Application of Real-Time Mud Gas Logging to Identify Additional Pay Zone in the Randegan Field, Northwest Java Basin

Abstract

The C1-C8 mud gas service was deployed in a recent development well to identify additional pay zones of commercial interest in the Randegan field located in the Java Basin. Twenty-five previous wells drilled to exploit the primary reservoir in Cibulakan Atas (CBA) Formation, were characterized as light oil. There is one secondary target in the lower part of CBA Formation, and two secondary targets in the Batu Raja Formation (BRF). With oil production from the primary reservoir declining with time, any
additional pay zone is a productivity enhancement.

Real-time mud gas measurement and analysis were performed as a mitigation plan to delineate zones of interest, with a qualitative assessment of fluid variations in the reservoirs penetrated. A gas system with high-speed chromatography was deployed on this well for superior quality and accurate detection of compounds from Methane through Octane, Benzene, Toluene, Carbon dioxide and Nitrogen. Gas ratios calculated from the measured compounds provided indicators in real-time in a continuous manner. Further analysis was made using cross plots combining gas ratios and drilling parameters, and the result was validated by comparing with offset Jatiasri’s well production using the same mud gas system.

Comparison of all zones of interest in the primary target reservoir and potential zones of interest made by real-time gas ratios and interpretative cross plots are proposed for advanced geochemical insights. The result of the mud gas analysis facilitated the wireline log evaluation process by highlighting the main zones of interest and reducing uncertainties in the selection of intervals for testing. A new zone of interest that was bypassed and not tested in previous wells was found to be potential hydrocarbon-bearing based on mud gas data. This zone was later confirmed with DST samples. The method combined with multiple gas ratios has proven to be reliable in this petroleum setting would also be considered as standard in the upcoming development wells in the area.

Real-time mud gas analysis reduces uncertainty by facilitating downhole log evaluation process, selection of intervals for DSTs, and increases net pay for production.