Periodic large-scale collapse phenomena and accidents of horizontal drilling in brittle shale formation are common. In this process, borehole rock suffers dynamic load originating from impact of bit, hit of drilling tool and chemical action of drilling fluid. Based on hypothesis of static load, traditional wellbore stability analysis model cannot reveal the real mechanism. Therefore, traditional counter-measures eventually fail to restrain periodic brittle shale wellbore instability.

In this paper, mechanics behavior and deformation mechanism of the brittle shale has been investigated in the dynamic triaxial compression experiments at varying confining pressure and soaking time. Moreover, analytical tools of high resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDS) and 3D visualization of ultra-high resolution CT scanner (Nano CT) etc. have been applied to observe and analyze the testing process.

The results indicate that mass ignored micrometer or even nanometer cracks, triggered by dynamic load, will not destroy the rock immediately, but accumulate and then aggravate the development of macroscopic fracture until rocks failure after a period of time. Consequently, the mechanism of strength reduction in this situation is not macroscopic fracture but micro-scale crack. Furthermore, the mechanical-chemical coupling model of brittle shale under dynamic load is established according to the quantitative relations among rock structure, interface state and strength. Based on the analyses above, counter-measures of Nano sealing technology to improve the strength of borehole rock is proposed.

Laboratory experiments show that Nano sealing can effectively strengthen wellbore stability in shale formation, whereas traditional highly macro sealing and inhibitive drilling fluid system cannot.

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