FOPAK Forecasting of architecture, seismic characteristics and flow behaviour in paleokarst reservoirs

Paleokarst reservoirs originate from dissolution (karstification) of soluble rocks and subsequent infill and collapse. Fluids percolating through fractures and faults as well and infiltrating porous beds can dissolve carbonate rocks and create connected systems cavities. This process can occur deep in the subsurface and be driven by hydrothermal fluids, but is best known as a near-surface process, involving surface water dissolving rock and forming caves and sinkholes. The shape and geometry of these cave systems is linked to rock properties, the orientation and spatial distribution of fault and fracture networks, climatic conditions and sea level. During burial, cave systems may gradually be filled in by sediments from the outside or by rocks originating from collapse of the roof. This produces a highly heterogeneous subsurface reservoir with properties and production behaviour that are very difficult to forecast.

The FOPAK project aims to improve our fundamental understanding of paleokarst reservoirs by studying if and how their petrophysical properties, seismic characteristics and production behaviour can be linked back to the configuration of the initial cave system.

We employ a series of 3D models of mapped caves representing different geometric scenarios. Collapse, infill and collapse- related changes to the size and shape of caves can be modelled using established principles of rock mechanics and sedimentology. Through a technique known as “forward seismic modelling” using these models as input, it is possible to simulate what these reservoirs will look like on seismic surveys. This will allow us to establish what level of detail can be resolved in seismic data and provide a means for finding and describing seismic characteristics of the different scenarios. Finally, production behaviour of the models will be investigated using a fluid flow simulator to test the reservoir performance for a range of predefined production and injection strategies.