基于精细回压控制的地层-井筒多相流耦合机理研究

Drilling engineering has been making progress in the area of deep-and complex-formations. Because the density windows of these formations are often narrow, lost circulation and overflow occur altogether from time to time. What is worse, poorly controlled gas invasion likely to bring about serious blowout accidents. So fluid flow and pressure control in the wellbore after the gas invasion has been a reasearch hotspot, with certain achievements already made. But the researches based on multiphase flow behavior in the wellbore, pressure distribution, pressure wave transmission and formation seepage in formation-wellbore coupling conditions after conducting precise backpress control,especially real-time evaluation of backpress on overflow control effect,have made little progress till now. On the base of theoretical analysis, experimental observation and computer simulation, the project aims at developing precise backpress control, multiphase flow in the wellbore, backpress transmission, formation seepage, real-time evaluation of well control effect, etc; respectively building the models of complex transient multiphase flow, pressure fluctuation and formation seepage in the wellbore and sovling the equations; revealing the mechnism of transition behavior of annular flow pattern , phase state, multiphase flow characteristic, pressure wave transmission and fluid seepage; conducting real-time evaluation on the control effect of precise backpress on formation overflow; establishing a dual balance theory of material and pressure and a method of precise backpress control in formation-wellbore coupling conditions, and provide the theoretical basis for safe and efficient drilling in deep-and complex-wells.

随着钻井方向逐步向深层及复杂地层发展，这些地层的安全密度窗口往往较窄，井眼漏失和溢流常常相伴发生，尤其是气侵如果控制不好很容易引发严重井喷事故，因此地层气侵之后的井筒流体流动、压力控制一直是研究热点，并取得一定研究成果。但在采取精细回压控制后基于地层-井筒耦合的井筒多相流动特征、压力分布、压力波传递及地层渗流等方面研究进展缓慢，特别是回压对地层溢流控制效果的实时评价研究甚少。本项目拟在理论结合实验、计算机模拟基础上，开展精细回压控制、井筒多相流动、回压传递、地层渗流和井控效果实时评价等方面的研究，分别建立井筒复杂瞬态多相流动、压力波动、地层渗流的模型和方程并求解，揭示环空流型与相态转变、多相流动规律和压力波传递、地层渗流机理，实时评价精细回压对地层溢流的控制效果，旨在建立完善的地层-井筒耦合的物质与压力双重平衡理论和精细回压控制方法，为深井、复杂井安全高效钻进奠定理论基础。

项目针对精细回压控制钻井中，井口回压在环空传播过程中的地层渗流及井筒多相流动的动态变化规律进行研究。建立了流体通过阀芯行程与阀门两端节流压降模型，分析表明节流阀存在阀门关度无效区间、有效区间和压力超调区间三个区间，有效调节区间内阀门关度与节流压降函数呈现指数关系，可用于控压钻井对井口回压的精细调节。基于岩心渗透性压敏实验，建立了回压控制下水平井溢流计算模型。建立了回压压力波在井筒中的传播速度和波动模型，揭示了回压在井筒中的传播特性：压力波波速随钻井液密度的增加而增大，随含气率、虚拟质量力系数的增加而呈现出减小的趋势，先急剧变化，之后变化幅度缓慢。实验分析了流体物性和井筒流动特性的压敏规律，随着压力增加，钻井液表观粘度、密度逐渐增大；同一回压下，液相排量相同时，摩阻压降随着气相排量的增大而增大，泡状流区增幅较缓，搅拌流区增幅较大；气相排量相同时，摩阻压降随液相排量的增大而增大；同一液相、气相排量下摩阻压降随回压的增大而增大。建立了精细回压控制的井筒多相流瞬态流动模型，采用ROE格式对方程求解，求解结果表明随着时间的增加井底压力逐渐减小，且回压越大井底压力越大。钻井液速度随井深的增加而减小，在同一井深处随着回压的增加而降低。项目研究成果为精细控压钻井在深部地层的进一步应用奠定了理论基础。