TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractIdentification of the correct cause of wellbore instability is the key to quickly solve the problem. This paper presents some results of tests conducted to check the behavior of preserved shale samples when immersed in different solutions, trying to identify the importance and relevance of the osmosis as well as the effect of other components in the mud, such as the emulsifier. Results showed that not only the salinity of the mud is important. Also, in some situations, rock swelling did not follow what states the osmosis theory, leading to a conclusion that other phenomena are relevant and with a similar order of importance.The paper also discusses the mechanism of micro-cracking with subsequent fluid penetration, and the use of a truly noninvasive fluid to avoid the instability. Field cases are reported, demonstrating the effectiveness of using such approach. The paper also presents the problems ofcurrent rig site monitoring tests that can induce wrong interpretation of the real cause of the problems.