An improved approach for quantifying the impact of geological uncertainty and modelling decisions on static and dynamic reservoir models - a case study from a giant fractured carbonate reservoir
Abstract
Carbonate reservoirs hold more than 60% of the world's oil and 40% of the world's gas
reserves. These reservoirs are characterised by geological heterogeneity, petrophysical
complexity, presence of naturally fractures, and mixed wettability, all of which contribute
to significant uncertainty in volumetric estimation and fluid flow behaviour within the
reservoir.
In this thesis, a comprehensive 3D multiple deterministic scenario workflow was applied
to compare and contrast how modelling decisions and geological uncertainties influence
this reservoir's volumetric estimates and flow behaviour. Specifically, the uncertainties
associated with the presence of fractures, the approach to reservoir rock type, and the
modelling of the initial hydrocarbon distribution were examined. This workflow was
applied to one of the giant complex carbonate reservoirs known to be fractured with a
thick transition zone in the Middle East. The most significant findings of this work
demonstrate that even minor changes in modelling decisions and reservoir rock typing
have a substantial effect on the saturation model, resulting in up to a 28% change in
STOIIP estimates, which may potentially mask the effect of other geological
uncertainties. These models were validated using repeated and randomised blind tests.
Such uncertainties must be carried forward in future reservoir management decisions and
when estimating reserves. Additionally, some of these reservoir models led to
significantly improved history match, especially for wells located in the transition zone
of the reservoir. The best history matches were obtained once sparse, fault-controlled
fractures were included in the reservoir model, using effective medium theory. The
presence of fractures specifically improved the history matching quality for wells located
close to the faults; these wells were very difficult to match in the past as fractures were
not considered by the operator.
This thesis demonstrates that a multi-deterministic scenario workflow is key to exploring
the appropriate range of geological uncertainties and that, equally importantly, the impact
of different modelling decisions (i.e., interpretation of the top structure, geostatistical
parameters, reservoir rock type approach, saturation approach, and presence of fracture)
must be accounted for when quantifying uncertainty during reservoir modelling. This is
particularly applicable to giant carbonate reservoirs, where relatively minor changes in
the workflow and data interpretation influence reserve estimates and subsequent history
matching and forecasts.