Estimating in-situ stress magnitudes from core disking

It is well known that elevated stress magnitudes will cause core disking during drilling. During recent site investigation drilling at Forsmark, Sweden, to depths of 1000 m localized core disking was encountered. The disking was observed in short (< 1-m long) sections of solid core, and as ring-disking in overcore cylinders. Three dimensional elastic numerical analyses were carried out to estimate the stress magnitudes required to initiates both ring- and solid-core disking. Three-dimensional fracture mechanics approach was used to investigate the relationship between disk thickness and disk shape as a function of stress magnitudes. A series of analyses were also carried out to determine the relationship between tensile strength and horizontal stress magnitudes. The results indicate that core disking can be used to constrain the stress magnitudes but that the tensile strength can have a significant impact on the stress magnitudes determined from core disking.