Optimising gas injection in carbonate reservoirs using high-resolution outcrop analogue models

Outcrop analogue studies for complex subsurface reservoirs have become increasingly common because they allow us to integrate reservoir characterisation with reservoir simulation. This enables us to correlate distinct geological features that can be observed in the outcrop but are normally upscaled in dynamic models, to complex flow phenomena present in real reservoirs. Hence we can construct static models that are better calibrated because they contain the key geological structures controlling the flow behaviour and translate them into dynamic models that are upscaled properly. In this study we use a high resolution simulation model of a middle Jurassic carbonate ramp outcrop from the High Atlas Mountains of Morocco, which can be regarded as an analogue for the Arab D formation, to investigate fluid flow processes during enhanced oil recovery (EOR). The outcrop analogue model contains a wide range of sedimentological and structural geological features, including patch reefs, mollusc banks, mud mounds and fractures. Our work aims to improve our understanding of the flow dynamics occurring during secondary and tertiary gas injection in complex carbonate reservoirs. We simulated gas injection as both, miscible and immiscible, where miscibility is determined by the minimum miscibility pressure (MMP) estimated from correlations. We compare continuous gas injection and water alternating gas (WAG) injection and decipher how they are influenced by matrix and fracture heterogeneities. The results show up to 7% incremental recovery by gas injection compared to secondary recovery due to the contact of un-swept zones and improved hydrocarbon displacement. We show how gas channelling along high permeability layers is mitigated by WAG injection and that detailed representation of small- and large-scale geological features such as fractures and high permeability streaks, leads to improved prediction of hydrocarbon recovery. Using this understanding, we optimise recovery by ensuring effective gas utilization, injection strategy and miscibility conditions.