De-Risking Al Bashair Overpressure in the Khazzan Field, Oman - Learning from First Well Control Event

Abstract The giant Khazzan gas field, onshore Oman has been on production since 2017 primarily from the Cambro-Ordovician paralic Barik Formation (avg porosity 6 pu). It covers an area of 2800 km2 and was originally interpreted to be a flat structure with a simple pore pressure and fracture gradient (PPFG). However, step-out Well A encountered the first well control event in Khazzan due to anomalous overpressure (1.56 SG, ~10100 psi) just below the Barik in the Al Bashair formation. The kick was successfully killed but with high NPT due to long killing operation which led to a differentially stuck pipe fish and ultimate side-track. The well control incident flagged limitations of offset wells and triggered a detailed investigation to derisk the Al Bashair overpressure given that all wells are required to TD in Al Bashair or deeper. An investigation was conducted to get to the root cause of the incident. Subsurface analysis was also carried out to identify the source of the overpressure in this well and predict potential overpressures likelihood in future wells in Block 61. The subsurface work involved fluid analysis (kick gas samples were acquired); mudlogging and log analysis, 2D pressure modelling, and geological and structural interpretation using a combined regional wells/seismic database from the Khazzan high to the Ghaba and Fahud salt basins (beyond Block 61). Understanding the kick controls enabled the creation of a Combined Common Risk Segment (CCRS) map for the Al Bashair overpressure. The investigation identified Well B, located outside of Block 61 and some 40 km from Well A that had encountered surprisingly identical overpressure in the Al Bashair. Both wells are located in a structurally complex area, that is next to a major bounding fault, the presence of structural closure and salt across the fault. Both wells also show unprecedented high CGR in the Al Bashair kick gas compared to the rest of Block 61 formation fluids, indicating a different source of kick gas. In this paper we investigate a list of possible overpressure scenarios: simple faulting, centroids, compaction disequilibrium, deep faults and reservoir cracking. In summary, the best-fit working model is that the Al Bashair isolated sands are oil charged through fault systems, and overpressure is created by volume expansion during oil-to-gas cracking. A CCRS map was finally generated to highlight areas of elevated risk to overpressure based on structural complexity. Important learnings came out from this first well control event in Block 61. The kick also required the successful first use of stress caging in Block 61. An integrated approach with a multidisciplinary team was undertaken: well control, well engineering, geomechanics, fluids and subsurface.