Overcoming Appraisal Challenges in Thailand: Real-Time Geomechanics with MPD and LWD in Shaly Sand Reservoirs

Abstract

Exploration drilling in regions with limited data and significant geological uncertainty poses numerous challenges, including wellbore instability, fluid losses and well control issues. These difficulties arise from the high uncertainty in determining a safe mud weight (MW) drilling window. This study presents a case from the shaly sand reservoirs of Thailand, where the successful drilling of an appraisal well was achieved by enhancing real-time geomechanics through the integration of Managed Pressure Drilling (MPD) and Logging While Drilling (LWD) technologies.

During the planning phase, the safe MW drilling window was estimated using limited data from nearby offset wells, which included drilling records, pore pressure measurements, and stress tests. Observed drilling issues in these offset wells, such as kicks, stuck pipes, and hole instability, highlighted significant uncertainty in the safe MW estimates. The integration of MPD and LWD provided a dynamic method for real-time data acquisition and analysis, enabling rapid adjustments to the drilling parameters and reducing wellbore instability risks.

The appraisal well reached total depth (TD), successfully penetrating multiple sand layers while continuously updating pore pressure and the drilling window in real-time. A Dynamic Formation Integrity Test (DFIT) was conducted in the first sand below the shoe of the 6-1/8" hole section to establish the open hole drilling window. Maximum pore pressure was updated at the end of the 8-1/2" hole section before drilling into new formations in the 6-1/8" hole section. Static pore pressure tests (SPPT) were performed during drilling in the target depths of the 6-1/8" hole section to monitor pore pressure trends. The LWD annulus pressure sensor provided bottomhole pressure measurements for comparison with MPD calculations during the MPD pore pressure and leak-off tests.

Simultaneously, MPD's front-end technology for SPPT during drilling was employed to calibrate real-time pore pressure analysis and adjust surface back pressure (SBP) while drilling in permeable sand formations. Collapse pressure estimation in unstable shale formations was achieved by observing tight spots during pump-off events. After mitigating these tight spots, SBP was adjusted based on the updated safe MW window, allowing the well to reach the target TD without issues related to influx, wellbore stability, or fluid loss.

This paper demonstrates the effectiveness of the integrated MPD and LWD methodology for real-time monitoring of pore and fracture pressures, detecting changes in formation properties, and assessing wellbore stability. Importantly, it highlights the potential for risk mitigation and the reduction of non-productive time in appraisal drilling, particularly in regions characterized by limited data and geological uncertainties.