Unearthing New Layers - Crosswell Seismic Redefines Seismic Imaging in Bunyu Field, Indonesia

Abstract

INTRODUCTIONTraditionally, measurements in oil & gas industry have inverse relation between formation vertical resolution and spatial coverage. The logs provide hi-resolution information but close to the wellbore while surface based measurements such as seismic & gravity provide field wide spatial coverage at coarser vertical resolution resulting in measurement gap at reservoir scale. Crosswell seismic bridges this measurement gap and provides enhanced imaging of the interwell space at reservoir scale (Figure 1).Crosswell seismic acquires two basic pieces of information; first, the direct arrivals between source and receivers are used to create a 2-D profile of seismic velocity (velocity tomogram), and second, reflection information from horizons above and below the source and receiver positions to create a structural image between the wells. The combined velocity and reflection imaging provides detailed information about the structure of a given reservoir as well as rock properties. Additionally, mode converted (S) processing will provide additional insight into the reservoir definition.The case study discussed here is from Bunyu field in East Kalimantan in Indonesia. It is a very difficult seismic area with high surface attenuation due to varying surface topography, unconsolidated weathering layer and presence of near surface coal seams. The formation is dominantly discontinuous sand-shale formations with intercalations of coal seams. Seismic interpretation becomes increasingly difficult as the surface seismic is not of high quality. Structurally, the acquisition area encompasses several major faults that create sub horst-graben features as shown in Figure 2. The map is primarily derived from well log information, since surface seismic data is of poor quality. Crosswell seismic was acquired over eleven (11) profiles, overlain on the structure map (Figure 2). The results of the profiles to be presented in this paper are highlighted in Figure 2 (right).The interwell spacing of these profiles ranged from 686 m to 1758 m, very challenging distances for the attenuative geological conditions. The profiles were chosen so as to maximize the coverage over the Bunyu field to help in refining the seismic interpretation and potentially to reduce the uncertainty in infill drilling decisions and field optimization. The seismic sections generated clearly show remarkable improvement over the existing seismic despite high seismic attenuation observed in this area. METHODOLOGYCrosswell seismic is an emerging technology that provides high resolution imaging of the subsurface at the reservoir scale and is used for delineating complex structure. The technique employs tomographic surveying, whereby a transmitter and receiver are deployed downhole in separate wells. As a result of the inherent source-receiver geometry, interwell velocity SUMMARY Surface seismic is fundamental information for reservoir mapping and in-fill drilling decisions. A good seismic is critical to successful development of a field. However, at times, the surf...