Multi-objective optimisation of Water-Alternating-Gas injection in fractured carbonate reservoirs : from theoretical considerations to application in a low permeability giant carbonate reservoir
Abstract
Water-Alternating-Gas (WAG) injection can be an efficient way to improve recovery factors in carbonate reservoirs. However, the complex geology of carbonate
reservoirs and the complexity of the WAG process itself create uncertainty when
predicting the efficiency of WAG. It is hence difficult to select engineering pa rameters such that recovery during WAG is optimised.
In this thesis, Polynomial Chaos Expansion (PCE) based proxy modelling is
coupled with the Non-Sorting Genetic Algorithm-II (NSGA-II) multi-objective
optimisation workflow to identify engineering parameters, i.e. well controls, that
enable us to optimise WAG injection in both, synthetic and real fractured carbonate reservoirs. The contribution from this thesis is the implementation, testing,
and application of an efficient framework to optimise WAG injection. In particular, this thesis aims to understand if a workflow that combines PCE-based proxy
modelling coupled with the NSGA-II algorithm can be applied effectively in
a real-world scenario. The application of this workflow becomes more difficult
as the physics of the recovery mechanism become more complex, the range of
engineering controls increases, and the reservoir geology becomes more complicated. This thesis therefore evaluates how the order of the PCE changes, how the
proxy models need to be enriched, and how the number of iterations need to be
adjusted in order to obtain reliable optimal WAG injection designs in different
geological reservoir scenarios. The geological scenarios range from a synthetic
analogue model for a Middle-Eastern fractured carbonate reservoir to a complex, giant, and faulted onshore carbonate reservoir in the UAE that comprises a
thick transition zone. Each geological scenario considers increasing levels of complexity and uncertainty with respect to reservoir characterisation. WAG designs
range from immiscible to pseudo-miscible (for the synthetic reservoir cases) to
fully miscible (for the real-field application).
As a key result, this thesis clearly illustrates the potential of integrating proxy
modelling and multi-objective optimisation with new reservoir characterisation
methods to identify WAG injection designs that have the potential to increase
reservoir performance in fractured carbonate reservoirs.