Shale preservation and testing techniques for borehole-stability studies

Shale rocks are known to be the primary troublesome formation that has plag ued drillers for many years. Attempts have been made to investigate the cha racteristics of shales, but results are limited because of the scarcity of test samples. Rapid deterioration of cuttings and cores has made data colle ction limited at best and a complete failure in many cases. It now appears that the shales become unsaturated as they dry out because of poor storage conditions, which result in false test results owing to the incorporation o f capillary pressure artifacts. This paper discusses results obtained with a specially preserved, highly re active shale core obtained at a depth of 4,500 ft in the North Sea. A previ ously developed method was used for the evaluation of shale cores to determ ine their level of saturation. This study shows that shales must be preserv ed at their native water content if accurate physical measurements are to b e made. Swelling data show that shales that are altered during handling (hydrated o r dehydrated) do not respond properly even when restored to their native hy dration conditions. They tend to experience excessive swelling compared to cores kept at their native water content. Results also showed that the North Sea core used in this study was maintain ed in a fully saturated condition during coring, retrieval, and storage. Th e technique used is discussed here.