Crosswell Seismic: Step Change in Seismic Imaging - A Case Study from Indonesia
Authors: Handayani Tri , Irwanzah Zeppy , Taslim M , Kurniadi Dede , Dogra Sanjeev , Nalonnil Ajay , Marion Bruce , Bryans Brad , Reyes Pedro Carrillo 
Published: January 2012

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.