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

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.