Crosswell Seismic Guided 3D Seismic Interpretation Results in Successful Infill Well Location in Bunyu Field (Indonesia)
Authors: Syarif Achmad , Irwanzah Zeppy , Handayani Tri , Dogra Sanjeev 
Published: September 2013

Abstract
Bunyu field in East Kalimantan, Indonesia is both a structural & stratigraphic play dominantly represented by discontinuous sand-shale formations. The field is on production since 1950 but production on a decline since 1976 reaching a minimum production of 1000 bopd in 2009. A surprise discovery well (X-18) in 2010 demonstrated significant future potential for this brownfield reservoir and hence a need for better understanding of reservoir. The existing seismic (2005) was of poor quality leading to uncertainty in reservoir mapping. Hence the objective was to provide a better sub-surface image to ultimately aid in defining new well locations. The use of crosswell seismic technology was chosen to minimize known attenuation effects produced by weathering layer, static corrections and the presence of shallow coal streaks in the area and provide hi-resolution data for successful infill drilling.
Data was acquired over 11 crosswell profiles to cover key portions of the Bunyu field in 2011. Two wells constitute a crosswell profile providing imaging between the wells. The inter-well distances (between 1000-1758m) achieved a world's first for the largest well separation in a commercial environment as well as in high attenuation formations. The crosswell data provided far superior seismic imaging revealing sub-seismic faults, new stratigraphic features which were not evident before. The available surface seismic (2005) was a time migration imaging and was of poor quality. Re-processing of surface seismic was undertaken that provided a little better imaging. Later, crosswell data was used for spectral equalization of surface seismic for integration. An integrated study incorporating crosswell seismic, re-processed surface seismic and logs provided a new structure for Bunyu field. Crosswell seismic profiles in the northern most part of the project area revealed a new fault which was not evident on surface seismic. This fault was tracked on the surface seismic which revealed a new fault block. In July 2011, Pertamina drilled a dry well X-28 (400m NW from producer well X-18) when the integrated study was on-going. The performance of this dry well (X-28) & other wells in the vicinity was well explained by the presence of this new fault delineated by crosswell seismic results. The presence of new fault necessitated moving the earlier planned well location (X-30) so that it fell within the new fault block.
In Aug 2012, Pertamina drilled the re-located well (X-30) in the new fault block. This well proved to be a success and confirmed the new interpretation guided by crosswell seismic. It shows hydrocarbon potential in four (4) layers. It produced 1655 bopd & 0.6 MMscfd of gas. The rest of the zones are still to be tested. The crosswell seismic thus reduced seismic uncertainty resulting in informed in-fill drilling. It has contributed significantly to millions of dollars of additional revenue and cost savings for Pertamina through production and avoiding drilling dry wells.