In development of mature oil fields using enhanced oil recovery (EOR) techniques, one of the challenges is to quantify remaining oil and to evaluate the potential gain of EOR in pilot studies. One of the proven technologies to estimate remaining amounts of oil is the single-well chemical tracer test (SWCTT). During such push-and-pull tests, oil/water partitioning ester partially hydrolyses to a non-partitioning water tracer. A time-lag in back-production time between the injected ester and the alcohol generated in-situ yields remaining oil saturation (ROS) through a simple relation. A similar time-lag technique is used in partitioning inter-well tracer tests (PITTs), where tracers are injected into injectors and sampled in producers. New and stable, oil-water partitioning tracers suitable for oil reservoir PITTs have been recently developed and field tested (SPE164059), allowing measurement of remaining oil saturation in inter-well regions.
This paper reviews methodology to assess oil saturation in both near-well and inter-well regions of an oil reservoir, and highlights the differences and similarities of partitioning near-well and inter-well tracer tests that can be used to evaluate the potential gain from an EOR-operation.
SWCTTs and PITTs target different scales of an oil field, as a SWCTT explores the near well zone, up to a few meters, and a PITT explore an inter-well region. In order to assess information on both these scales we propose a systematic procedure for oil saturation measurement, using partitioning tracers. The procedure involves use of PITTs and systematic residence time distribution (RTD) interpretation of tracer production curves to extract information about remaining oil saturation and the distribution of this saturation in an oil field. The procedure is validated using the tracer data recently reported by Viig et al. (SPE164059).