Unveiling Reservoir Potential With World's First 360° Wireline Microresistivity Imaging in Oil-Based Mud for 12.25-Inch Borehole

Near-field exploration campaigns can prolong the operational life of existing field production facilities by uncovering additional hydrocarbon volumes. Typically, exploration wells target primary, well-understood reservoirs while also investigating secondary reservoirs with potential that have not been fully realized. Historically, secondary reservoirs have been overlooked because of the focus on primary targets, limited data acquisition to reduce costs, and incomplete geological knowledge. Wells drilled for primary reservoirs often produce insufficient data from shallower formations, leading to uncertain results. Budget constraints often influence decisions on data acquisition, such as the use of electrical logs and whole core acquisition. Although reducing initial costs may seem helpful, it can increase uncertainty about the reservoir's full potential. However, new technologies are helping to bridge these knowledge gaps. This case study focuses on a North Sea well that targeted a shallower, secondary reservoir alongside the primary reservoir, with particular emphasis on overcoming challenges related to incomplete data acquisition and subsurface imaging. The well, drilled with oil-based mud (OBM) was targeting both a secondary, underexplored reservoir and the main reservoir. Constraints related to formation pressure required multiple iterations of the well design, resulting in the two reservoirs being drilled with different hole sizes. Initially, consideration was given to core acquisition in the 12.25-inch hole, but uncertainty about the top of the secondary reservoir made it difficult to place the core optimally. A dedicated by-pass core was considered but not included in the final drilling program. To compensate for this lack of coring, advanced wireline logging was chosen instead. Standard electrical imaging tools provide high-resolution data, but limited wellbore coverage in a 12.25-inch hole (< 64%) posed challenges. Key geological features, such as stress-related structures (e.g., breakouts, induced fractures), or asymmetric features like climbing ripples, could have remained hidden in gaps left by the imaging tools. To address this, a dual-tool technique was employed, using an electrical imaging tool to capture a full 360-degree circumferential image of the borehole. Data was acquired at a 0.24-inch vertical and 0.13-inch horizontal resolution, allowing better visualization of reservoir features. Besides the tool's sensitivity to formation and mud-filtrate permittivity, a novel Hayman interpretation scheme was applied, enabling detailed interpretations of porous and permeable intervals. Combined with measurements like magnetic resonance or acoustic data, these high- resolution images contributed significantly to reservoir characterization. In combination with seismic data, the images will help to identify slumping and other architectural elements that might influence internal reservoir communication and assist in future reservoir development. Operational Insights, the dual-tool imaging configuration improved decision-making and aided optimal placement of formation pressure points and sidewall cores (SWCs). It also helped identify cemented beds, clay-rich gravity flow beds and nodules, which can lead to non-representative sampling results. Further, the enhanced wellbore coverage reduced the risk of missing stress induced fractures, improving operational outcomes. Geological and Reservoir Insights, the 360-degree imaging, provided detailed reservoir architecture, aiding the identification of sedimentological structures and mapping reservoir facies. It enabled precise extraction of paleocurrent azimuths and identification of erosional features, revealing depositional environments. It also distinguished between cemented beds and nodules, of which the former act as flow barriers, affecting vertical permeability. This enhanced imaging improved understanding of the reservoir's stress regime, aiding future well planning. SWCs can be placed in the correct geological setting during the post-well evaluation. The images provide critical insights into reservoir connectivity and geometry. In summary, advanced