Re-Evaluating Contributions from Thin Bedded Reservoirs: Integrated Reservoir Modelling of the Greater Dolphin Area

Abstract

The Dolphin Field has been producing gas since 1996, however predicting in place volumes, reserves and forecasting production has been a challenge since field inception. The fact that in place estimates have increased significantly since development sanction highlights that a range of geophysical, geological and petrophysical uncertainties are associated with the field. Historically, static volumes have been smaller than dynamic volumes estimated from material balance. The explanation of this difference traditionally related to uncertainty in contact depth (given the minimal data on contacts), that adversely caused poor predictions of water production in the historical models. Many of the reservoir units within the Greater Dolphin Area (GDA) are characterised by a heterolithic deltaic succession of centimeter scale very-fine sandstone, siltstone and mudstone. Given the thin-bedded nature of the reservoir, conventional wireline-logging tools lack the resolution to accurately resolve many of the static parameters including water saturation. However, based on the available PLT data, it is believed that these thin-bedded intervals generally contribute to the production from the wells and hence to the fluid flow in the reservoir. A new static and dynamic reservoir model of the GDA has been built that integrates and incorporates new seismic interpretation, petrophysical recharacterization, revised geological and reservoir engineering concepts, and eventually history matching to production data. A key component of this new model build has been integrated modelling iterations amongst different disciplines from new petrophysical interpretations through to dynamic simulation. Initial iterations used a conventional formation evaluation method and resulted in simulations that showed accelerated pressure drops (compared to production data) as a result of failure to capture flow from thin-beded intervals. An alternative petrophysical methodology that aims to better estimate water saturation within thin bedded intervals has been incorporated into a new workflow to account for the thin bed volumes. The new thin bed simulation model results in greater gas contributions from the thin-bedded intervals and helps overcome the historical shortage of static volumes required to achieve a pressure match.