Effect of Rock Strength Criterion on the Predicted Onset of Sand Production

The variation of sanding onset prediction results with the selection of one or another rock strength criterion is investigated. In this paper, four commonly used rock strength criteria in sanding onset prediction and wellbore stability studies are presented. There are Mohr–Coulomb, Hoek–Brown, Drucker–Prager, and modified Lade criteria. In each of the criterion, there are two or more parameters involved. In the literature, a two-step procedure is applied to determine the parameters in the rock strength criterion. First, the Mohr–Coulomb parameters like cohesion $S_{\mathrm{o}}$ and internal friction angle ${\varphi }_f$ , are regressed from the laboratory test data. Then, the parameters in other criteria are calculated using the regressed Mohr–Coulomb parameters. It is proposed that the best way to evaluate the parameters in a specific rock strength criterion is to perform direct regression of the laboratory test data using that criterion. Using this methodology, it is demonstrated that the effect of various rock strength criteria on sanding onset prediction is less dramatic than using the commonly used method. With this methodology, the uncertainties of the effect of rock strength criterion on sanding onset prediction are also reduced. Through this study, it is also demonstrated that a sanding onset prediction problem cannot be properly solved by adopting strength criteria that are not influenced by the intermediate principal stress if laboratory test data indicate rock failure is dependent on intermediate principal stress.