沿井筒方向变化的CO_2分压对油管服役时间的影响规律研究

开展沿井筒方向变化的CO_2分压下油管服役时间规律研究,有助于最大限度地延长油管使用年限。为此,将CO_2分压引入Q/HS14015标准腐蚀速率模型中,结合坐封、生产和开发过程中油管服役工况,建立了以油气产量和腐蚀环境为参数的井筒CO_2分压耦合计算模型。应用该模型的计算结果表明:①沿井深方向CO_2分压呈多项式分布,同一井深处,产量越大,CO_2分压越小;②与实测井筒CO_2分压相比,井底段CO_2分压计算值吻合度高,井口段CO_2分压受温度、井筒压力和CO_2摩尔含量降低影响,计算值误差较大;③相同CO_2分压下,腐蚀速率随温度的增加先增大后减小;④CO_2分压与服役时间内油管强度呈反比;⑤一定井深下,相对于温度,CO_2分压对腐蚀后油管强度影响更明显。结论认为:以井筒CO_2分压为基础,结合腐蚀速率和油管坐封、生产和开发过程中所受外挤、内压、拉力服役工况预测油管服役时间的方法,能够进一步优化CO_2腐蚀环境下的油管选材,节约油气井建井成本,在生产中的应用效果也证明了该方法的可行性。

Tubing service time reducing laws under CO2 partial pressure of wellbore

To exert the tubing performance to the utmost, it is necessary to analyze the service time laws of tubing under CO2 partial pressure which changes along the wellbore. In this paper, CO₂ partial pressure was introduced into the Q/HS14015 standard corrosion rate model. A coupled calculation model of CO₂ partial pressure with oil and gas production rate and corrosion environment as parameters was established based on the service performance of tubing in the process of setting, production and development. The calculation results by this model indicate that: (1) CO₂ partial pressure along the wellbore direction is distributed in a multinomial pattern. In the same depth, the higher the production rate is, the lower the CO₂ partial pressure. (2) The calculated CO2₂partial pressure of the bottom hole is highly coincident with the measured value while that of the wellhead section is more deviated from the measured value due to the decrease of temperature, pressure and CO₂ mole content. (3) Under the same CO₂ partial pressure, the corrosion rate increases and then decreases with the increase of temperature. (4) CO₂ partial pressure is inversely proportional to the tubing strength during its lifetime. (5) In a certain depth, CO₂ partial pressure is more influential to tubing strength after the corrosion than temperature. It is concluded that the method to predict tubing service time based on the CO₂ partial pressure of wellbore, combined with the corrosion rate in the working conditions with the internal, external and tension force in the process of tubing setting, production and development can optimize the tubing material selection under the CO₂ corrosion environment and reduce the construction cost of oil and gas wells. It is actually proved to be feasible in production application.