Pre-Drill Geomechanical Modelling and Wellbore Stability Analysis for Successful Drilling in a Highly Overpressured Shale Zone and Potential Losses Carbonate Zone - A Case Study from West Tuban Block, East Java, Indonesia

Abstract

Abstract A high pressure well with a high risk of encountering a loss zone is a challenging drilling operation and requires a proper pre-drill study incorporating drilling practices, an understanding of the subsurface, and geomechanical modelling. This paper presents a case from the West Tuban Block. This block is within the East Java Basin, one of the prolific basins in Indonesia and a proven hydrocarbon reserve. The major challenge is drilling through the shale overpressure zone and the carbonate loss zone safely and successfully. This paper describes the building of a pre-drill geomechanical model, the subsequent wellbore stability analysis and how the results and recommendations were applied during drilling. The geomechanical model was built using data from seven offset wells and also incorporated regional knowledge. The offset well review of the drilling experiences showed that the key challenges that will need attention are significant drilling problems such as pack-off and tight hole while POOH, RIH with casing and while wireline logging. Added to this are instances of losses and gains while drilling the carbonate reservoir section. The geomechanical modelling process involves using petrophysical, geological, geophysical and drilling data to constrain the magnitudes of the overburden, pore pressure, rock mechanical properties and the two horizontal principal stresses. It is also important to constrain the azimuth of the maximum horizontal stress. The final model is verified using stress-related drilling problems and observations of wellbore failure in shale interpreted from caliper data. The resultant geomechanical model suggests that the planned well is associated with a strike-slip stress regime. The magnitudes and azimuths of the stresses play an important role in successful drilling because the well is directional in the reservoir section with a maximum inclination ∼26°. The wellbore stability analysis was used to optimise mud weights for each hole section of the planned well. Recommendations were also made regarding additional data gathering (cores for conducting rock tests, extended leak-off tests, full suite of logs, etc.) to reduce uncertainty in the geomechanical model. The mud weight and drilling practice recommendations, that were an outcome of the study, were followed meticulously by the drilling team so the well was able to safely overcome the overpressure zone and the loss zone. The effort contributed to the operator's success in the onshore development campaign with timely execution within budget and no reported HSE events.