Integration of High-Definition Karst Mapping and Quantitative Porosity Analysis from Borehole Image to Improve Well Placement in Complex Carbonate Reservoir of East Java Basin
Authors: Azwar Olivia , Permanasari Dian , Ardikani Nadia , Adani Nabilah , Kusuma Didit , Abrar Budi , Nordin Taufik , Widianto Tonny Fadjar , Priyanka Elvara , Roosa Figa , Soenarwi Isra Febrianto 
Published: October 2022

The CD Carbonate in X-Field, East Java Basin has multiple reservoir targets with varying flow units, predominantly because of secondary porosity development from karst processes. Each of these reservoirs are relatively thin and the continuity of the karst within the reservoir is random. The amount of producible hydrocarbon will rest on the length of the drain section along the reservoir and the amount of karst intersected by the trajectory. Thus, maintaining the trajectory inside the carbonate reservoir while targeting the karst position is important for productivity.

To maintain the production in the field, a new platform was built to the southeast of the first platform, aiming for a new development area. Five horizontal wells were planned as part of the second development phase. The first horizontal well is the most challenging one because of the high uncertainty of the structural dips along the lateral length triggered by the low seismic resolution and the limited nearby offset wells for control points. Moreover, the offset wells show inconsistent log properties that complicate the correlation to land into the targeted karst level within the reservoir. Initially, a pilot hole was planned to reduce the landing uncertainties; however, it was being challenged for cost efficiency. Therefore, a well placement strategy was proposed instead, including landing and geosteering using a new logging-while-drilling (LWD) combination of advanced high-definition reservoir mapping technology, high-resolution laterolog borehole imaging technology, and a multifunction LWD tool.

In this paper we demonstrate the complete preparation of the well placement project, strategy, and evaluation using this new LWD combination for better interpretation of the reservoir. The deeper reading and higher resolution of the new reservoir mapping technology have permitted continuous mapping of the target reservoir, which typically has 35- to 50-ft thickness, to reduce the structural uncertainties from seismic. For the first time, it successfully revealed the karst network within the reservoir with greater details, identified by a blue-vein color spectrum of the resistivity inversion caused by seawater invasion or clay-filled karst. This high-definition karst mapping has helped to land the well precisely at the target karst sweet spot, improving the understanding of the karst characters along the trajectory, and providing higher confidence in the real-time geosteering decision. The high-resolution borehole image revealed the carbonate rock texture and karst/vugs appearance on a smaller scale, which was used to analyze the secondary porosity distribution and contribution along the trajectory using a quantitative image-based porosity analysis method.

By integrating the high-definition reservoir mapping inversion interpretation and porosity analysis from a high-resolution borehole image, we were able to bridge the gap from seismic to reservoir scale, and finally to link the karst scale down to the vug pore sizes, for a better geological understanding and an improved geosteering strategy in the field.