北极永冻区钻井地层压力预测方法

A formation pressure prediction method for well drilling in the Arctic permafrost region

北极地区油气资源潜力巨大,但面临地质条件复杂,生态环境敏感,施工作业窗口期短等困难,其中由于极地地区地层压力难以预测为该区钻井开发带来极大的挑战。为此,以北极喀拉海海域地区为研究对象,基于Eaton法并同时考虑永冻层密度和声波偏移的影响,提出了修正后的地层压力预测模型,建立了该地区地层三压力剖面。研究结果表明:(1)极寒条件下,永冻层岩石裂隙中的充填物会增加岩石的强度,有利于提高上部层破裂压力,而解冻后岩石强度下降,岩石弹性模量会明显下降,塑性变形能力显著增强,沉降变化幅度介于49.5%~72.3%;(2)第四系—白垩系上部500 m左右厚度的永冻层使声波对数随井深的拟合曲线偏离正常趋势线,根据建立的地层压力模型计算得到研究区块地层压力系数范围在0.87~1.23;(3)修正模型预测钻井剖面安全钻井液密度窗口范围介于1.20~1.75 g/cm^(3),地层破裂压力当量密度预测值与地漏实验取得的实际值平均误差为3.3%。结论认为,该方法预测可以有效预测北极永冻区的地层压力,能满足钻井工程的精度要求,为该区后续极地钻井提供了重要支撑,具有十分重要的现实意义。

The Arctic region is immense in oil and gas resources,but it faces the difficulties of complex geological conditions,sensitive ecological environment and narrow construction time window,and especially the formation pressure in the Arctic region can be hardly predicted,which brings a great challenge to well drilling and development in this area.Taking the Arctic Kara Sea area as the research object,this paper proposes a modified formation pressure prediction model based on the Eaton method,by considering the influence of permafrost horizon on density and acoustic migration.In addition,the three-formation-pressure profile of this area is established.And the following research results are obtained.First,in extremely cold conditions,the fillings in the rock fractures of permafrost horizon can increase the rock strength,which is helpful to increase the fracturing pressure of the formation above it.After thawing,however,the rock strength decreases,the elastic modulus reduces greatly and the plastic deformation capacity increases significantly,with the subsidence variation amplitude ranging from 49.5%to 72.3%.Second,the permafrost horizon about 500 m thick above Quaternary-Cretaceous leads to the deviation of the fitting curve of acoustic log with well depth off the normal trend line.And the formation pressure coefficient of the study area is calculated by using the established formation pressure model,which is in the range of 0.87-1.23.Third,according to the prediction results by the modified model,the safety drilling fluid density window of drilling profile ranges from 1.20 g/cm^(3)to 1.75 g/cm^(3).And the average error between the equivalent density of formation fracturing pressure predicted by the modified model and the actual value measured in the formation leakage experiment is 3.3%.In conclusion,this method can effectively predict the formation pressure in the Arctic permafrost region and meet the required accuracy of drilling engineering,which provides an important support for the subsequent drilling in the Arctic region and is of great practical significance.