Because of high cost and pollution of oil-based drilling fluid, the water-based drilling fluid is increasingly used now. However, bedding planes and micro-cracks are rich in shale formation. When water-based drilling fluid contacts formation rock, it causes the propagation of crack and invasion of drilling fluid, which decrease shale strength and cause wellbore instability. In this paper, we analyzed influence of water-based drilling fluid on shale strength and failure mode by mechanics experiment. Based on those experimental results, considering the effect of bedding plane and drilling time, we established modeling of wellbore stability for shale formation. The result from this model indicates that in certain azimuth of horizontal well, collapsing pressure increases dramatically due to shale failure along with bedding plane. In drilling operation, those azimuths are supposed to be avoided. This model is applicable for predication of collapsing pressure in shale formation and offers reference for choosing suitable mud weight.
Acknowledgments
This research was Supported by the United Fund Project of National Natural Science foundation of China (Grant No. U1262209), the Open Fund (Grant No. PLN0906) of Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (Southwest Petroleum University) and Basic research Project of Sichuan Province (Grant No. 2014JY0092).
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