Impact of Sedimentologic Heterogeneity on Gas-oil and Water-oil Displacements in the Fluvial-Deltaic Pereriv Suite Reservoir of the ACG Oilfield, South Caspian Basin

Kevin Choi¹, Matthew Jackson¹, Gary Hampson¹, Alistair Jones², and Anthony Reynolds^2
1Imperial College London, London, United Kingdom
²BP, London, United Kingdom

The ACG oilfield is located in the offshore Azerbaijan sector of the south Caspian Basin. It comprises the Azeri, Chirag and Gunashli accumulations, which cap an elongate anticline. We focus on the Azeri accumulation, which is being developed by down-dip water injection, with up-dip gas injection on the more steeply dipping central north flank. The major reservoir interval within Azeri, the Pliocene Pereriv Suite, is characterized by laterally continuous layers of variable net-to-gross (NTG) deposited in a fluvial-deltaic environment.
We have investigated the impact of large- to small-scale heterogeneity on recovery from the Pereriv Suite reservoir, using high-resolution models derived from outcrop-analog and subsurface data. We find that the principal controls on oil recovery are (i) local variations in NTG within low NTG (<50%) reservoir layers, and (ii) the degree of communication between low NTG layers and adjacent high NTG (>85%) layers. Heterogeneity within high NTG reservoir layers has only a small impact on recovery; the principal controls are channel-fill sandbody stacking and orientation, the presence of mud-clast lags at channel bases, and the vertical-to-horizontal permeability ratio of channel-fill facies. Heterogeneity within low NTG layers is much more significant, and recovery is primarily controlled by channel-fill sandbody stacking and sinuosity. These findings are also relevant to the overlying low NTG Balakhany reservoir in the Azeri Field.
In general, the same significant heterogeneities impact on water and gas displacements, because both are stable at the planned production rates; the water displacement is mobility stable, while the gas displacement is gravity stable.

AAPG Search and Discovery Article #90072 © 2007 AAPG and AAPG European Region Conference, Athens, Greece