Optimizing Miscible Injectant (MI) Composition for Gas Injection Projects

Abstract

Abstract Many gas injection projects can flexibly design injectant gas compositions through sources of gas and enriching fluids. In such instances, optimization of the injectant composition for project efficiency becomes important and necessary. Based on experiences from BP gas injection projects, we present a procedure to optimize miscible injectant (MI) compositions for field projects. This procedure emphasizes an integrated interpretation and screening from fluid PVT, facility processes, prediction of minimum miscibility conditions to 1–D and field scale compositional model results. To demonstrate the factors that affect design of an optimal MI, we presented two field processes, which have contrasting mass-transfer mechanisms: one a vaporizing-dominated mechanism and the other condensing-vaporizing. For a vaporizing-dominated displacement process, slim-tube MMP is not sensitive to MI compositions. However, different MI compositions, having the same MMP (minimum miscibility pressure), do not necessarily have the same field-scale recovery efficiency. Analysis and understanding of the role of each MI component in the slim-tube mass transfer process gives important insight to optimize the MI composition. Enriching MI above the slim-tube MME (minimum miscibility enrichment) or MMP brings about additional EOR oil in a field process. The key mechanism for the above to occur is through further reduction of residual oil in the MI swept reservoir portions. The additional oil recovery is significant; yet the benefit must be evaluated with respect to MI supply and its utilization. MI compositions can affect surface oil rate through different mass-transfer mechanisms occurring at facility separations. Such an effect is particularly notable for volatile and light oil systems. The optimal MI must be assessed in terms of incremental EOR oil recovery, net MI efficiency and MI cost. Considerations from field processes, e.g. facility constraints and effects of reservoir processes, may therefore alter the optimal MI derived from 1-D slim-tube processes. Introduction Enhanced oil recovery (EOR) through gas injection has been in practice in the Industry for nearly half a century, probably since the pioneering works and patents by Whorton and Kieschnick1,2 during early to mid 1950s. Since then, there have been significant advances in technology in this area. The technical progress has made many field gas EOR projects successful in technical and economic terms. In recent years, there are important advances in fluid thermodynamics, calculations of minimum miscibility conditions, reservoir characterization and reservoir simulations in general. All of the above are relevant to evaluations of field gas EOR projects. There does not seem to exist in the literature a work that combines the above new technical advances and presents an integrated investigation of MI composition versus EOR efficiencies. A good knowledge on how to design an MI for gas injection project is critical from technical and economic points of view. This paper re-examines the gas EOR processes with emphases on obtaining and interpreting results from laboratory, one- dimension slim-tube, field-scale modeling and facility separation to develop criteria for optimizing the MI (miscible injectant) compositions for field projects. The work has two main objectives:Understand the important mechanisms that govern field-scale gas injection processes; andDevelop guidelines to design MI compositions for field gas injection projects.