Maximising Value of Gas Tracer Surveillance via Simulation

Abstract Tracer surveillance is an important part of the integrated field surveillance programme. It becomes an increasingly important tool in evaluation of reservoir flow performance and reduction of geological uncertainties. Literature surveys show that gas tracer surveillance combined with subsequent simulation modelling are rarely studied in gasflood projects and more attention is required to maximize the value obtained from the data. This article presents results of modelling gas tracer behaviour in Azeri field, part of the supergiant Azeri-Chirag-Gunashli mega structure, and its contribution to surveillance interpretations and history matching. The main purpose of simulating the tracer surveillance in Azeri model was to help explain some ambiguous tracer results, to improve the static model, and also to confirm the behaviour of the crestal fault. The study confirmed that the initially sealing crestal fault became leaky in 2008. The static model was modified to match the observed tracer behaviour, which helped improve the overall history matching and reservoir management. Also, the study helped filter out anomalous tracer results as there are often cross contamination risks during sampling and analysis. Introduction The Azeri Field is part of the Azeri-Chirag-Gunashli (ACG) mega structure located in the South Caspian Sea, 100 km east of Baku. The trap, which forms the giant ACG Oil Field, is a NW-SE trending, steeply dipping thrusted anticline. Within structural closure there are a number of crestal faults oriented primarily along strike. The depletion mechanism is based mainly on crestal gas injection in the north supplemented by peripheral water injection in the south flank. Production commenced in 2005, and gas injection commenced in 2006. The Azeri Field has one of the biggest gas injection schemes (800+ mmscf/d, some wells injecting up to 300 mmscf/d) among the fields which are operated by BP and it is the first gasflood project in the Caspian Sea basin. There are 6 gas injector wells all located in the Central Azeri which inject gas into the initial gas cap.[1]. Gas Tracer application in oil field is systematically described by B. Zemel [2]. A wide review on usage of tracers in various fields across the world has extensively been discussed by Y. Du and L. Guan (2005). [3]. According to the literature survey conducted in this paper, gas injection tracer simulation was not widely used and applied in gas flood projects. Only in one case tracer simulation study has been carried out and applied in gas storage project in Snorre Field. [3,4]. There are only few papers published after 2005 which cover both tracer application in field development and its simulation [5–7]. The tracer program involves adding chemical tracer to gas injection wells and monitoring gas production for tracer breakthrough and its concentration. Tracer surveillance program in Azeri was implemented in 2007 to 1) track the gas flood fronts, 2) confirm path and spread of the gas to East and West Azeri, and 3) reduce reservoir and geological uncertainties. The sampling program in the offset producers was initiated 18 months after the first injection of the tracers. Field Observations Dynamic surveillance data, collected after initial production, showed that the extensive fault interpreted from the seismic data that separates the north and south flanks is originally sealing, and therefore the injected gas in the north wasn't expected to move to the south. The continuous gas injection in to the North Flank created pressure differential between the North and South flanks. The pressure data in 2008 showed a sudden increase in south flank pressure which has been interpreted as the sealing fault being broken. This interpretation was also supported by history matching.