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Modeling Pore pressure, Fracture pressure and Shear Failure gradients in the Takouazet Field, Illizi basin, Algeria: Implications for wellbore stability, drilling and completion design

Rafik Baouche

This work presents the assessment of overburden stress, pore pressure and fracture pressure distribution in the Paleozoic stratigraphy in Takouazet field, eastern Illizi basin, Algeria. We studied a vertical exploratory well, where significant hydrocarbon discoveries have been established from Devonian and Ordovician sandstone reservoirs. Overburden stress, interpreted from density log has an average 1.07 PSI/ft gradient. Pore pressure from sonic log reveals hydrostatic condition (0.43 PSI/ft) in Devonian and Ordovician units, whereas abnormal pore pressure with a maximum 0.66 PSI/ft gradient has been recorded against marine Silurian shale. Fracture pressure has been modeled from both effective stress ratio models as well as Poisson’s ratio based approach. Rock property based fracture pressure estimate is more sensitive to lithological variations and hence used for determining safe drilling mud window. Estimated pore pressure and fracture pressure have been calibrated with direct downhole formation pressure measurements and leak-off test (LOT) respectively. Shear failure gradient using Mohr-Coulomb rock failure criterion detected the minimum allowable mud pressure to avoid wellbore instabilities and the result has been validated with caliper log observations. Based on interpreted pressure gradients and image log analyses, optimum drilling (mud and casing design) and completion policies have been recommended for safe and successful operational planning.