Handil is a mature oil and gas field discovered in 1974 and developed since 1975. The field extends on a 40km2 area and includes hundreds of stacked hydrocarbon accumulations.

Firstly produced by natural depletion, water injection was then implemented as an improved oil recovery mechanism to sustain the oil production and enhance the waterflooding.. From 1995 to 2007, tertiary recovery mechanism was implemented with gas injection. In 2007, with increasing oil price it has been decided in 2007 to resume EOR opportunities in Handil, an Alkali-Surfactant-Polymer project has been initiated. A pilot project of Chemical EOR Injection had been initiated in 1980-s but had to be suspended few years later due to technical reasons and low oil price.

In order to evaluate this EOR method potential, remaining stake evaluation was done with different residual oil saturation measurements (coring, logging, SWTT). First analysis lead to very low oil residual saturation: Gas effect with possible tertiary flooding and fresh water flooding are the main hypotheses to explain these results.

A screening of the EOR techniques applicable in Handil ended up with the suggestion to test surfactant polymer injection into a wareflooded reservoir. Therefore as a first step an assessment of residual oil saturation was critical.

To properly evaluate the volume at stake it was first essential to determine range of remaining oil saturation into water flooded reservoirs. It has been decided to use two different approaches: a first one via existing wells based on the technique of Single Well Tracer Test (SWTT), the second one being coring of the reservoir nearby the tested area investigated by the SWTT. Both approaches gave similar results. However preparation of a SWTT campaign in a mature field environment was challenging.

Facing the importance of the measurement, long preparation time was taken to ensure that residual oil saturation values found are reliable and representative. Due to field maturity where most of existing well integrity is questionable, requirement to perform integrity test of the well candidate was compulsory. A hydraulic test and a mutifinger imaging tubing run were done ending with what was assumed to be the qualification of 4 wells out of 5 initial candidates.

Indeed SWTT have been successfully performed into reservoirs accessible via the some qualified existing old producers. However interpretation of the data raised several questions.

Consequently it has been decided to investigate whether tracers had been placed into the right reservoir. Investigation was performed using Water Flow Log technique that helped understanding where the tracers went and also the well integrity status behind casing. Three (3) Water Flow Logs have been run into the wells with uncertainties. Combination of the hydraulic test, multiimaging tool and water flow log, helped discarding 1 (one) out the 4 (four) wells due to unexpected communications behind casing. Thanks to these operations EOR project could go forward with reliable remaining oil saturation measurement, key value for project design, and with a certified qualifying procedure to be applied on any future existing old well susceptible to be used by the project.

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