Numerical simulation and optimisation of foam-based EOR in naturally fractured carbonate reservoirs

Abstract

Carbonate reservoirs hold more than half of the world’s remaining oil reserves and around 40% of the world’s remaining gas reserves. Producing hydrocarbons from carbonate reservoirs can be very challenging as carbonate reservoirs are often highly heterogeneous, fractured, and with a tendency to be mixed- to oilwet, all resulting in low recoveries from primary or secondary recovery schemes. Naturally occurring fractures are often the main flow paths in such systems, resulting in poor areal sweep, gravity override and viscous fingering. Foam injection is one way to mitigate such production problems by reducing the flow in the high-permeability fractures, diverting flow into the less permeable rock matrix, or by inducing a viscous pressure gradient to overcome gravity override, all of which increases the overall sweep efficiency. This thesis quantifies and compares the impact of uncertainties associated with the different foam models to the uncertainties inherent in the heterogeneity of a fractured carbonate reservoir by means of reservoir simulation in a reservoir analogue model for the Arab D formation. The results show that geological heterogeneity, not the uncertainty in the foam model itself, along with operational conditions such as injection rate and surfactant-to-gas ratios appear to be the most important uncertainties when implementing a foam flood. Higher fracture connectivity (higher fracture density) led to poorer foam flood performance in general, since the high permeability pathways allowed gas to streak through the reservoir and reduced mixing with the surfactant. In addition, the injection rate was observed to be a key parameter for optimising a foam flood where gravity override is already present in order to increase the mixing between gas and surfactant and subsequent generation of foam. The results also showed that foam is always more profitable than a waterflood for this fractured carbonate reservoir, indicating that the economic risk of a foam injection may indeed be lower than expected.