大26井欠平衡钻井后期出现井壁不稳定问题分析

大26井采用聚磺钻井液进行欠平衡钻井,前期钻井施工顺利,但在欠平衡施工完毕之后,出现了井壁持续掉块现象,不得不通井划眼,电测难以到底.本文通过分析得出,造成大26井井壁坍塌及其后的井下复杂情况的根本原因是起钻所需平衡压力与实钻井底水静压力之间存在压差.并且认为如果欠平衡钻进期间井下流体有显示,施工在控压状态下进行,则情况会好一些,因为控制回压实际上等于提高钻井液水静压力,对井壁稳定有利.跟踪几口实施欠平衡钻井的井得知,井下有显示、控制了回压、带压钻进的井没有出现井壁不稳定问题,完井后其它施工安全、顺利.

Analysis on wall failure during later under balanced drilling in Da26 well

Da26 well is drilled under-balanced, using polysulfonate drilling fluid. No problem appeared until continual sloughing occurred after under balanced drilling. The operator had to ream the hole many times, and what was more difficult was that the log tools could not run to the bottom. Based on careful analysis, it is considered that the root causing the complex accident is the pressure difference between the balance pressure needed and the actual water static pressure down hole. It was thought that it would be better if there was fluid in-show down hole during the under balanced drilling process, or the operation was under controlled back pressure, because to control the back pressure was actually to increase the static pressure of drilling fluid, which is benefitable for hole stabilization. Investigation on several wells drilled with under balanced drilling technology showed that the wells with fluid show and controlled back pressure, no wall failure phenomenon appeared during completion and continued operations.