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Abstract The effect of different drilling fluids on the strength behavior of Kuqa shale is studied based on triaxial rock mechanical test. Combined with existing drilling fluid performance evaluation methods，stress-strain curve shown by shale after immersion in different drilling fluids for 24 hours and the rock mechanical performance，rock deformation and failure laws，the existing drilling fluid is selected and optimized. One drilling fluid with better stabilization to this kind of shale formation is obtained. Besides，the formation collapse pressure change in different drilling fluids is also calculated and analyzed by testing rock strength before and after immersion in those systems. The findings achieve remarkable wellbore stabilization and speedup effect when they are applied to drilling field practice. Results show that present drilling fluid will remarkably decrease rock strength，and the shale strength performance can be regulated and changed by optimizing the drilling fluid. Rock strength performance research is applied to existing shale drilling fluid evaluation. It can not only induce the chemo-mechanical coupling between shale and drilling fluid initiatively into the optimization procedure of drilling fluid system，but also can establish theoretical and methodological basis to restrain the uncertain effect to shale wellbore caused by coupling chemo-mechanical radically. The research combines the optimization procedure of drilling fluid with wellbore stability. It establishes the basis to obtain collapse pressure of target shale formation and achieve safe，effective drilling with low density drilling fluid. Deformation failure law showed by shale after contacted with different drilling fluid is the important criteria to shale formation drilling fluid stabilization and wellbore performance optimization. The stress-strain curve of shale before and after being immersed by different drilling fluids can lead to achieve evaluation and optimization of drilling fluid performance. The shale strength behavior and collapse pressure are adjustable by optimizing drilling mud components.
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Received: 17 August 2008
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