Mechanisms of Borehole Wall Destabilization in Que’er’Que’ke Formation in Shunbei Oil and Gas Field and Measures Dealing with the Borehole Wall Collapse
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摘要: 塔里木盆地顺北油气田却尔却克组钻井过程中井壁失稳事故频发,严重影响了施工进度。针对顺北油气田X井却尔却克组地层井壁失稳的机理,开展了岩石崩解实验、点载荷实验、X衍射、扫描电镜实验研究。结果显示,岩石矿物组分中黏土含量大(15%~35%),其中伊利石含量在60%左右,伊蒙混层主要在10%~30%,吸水率小(2%),崩解率大(5%),岩石属于典型的硬脆性灰质泥岩。室内实验表明,在清水和水基钻井液条件下,岩石强度表现出较强的非均质性和强度弱化(岩石单轴强度降低在10~40 MPa之间),但在油基钻井液下,岩石强度几乎无弱化影响,并且油基钻井液对微裂缝起到较好地封堵作用。此地层水基钻井液下井壁失稳机理一方面是硬脆性泥页岩中的膨胀性黏土矿物吸水膨胀,强度弱化,失去有效支撑;另一方面微裂缝在水力尖劈作用下,井壁岩石崩落垮塌失稳。油基钻井液中乳化剂、润湿剂等表面活性剂特殊结构可以对泥页岩中裂缝微裂缝进行有效封堵,减少水化和水力尖劈作用,实现井壁稳定。现场在第三侧钻井眼水基钻井液井壁严重失稳条件下,改用油基钻井液,顺利钻完计划进尺,大幅度减少井壁失稳损失。Abstract: Frequent occurrence of borehole wall collapse in drilling the Que’er’Que’ke formation in Shunbei block in Tarim Basin has seriously affected the progress of the drilling operation. Laboratory experiments, such as rock disintegration test, point load experiment, X-ray diffraction, and SEM, were performed to find out the mechanisms of borehole wall collapse in drilling the well X in Shunbei block. It was found that the rock sample taken from the Que’er’Que’ke formation has high content of clays, of which 60% is illite and 10%-30% is illite/montmorillonite mixed layer. The rock sample has low percent water absorption (2%) and high ratio of disintegration (5%), and is a typical hard and brittle calcareous claystone. In laboratory experiment, the rock sample in fresh water and water base mud showed strong heterogeneity and reduced strengths (uniaxial strength of the rock sample in fresh water and water base mud was reduced by 10-40 MPa.) In oil base mud, the strength of the rock sample was almost not weakened. Furthermore, oil base mud can even seal the micro fractures in the rock. When drilling with water base drilling fluids, collapse of the borehole wall resulted from water absorption by the clay content in the formation and hydraulic wedging of the micro fractures developed in the formation. Water absorption by the clay content in the formation decreases the strength of the formation rocks, and the rocks can thus not be supported by their strengths. Hydraulic wedging accelerates the disintegration of the rocks. When drilling with oil base muds, the emulsifiers and wetting agents in the muds can effectively seal off the micro fractures in the rocks, thereby stabilizing the borehole wall through weakening of the hydration of the rocks and mitigation of hydraulic wedging. In the third sidetrack of the well, a water base mud was first used and severe borehole wall collapse happened. Then oil base mud was substituted for the water base mud, and the well was finally successfully completed with borehole wall collapse brought under control.
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Key words:
- Hard and brittle claystone /
- Borehole wall collapse /
- Oil base mud /
- Clay mineral /
- Micro fracture
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