This paper discusses the applications of gas and water tracers in the Central Fault Block (CFB) of the Snorre Field in the North Sea. An extensive tracer program was initiated in 1993 to improve the understanding of the flow dynamics in the field. The CFB of Snorre has been produced by WAG injection since 1994. The tracer program has involved 3 injectors and 4 producers. Tracers applied were perfluorinated hydrocarbons (PFCs) and SF6 as gas tracers. As water tracers tritiated water (HTO), chemical SCN- and radioactive S14CN- have been applied in addition to the new tracer 4-fluoro benzoic acid (4-FBA). Results from this integrated tracer study have improved the understanding of fluid flow and WAG injection efficiency in the reservoir. The tracer program will be continued and will also be expanded to other fault blocks.

It is well known in the petroleum industry that tracer data can provide valuable information on reservoir characteristics and fluid flow performance. However, a prerequisite for obtaining reliable information is careful planning, design, and field implementation. The application of gas tracer technology for improving the reservoir description and optimizing the RKF Field miscible gas injection project was presented earlier. The current paper compliments the earlier work by discussing issues related to design and field implementation of the tracer program, including definition of the objectives, determination of tracer types and amounts, and evaluation of sampling and detection techniques. In this paper we document the lessons learned and propose best practices based on our experience with tracers in the RKF Field, Algeria. A critical element is integration of the field operator and Research Centre personnel with the subsurface team during all phases of the tracer program. This integration has been very important in obtaining good quality data in the RKF project. We also review common mistakes and bad practices that could occur in the absence of a robust tracer design program. The importance of analytical calculations, simulation, sampling frequency and the recycling of tracers are also among the design topics that are addressed. The proposed best practices can be applied by field operators in the design of future tracer projects and can also be used to identify the causes of tracer monitoring problems in existing projects. We hope that this paper will be of use to those managing or planning tracer projects. Introduction The Rhourde El Khrouf (RKF) Field is located in Block 406A of the Berkine Basin, 300-km southeast of Hassi Messaoud1. The field has been producing under partial pressure maintenance by miscible gas injection from the TAGI (Trias Argilo-Gréseux Inférieur) since 1996. Deeper volatile oil and retrograde gas condensate reservoirs provide makeup gas for the project. Tracer technology has been successfully applied in RKF. It is demonstrated that reliable tracer data can be obtained through careful planning, design, implementation, and monitoring. A key element of success is integration of the project team to include the disciplines of reservoir engineering, geosciences, field operations, and tracer specialists. This paper will discuss the integrated workflow and lessons learned, and will propose best practices based on the successful gas tracer project implemented in RKF field2. Background Tracer technology has been used for hydrocarbon reservoir characterization for more than 50 years3. Tracers can render information that is almost impossible to obtain with other methods, such as identifying flow paths, breakthrough times from injector to producers and estimations of the interwell oil saturation. Tracer technology is also used in the subsurface in single-well applications for mass balance calculations and saturation measurements. At the surface, tracers have been used in separator efficiency tests, and in transport lines and refinery surveys4,5. Tracer can provide very reliable information on fluid flow characteristics in secondary or tertiary recovery processes. Tracers reflect the reservoir dynamics and can be injected any time at the start of the injection or later to obtain the fluid flow paths and velocities in the reservoir. The type of tracer information acquired depends on the objectives for the application. Some objectives could include:detecting and documenting breakthrough times;mapping flow paths and performing analytical calculations; andmore advanced analyses using reservoir simulation to improve reservoir description and optimize reservoir management. Recent developments in tracer technology, especially in the areas of sampling and laboratory analysis techniques, have made it easier and less expensive to undertake a tracer project. This does not mean that it is not a complex procedure. On the contrary, based on our experience, a successful tracer project implies special technical, cost, and environmental considerations.

No abstract

An interwell gas tracer test provides very useful information not only to improve reservoir description but also to characterize flow dynamics. Approaches to tracer application can range from conventional qualitative interpretation to quantitative analysis including numerical simulation. Our aim is to optimize miscible gas injection efficiency by correct calculation of gas injection allocation based on mass balance of the injected tracer (how much of an injected tracer is produced in the various production wells). Since perfluorocarbon gas tracers are partitioning tracers, and the capillary tube sampling technique only allows measurement of tracer concentration in the gas phase, knowledge of the partitioning coefficient (Kd) of the gas tracers at sampling conditions (wellhead pressure and temperature) is required. We successfully developed and implemented experimental design, equipment set up and procedures to measure a set of consistent Kd values for the oil and the gas mix for all commercial available gas tracers. Subsequently, we integrated the Kd values with tracer and production data and estimated the correct recovery of the gas tracers. The next step was to estimate the gas injection allocation to optimize gas injection patterns and to provide input for improving the simulation history match. Our approach is a further step in quantitative analysis of tracer data and can be applied to provide valuable information to optimize gas injection and oil production.

No abstract