Abstract

The ACG Oilfield caps an elongate anticline with three culminations - Azeri, Chirag and Gunashli - and is located in the offshore Azerbaijan sector of the south Caspian Basin. This study focuses on Azeri in the south-east of the structure, which has over 8 billion barrels of oil in place. The major reservoir interval, the Pliocene Pereriv Suite, is characterized by laterally continuous layers of variable net-to-gross (NTG) deposited in a fluvial-deltaic environment. Azeri is being developed by down-dip water injection, with up-dip gas injection on the more steeply dipping central north flank. At the planned offtake rates both recovery mechanisms are expected to be stable. However, these predictions are based on reservoir models which do not explicitly capture the full range of geologic heterogeneity present in the Pereriv Suite reservoirs.

We report the first detailed assessment of the impact of large- and intermediate-scale heterogeneities on flow. Experimental design techniques have been used to rank the impact of different heterogeneities. A key finding is that communication between adjacent high and low NTG reservoir layers significantly improves recovery, providing pressure support and a route for oil production from sandbodies within the low NTG layers which would otherwise be isolated. Heterogeneity within high NTG layers has only a small impact on recovery, but heterogeneity within low NTG layers is much more significant. In most cases, the same significant heterogeneities impact both water and gas displacements, because both displacements are stable at the planned production rates.

The results are applicable to Azeri, and to similar reservoirs in the Caspian Basin. They also represent the first comparison of water-oil and gas-oil displacements in fluvial-deltaic reservoirs using 3D geologic/simulation models derived from outcrop and subsurface data.

Introduction

The giant Azeri-Chirag-Gunashli (ACG) Field occurs in a large, elongate anticlinal structure located in the offshore Azerbaijan sector of the south Caspian Basin (Fig. 1A). The structure has steeply dipping limbs and contains three culminations (Azeri, Chirag and Gunashli). This paper focuses on the Azeri accumulation in the south-east of the ACG structure, which contains an estimated 8 billion barrels of oil in place. The ACG field development project is one of the largest current energy projects (US$20 billion aggregate) in the world. The Azerbaijan International Operating Company, operated by BP, has been awarded a 30 year production license for ACG which expires in 2025, and plans to bring total production in ACG to 1 million bbls oil/day by 2008.

The oil fields of the South Caspian Basin (Fig. 1A) have reservoirs in the thick (up to 7000 m) latest Miocene to early Pliocene strata of the Productive Series (Fig. 2A). These strata record multiple, high-frequency cycles of deltaic shoreline advance and retreat in response to fluctuating lake levels in the isolated South Caspian Basin 1–4 (Fig. 1B). The resulting Productive Series stratigraphy is strongly layered (Fig. 2A), and sandstone-bearing intervals are bounded by laterally extensive mudstones 1–4. Productive Series sandstones in the northern part of the South Caspian Basin (including the ACG Field) are quartz-rich, well rounded and well sorted, indicating deposition by the paleo-Volga River and Delta 5 (Fig. 1B).

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