Reservoir Characterization of Horizontal Wells in CD Carbonate Formation with Advanced LWD Technology

Abstract

Abstract The CD Carbonate of Ngimbang Formation in East Java basin was deposited in early Oligocene and divided into 5 reservoir targets for development. These reservoir targets were subdivided into different flow units, predominantly due to secondary porosity resulting from karst processes. Identifying these karst zones therefore became important for formation evaluation. Classic high resolution borehole images, combined with sonic, density and neutron logs, and NMR are the most commonly used tools to identify secondary porosity. However, due to the horizontal well design, conventional wireline logging was no longer possible thus Logging While Drilling (LWD) has become the primary data acquisition method. The LWD tool selection criteria have to meet both the geosteering and formation evaluation objectives by considering all drilling challenges. Even though all these measurements are currently available with LWD technology, the complexity of the long horizontal well trajectories and associated drilling risks have been assessed as being too high to run all the LWD services. Therefore, a different technique is required to identify karst zones and to determine saturation along the deeply-invaded karst intervals. A statistic-based correlation technique using principal component variance between advanced measurements from the multifunction LWD tool such as sigma, neutron capture spectroscopy, and standard measurements such as gamma ray, neutron porosity, density, caliper, and resistivity invasion profile is used to show an indication of karst- or matrix-dominated porosity. The spectroscopy provides accurate lithology volumes and information on grain properties. A simultaneous sigma–resistivity inversion technique is used to determine Archie's parameters in zones where the water salininty is known. The applicability and limitations of the technique are discussed for this particular reservoir environment. The combination of all these techniques along with information from state-of-the-art reservoir mapping technology has provided more complete characterization of the reservoir along the horizontal section.