Restrack offers oil saturation measurements in near-well regions, using a Single Well Chemical Tracer Test. The method was used successfully to estimate oil saturation in the Heidrun field on the Norwegian Continental Shelf (NCS).
SWCTT operation in the Heidrun Field
The operator (Statoil) of the Heidrun field in the Norwegian Sea wanted to measure oil saturation in the near-well region to evaluate an EOR-method.
For safety reasons, the SWCTT operation was performed in a remote location on the platform with Arctic mid-winter weather conditions. Detailed planning in close collaboration with the operator was necessary to ensure a secure and effective operation in the challenging conditions. Frequent sampling and continuous on-site analysis secured reliable tracer curves that were afterwards interpreted to determine saturation values.
How the single-well chemical tracer test works
The method works by injecting a reacting partitioning tracer (ester) that produce a non-partitioning tracer (alcohol) in-situ. During back-production, the ester alternates between the moving water phase and the immobile oil phase and is slowed down, compared to the alcohol. This alternation (denoted partition coefficient) is fixed for a given oil/water system and is measured in the lab beforehand.
Oil saturation from production curves
Oil saturation in the test-zone is found from the time-lag between production of the ester and the alcohol in the producer (see figure below), and the partition coefficient. Oil saturation (Sor) is given by the expression: Sor = (t2-t1)/(t2+t1(K-1)), where t1 and t2 are the retention times of the non-partitioning and partitioning tracer, respectively, and K is the partition coefficient.
Planning & interpretation using Artsim tracer simulation
Simulation of SWCTT requires that the simulation tool can handle hydrolysis reactions, such as the reaction for ethyl acetate:
CH3COOCH2CH3 + H2O ↔ CH3CH2OH + CH3COOH
Previously this required complex reactive transport modeling tools to handle the hydrolysis reaction and could not be done with black-oil simulators. Restrack’s in-house ARTSim tracer simulator solves this challenge and makes SWCTT simulation possible using existing black-oil simulation cases.
To achieve this, ARTSim exploits the fundamental property that tracers do not affect the fluid transport (but are affected by it). ARTSim reads fluid simulation output from Eclipse and calculates tracer reaction and transport separately.
More case studies
A significant tracer program (with more than 50 individual injections) has been executed at Snorre. These tracer data were used in an integrated process to map reservoir flow that confirmed interpretations of water fronts indicated by 4D seismic dimming.» Read more
Water tracking information from Restrack’s inter-well tracer technology in a North Sea reservoir revealed the source of a massive water breakthrough at the producer.» Read more
Restrack is proud to introduce new tracers suitable for measuring inter-well oil saturation at reservoir conditions. We believe that with the new tracers, the partitioning inter-well tracer test (PITT) could become the standard method to identify EOR targets and to evaluate the performance of EOR operations.» Read more