【作者】 王志远；

【导师】 孙宝江；

【作者基本信息】 中国石油大学 ， 油气井工程， 2009， 博士

【摘要】 含天然气水合物相变的环空多相流流型转化机制的研究,是深水油气及天然气水合物藏钻探过程中,井筒内流体力学计算的关键问题之一。深水钻探时,由于井筒中存在天然气水合物相变问题,使得目前环空多相流动理论无法满足工程需求。此理论的突破,对我国深水油气及水合物资源的开采具有重要的意义。本文通过实验和理论分析,得到了天然气水合物相平衡条件、生成速率以及分解速率的计算方法;深水条件下,井筒中天然气水合物的相变模拟计算表明:水合物的生成速度要远大于其分解速度,天然气一旦进入井筒中水合物生成区域,会在很短的时间内相变为水合物。井筒内温度会对水合物相变产生很大影响。利用自行研制的承压、大直径的“含天然气水合物相变的环空多相流流型转化模拟系统”进行了流型转化实验研究,得到结论如下:随着注气量的增大,流型变化历经泡状流、弹帽泡状流、弹帽沫状流和沫状流四种流型,而没有发现段塞流,并探讨了段塞流未出现的原因。随着混合相雷诺数的增加,泡状流失稳时的临界截面含气率呈现先下降再升高的趋势,因为流动的湍流度对气泡的聚并有正反两方面作用。一方面,它使得气泡的碰撞机率增高,对气泡的聚并有利;另一方面,它使得两气泡相遇的特征时间变小,对气泡的聚并不利。混合相雷诺数较小时,有利气泡聚并的方面起主要作用,而混合相雷诺数超过一定数值后,不利于气泡聚并的方面起主要作用。大直径环空中泡状流失稳时,混合相雷诺数较高,湍流强度较强,脉动较大。由于强湍流作用使气泡聚并难以发生,或由于涡旋的搅动使已经形成的大气泡被打碎,因此大直径环空两相流中因强湍流运动和大尺度涡旋形成,使Taylor泡的形成或气泡群的聚并受到抑制,难以形成段塞流。注气扰动频率与空隙率波的控制频率接近的情况下,空隙率波具有最大增长率;注气扰动使得空隙率波的波动程度增加,使泡状流失稳时的临界截面含气率降低;当环空中流型处于弹帽泡状流时,少量的注气就会引起流型的转化;施加注气扰动,空隙率波的传播速度呈现二次曲线增长的趋势;注气扰动加速了泡状流的失稳。井筒压力的升高,会使得空隙率波的最大增长率降低,波动程度减小,气泡的滑脱速度降低,泡状流失稳时的临界截面含气率升高,井筒压力变大空隙率波传播波变慢,井筒压力对泡状流的失稳起到抑制作用。通过Kolmogorov熵计算表明:气液两相流的空隙率波动存在非线性特性。扰动使得空隙率波的非线性特征充分表现,从而导致空隙率波传播特性的变化。针对含天然气水合物相变的环空多相流动特点,增加天然气水合物相变项,得到质量守恒和动量守恒方程;增加天然气水合物分解热项,得到能量方程。通过理论分析完善了含天然气水合物相变的环空多相流动模型。通过所编制的“含相变的环空多相流动分析及井控模拟软件系统”,模拟计算表明:钻进期间,流量越小、抑制剂浓度越低、泥浆入口温度越低以及水深越大均会使水合物的生成区域变大;停钻期间,水合物的生成区域会随着停钻时间的增长而增大;压井期间,水合物的生成区域会随着节流管线内径的减小而增大。溢流期间,天然气水合物的相变使得环空内的气体体积分数减小,井底压力增加,关井套压降低,泥浆池增量降低,环空内的压力分布发生改变;由于环空气体体积分数和泥浆池增量的减小,增加了深水井涌的早期检测的难度;由于水合物相变,关井套压不能真实反映气侵的严重程度;对于一定精度的泥浆池检测设备,发现井涌时,储层渗透率低时井筒内生成水合物的可能性要大于储层渗透率高的情况。压井期间,气体进入尺寸较小的节流管线后会迅速膨胀,使得井筒内静液压力迅速降低。为了能够使井底有足够的压力,需要快速的调节节流阀,维持一定的节流压力,因此在深水压井时节流压力的调节速度要高于陆地井控。更多还原

【Abstract】 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.更多还原