An Application of Chemical Tracers In Monitoring Injection In Vertical Hydrocarbon Miscible Floods
Authors: Carr L. , Wong F. , Nagel R. , McIntyre R. , Rosenegger L. 
Published: March 2010

Chemical tracers have been utilized to effectively monitor injected fluid movement in two vertical hydrocarbon miscible floods in the Rainbow Field, Alberta. By placing small slugs of chemical tracers, consisting of sulphur hexafluoride (SF6) and chlorinated fluorocarbons (CFC) or halocarbons within the injectants, the spreading and development of chase gas and solvent banks were monitored, and in some cases, reservoir flow channels identified. In addition, geological units were confirmed and well workover planning improved, resulting in increased cost effectiveness in operating the miscible floods.

The chemical tracer program commenced with the first field application conducted in the Rainbow Keg River "A" (R.K.R.A.) Pool secondary miscible flood on January 31, 1991, where chase gas injection into two wells was traced.

Subsequently a program of tracing chase gas and solvent was initiated at one well location in November, 1991 in the Rainbow 1 Keg River "B" (R.K.R.B.) Pool tertiary miscible flood, and later at six additional chase gas and solvent co-injection wells. This paper presents the design of the tracer programs, the significance to the operation of the miscible oil recovery schemes and alternative tracers considered for the future.

Introduction

Husky Oil operates seven mature secondary and three tertiary miscible flood enhanced oil recovery projects in the Rainbow Field of northwest Alberta (Figure 1). Cost effective operation of these floods requires that solvent and chase gas sweep a significant extent of the reservoir without being produced at high rates. If miscible solvent bypasses large portions of the reservoir due to channeling, recovery efficiency will be lower than expected. Maintaining cost effective operation of hydrocarbon miscible flooding has required the development of extensive tracer programs designed to track and optimize solvent distribution in the reservoir as well as detect breakthrough at the very early stages. Previously, Husky had used radioactive tracers to monitor solvent bank placement in the R.K.R.B. Pool(1). Following the successful conclusion of this program and others in Rainbow Lake, other tracers were researched with the expectation that more of the analysis could be conducted on site and with reduced cost. More recently, Husky has used several Freon" haIocarbons and SF6 as chemical tracers to monitor chase gas and solvent injection in both the R.K.R.A and B Pools.

FIGURE 1: Field location map. Illustrations available in full paper.

Tracing injected chase gas and miscible solvent is used to identify the distribution of these fluids, both regionally and vertically, in the miscible floods. This enabled identification of the source of early solvent and chase gas breakthrough due to reservoir channeling, and allowed for early remedial action.

Tracers less expensive than radioactive tracers, both from an initial cost and a per sample analysis cost basis were sought. They had to be detectable and accurately measurable at extremely low concentrations in the field lab and be readily handled by the field operations group. Finally, they could not be native to the oil reservoirs or have adverse rock or fluid adsorption or partitioning characteristics.