Modified Cam-Clay Model for Large Stress Ranges and Its Predictions for Geological and Drilling Processes

We modify the Modified Cam-Clay (MCC) model for large stress ranges encountered in geological applications. The MCC model assumes that the friction angle (phi) and the slope of the compression curve (lambda) of a mudrock are constant and thus predicts constant values for the lateral effective stress ratio under uniaxial, vertical strain (K-0) and undrained strength ratio ( Su sigma ' v). However, experimental work shows that lambda, phi, and Su sigma ' v decrease and K-0 increases substantially with stress over large stress ranges (e.g., up to 100 MPa). We incorporate the stress dependency of lambda and phi into the MCC model and use the new model to predict Su sigma ' v and K-0 ratios. The modified model, with only one additional parameter, successfully predicts the stress dependency of these ratios. We encode the modified model and use it for finite-element analysis of a salt basin in the deepwater Gulf of Mexico. The stresses that the new model predicts around salt differ significantly from those predicted using the original model. We incorporate the stress dependency of the friction angle into the analytical models developed for critical tapers, wellbore drilling, and the stability of submarine channel levees. We show that the decrease of the friction angle with stress (1) results in a concave surface for critical wedges, (2) shifts the drilling window to higher mud weights and makes it narrower for a vertical wellbore, and (3) causes deep-seated failure of submarine channel levees at lower angles. Our study could improve in situ stress and pore pressure estimation, wellbore drilling, and quantitative understanding of geological processes.