Drilling a Deep-Water Well in a Subsalt Structure in Mexico
Summary
In the Gulf of Mexico, large oil and gas reservoirs are associated with sub-salt structures and are very attractive for potential hydrocarbon reservoirs. In 2009, Pemex pursued drilling a prospective location identified with 3D seismic located in a subsalt structure in the Tertiary formations. The Kabilil-1 well was located in 740 m water depth and the plan was to drill to the Lower Miocene and Upper Oligocene alluvial sediments. The well was drilled with the Ocean Voyager (3rd generation) in 94 days and reached a TD of 5350 m.
Although sub-salt drilling is a challenge in itself, it is not uncommon for operators in the deep-water Gulf of Mexico (GOM). The Pemex Kabilil-1 well was a deep-water subsalt first for Pemex in this challenging environment. The decision was taken to avoid the drilling risk through the salt and the trajectory was planned to go around the salt flank. This alternate strategy also posed challenges such as drilling through the sheared zone (rubble zone) that is commonly found underneath salt tongues or adjacent to salt diapirs where there is always a risk of getting stuck. In the well preparation phase, a finite-element numerical model was employed along with a 3DMEM simulator (Stonefish and Visage) to predict the effects of the initial in-situ stresses and induced increases in stress as a result of the salt intrusion. The induced stresses increase near the salt intrusion and result in subsalt overpressure, pressure reversion formations, and wellbore stability problems. The pre-drill model was updated in real time using the Stonefish-RT geomechanics real time software that has the ability to assimilate logging-while-drilling sensor data (sonic, resistivity, density, porosity, seismic while drilling) to allow calibrating the pore pressure and breakdown pressure against pre-drill model.
According to the pre-drill geological well prognosis, the salt proximity to the well was within 200m of the well trajectory, but reliable calibration was needed to verify top of salt and salt flank proximity in real time. In an effort to reduce the geological uncertainties, the SVWD-seismic (Seismic Vision While Drilling) while drilling was employed to look ahead of the bit to determine the salt distance to well in real time. This service was acquired for the first time in Mexico deep-water basins and was employed to mitigate the drilling risk associated with drilling near salt domes.
