High-Resolution LWD Image Data Acquisition Improves Subsurface Insight in Mature Field of Limestone-Claystone-Basement Formation Sequence, A Case Study from the Sunda Basin, Indonesia

Abstract

The main challenge in offshore Southeast Sumatra, as one of Indonesia's mature fields, is to increase production in existing assets. The approach taken by the operator is through side-track drilling from historical wells, planned as a series of infill development wells, whilst evaluating new potential plays. This case study examines the logging-while-drilling (LWD) data acquisition in the Krisna-XX well in the Krisna field, which was designed to drain the remaining reserves from the Lower Baturaja limestone formation in the Sunda basin and maximize undrained area oil potential. Based on simulation analysis, the estimated reserves in the Lower Baturaja were 581 MBO with an initial rate of 426 BOPD. Additionally, the completion strategy required a rat hole to be drilled into the basement, which elsewhere in the basin can be hydrocarbon bearing. The first two wells drilled in the Krisna Field experienced severe mud losses while drilling, which had caused significant invisible non-productive time (NPT). The losses were associated with potential natural fracture swarms in the limestones and basement formations, but the fracture apertures were below the resolution of the LWD density image tool used in those wells. Hence, to refine evaluation of the fractures, an azimuthally focused resistivity tool was utilized in the third well (Krisna-XX), providing omni-directional laterolog resistivity and high-resolution resistivity images. The high-resolution resistivity tool was run in combination with quad-combo LWD tools to drill and log the high-angle (>70° inclination) 8.5-in. reservoir section. While drilling both the reservoir and basement intervals, this well also experienced high dynamic loss rates of water-based drilling fluids, which were treated with a lost circulation material, minimizing impact on the reservoir productivity. The well was drilled to total depth (TD) without incident. The aim was to utilize borehole imaging memory data acquired from this section as a benchmark for examining the current state of the Sunda Basin's formation. Post-run analysis of the memory image data yielded excellent quality high-definition images over the interval of the carbonate reservoir, coal, claystone, and basement sequences, improving knowledge of the Sunda Basin's detailed geological structure. This result helped the operator to understand the reservoir facies characterization of the Lower Baturaja formation, while overcoming potential fluid-loss conditions of up to 250-300 bbl/hr. The images clearly showed sedimentary structures, natural fracture networks, drilling induced fractures and vugs. Subsequently, by comparing against actual production (which has reached up to 1,842 BOPD), analysis of weak zones will improve reservoir insight to be incorporated into future subsurface models. In addition, high-resolution images combined with pressure-while-drilling data complements drilling operations analysis. Correlation to the intervals of fluid loss has allowed the operator to improve time and cost efficiency of the drilling operation and forthcoming planning.