Enhanced Aquifer Pump-Off Strategy for a Steam Flooding Development, A West Field, Sultanate of Oman

摘要

One of the key challenges of the "A" West reservoir thermal development is the presence of an active bottom aquifer. If unmanaged, high aquifer pressures relative to the oil reservoir would have a detrimental effect on the ongoing thermal Enhanced Oil Recovery (EOR) process. Consequently, the "A" West development strategy includes an Aquifer Pump Off (APO) system. The key APO management objectives are: 1. Lower the reservoir pressure and hence: a. Improve injectivity. b. Improve heat efficiency (more latent heat). c. Prevent quenching of the injected steam. 2. Secure the feed water for the steam generation. In order to meet the above objectives, the following methods were used to evaluate the targeted pump off rate: 1. Material Balance analysis. 2. History matched full field model. 3. History matched regional aquifer model, whereby several aquifer pump-off scenarios have been explored to further deplete the aquifer pressure in "A" West. A high APO capacity is initially required for an accelerated aquifer pressure depletion to match the currently low reservoir pressure caused by historical cold production of the upper half of the reservoir. This initial APO capacity is only required until the target aquifer pressure is achieved, after which, aquifer pump off rate will be gradually reduced to avoid oil cusping into the aquifer. To monitor oil cusping, water samples are collected from APO wells and facilities and analyzed for oil contamination. Moreover, three observation wells were drilled for real time reservoir/aquifer pressure gradient monitoring. These observation wells will also ensure an optimum pressure differential between the reservoir and the aquifer. The produced APO water is partly used as feed water for steam generation, with the remainder relocated to a shallow aquifer via a separate relocation system. To ensure oil free water, APO wells are located 100m below the OWC. In order to avoid well integrity issues and heat losses through the produced water, well trajectories have been designed to evade penetrating the steamed formation. An additional opportunity was realized by re-routing a portion of the excess water to a northern water-flooding project to maintain its reservoir pressure. This paper will focus on the integrated APO strategy, which meets the reservoir management objectives of the "A" West steam flooding project, and the PDO water management strategy.