Stability Analysis in Shale using the Mohr Coulomb Failure Criterion

Instability problems in shale formations are closely related to the ‘bulk properties ’ of shales such as strength and deformation as a function of porosity, water content, clay content, composition of the shale, and compaction rates. The bulk properties of the drilling fluid such as the concentration of the continuous phase of the mud, the additives used, the chemical composition etc are equally important. This research demonstrates the use of Mohr-coulomb failure criterion in selecting a safe mud weight. Using the Mohr-Coulomb failure criterion and assuming a range of values for the minimum horizontal stress from 2000-6000 psi, the Mohr-Coulomb failure envelope was created. The failure envelope describes shear failure in rocks. The rocks to the left of the line in figure 7 are considered to be unstable while those to the right are stable. The linear elastic theory was applied to determine the horizontal stresses because no direct measurements of the horizontal stresses of the reservoir were available. Further assumption that the tensile strength is negligible helps us to derive relationships of mud weight against well deviation for the Mohr Coulomb failure criterion. The stability curve shows the minimum mud weight required to prevent hole collapse as 10ppg and the maximum mud weight to prevent tensile fracture to be 21ppg.The wellbore will be stable in that region