An oil field operator developed an offshore field using sub-sea extended reach wells from one common sub-sea template. The wells were completed in both the heel and mid/toe sections to target two discrete reservoir zones lying parallel to each other separated by a possible sealing fault. Waterflooding using four injector wells was planned as part of pressure maintenance and hydrocarbon displacement. Due to varying permeability within the formation it was essential that injection water management was optimised to ensure minimal water breakthrough and maximise hydrocarbon recovery. Chemical tracers were used in each injection well zone to monitor source of water breakthrough and verify no communication between the two discrete neighbouring formations. Figure 1 shows the position of each well within the reservoir.
Reservoir parameters and fluid flow data was used to determine worst case pore volume dilution of the tracer when flowing through the reservoir. Due to the use of a common flow line to the FPSO a secondary dilution factor was taken into consideration due to mixing of production water from all other wells. To minimise well testing requirements, a sampling strategy was formulated to only test individual well flow lines suspected of water breakthrough once tracer detection had occurred within the common production flow line.
Tracerco mobilised to the field and injected approximately three barrels of each chemical tracer over several days.
In order to ensure that the correct tracer entered the correct well position, all other water injection wells sharing the common sub-sea template were shut-in. In addition, either the heel or the toe of individual wells was closed to eliminate flow of tracer into the wrong section of the well being targeted. The tracer was displaced into the main flow line of the FPSO over one hour. The flow line was then flushed for three surface and downhole tubing volumes to ensure that all tracer had entered the correct reservoir position. This was repeated for all eight tracers.
Sampling and Analysis
Sampling was carried out by the operator with samples taken once every week. Tracer analysis was carried out using sensitive gas chromatography systems. Upon detection of tracer within produced water samples, production wells most likely to be producing water containing tracers were sampled and analysed. It was then possible to isolate water breakthrough at each production well to a specific water injector over time.
Interpretation of Results
The tracer elution curve from well B in figure 1 is shown below together with volume water production over time.
Confirmation of the presence of tracer that was injected into the toe section of water injector two allowed the operator to reduce water injection rates into the toe of well two. The result can be seen by looking at the reduction in the rate of water production increase a little time after this change in water balance was made. Identical treatment of every production well allowed the operator to constantly monitor and change their water injection rates in each of the injector wells to minimise water production and maximise hydrocarbon recovery.
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