Characterization of pore pressure, fracture pressure, shear failure and its implications for drilling, wellbore stability and completion design - A case study from the Takouazet field, Illizi Basin, Algeria

This work presents the assessment of overburden stress, pore pressure and fracture pressure distribution in the Paleozoic stratigraphic units in the Takouazet field, eastern Illizi basin, Algeria. We studied a vertical exploration well, where significant hydrocarbon discoveries have been established in Devonian and Ordovician sandstone reservoirs. Overburden stress interpreted from density log has an average 1.07 PSI/ft (24204.04 Pa/m) gradient. Pore pressure derived from sonic log reveals that hydrostatic conditions prevail in Devonian and Ordovician units, whereas abnormal pore pressure with a maximum 0.66 PSI/ft (14929.60 Pa/m) gradient was recorded against the marine Silurian shale. Fracture pressure was modeled using effective stress ratio models as well as a Poisson's ratio based approach. Rock property based fracture pressure estimate is more sensitive to lithological variations and hence this estimate was used for determining safe drilling mud window and casing placements. The estimated pore pressure and fracture pressure were calibrated with direct downhole formation pressure measurements and leakoff test (LOT) measurements, respectively. Shear failure using Mohr- Coulomb rock failure criterion detected the minimum allowable mud pressure to avoid wellbore instability issues and the result was validated with caliper log observations. It indicated that solid production from the reservoir is very likely during initial production from the reservoir and a solid control mechanism has been suggested. Also widening of breakouts and deteriorating wellbore interpreted from time lapse formation imaging logs strongly suggest against open-hole completion. Based on interpreted pressure gradients and image log analyses, optimum drilling (mud and casing design) and completion policies were recommended for safe and successful operational planning.