Shale sloughing and swelling are the two major problems encountered when drilling through shale formations. Some areas are characterized by shale sections containing bentonite or other hydratable clays, which continually absorb water, swell and slough into the hole. These types of formations are known as heaving shales. This problem, if left uncombated, results in high cost of drilling the hole and significantly, to other hole problems (pipe sticking, excessive solid buildup in the mud and hole bridging), and sometimes abandoning the well because of the difficulty of reaching the targeted or anticipated pay depth. Drilling through shale formations can, many a time result to hole stability problems and may be aggravated the more when drilling through more than one kind of shale lithology.
The existing literature addresses possible solution techniques to combat the shale problems based on laboratory studies but these techniques neglect the existence of shale heterogeneities which may cause sloughing and swelling while drilling the same shale formation.
This paper examines the physical appearance of various shale samples with a view to (a) determining the physical properties which contribute to the shales' undesirable properties, and (b) proposing methods to minimize their effects on drilling performance. Five field cases are presented where sloughing and swelling shales were encountered. These cases reveal that the heterogeneity of the shale formations can cause difficulties in controlling the sloughing and/or swelling problems because of the variations in shale composition, ion exchange capacity, formation water content and shale strength.