Abstract

Drilling horizontal wells in shallow and poorly consolidated reservoirs in deepwater scenarios involves risky operations due to the narrow mud weight window. Risks include severe drilling fluid losses, wellbore instability, and fault reactivation, which, in the worst case, may connect reservoir to the sea floor.

This work presents a case study of risk reduction based on geomechanics, which includes concepts of fault reactivation while drilling, permitted plastified area around the wellbore, and fit-for-purpose data acquisition, which allowed a recalibration of the model and timely changes on the drilling plan.

The study started with a full 3D geomechanical characterization, which is an advanced way to determine stress distribution in a field, in particular, along the faults. Based on this study, it was possible to locate and avoid zones of higher risks of losses and fault reactivation, mitigating drilling risks.

From the study, it was also possible to identify the main uncertainties of the model, which allowed a fit-for-purpose data acquisition plan. The most important missing information was a calibration point for the minimal horizontal stress in the reservoir. Previous drilling experience in the area and geomechanics modelling were not conclusive about losses mechanisms, and the upper limits for horizontal drilling were also not clear. In addition, borehole instability had been shown to be an issue on offset wells; therefore, lower limits for drilling were unclear too.

It was decided to drill a pilot hole down to the reservoir, set a packer in the caprock and perform a series of minifrac tests. The measured minimal horizontal stress in the reservoir was revealed to be lower than initially expected, which implied the need to recalibrate the model and make important adjustments to the drilling plan. The model was recalibrated and the safe mud weight window was found to be even narrower. It was identified that a lower and unprecedented mud weight had to be used in the horizontal section, which was an additional risk. To evaluate this risk, the concept of permitted plastified area around the wellbore was used, and a lower mud weight was selected under a risk analysis manner.

Based on the study, drilling risks were mitigated and horizontal drilling was performed successfully, with minimal losses and controlled wellbore collapse.

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