鄂尔多斯盆地延长组页岩气井壁稳定钻井液

针对鄂尔多斯盆地延长组页岩气水平井钻井过程中存在的裸眼井壁失稳问题,对该组地层进行理化性能分析,明确了导致页岩井壁失稳的主要因素,即钻井液滤液沿地层微裂缝进入地层引起伊蒙混层膨胀,岩石强度降低,为此提出了采用油包水乳化钻井液的技术对策。在此基础上,通过室内实验优选了一套页岩水平井油包水乳化钻井液。室内性能评价实验表明：该体系可加重至1.60 g/cm3,破乳电压高于800 V,抗温120℃,封堵突破压力为7.28 MPa,浸泡后对页岩强度影响较小,能够有效抑制页岩膨胀和封堵裂缝性地层,对于解决该区页岩气钻井过程中的井壁失稳问题具有指导意义。

Drilling fluid for wellbore stability of shale gas in Yanchang Formation, Ordos Basin

Wellbore instability is a main problem during the drilling of shale gas horizontal well in Yanchang Formation,Ordos Basin.Through analyzing the physical and chemical properties of the formation in this paper,the main factors causing shale wellbore instability are obtained,that is the decrease of rock strength has a direct relationship with I/S ratio expansion caused by the fact that the filtrate of drilling fluid goes into the formation along the micro fracture.Therefore,a technical countermeasure of using water in oil emulsified drilling fluid is put forward.A new system of emulsified drilling fluid for shale gas horizontal well is developed through a large quantity of laboratory experiments.The experiment results show that emulsion-breaking voltage is more than 800 V and the drilling fluids with high plugging breakthrough pressure of 7.28 MPa can be weighed to 1.60 g/cm3 and resist 120 ℃.Also the compressive strength of shale cores changes little after soaking in the emulsified drilling fluid,which can effectively inhibit shale expansion and plug the fissured formation so as to directly solve the wellbore instability problem during the drilling of shale gas horizontal well in this region.