Leveraging Look-Ahead Technology to Manage Subsurface Uncertainty in High-Impact Exploration Drilling

Abstract

Abstract The challenges of safe and cost-effective well delivery are often magnified in an exploration setting by increased subsurface uncertainty and reduced well control. Exploration of carbonates, in particular reef limestone plays, can further escalate these challenges. Penetrating such reservoirs with inappropriate mud weight can lead to extensive mud losses. Exploration well objectives often entail a comprehensive reservoir characterization program; thus, not only is casing placement decisive but also casing size such that the data gathering objectives can be met to ensure the asset development timeline and scope. The objective of this paper is to present how incorporating look-ahead technology in an exploration campaign overcame these challenges. The exploration wells in the Papua region were targeting a reef limestone in the Kais formation. The usual exploration practice has been to use log correlation, cutting samples, and drilling breaks but it often yielded to mud losses due to the lack of significant markers above the Kais formation. The exploration well objectives were comprised of a full interval coverage of openhole logging plus a well testing program for accurate reserve calculation. To meet the objectives required drilling a 12.25-in. hole section through the overlying shale and ideally setting the 9-5/8-in. casing ~3 to 5m TVD above the pinnacle carbonate reef; thereby, avoiding potential losses associated with penetrating the reservoir. Based on these challenges, logging-while-drilling technology with the capability to detect the reservoir of interest ahead of the bit was required. A real-time look-ahead technology, first introduced and applied for Pertamina exploration wells in the Sulawesi area, was chosen for this exploration drilling campaign. Real-time electromagnetic (EM) look-ahead technology, which uses a deep directional EM technique to detect changes in resistivity ahead of the bit, can detect the top Kais formation as early as 10 m ahead of the bit and differentiate between limestone stringers and the reservoir. While drilling, comprehensive integration of real-time look-ahead measurements, cutting samples, and seismic interpretation were the key factors for success. Real-time look-ahead measurements successfully identified the Kais formation 10-m TVD ahead of the bit, allowing the wellsite team to set the 9-5/8-in. casing optimally. This technology and associated novel workflows were used in the first two exploration wells in the Papua drilling campaign to set casing prior to entering the carbonate reservoir. In both wells, the casing was set at the ideal depth above the Kais formation. In the first well, the top Kais was detected 10 m deeper than the predicted depth. In the second well, the top Kais was detected 50 m shallower than the expected depth, showing the high level of uncertainty in this area. The close collaboration between Pertamina and SLB was key to dealing with this complex geological setting and meeting the subsurface and drilling targets.