Management of injected nitrogen into a gas condensate reservoir

Abstract

This study investigates the means of deferring the breakthrough of injected N2and alleviating the impact of such on production rates and specifications as well as minimizing the required changes to the gas processing facilities. This aimed at assisting the ongoing efforts to transfer the Cantarell experience to Abu Dhabi, where large amounts of N2gas will be generated and injected into a large gas condensate reservoir to partially substitute the recycling of lean gas. This will bring forward the opportunity to exploit lean gas by securing base load supplies before the start of reservoir blowdown, compared to the peak shaving approach currently practiced. Managing N2breakthrough starts by better understanding the pattern at which N2injection spreads into the gas accumulation. Based on the findings of initial subsurface and plant simulations carried out in 2008, N2breakthrough in Abu Dhabi might be possibly deferred by segmenting the reservoir into a rich N2region and lean N2region. The approach assumes no thief zones will be faced and no channeling of N2injected between the two regions is taking place. N2is injected in the north region of the reservoir. The production of that region will be segregated and fed to a gas processing plant of lower NGL (natural gas liquid) recovery, which essentially takes longer time to start suffering the deterioration of residue gas (gas mixture resulted after separating NGL) quality. The residue gas use can be limited to re-injection where the effect of below specification LHV (Low Heat Value) would not be an issue. The rest of the reservoir feeds another gas processing plant of higher NGL recovery level from which an amount of residue gas equivalent to that of the injected N2will be rerouted to the sales network. This scenario will significantly delay as well as downsize the requirement of a N2rejection plant. There is technical and certainly economical advantage of deferring the installation of costly N2rejection units. Such a requirement can be entirely eliminated if the sales gas specification can be relaxed considering blending with other gas streams of higher LHV, and in collaboration with gas customers, i.e. assessing their capability to tolerate feedstock of lower specifications. It must be noted that such school of thinking may not necessarily be eventually embraced. The chosen scenario will also depend on the final configuration, i.e., wells grouping and gas gathering, of the ongoing project.