A Multidisciplinary Approach in Understanding the Challenges in Drilling Infill Extended-Reach Wells in the Depleted Stag Field, Northwest Shelf, Australia—A Case Study

Abstract The Stag oil field is a shallow, highly unconsolidated, highly permeable sandstone reservoir that was discovered in 1993. Currently, the Stag Field is depleted, with 51 wells drilled in the field. While drilling two extended reach drilling (ERD) wells in 2022, the Stag 51H experienced two incidents of severe total mud loss that impacted the efficiency of the drilling operation. In 2025, potentially one or two more ERD wells are planned to be drilled. One of the ERD wells may use casing while drilling (CWD) technology based on the experience from previous experiences in the field Therefore, it is important to understand the reasons for these total losses and construct a robust, calibrated geomechanics model so that steps to mitigate drilling risks could be taken when drilling the planned wells. A detailed geomechanics study was undertaken to understand the reasons for these total mud-loss drilling events. A predrill, 1D mechanical earth model (1D-MEM) was constructed in three offset wells incorporating the leakoff test (LOT), rock test data, petrophysical logs, and drilling experience. This predrill 1D-MEM was propagated to the actual trajectory of the recently drilled horizontal wells to understand the drilling experience, especially the mud losses in Stag-51H. There were some very insightful inferences from the geomodeling work that provided possible reasons for the total losses. We found that the change in stress magnitudes due to depletion, does reduce the fracture gradient (FG). The equivalent circulating density (ECD) did go above the modelled FG but was still below the fracture propagation pressure (FGrow) estimate (Ito et al. 2001). The stability of the faults present in the legacy operator static model were analyzed for possible reason for losses. We found that the depletion significantly decreases the ECD limit at which these faults, if intersected, could become critically stressed, causing total mud losses. If the two wells did intersect faults, then these critically stressed faults in depleted conditions could explain the total mud losses. However, two predepletion horizontal wells, drilled in the same orientation as the recently drilled wells, did not experience total losses. Why would this be? Could the geomechanics model constructed explain the reasons? In this paper we explain our findings. There is an increasing shift in drilling infill wells in severely depleted formations. An integrated geomechanics approach was adopted to analyze these mud-loss events when drilling through depleted formations. The learnings from this case study could form a template for other operators to predict these incidents and take mitigation steps when drilling into depleted reservoirs.