Tight sand gas reservoirs in west Sichuan basin are deeply buried, about 3 to 5 kilometers. They are easily damaged during conventional drilling, and the drillability of the mid-depth sand shale layers is poor. Air drilling and nitrogen drilling are selected to improve ROP and protect formation. However, the borehole stability is a big problem through several pilot gas drillings. In this paper, combined lab investigations with pilot drilling information analysis, it is found that the complex geologic conditions and drilling operations lead to at least four kinds of borehole instability. First, the lithology varies frequently in longitudinal section and the weak argillutite alternate layers rate more than 50% thickness of the whole profile, which leads to mechanic instability problem. Next, drilling open of abnormal high pressure gas layers leads to second wellbore stress distribution, which results in sandstone avalanche and adjacent argillutite layer collapse. Third, wellbore enlargement is common due to breakage of deep friable hard shale layers. At last, aqueous phase invasion due to drilling operation or formation water production from lower layers leads to swelling and weakening of argillutite formation, which results in borehole collapse and sticking. According to such mechanisms, gas drilling borehole stability prediction methods were presented and proved to be helpful to successful gas drillings in this area.

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