Various techniques and drilling practices have been developed to remedy drilling problems owing to shale swelling; however, a quantitative estimation of shale swelling potential has been difficult to establish in the oilfield. The cation exchange capacity is one parameter that has been shown to control the swelling of shales both qualitatively and quantitatively. A correlation derived from basic physical principles for the cation exchange capacity (CEC) of shale formations using common log data of temperature, resistivity, and sonic slowness has been modified using available core and cuttings test data. Cation exchange capacity is then used to distinguish problematic, smectite-rich shale formations from the trouble-free, illite-rich formations using a threshold value. An empirical relationship that involves calculated values of CEC and thresholds is then developed to estimate the osmotic pressure resulting from an imbalance of water activities—in effect, estimating a higher mud weight needed to stabilize the well. This methodology was applied to a well drilled in the North Sea combined with mechanical wellbore-stability analysis and shows good agreement with wellbore enlargement and well events. The technique has multi-fold benefits, including identifying problematic drilling intervals owing to the abundance of smectite, estimating the minimum mud weight necessary to prevent drilling problems owing to shale swelling, and ultimately determining whether a well is drillable with a water-based mud system.
Wellbore instability is a challenge when drilling through shale formations with a water-based mud. Shale instability may lead to stuck pipe, shale sloughing, and a low-quality or even a lost wellbore. Causes of wellbore instability in shales may be separated into two main categories—mechanical and chemical.
Mechanical wellbore instability in shales results from mechanical failure of weak rock formations owing to stresses in the vicinity of the wellbore. Shear failure along weak bedding planes—an example of strength anisotropy that often afflicts shale formations—also falls into this category.