rate, however, slug flow can not be found. And the reasons are discussed. The critical void fraction of bubble flow destabilizing has an ascent trend after an initial decline with increase of total Reynolds number. The reason is that the turbulent intensity has a pro and con effect on bubble coalescence. On the one hand it increases the chance of bubble collision which is good for bubble coalescence, and on the other hand it reduces the characteristic time to meet each other for two bubbles which is a disadvantage for bubble coalescence. And the advantage aspect predominates with small total Reynolds number, however, the result is contrary when total Reynolds number beyonds a certain value. When bubble flow destabilizes, the total Reynolds number is very high in large size annulus, and there are great turbulent intensity and pulsations in annulus. It is possible that great turbulence makes it difficult for bubbles to coalesce or vortex agitation makes the large bubble already formed destroyed, which suppresses Taylor bubble formation or bubble clusters coalescence. So slug flow can not happen.Void fraction wave has a maximum growth rate when gas injection disturbances are applied with a certain frequency. The fluctuation extent of void fraction wave increases and the critical void fraction of bubble flow destabilizing decreases due to gas injection disturbances. When the present flow pattern is cap-bubble flow, the flow pattern transition is likely to happen after adding a little more gas. Void fraction wave speed increases with a trend of quadratic after gas injection disturbances applied. The gas injection disturbances hasten destabilizing of bubble flow. With wellbore pressure ascending, the maximum growth rate of void fraction wave decreases, and fluctuation extent of void fraction wave decreases, and bubble slip velocity slows down, and void fraction wave speed slows down. It all adds up to that wellbore pressure suppresses bubble flow destabilizing. Void fraction wave of two phase flow has a non-linear quality. The non-linear quality of void fraction wave becomes obviously with disturbances, which causes flow pattern transition.The multiphase flow governing equations are established through theory analysis. Mass conversation and momentum conservation equations are obtained with adding gas hydrate transition term. And energy conversation equations are obtained considering decomposition heat term of gas hydrate transition. The numerical simulation is applied by software system "Analysis of annular multiphase flow considering gas hydrate phase transition and well control simulation software system". The conclusions are drawn as follows:Hydrate formation area is getting large with decrease of circulation rate or inhibitor concentration or inlet temperature of drilling fluid, or with increase of water depth or rig downtime, or with decrease of choke line size during well killing. Annular void fraction, pit gain, and shut down casing pressure decreases, and bottom hole pressure increases, and annular pressure profile changes due to gas hydrate phase transition after overflow. It is more difficult for early detection of gas kick due to the decrease of annular void fraction and pit gain. Shut in casing pressure can not reflect the truth of gas kick due to hydrate formation. When gas kick is monitored by pit gain detection equipments with certain accuracy, the possibility of hydrate presence with lower gas production rate is much greater than that with larger production rate. Gas expands greatly after entering small size choke line, which reduces hydrostatic pressure rapidly during well killing. In order to maintain enough bottom hole pressure, throttle valve should be adjusted quickly and choke pressure reaches some extend. Therefore, the speed of throttle valve adjustment is faster off shore than on shore.