Physicochemical Characteristics of Hard Brittle Shale and Associated Wellbore Instability
Wellbore instability in hard brittle shale is a key problem restricting the effective development of deep resources and unconventional shale gas resources which has not been solved effectively. Based on this, different experiments including x-ray diffraction, scanning electron microscope, hydrate swelling test, rock mechanics test were conducted in order to understand the components and microcosmic structure of the hard brittle shale and its physicochemical characteristics comprehensively. In addition, the properties of the collapsing block from the borehole were analysed through comparison before and after soaked in water. The results showed that unlike soft shale, the content of water sensitivity mineral such as montmorillonite within hard brittle shale was low and swelling capability due to water absorption was consequently low, but internal micro-fractures were developed, so the main reasons of wellbore instability are possible not chemical factors, but that the crack propagation and strength weakened. In drilling engineering, effective countermeasures are mainly including two categories; one is improving drilling fluid properties such as sealing, rheological property, wettability to slow down the drilling fluid invasion into micro-cracks within hard brittle shale, and the other is increasing the strength and reducing the stress acting on the rock surrounding wellbore through increasing drilling mud column pressure, reducing disturbance and the impact of drilling tool, optimizing well trajectory and so on.
KeywordsHard brittle shale Physicochemical characteristics Wellbore instability
The authors gratefully acknowledge the support of National Natural Science Foundation of China (Grant No. 51604225 and Grant No. 51604224).
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