龙门山地震断裂带地应力分布及其对井壁稳定的影响———以WFSD-2 井为例

[摘 要]龙门山地震断裂带是我国最为强烈的地震带之一,地层破碎、地应力异常,钻孔缩径造成孔内事故频发,井壁稳定问题十分突出。本文介绍了汶川地震断裂带科学钻探施工中所发生的孔内事故情况,并通过对地层应力数据的统计分析,得出龙门山地震断裂带最大水平应力和最小水平应力随深度变化的回归曲线,最大水平应力梯度为4. 52MPa/100 m,最小水平应力梯度为2. 51 MPa/100 m。以WFSD-2 井为例,分析了龙门山地震断裂带地层应力、尤其是断层泥应力对井壁稳定的影响;针对膨胀性地层,介绍了一种通过泥浆密度微调现场测定地层坍塌压力和破裂压力的简易方法。

In-situ Stress Distribution and Its Effects on Borehole Stability in theLongmenshan Earthquake Fault Zone

Abstract:The Longmenshan earthquake fault zone is one of the most active fault zones in China. It is characterized by fractured formation and anomalous in-situ stress which cause Frequent down hole drilling accidents due to formation expanding. The borehole stability is a serious problem. This paperpresents the down hole drilling accidents that occurred during the scientific drilling (WFSD at the Wenchuan earthquake fault zone. The statistics and regression analysis of the in-situ stress produced the curves that describe the maximum and minimum horizontal stresses changing with depth. The maximumhorizontal stress gradient is about 4. 52MPa/100m, and the minimum horizontal stress gradient is about 2. 51MPa/100m. Taking the borehole WFSD-2 asan example, the effects of stress, especially gouge stress on borehole stability is analyzed. A simple method is introduced that the formation caving pressure and fracture pressure are measured by density fine adjustment of drilling fluid applied to formation expanding and diameter shrinkage.