The collapse cycle of brittle shale is related to many factors, among which the strength weakening after contact with drilling fluid is an important factor, and the degree of strength weakening is closely related to the degree of rock damage. In order to study the influence of core damage on borehole collapse, shale cores were first loaded with different loads before the peak strength and then unloaded, and then the strength tests were carried out after immersing in the drilling fluid, through which a model of the influence of shale damage on the strength weakening and a collapse cycle evaluation method considering shale initial damage were established. The study results show that initial damage increases the needed drilling fluid density to maintain wellbore stability in subsequent drilling, and shortens the collapse cycle. Therefore, in complex brittle shale drilling, proper drilling fluid density should be selected before drilling to avoid serious initial damage.

1. Introduction

During the drilling process, the drilling fluid column pressure replaces the support of the drilled rock, and the stress around the borehole is redistributed, resulting in stress concentration. Under the action of stress, the rock around the wellbore will be deformed or even destroyed, which causes wellbore instability. According to statistics, more than 90% of the wellbore instability in oil and gas drilling occurs in shale formation, among which the hard brittle shale formation accounts for 2/3 (Wang et al., 2006; Lu, 2011). The typical falling blocks from hard brittle shale formation are shown in Fig. 1. These blocks do not stick when exposed to water, but they flake easily. Some of the blocks are schistose, which is the characteristic of layered shale.

The research on the failure mechanism at home and abroad shows that the failure of brittle rock is mainly caused by the initiation, propagation and intersection of internal cracks during loading and unloading (Zhu et al., 2008). For fractured brittle formations, increasing the plugging capacity of drilling fluid can slow down the invasion and pressure transfer, but this method alone cannot maintain wellbore stability (Ottesen, 2010). In addition, the existence of micro-cracks in hard brittle shale makes the wellbore instability correlated with time and causes periodic collapse (Han et al., 2018).

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