Integrated Approach for Understanding Overpressure Mechanism and Dynamic Pore Pressure Calibration Using Managed Pressure Drilling (MPD)

Abstract As more exploration prospects move to deeper, high temperature high pressured reservoirs (HPHT), accurate pore pressure prediction is crucial for well planning. In such challenging environment, understanding pore pressure generating mechanism and pore pressure prediction plays a key part, particularly when no analogues are available for calibration purposes. This paper presents methodology and best practices to attune the existing pore pressure prediction methods using dynamic pore pressure data derived from Managed Pressure Drilling (MPD). Presented methodologies includes maintaining desired bottom hole pressure (BHP) by surface Back Pressures (SBP). This was achieved by conducting dynamic flow checks and adjusting the dynamic mud weights (MWs) based on the response of flow behavior in trip tank. The bounds of the pore pressure are further tuned based on the response from gas behavior and drilling Equivalent circulating density (ECD)- Equivalent static density (ESD) contrasts. The dynamic formation integrity tests were also conducted successfully to estimate the upper limit of the drilling operating window. The petrophysical logs and pressure data from MPD were used in understanding the overpressure generation. The conventional pore pressure estimation methods helped in identifying the depth of overpressure depth but failed to constrain the magnitude and rate of increase of pore pressure which was close to ~17.2 ppg. Hence to re-calibrate the model, pore pressure in shales were inferred from dynamic pressure tests using MPD. This was done by regular flow checks followed by monitoring the gain/return in flow and gas trends. Accordingly, the required back pressure (~a maximum of 600-700 psi) was applied in steps from the surface to balance the well plus keeping the safety margin. The pore pressure ramped up below the hydrostatic pressure depth to more than 17 ppg within 200-300 m of depth interval. The dynamic FIT was also performed with MPD in steps with increasing back pressures to define the maximum allowable mud weights and ECD. This enables dynamic adjustments to the drilling settings in response to dynamic pore pressure measurements and dynamic formation integrity data. The pore pressure derived from MPD was used in velocity-effective stress cross plot to understand the overpressure mechanism. The analysis verified the depth of unloading surface in the overpressured zone which is consistent with 120°C temperature threshold. Proper application of these methodologies and procedures helped in HPHT well drilling plan with minimal offset geological and petrophysics information. MPD allows reasonable accuracy of the pore pressure prediction without compromising well integrity and operational safety.