Impact of heterogeneity on flow in fluvial-deltaic reservoirs : implications for the giant ACG field, South Caspian Basin

• Main Author: Choi, Kevin
• Published: Imperial College London 2008
• Subjects: 622
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.487298

The Azeri, Chirag and Gunashli (ACG) oilfield is located in the offshore Azerbaijan sector of the south Caspian Basin. This dissertation focuses on the Azeri Field which has over 8 billion barrels of oil in place. The major reservoir interval is the Pliocene Pereriv Suite, which is characterized by laterally continuous layers of variable net-to-gross (NTG) deposited in a fluvial-deltaic environment. The Azeri Field is being developed by both down-dip water injection and up-dip gas injection. This dissertation uses high-resolution models, derived from outcrop analogue and subsurface data, in conjunction with experimental design techniques, to rank the impact of different geological heterogeneities on recovery by both displacement mechanisms. Firstly, the impact of reservoir-unit scale heterogeneities on water and gas injection is assessed using a simplified fluids PVT description. At this level, the principal controls on oil recovery are the factors that affect sandbody connectivity and sweep efficiency. Secondly, the impact of reservoir- and genetic-unit scale heterogeneities on gas injection in high NTG intervals is assessed. The genetic-unit scale heterogeneities that control the approach to vertical equilibrium (VE) are found to be the principal controls on recovery. Their impact is large when gravity forces dominate the mobility unstable displacement, but decreases with increasing production rate as viscous forces dominate and the displacement moves out of VE. The presence of laterally extensive shales is also important, because they change the geometry of the reservoir layering and yields higher recovery than equivalent homogeneous models. The same rank order of key heterogeneities is obtained for simulations with simplified and a more realistic PVT description. Finally, the results are applied to develop a methodology to identifY the upper and lower boundaries on predicted recovery using a simple analytic approach that incorporates some geological heterogeneity.