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Waterflood monitoring with tracers
Waterflood monitoring is an important aspects of Improved Oil Recovery for ensuring that the displacement is proceeding optimally and for ensuring good sweep of the reservoir. An effective technique for such monitoring involves adding selected chemicals (tracers) in the injected water and analyzing the produced water for their presence. Tracers help to assess the path that the injected water is taking from injection wells to producers in the existing patterns. This type of monitoring program follows the approaches and methods described below.
Tracer materials are introduced into injection water to establish the inter-well flow pattern. Application of tracers requires introduction of reservoir compatible chemicals into the injection water and monitoring their presence in the production fluid at the target producing wells throughout the field. Analysis of the resulting tracer concentration versus time curves from the individual producing wells enables inter-well flow characteristics to be determined so that improvements can be made to increase injection fluid sweep efficiency of the hydrocarbon reserve.
The information gained from the tracers study includes:
- Presence of fault block & channel communication
- Determination of the source of produced water at all target production wells
- First tracer breakthrough time
- Percentage of tracer and hence the percentage of injection water flowing from one injector to a specific production well
- Average tracer transit time from each injector to production wells allowing the injector to producer relationship volumetric sweep efficiency to be calculated
The tracer chemicals must possess properties that will ensure same propagation behavior as the bulk carrying medium - water. Any deviation may generate misleading information. Therefore, the tracers must have the following properties:
- No reactivity with substances present in the reservoir and stable at reservoir conditions of temperature and pressure
- Will not undergo adsorption/absorption or exchange with the formation in the reservoir system.
- Produces a clear, unambiguous response during the analysis
The most reliable tracer for waterflooding studies is tritium in the form of tritiated water. This tracer is chemically identical to the medium that is being traced and would be expected to show similar reservoir characteristics. A further advantage in the use is that it poses relatively few radiological problems since it emits only low-energy Beta radiation. Considerable quantities of this tracer can be injected without the necessity to shield personnel from the radiation.
Besides tritium, fluorinated benzoic acids are strongly recommended. A number of these materials are stable in the harsh environments within oil reservoirs with negligible partitioning into the oil phase, they are resistant to both aerobic and anaerobic bacteria, and temperatures up to 150oC, as well as compatible with a range of reservoir types.
Application of tracers in inter-well monitoring requires unequivocal identification of fluid breakthrough in some wells in a field. This requires the field to be simultaneously traced. In most cases, this can only be achieved by using one unique tracer species per well.
Sampling is the most important step since unless the produced liquids are tested in a timely manner, and the results will be unreliable. The water sample must be representative of the water of interest. the sample transfer lines and sample containers must be corrosion resistant and clean. The transfer should be with the line inserted to the bottom of the container with the flow at a low rate that prevents splashing or agitation that will aerate the water. Some waters tend to precipitate solids on the release of pressure, and where this may occur, acid can be added to the sample container before sampling.
Therefore, the following is recommended:
- Water samples shall be collected under the producing conditions.
- Samples shall be appropriately labelled so that the sample can be identified, and with all necessary data that will be stated in the report.
- Due care should be taken during the sample packing and shipping.
Tracer monitoring – produced water sampling
The tracer should be injected rapidly and safely. To monitor tracer progress, samples of produced water will be required to be taken on a routine basis. It is necessary to ensure that the samples are taken at recommended frequency and procedure. Sampling should be undertaken in a manner that eliminates the potential for cross-contamination of produced water samples between wells. It is necessary to rinse the sampling bottles with production fluids from the well to be sampled. If using a test separator, it is necessary to purge it with produced fluids from the well to be sampled for a period of time, to purge fluids from other wells out from the test separator.
Sampling frequency from the target producing wells depends on conditions, and it can start at once every two weeks. To reduce the analytical cost to a minimum, one in every four samples taken from the individual target well should be analyzed with the remaining samples stored. If the tracer is found, then all stored samples can be analyzed to obtain a detailed tracer production curve. Conversely, if no tracer is found, stored samples can be discarded.
As small quantities of tracer are injected, it is necessary to develop a sensitive analytical technique to measure low-level tracer species in the presence of naturally occurring radioactive isotopes.
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- Sampling and analysis of produced water
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