Defining the Mud Weight Window While Drilling Through Depleted and Overpressured Reservoirs in a HPHT Well

Abstract The formation fracture gradient (FG) in a high pressure, high temperature (HPHT) well that has also undergone depletion can be markedly different to a well that is hydrostatically pressured. The consequence is a narrow mud weight (drilling) window, and this can pose a significant challenge for such wells. In certain circumstances, the FG (i.e., the mud pressure at which losses occur) is not the same as the minimum horizontal stress (Shmin). Therefore, instead of Shmin being the upper bound of the mud weight window, it is the FG, which in turn could be the fracture initiation pressure (FIP) or could be the pressure required to induce losses into natural fractures. The FIP is a function of wellbore trajectory in contrast to Shmin which is independent of wellbore trajectory. The magnitude of Shmin in the offset wells was calibrated using quality-checked leak-off tests (LOTs) conducted at casing shoes and from dynamic leak off tests (LOTs) under managed pressure drilling (MPD). A depth continuous Shmin profile was calculated using an effective stress ratio fitted to the calibration points. To conduct the investigation into the effects of depletion on the magnitude of the FG, a fieldwide geomechanical model was constructed using geological and geophysical data, petrophysical logs, drilling data and the results of laboratory rock mechanical tests on core samples. The resultant geomechanical model shows that planned wells in the field are subject to extremely narrow mud weight windows if Shmin is considered as the FG. This is because high mud weights are required to support overpressured shale in the deeper section and at the same time the magnitude of Shmin is reduced significantly in reservoir sandstones due to production induced depletion. Given these expected challenges, the geomechanical model was used to calculate the FIP as an alternative estimate of the FG. The variation of the FIP with borehole trajectory was also investigated for depleted conditions. This approach was substantiated with offset well drilling experiences, particularly mud loss events. Modelling showed that losses due to higher equivalent circulating densities (ECDs) in the depleted sandstone reservoirs were consistent with the depths of lower FIP rather than the magnitude of Shmin. Therefore, it is possible that elevated ECDs triggered losses due to the creation of fractures in these depleted sandstones. It is noted that in the undepleted shale sections losses were not observed even when the ECD exceeded Shmin suggesting that the FG is largely controlled by the FIP. If natural fractures exist in the reservoir, drilling mud may be lost into these fractures. Fluid conductivity can be enhanced by shear slip with a bottomhole pressure (BHP) less than both Shmin and the FIP. A Mohr-Coulomb sliding criterion was used to identify the population of natural fractures in the reservoir most susceptible to shear slip. Results indicate that inclined fractures may become critically stressed during drilling (prone to slip) and may lead to fluid losses with the BHP less than the FIP. However, if any natural fractures are present in the virgin pressured shale sections, these fractures should remain stable with an ECD close to the FIP. The results of this study led to revisions of the planned well trajectories to take advantage of higher FIP, relative to Shmin, to reduce the risk of fluid losses into natural fractures. The modelling and the modifications to the well designs enabled the operator to successfully drill through the challenging HPHT narrow mud weight window setting.