Abstract
Most shale rocks which contain an appreciable fraction of reactive clays (e. g. Montmorillonite) will adsorb drilling mud filtrate (water+ions) and cause unstable drilling conditions. When contacted with the mud filtrate, these shales will swell, creating a soft, swollen zone around the wellbore, therefore, the natural mechanical properties or the strength of the swollen shales will decrease causing serious hole problems such as undergauge hole, stuck pipe, overpull on trips, and several other problems. Thus swelling stresses and rock strength reduction must be included in any attempt to effectively model shale mechanical properties after interaction with drilling fluid filtrate. In this study shale swelling stresses were integrated into the prominent Mohr-Coulomb failure criterion and therefore a new form of this criterion has been introduced which combined the natural mechanical properties with swelling stresses to predict the in-situ strength of shales when invaded by the drilling fluid filtrate. The modified failure criterion was verified experimentally.