https://jrisetgeotam.lipi.go.id RISET Geologi dan Pertambangan Indonesian Journal of Geology and Mining 108 Research article Wireline logs vs. drilling events: Which one to believe in implying subsurface pressure? Riset Geologi dan Pertambangan Indonesian Journal of Geology and Mining Vol.31, No 2, pages 108–116 doi: 10.14203/risetgeotam2021.v31.1176 Keywords: Subsurface pressure, Wireline logs, Drilling events, South Sumatra Basin. Corresponding author: Agus Mochamad Ramdhan E-mail address: agusmr@gl.itb.ac.id Article history Received: 2 July 2021 Revised: 14 October 2021 Accepted: 25 October 2021 ©2021 Research Center for Geotechnology - Indonesian Institute of Sciences This is an open access article under the CC BY-NC-SA license (http://creativecommons.org/ licenses/by-nc-sa/4.0/). ABSTRACT As generally known, subsurface pressure can be implied using both wireline logs and drilling events. However, there may be a case where wireline logs and drilling events do not indicate the same subsurface pressure. Data from four vertical wells located in the South Sumatra Basin, Indonesia, were analyzed as a case study. Two wells, Wells A and D, encountered high overpressured zones, confirmed by drilling events and wireline logs data. The two others, Wells B and C, only encountered low overpressured zones, inferred by the relatively low mudweight used during the drilling. However, the wireline logs of Wells B and C show a reversal as Wells A and D. There are two hypotheses to explain the condition in Wells B and C. First, the wireline logs reversal is due to shallow carbonate cementation. Second, Wells B and C were drilled in an unintentional underbalanced condition. The method used includes XRD, SEM, and titration analysis. The results show that the first hypothesis is false, while the second is true. It may be due to some missing information related to drilling events in the final well reports of Wells B and C. INTRODUCTION Subsurface pressure anomaly, i.e., overpressure, can cause problems during the drilling. Lumpur Sidoarjo (LUSI) in the East Java Basin is the best example in Indonesia of how serious the problem can be. The subsurface pressure can be measured directly using pressure instruments such as Repeat Formation Tester (RFT) and Modular Formation Dynamic Tester (MDT) or indirectly based on wireline logs, mudweight, or drilling events like kick and gas while drilling. Deviated sonic and resistivity logs from the normal compaction trend in a certain depth interval below the surface commonly indicate an overpressured zone. The presence of drilling events, such as a significant increase of mudweight, kick, and increasing gas content, can also indicate the Agus Mochamad Ramdhan1, Arifn1, Rusmawan Suwarman2 1 Department of Geology and Department of Groundwater Engineering, Institut Teknologi Bandung, Bandung, Indonesia 2 Department of Meteorology and Department of Groundwater Engineering, Institut Teknologi Bandung, Bandung, Indonesia