Single-Well Chemical Tracer Test Experience in the Gulf of Guinea to Determine Remaining Oil Saturation

Abstract

Abstract The Single Well Tracer Test (SWTT) method has proved to be a reliable technique to measure Remaining Oil saturations (ROS) consistently. The technique is based on the chromatographic separation of two tracers, a tracer that partitions into the oil and that hydrolyses in the reservoir, to generate the second tracer, which remains exclusively in the water. Both tracers are produced through the same well in an interval of a few days and the ROS is determined by means of the difference in their arrival times at the wellhead. The SWTT technique was first implemented within Total in 2007 in the Handil field, Indonesia, and was recently employed by the group for ROS determination in an offshore field located in the Gulf of Guinea in West Africa. This paper focuses on the results of the Single-Well Chemical Tracer Test (SWTT) carried out for this offshore field and how the information was used to better assess the reservoir's current ROS. Total's SWTT interpretation methodology allowed the group to distinguish the presence of moderate to low ROS values in the swept layers of the investigated interval, while there was a higher uncertainty in ROS in non perforated layers. This paper presents the SWTT results obtained in this West African field together with the detailed laboratory campaign that was carried out to complement field SWTT information. The paper also details the SWTT interpretation carried out by Total and the learned lessons obtained from this offshore SWTT application. Introduction Knowledge of Remaining Oil Saturations (ROS) in mature oilfields is one of the key parameters that help assess reservoir evaluation and eventual reredevelopment schemes. In this domain, the Single-Well Tracer Test (SWTT) has been reliably been used since 1971 (Ref. 1) to provide this information. The technique is based on the chromatographic separation of two tracers, a tracer that partitions into the oil and that hydrolyses in the reservoir, to generate the second tracer, which remains exclusively in the water. Both tracers are produced through the same well in an interval of a few days and the ROS is determined by means of the difference in their arrival times at the wellhead (ß) as shown in Figure 1. Single Well Tracer Test (SWTT) design and interpretation relies on two fundamental constants that need to be determined experimentally: the hydrolysis rate constant of the tracer (kH) and the partition coefficient (Kd). The hydrolysis rate constant of the partitioning tracer needs to be ascertained before the field operation, to ensure that a sufficient amount of tracer will be injected and transformed in the reservoir to obtain a good quality SWTT signal. It also allows fine tuning of the chemical analysis technique to guarantee that measurements will be done with enough precision during the flowback period of the test. The partition coefficient of the tracer (Kd) is a thermodynamical parameter that is directly linked to the determination of the ROS. It has therefore to be measured before the operation at reservoir conditions and using live oil and production water to verify that the chosen tracer will have enough sensitivity to measure the remaining oil level that is expected for the candidate reservoir.