Fill in The Gap Between Seismic and Borehole Imaging Scale With Highdefinition Karst Mapping for Improved Geosteering Strategy
Authors: Kusuma D.P. 
Published: March 2023

The CD carbonate of the Ngimbang formation in the X-Field, East Java Basin, has multiple reservoir targets with varying flow units due to secondary porosity from karst processes. Each of these reservoirs are thin, and the continuity of the karst within the reservoir is quite random. The amount of producible hydrocarbon will depend on the length of the drain section along the reservoir and the amount of karst intersected by the wellbore trajectory. Thus, maintaining the trajectory inside the carbonate reservoir while mapping and targeting the karst position is especially important for hydrocarbon productivity. Many horizontal and high-angle wells were drilled from the first offshore platform in this field as part of the phase one development plan. To maintain production in the field, a new offshore platform was placed south of the first platform, targeting new development areas in the field. Five horizontal wells were planned as part of the second development phase. The first horizontal well to be drilled in the second phase was the most challenging well because of the high uncertainty in the structural dips along the lateral length caused by the low-seismic resolution data and the limited number of nearby offset wells for control points. Moreover, the offset wells had inconsistent log properties, which complicates for correlating the well landing in the targeted karst level within the reservoir. Initially, a pilot hole was planned to reduce the landing uncertainties; however, drilling the pilot hole was being challenged for cost efficiency. As a result, a geosteering strategy was proposed instead, comprised of landing and geosteering using a new logging-while-drilling (LWD) combination of a new high-definition reservoir mapping technology, high-resolution laterolog borehole imaging technology, and multifunction LWD tool. This paper presents the complete preparation procedure of the geosteering project, strategy, and evaluation using this new LWD combination for improved reservoir interpretation. The deeper reading and higher resolution of the new reservoir mapping technology enabled continuous mapping of the target reservoir boundaries that reduced the structural uncertainties in the seismic measurements, and for the first time, revealed the karst architecture within the reservoir with greater details. This high-definition information helped to land the well precisely in the target karst sweet spot, improved the understanding of the karst characters along the trajectory, and provided greater confidence in the real-time geosteering decision. The high-resolution borehole image revealed the carbonate rock texture and karst and vugs appearance on a smaller scale, which was used to analyze the secondary porosity distribution and contribution along the wellbore trajectory. Integrating this additional information while drilling allowed for bridging the gap from seismic to reservoir scale, and finally linking the information to the high-resolution borehole image for improved geological understanding; therefore, improved geosteering strategy in the field.