Deciphering the fundamental controls of flow in carbonates using numerical well-testing, production optimisation, and 3D high-resolution outcrop analogues for fractured carbonate reservoirs

Abstract

Carbonate reservoirs contain a significant portion of the world's proven hydrocarbon reserves but are challenging to produce due to their complex lithologies, structural heterogeneities, and neutral to oil-wet nature. Increasing recovery requires a better understanding of how different recovery processes respond to the heterogeneities inherent to these reservoirs. This will contribute to the design of appropriate engineering solutions which extend the life of mature fields and develop green fields more effectively. We use a high-resolution 3D outcrop model of a Jurassic carbonate ramp in order to perform a series of detailed and systematic flow simulations. The aim is to test the impact of small- and large-scale geological features on reservoir performance and oil recovery. The outcrop analogue model is of excellent quality comprising a wide range of diagenetic and structural features, including discontinuity surfaces, mud mounds, mollusc banks and fractures. Flow simulations are performed for numerical well-testing and secondary oil recovery. Numerical well-testing allows us to generate synthetic but systematic pressure responses for different geological features observed in the outcrops. This allows us to assess and rank the relative impact of specific geological features on reservoir performance. The outcome documents that, due to the high level of matrix heterogeneity, most diagenetic and structural features cannot be linked to a unique pressure signature. Instead, reservoir performance is controlled by sub-seismic faults and mollusc banks acting as thief zones. Numerical simulations of secondary recovery processes reveal strong channelling of fluid flow into high-permeability layers. This is the primary control for oil recovery. However, appropriate reservoir engineering solutions such as optimising well placement and injection fluid can reduce channelling and increase oil recovery.