TX 75083-3836, U.S.A., fax 01-972-952-9435.
AbstractMany wellbore instability problems are being reported during development drilling at an offshore field in Saudi Arabia. It is a sandstone reservoir with shale intercalations at a depth of about 6000 ft. A few wells had to be abandoned because of stuck pipe and hole pack-off problems. Quite a number of wells had to be sidetracked, and drilled using oil-based mud (OBM).The aim of this study is to identify the causes of the instability problems encountered in this field. The well plan, daily drilling report, well completion report, and various logs of each well are carefully studied. The frequency of instability problems, length reamed and the mud weight used are being analyzed with respect to wellbore azimuth, inclination and TVD. A computer program is developed using linear elasticity behavior of the rock material and the Mohr-Coulomb failure criterion. The model predicts the lower and the upper bounds of the mud weight that will prevent the borehole from compressive and tensile failure, respectively. Thus, we generate the stable mud weight window using rock mechanical and in-situ stress data pertaining to the problem wells where instabilities have occurred. Then we compare the mud weight used during drilling to the predicted mud weight window. Besides the Mohr-Coulomb failure criterion, we have used three modes of the Drucker-Prager criteria in order to compare the results.An approach that combines statistical analysis and constitutive modeling to identify the causes of instability and predict safe mud weight window, is being used for the first time to the knowledge of authors. Some interesting results of the study indicating the possible causes of the wellbore instability are presented and discussed.