Abstract

For gas condensate reservoirs with high condensate content, cyclic dry-gas reinjection is usually adopted to improve recovery. But cyclic gas reinjection gradually changes the composition and distribution of reservoir fluid, which causes much more difficulties for dynamic evaluation of this kind of reservoirs, especially for the reservoirs with strong bottom or edge aquifer support. This paper presents the dynamic evaluation on dry-gas reinjection in a gas condensate reservoir with strong aquifer support.

The method mainly involves pressure transient analysis and rate transient analysis (RTA). Firstly, pressure transient data are interpreted, and reservoir properties and its variation are evaluated. Then all wells are classified into different types based on well test log-log curve shape and its change. Furthermore, all producers are also analyzed based on RTA, and RTA type curve characteristics and diagnostics for different type of wells are also determined. Finally, results and understandings interpreted from surveillance data are analyzed and compared, from which reasons for well performance variation are determined.

This new method has been applied to Y gas condensate reservoir in China. The massive cyclic dry-gas injection of Y reservoir started in 2001, and current gas condensate recovery is higher than 40%. A total of 145 pressure build-up tests were conducted since then. Through detailed interpretation, reservoir properties and their variation behavior, such as permeability, skin factor, aquifer distance, etc. were evaluated. Based on analysis of the results, producers are classified into four types: performance no change type, condensate banking type, aquifer influx type, and injection gas breakthrough type. Furthermore, the 4 types of producers are also identified based on the type curve diagnostic of rate transient analysis. Through detailed comparison of results from the two methods, the reasons for the difference of different type of wells are analyzed and determined in combination with other production data and surveillance data. The understandings are used for cyclic gas injection adjustment, which improves the performance of Y gas condensate reservoir.

This systematic technique has been successfully applied to Y gas condensate reservoir in Tarim Basin of China, which properly and correctly evaluates the reservoir properties around both producers and injectors. It also provides a reliable method for decision making of cyclic gas injection adjustment, such as producer conversion, well infilling, etc., which is helpful for effective and economic development of this kind of reservoir.

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