Exploring the Interwell Reservoir Space with Crosswell Imaging and Interpretation: An Example of Multi Scale Data Integration from Indonesia
Authors: Handayani Tri , Irwanzah Zeppy , Taslim M. , Kurniadi Dede , Dogra Sanjeev , Nalonnil Ajay , Carillo Pedro 
Published: May 2016

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).