Abstract:

The coexistence of influx and loss in different formations represents the most complex scenario in well control. Accurate prediction of the gas-liquid two-phase flow behavior and pressure distribution within the wellbore is essential for effectively dealing with such high-risk incidents. In this study, a one-dimensional coupled gas-liquid two-phase flow model of the wellbore and the formation is developed under conditions of simultaneous influx and loss. By adjusting the distributions of pore pressure and fracture pressure to alter the relative positions of the influx and loss zones, the upper-influx/lower-loss and lower-influx/upper-loss scenarios are simulated. Additionally, by specifying different fracture pressures for the loss zone, the varying resistance of the formation to drilling fluid loss is simulated. The model is solved using a finite-difference numerical method, yielding the transient distributions of gas holdup, pressure, and velocity along the well depth under different operating conditions. The simulation results indicate that the fracture pressure of the loss zone is a key factor governing the flow regimes within the wellbore. When the fracture pressure is sufficiently high, the influx does not evolve into more complex conditions. At lower fracture pressures, a transition from influx to loss occurs within the wellbore. If a weak section with extremely low fracture pressure exists, a subsurface blowout characterized by the coexistence of influx and loss may develop. Under the upper-influx/lower-loss scenario, if the loss zone at the bottomhole has a relatively high fracture pressure, the influx within the wellbore ceases and the system evolves into a no-influx/no-loss state, with the shut-in casing pressure continuing to increase. When the fracture pressure of the loss zone is lower, a transition from influx to loss occurs, and the shut-in casing pressure first increases and then stabilizes. When the fracture pressure is further reduced, a subsurface blowout characterized by simultaneous influx and loss develops. The shut-in casing pressure initially increases and then rises slightly, and the inflection-point pressure is lower than the stabilized value in the influx-to-loss transition scenario. Under the lower-influx/upper-loss scenario, as the fracture pressure of the loss zone decreases, the flow regimes within the wellbore successively evolve from no-influx/no-loss, to influx-to-loss transition, and finally to simultaneous influx and loss. The variation of shut-in casing pressure is closely related to the fracture pressure. A lower fracture pressure corresponds to a lower inflection-point value of the casing pressure. In the no-influx/no-loss and influx-to-loss transition cases, a finite-length contaminated drilling fluid section detaches from the bottom and migrates upward. In contrast, under simultaneous influx and loss, a continuous contaminated section forms above the influx zone, while a high gas-fraction section with nearly constant gas holdup exists below it.

Key words: kick and loss in different formations, well control, gas-liquid two-phase flow, wellbore, numerical modeling

摘要：

喷漏异层是井控中最复杂的情况，准确预测井筒中气液两相流的流动状态和压力分布是处理此类风险事故的关键。本文建立了喷漏同存条件下，井筒与地层耦合的一维气液两相流模型。通过调整孔隙压力和破裂压力的分布改变喷层和漏层的相对位置，模拟上喷下漏和下喷上漏。另外，通过设置不同的漏层破裂压力，模拟地层的不同抗钻井液漏失能力。本文采用有限差分数值模拟方法对模型进行求解，得到不同工况下井筒内部含气率、压力和速度沿井深的瞬时分布。数值模拟结果表明，漏失层位破裂压力的大小是井内出现不同流动状态的关键因素，破裂压力足够高时井内溢流不会向复杂情况发展。当破裂压力较低时，井内会出现溢漏转换。而如果存在破裂压力极低的薄弱井段，井内则会发生喷漏同存的地下井喷。在上喷下漏情况下，若井底漏层有较高的破裂压力，井内的溢流将停止，发展为停喷不漏的状态，关井套压持续增加。漏层破裂压力较低时，井内出现由喷转漏的状态，关井套压呈现先增大而后保持不变的变化规律。当漏层破裂压力更低时，井内会形成边喷边漏的地下井喷状态。关井套压先增大而后小幅上升，套压拐点小于喷转漏情况下的稳定值。上漏下喷情况下，随着漏层破裂压力的降低，井内出现的流动状态依次为停喷不漏、由喷转漏和边喷边漏。关井套压变化趋势与破裂压力相关。破裂压力越低，套压拐点值越低。停喷不漏和由喷转漏时，井内出现有限长污染钻井液段离底滑脱的现象。边喷边漏情况下，喷层以上形成连续污染段，在喷层之下则存在含气率为常数的高含气段。

关键词: 喷漏异层, 井控, 气液两相流, 井筒, 数值模拟

CLC Number: 

• TE249

• X937