Design, Implementation and Interpretation of an Interwell Waterflood Tracer Project in a Stacked Naturally Fractured Reservoir in the Sultanate of Oman: A Best Practice Case Study

Abstract The paper discusses the conceptualization, design and implementation of an interwell waterflood passive chemical tracer study in stacked naturally fractured carbonate reservoirs, in a Field in the Sultanate of Oman. The reservoir structure is an almost undisturbed directionally trending anticline, intercalated by several fault zones at the southern flank, running parallel to the structural axis. The shaly intercalations intercepting the chalky reservoir add another layer of complexity to the fractured reservoir system. During the 1960s, the field was first put on fracture spurt production which then rapidly declined by the 1970s. Thereafter, a peripheral water injection program was introduced followed by a gas oil gravity drainage to stabilize production rates. To manage the uncertainties arising out of the complex geology and develop the field in the best techno-economic fashion, it was imperative to gain understanding of the fluid flow dynamics operating within the reservoir that was dominated by the complex fracture-matrix system. Over decades, chemical tracers have been a proven and reliable source to gather such information. Over the past years, several studies using interwell passive water tracers were conducted in the field, covering various areas of interest. The design of the tracer study in a fractured system is more complex than designing the same for simple hydrodynamic applications. Complexities including preferential fluid influx through layered reservoirs, competing fracture matrix movement and relatively faster breakthroughs amongst other things required to be considered. Connectivity between overlaying reservoirs was suspected to affect flow dynamics but not confirmed in water flooded areas. In this case, another layer of complexity was added due to re-injection of produced water that required meticulous understanding of the pseudo breakthroughs. The paper discusses the challenges in design, implementation, interpretation and simulation when carrying out a passive chemical tracer study in a fractured reservoir and the best practices adopted to counter them. The case study also discusses immediate oil gain, the qualitative and quantitative results obtained from two large scale tracer studies involving fourteen injectors and 36 producers and six injectors and nineteen producers carried out in recent years and explains the results in light of mean residence time, swept volume, sweep efficiency and heterogeneity indices. Additionally, the study also discusses numerical tracer simulation modelling. To date, available literature only explores the concept of complex tracer studies in fractured reservoirs through numerical simulation models. To the best of the authors' available information, this study is pioneering in the provision of best practice guidelines when designing such a study with respect to volume assessment, operational challenges, design of customized sampling and analysis plans, qualitative and quantitative interpretation of the data obtained as well as modelling aspects.