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Designing single well chemical tracer test for residual oil
Using the single well chemical tracer (SWCT) test avoids the problems of too-wide well spacing and excessive tracer dispersion caused by layering that can occur with well to well tests. In the SWCT test, the tracer-bearing fluid is injected into the formation through the test well and then produced back to the surface through the same well. The time required to produce the tracers back can be controlled by controlling the injected volume on the basis of available production flow rate from the test well.
Flow of tracers during a well test
In a single-well test, tracers injected into a higher-permeability layer will be pushed farther away from the well than those in a lower-permeability layer, as indicated in Fig. 1a; however, the tracers in the higher-permeability layer will have a longer distance to travel when flow is reversed. As the tracer profiles in Fig. 1b show, the tracers from different layers will return to the test well at the same time, assuming that the flow is reversible in the various layers.
Flow reversibility in a single-well test is desirable for the reason just explained, but it complicates residual oil (Sor) measurement. The simple strategy of injecting two tracers with different partition coefficients does not work in this case. As Fig. 2 shows, tracers A and B separate during injection, just as in a well-to-well test, and at the end of injection, tracer B will be farther from the wellbore than tracer A because the presence of the residual oil retards A. During production, then, tracer B has farther to travel to return to the well. The separation achieved during injection is reversed during back production, so that A and B arrive back at the wellbore at the same time (if they were injected at the same time). No measurement of Sor is possible using Eqs. 1 and 2 because no information can be obtained about the ratio of velocities of A and B.
Possible ways to avoid reversibility problem
One possible way of avoiding this reversibility problem is to generate the second tracer in the formation instead of injecting it. The steps are:
- Inject tracer A and push it into the target formation, as described above
- Stop flow to allow part of the injected A to react, forming tracer B in the same pore space where A is located, after the reaction time, A and B are together
- The fluid is then produced back into the test well
A and B must separate if their equilibrium distribution coefficients (KA and KB) are different and if residual oil is present. This concept is the basis for the SWCT test patent.
The practicability of this method depends on finding suitable tracers. The demands on partitioning tracer A are especially severe:
- Available in reasonable quantities at high purity
- Not present in the reservoir fluids
- Easily measured at low concentrations in water
It must have an appropriate KA for the oil, water, and temperature of the target field, and, most importantly, it must react at a rate that allows formation of enough (but not too much) of a suitable tracer B. KB must be different from KA, and tracer B also must be measurable at low concentrations in the produced water, and not be present in the reservoir fluids.
A methyl, ethyl, or propyl ester of formic or acetic acid has proved suitable in every reservoir tested. These simple chemicals are sufficiently soluble in water, and have an appropriate range of K values and reaction rates. They are relatively inexpensive and nontoxic at the concentrations used, and they react with water to produce alcohols, which are not found in crude oils and can be detected readily in the produced fluid.
For best results, choose an ester with a retardation factor (βe) in the optimum range (0.5 < βe < 1.5). This requires that Ke be in the range 0.5(1.0-Sor)/Sor<Ke<1.5(1.0-Sor)/Sor. Use the best available estimate of Sor to fix this range. Then choose the optimum ester using available correlations for the dependence of Ke on temperature and water salinity.
Two esters (e.g., methyl acetate and ethyl acetate) can be used simultaneously to give two different depths of investigation for Sor in the same test. This multiple-ester test design has become increasingly popular in recent years.
The design and implementation of a SWCT test for Sor is straightforward. Certain facts about the target formation are needed to begin test design. Some essential reservoir properties include:
- Oil cut of the test well
- Reservoir temperature
- Reservoir lithology
- Production rate
- Test interval size and average porosity
- Formation water salinity
Oil cut of test well
Candidate wells for Sor measurement must be able to produce formation water to the surface. The produced fluid should be nearly all water to ensure that the test interval is at or near Sor. In cases where the test interval produces high oil cut, water can be injected into the test interval before testing to water-out the zone before tracers are injected.
Reservoir temperature dictates which esters are suitable for the SWCT test. The formate esters hydrolyze approximately 50 times faster than do the acetate esters, and are used in the reservoir temperature range of 70 to 135°F. The slower-reacting acetate esters generally are used in the 130 to 250°F range.
SWCT testing has been done in a variety of test conditions. In sandstone reservoirs, SWCT tests give satisfactory results for a wide range of test designs. Test timing, total injected volume, and the ester used can vary considerably for the same zone, with little effect on test interpretability.
However, SWCT tests in carbonate formations require much more precise design. In a given carbonate test zone, subtle changes in test design can cause significant variation in the tracer profile shapes. Experience with carbonate test designs has shown they require significant tailoring to overcome the dispersed nature of the production profiles generally present in carbonate test results. The reason for this dispersed nature is that the assumption of local equilibrium is not always valid for carbonate reservoir tests.
The production rate of the candidate well controls the test size or volume to be injected. The amount of water that can be produced in one day is a normal test volume; two days’ production is a practical upper limit. In normal productivity reservoirs, the injection is sized to give a 15- to 20-ft depth of investigation into the formation. The injection rate of the SWCT test usually is approximately the same as the well’s production rate. Care must be exercised to avoid fracturing the formation during test injection.
Test-interval size and average porosity
SWCT test can be carried out on cased and openhole completions. Estimated interval size and porosity are used to calculate a theoretical radius of investigation for the design injection volume, although Sor results are not dependent on either. Interval sizes of from 1 to 100 ft have been tested with satisfactory results. Generally, testing on 10- to 50-ft zones generates more definitive results.
Because it normally avoids chemical incompatibility, water that is produced from the test well generally is used to carry the chemical tracers into the reservoir. Because Ke depends on it, water salinity also is a factor in choosing the best ester for the SWCT test. To avoid difficulties during injection of the tracers, closely examine the water quality (especially suspended solids) and any precipitation tendencies.
After selecting the ester and sizing the test volume, determine the field-test location, production method, and safety requirements. Then, schedule and implement the test.
Four lift methods have been used in SWCT testing. They are:
- Free flow
- Electric submersible pump
- Rod pump
- Gas lift
These lift mechanisms are listed roughly in order of desirability, but all are satisfactory.
Before the field test, the candidate well should be produced long enough to:
- Establish the oil cut
- Measure the stabilized production rate
- Clean up the tubular goods in the completion
Then accumulate produced water for the upcoming test in clean tanks near the well. Position the portable laboratory/pumping system near the test well.
Pure tracer chemicals usually are delivered to the wellsite in 55-U.S.-gal drums. The tracers either can be batch-mixed with formation water before injection, or continuously metered into the water during injection. Fig. 3 shows a schematic for a typical field setup for continuous metering of chemical in the injection water. For batch mixing, the tank can serve as the mixing vessels. With either batch-mixing or continuous metering, filter the water to one-micron or higher quality to prevent plugging when the fluids enter the reservoir.
After a short period for analytical equipment checkout, inject the chemical solution of ester and material-balance tracer and push it according to test design. Samples of the injection water should be analyzed periodically to verify tracer concentrations, and volume, rate, and pressure information should be monitored carefully throughout the injection. Be careful not to part the formation by exceeding the fracture gradient.
Once the injection is complete, the well is secured for the planned shut-in period. When the shut-in period is over, the well is placed on production. Fig. 4 shows a general schematic for the production phase of a SWCT test. The produced water flows through a portable separator (if necessary) to the storage tanks on location, where its volume is carefully measured. Production volume also can be measured using a field production test separator, if one is available.
During production, water samples should be taken near the wellhead and analyzed on location for tracer concentrations. On-site chemical analysis is necessary to gather data that are accurate for the time of production, whereas sending the samples to a service laboratory for analysis would allow additional hydrolysis of the ester to take place during transport. At the time each sample is taken, the total production volume is recorded and plots of tracer concentration vs. volume produced are generated. These tracer concentration profiles are the essential field data for the SWCT test.
Because of dispersion, the total produced volume required normally is two to three times the injected test volume. Injected volume usually is one day’s production, and two to three days normally are required for the back-production phase of an SWCT test.
Example applications of the single well chemical tracer test are given in the following additional articles:
- Interpreting data collected during a chemical tracer test
- Simulation of single well chemical tracer tests
- Deans, H.A. 1971. Method of Determining Fluid Saturations in Reservoirs. US Patent No. 3,623,842.
- Deans, H.A. and Majoros, S. 1980. The Single-Well Chemical Tracer Method for Measuring Residual Oil Saturation, Final report, Contract No. DOE/BC20006-18. Washington, DC: US DOE.
- Carlisle, C.T. et al. 1982. Development of a Rapid and Accurate Method for Determining Partition Coefficients of Chemical Tracers Between Oils and Brines (for Single-Well Tracer Tests). Contract No. DOE/BC/10100-4, US DOE, Washington DC (December 1982).
- Deans, H.A. and Carlisle, C.T. 1986. "Single-Well Tracer Test in Complex Pore Systems," paper SPE/DOE 14886 presented at the 1986 SPE/DOE Symposium on Enhanced Oil Recovery, Tulsa, 20–23 April.
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