地层H2S侵入时井筒压力变化规律及控制研究

随着我国酸性油气藏勘探开发的深入,处于对井控安全的考虑,需对酸性气体侵入后井筒多相流动及相态转变规律进行研究。针对H₂S特殊的物理性质,并考虑其在井筒内相态变化,建立了钻井过程中H₂S侵入时井筒流动与传热的数学模型。将井筒传热、压力与H₂S物性参数耦合迭代计算,给出了求解方法并编写程序进行数值计算。计算结果表明:井口回压较小时,H₂S在环空上升过程中由液态转化为气态,相态转变点上部为气液两相流,其压力梯度较小,下部为井筒单相流,其压力梯度较大。H₂S侵入速度对环空压力和相变井深均有影响。随着侵入量增大,井底压力先急剧减小,后基本保持不变,而相变井深先增大后减小。井口回压对井底压力影响较大。随着井口回压增大,井底压力增大,但影响程度逐渐减小。井口回压不仅可以控制井筒是否发生相变,而且对相变井深位置影响十分大。对是否考虑传热对相变井深和井底压力的影响进行了对比分析。研究对提高酸性油气藏开发勘探安全具有一定指导意义。

Principles of wellbore pressure variation and control during formation H2S influx

The growing number of acid reservoirs were explored and developed in the China and the increasing necessity for well control safety has been forcing the oil and gas industry to study the principles of wellbore multiphase flow and phase transition of drilling during acid fluid invasion. Based on the theories of pressure drop and heat transfer for liquid-gas two phases flow in wellbore, a mathematical model for wellbore flow and heat transfer, in which the special physical properties of supercritical fluid (SC-F) and its phase transition were considered in the wellbore, was established during formation SC-F influx, and a solution method of the model was proposed and program was compiled for the numerical calculation. The results indicate that H2S transforms from liquid to gas state during its rising in the annulus when backpressure is small, and the flow in the annulus above the phase transition point is two-phase flow, while the flow in the annular below the phase transition point is single-phase flow and the pressure gradient is large. The pressure and depth of phase transition are both influenced by H2S influx rate, with the increase of H2S influx rate, the bottom-hole pressure decreases rapidly at first and then remains steady but the depth of phase transition increases at first and then decreases. The bottom-hole pressure increases with the increase of backpressure, but the amplitude of affecting becomes small. The backpressure not only controls the occurrence of phase transition but also greatly affects the depth of phase transition. Bottom-hole pressure and depth of phase transition influenced by heat transfer are analyzed in the paper. The study is of certain significance for improving the safety of acidic oil & gas reservoirs exploration and development.