Crosswell seismic provides high resolution imaging of the subsurface at the reservoir scale. The technology is used for delineating complex structure and monitoring the effectiveness of hydrocarbon recovery using time-lapse techniques. The technique employs tomographic surveying, whereby a transmitter and receiver are deployed downhole in separate wells providing interwell velocity profiles and reflection seismic from direct wave and reflection processing respectively. In the Bunyu Field in Indonesia, the surface seismic quality is degraded due to a highly attenuative weathered layer, difficult surface access issues and the presence of coal layers. In order to optimize the field development and boost recovery, enhanced seismic is seen as a fundamental requirement to make informed in-fill drilling decisions.Bunyu Field in East Kalimantan is primarily a discontinuous sand-shale sequence with shallow coal layers. The existing 3D seismic is of poor quality due to varying topography, the highly attenuative weathering layer and shallow coal layers making reservoir interpretation very uncertain. The field has been producing since 1950 with production declining over the years. The field development plan got a major boost last year (2010) when Pertamina were pleasantly surprised with the drilling of a 5000 bbls/day well. Another well drilled nearby (300 m) did not encounter this reservoir but had hydrocarbon shows from another zone, highlighting the complexity of the problem and uncertainty in reservoir understanding. Pertamina thus needed targeted in-fill drilling to optimize the field recovery which was difficult using the current surface seismic. A crosswell seismic solution was proposed for informed in-fill drilling and field optimization.A total of eleven (11) crosswell seismic profiles were acquired at Bunyu, with the objective to provide a better seismic image of the reservoir, dominantly represented by sand-shale formations, with the final aim of identifying new in-fill locations. The reward will be to locate bypassed pay areas such as those tapped by the recently drilled 5000 bbls / day well, thus providing significant future potential for this brownfield reservoir. Crosswell seismic technology was chosen to avoid the issues discussed above.In this paper, some of the key results will be discussed with the focus on enhanced resolution. Initially, a brief description of the measurement theory and its capabilities is provided, followed by a description of the processing workflow and finally a discussion of the acquired results. Two main products were obtained from each profile: tomographic velocities and reflection images for both compressional (P) and Shear (S) arrivals.Tomographic velocity images showed high correlation with sonic velocities extracted from acoustic logs available in the area. The crosswell seismic reflection profiles show improved seismic imaging in comparison to the surface seismic previously acquired in the area.
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...
Much has written about seismic interpretation and characterisation of the subsurface and until recently, this topic has focused on the traditional techniques such as surface seismic and vertical seismic profiling (VSP). One critical limitation in combining these with well based measurements is the lack of understanding as to what happens to the seismic signal both away from the well and at a finer resolution (reservoir space) than the surface based methods. Crosswell seismic helps to bridge the gap in both directions; log property mapping and upscaling these to field wide scale and seismic property mapping or downscaling surface seismic properties for well correlation. By nature of the measurement, crosswell provides higher resolution images of the interwell region that augments the seismic characterisation of the subsurface. Presented in this paper, is an example of crosswell seismic imaging from a mature onshore field in Indonesia and the current work related to integating the various multi scale data sets. The extension to seismic attribute analysis and correlation with known interpretation in the field is also discussed. Introduction Traditional well log data provide high-resolution measurements of the formation very close to the wellbore. Surface-based methods provide greater spatial coverage but coarser resolution. Crosswell seismic provides images of the interwell space at reservoir scale, hence bridging the measurement gap between the existing scales (Figure 1). The Bunyu Field in East Kalimantan is primarily a discontinuous sand-shale sequence with shallow coal layers. The existing 3D seismic is of poor quality due to varying topography, the highly attenuative weathering layer and shallow coal layers making reservoir interpretation very uncertain. The field has been producing since 1950 with production declining over the years. The field development plan got a major boost last year (2010) when Pertamina were pleasantly surprised with the drilling of a 5000 bbls/day well. Another well drilled nearby (300 m) did not encounter this reservoir but had hydrocarbon shows from another zone, highlighting the complexity of the problem and uncertainty in reservoir understanding. Pertamina thus needed targeted in-fill drilling to optimize the field recovery which was difficult using the current surface seismic. A crosswell seismic solution was proposed for informed in-fill drilling and field optimization. A total of eleven (11) crosswell seismic profiles were acquired at Bunyu (Figure 2), with the objective to provide a better seismic image of the reservoir, dominantly represented by sand-shale formations, with the final aim of identifying new in-fill locations. The reward will be to locate bypassed pay areas such as those tapped by the recently drilled 5000 bbls / day well, thus providing significant future potential for this brownfield reservoir. Crosswell seismic technology was chosen to avoid the issues discussed above (Handayani et al. 2011).
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