Karst Identification and Impact on Development Plan
Authors: Bourdon Laurent , Coca Spencer , Alessio Laurent 
Published: December 2007

Karstification (carbonate rock dissolution during sub aerial exposure), although frequent in modern analogues is often underestimated and not properly modeled in carbonate petroleum reservoirs. Poorly understood in cores and borehole image logs, too often the presence of rock property enhancing Karst networks is rather attributed to elusive high permeability faults or fractures.

The F6 field, located in the Central Luconia carbonate province (offshore Sarawak, Malaysia) is the largest gas field within the Sarawak Shell portfolio. The reservoir is an elongated carbonate build-up of Miocene age characterised by steep flanks, a generally flat top and a central pinnacle. So far, the field was modelled as a layered carbonate reservoir with laterally homogeneous properties. Gas is contained in three major zones, namely the highly porous Zone1, which forms the central pinnacle, the tighter Zone2, which extends over the central half of the build-up, and the good quality Zone3, which extends over the whole build-up. Commissioned in 1987, the field is now maturing, with over half the reserves produced.

A 3D seismic survey was acquired in mid 2002 to support an infill drilling campaign and a potential field re-development. Initial coherency seismic attributes used for the structural interpretation suggested an extensive dendritic Karst network within Zone3. Also initial 3D dynamic simulation suggested the need for enhanced properties in Zone3 and possibly higher volumes to match the combination of contact rise and pressure data in the field.

Karst networks are often poorly expressed features on 3D seismic due to their inherent complex shapes and lateral variability. In order to image the Karst, special post-processing attributes based on coherency, phase or frequency algorithms have been tested. None of these attributes would give a full image of the Karst (mostly due to the rapid lateral variation in thickness). Therefore, these attributes were then combined using a multi-attributes volume interpretation workflow in order to build a realistic Karst model. The model was then tested against drilling operational data (mud losses). The multi-attributes interpretation allowed for the merger of different Karst "signatures" while the 3D volume based approach allowed for the spatial definition of complex geometries. Karst network geometries were subsequently incorporated into a deterministic reservoir model and risked properties could be tested and further refined against 15 years of production history in an iterative dynamic simulation workflow.

This detailed reservoir architecture mapping based on 3D seismic had considerable impacts on the F6 field projects: Large (open) Karst features, combined with a depleted reservoir pressure, could create major drilling problems. Therefore, strategies had to be defined by the subsurface and drilling team to mitigate the risk of Karst-induced major losses while drilling the planned development wells.Although not fully sampled by wells, the Karst network effective properties could be roughly derived from seismic then refined during dynamic modelling by history matching of multiple scenarios. This work is still on going and will be used to screen re-development opportunities to safeguard and increase reserves.