Case studies

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

SWCTT-Restrace-Tracer-bw

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.

Restrack-tracer-SWCTT

Illustration of the single-well chemical tracer test (SWCTT) principle. During a SWCTT, ester is injected into the formation. Parts of the ester react with water (hydrolyze) to form alcohol. During back-production (see illustration) the partitioning ester lags behind the alcohol and the time-difference is directly related to oil saturation in the formation.

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.

Prod1WebFig02

The time lag of the partitioning oil/water ester tracer is used to calculate oil saturation.

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.

Restrack-tracer-SWCTT-simulation

Simulation results for Ethyl acetate ester (EtAc) and Ethanol (EtOH) concentrations, obtained on an Eclipse simulation case using the SWCTT option in Restrack’s ARTSim tracer simulator. During back-production, the EtOH tracer moves faster towards the producer in the center than EtAc.

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