酸性气体侵入井筒在钻井液内赋存状态及对瞬态流动影响研究

Sour gas reservoirs are generally marine sediments with complex geological condition, and as formation pressure of sour gas reservoirs is difficult to accurately predict and drilling fluid density window is narrow, overflows and kicks are frequently encountered in the sour gas reservoirs. Sour gas is sensitive to pressure and temperature, and solubility of sour gas is very large; High content of acidic component in sour gas may lead to phase transition. Kick, blowout and other serious drilling accidents may easily occur during drilling operation in terms of the short response time. .A test device for occurrence state of sour gas in drilling fluids is developed, The expression of occurrence state of sour gas in drilling fluids will be derived through theoretical analysis and test results. The transient multiphase flow model during sour gas invasion is established considering the coupling effect of wellbore flow, wellbore heat transfer, and formation non-Darcy seepage field and phase transition of high content of acidic component in sour gas. Wellbore transient flow affected by occurrence state of sour gas is analyzed through pipe flow experiments for gas cut simulation which revise and improve the established model. The computational accuracy of model will be hopefully enhanced by oilfield measured data. .The mechanism and its influence factors of transient flow based on the improved model are also presented during sour gas invasion. The study will provide guidance for drilling design and well-control operation of sour gas reservoirs.

酸性气藏一般位于海相沉积、地质条件复杂，地层压力难以准确预测，密度窗口极窄，钻进过程中极易发生气侵；且酸性气体对压力温度敏感，溶解度变化很大，气侵后监测难度大，且高含酸性组分气体在近井口处可能发生相态转变，井控操作反应时间短，易造成井涌、井喷 等钻井事故。.本项目在改进设计酸性气体在钻井液内赋存状态试验装置基础上，通过试验结果与理论分析得到酸性气体在钻井液内赋存状态的定量表达式；基于此，将井筒传热、流动与地层非达西渗流耦合，并考虑高含酸性组分的相态转变，建立酸性气体侵入井筒瞬态流动数学模型并进行数值计算模拟分析；开展变侵入速度连续气体多相管流实验，实验分析赋存状态对井筒瞬态流动影响，并对所建数学模型进行对比校正和改进；同时利用部分油田井控现场数据对模型进行校对和提高。.利用改进完善后的模型揭示酸性气侵井筒瞬态流动机理并系统分析其影响规律，为酸性气藏钻井工艺设计和井控安全提供技术指导。

酸性气藏一般位于海相沉积、地质条件复杂，地层压力难以准确预测，密度窗口极窄，钻进过程中极易发生气侵；且酸性气体对压力温度敏感，溶解度变化很大，气侵后监测难度大，且高含酸性组分气体在近井口处可能发生相态转变，井控操作反应时间短，易造成井涌、井喷等钻井事故。.本项目在改进设计酸性气体在钻井液内赋存状态试验装置基础上，通过试验结果与理论分析得到了酸性气体在钻井液内赋存状态的定量表达式；基于此，将井筒传热、流动与地层非达西渗流耦合，并考虑高含酸性组分溶解赋存特性及相态转变，建立了酸性气体侵入井筒瞬态流动数学模型并进行数值计算模拟分析；开展气侵井筒多相管流实验，并结合以往文献中全尺寸管流实验数据验证了模型的准确性，并对所建数学模型进行了校正和改进。.研究表明：CO2流体在钻井液和清水随温度升高其赋存量呈先降低后升高趋势，赋存量与压力近乎呈线性关系，CO2流体在钻井液内赋存量明显高于在清水内。酸性气体由于其密度较大并且大量气体以溶解气形式存在，在气侵初始时刻井筒压降较小，游离气体积分数较小而溢流时间较长，因而酸性气体侵入更加隐蔽，不易被检测；但靠近井口处随着温度压力降低酸性气体体积迅速膨胀，并且酸性组分溶解度迅速降低，大量析出为游离气，导致井筒压力迅速降低，若处理不当会引起严重的井控事故。水平井在溢流初始时刻气体井筒压降很小，不易被发现，发生溢流时更加具有隐蔽性和危险性。酸性气侵时对井筒压力影响最大的为注入排量，其对溢流期间整个过程都有影响；其次为井口回压，随井口回压增大井底压力增幅远大于井口压力增幅，能有效控制溢流；而储层渗透率和打开储层厚度的影响类似，主要影响主井底气体侵入速度方面；而初始压差的影响相对最小，仅对溢流起诱导作用。.利用改进完善后的模型揭示了酸性气侵井筒瞬态流动机理并系统分析了其影响规律，为酸性气藏钻井工艺设计和井控安全提供技术指导。