In late 1990, we proposed the following procedure for conducting the radioactive tracer survey (RTS or as we prefer RATS) as a mechanical integrity test (MIT) on certain very shallow wells in Western Kentucky. Said proposal was made to US EPA Region IV because Kentucky has never obtained primacy under the underground injection control (UIC) program, and remains a direct implementation (DI) state, under the thumb of EPA. Our RATS MIT procedure received approval from Region IV EPA on January 23, 1991, for injection wells located in Easton and Haynesville Fields in Western Kentucky. Interestingly, Region IV EPA distributed copies of our procedure to members of the regulated community as a recommended RATS MIT regimen (without permission from this company). US EPA Region IV has a history of resistance to the use of RATS MITs, even going so far as to make negative formal written comments while approval of the RATS as an alternative MIT was pending (there was a public comment period after the first Federal Register notice was published in 1987). Region IV was the only EPA region to comment, and the only commenter to encourage a delay of the approval, though no delay was granted (sadly there were only seven commenters, including Region IV).
The RATS procedure that follows was created for use as an MIT for shallow casing injectors (wells completed without tubing and packer). This RATS MIT procedure does not include the use of a modified crossflow check in shut-in wells with positive wellhead pressure, an incredibly sensitive method for the detection of casing or tubing leaks (its sensitivity is surpassed by no other MIT). The use of a modified crossflow check should always be considered when RATS logging for mechanical integrity purposes; in some cases it is the very best MIT methodology available.
A different procedure involving the testing of system integrity for a small group of very low risk shallow injection wells equipped with tubing and packer, where the packer would remain seated during the RATS, was dubbed the "Variant RATS". Region IV EPA incorrectly asserted that the Variant RATS was prohibited by the UIC regulations (it is now known said procedure has been used in US EPA Region VIII, and elsewhere, for many years). The Variant RATS wound up at the center of civil litigation filed by EPA in US District Court. In the end, after years of litigation costing both sides obscene amounts of time and money, Region IV EPA finally agreed to allow the Variant RATS, including the modified crossflow check methodology for those shut-in wells with positive wellhead pressure.
For a more general discussion of this technology see our RATS overview.
Suggested Procedure for Very Shallow Wells in Western Kentucky
1. Rig up on location, installing lubricator and pressure control equipment. Establish the radiation area and implement all required radiation safety procedures. Run a dummy tool if deemed prudent.
2. Determine and record the injection rate and pressure from current records. Verify the wellhead pressure with a gauge reading and verify the injection rate with a water meter 1/100 barrel sweep hand calculation (see Table I). The relatively low injection rate of the Haynesville-Easton-Fordsville area shallow injection wells allows an RTS to be conducted at normal injection rates and pressures; in most cases, it should not be necessary to periodically cease injection for staging purposes.
3. Record all pertinent well data for inclusion on the log heading and pigtail.
4. Run a normal sensitivity Gamma Ray correlation log with Casing Collar Locator (CCL) from total depth (TD) to surface. This run and all subsequent runs should be in the range of 15 to 30 feet/minute; 20 feet/minute with a Time Constant of 3 is a good combination. The beginning and ending times should be indicated on this and all subsequent log runs; a time marker (minute marks) may be included if available. 3-5 minute statistical checks in both shale and sand may be optionally run.
5. Drop back to TD and run a reduced sensitivity Gamma Ray background log with CCL to surface. This and all subsequent tracer runs must be made at the same logging speed and sensitivity settings. Again, 3-5 minute statistical checks in both shale and sand may be optionally run.
6. Using a small lubricator(s), inject one or two radioactive tracer slugs, each consisting of at least 1.5 millicuries (mCi) of water soluble Iodine 131 (I -131) into the well; note the time of injection of each tracer slug. If the dual tracer slug technique is employed, the time between tracer slugs should be selected to provide a few feet between the slugs (See Table II). This quantity/intensity of tracer material enables detection behind tubing and casing strings with a Geiger-Mueller detector tool (smaller amounts of tracer may be used if scintillation detector tools are available). In the alternative, the radioactive tracer slug(s) may be introduced with an injector (ejector) tool, however this procedure was specifically designed to eliminate the use of said tools, thus avoiding the risk of false positives resulting from a leaky tool.
7. With the recorder switched to time drive, record the tracer slug(s) very near surface as they pass the detector. Drop below the tracer slug(s), switch the recorder to depth drive, and log through the tracer slug(s) as near the surface as possible. This step establishes tracer slug size(s) (and distance between slugs when the two slug method is used).
8. Drop to the first station at either 50 or 100 feet (50 foot stations should be used when fluid velocity allows sufficient time), switch the recorder to time drive, and record the tracer slug(s) as they pass. As soon as the tracer slug(s) clear the detector, drop below the tracer slug(s), switch the recorder to depth drive, and log through the tracer slug(s) to surface.
9. Drop to the second station and repeat the procedure outlined in Paragraph 8. Log to 25 feet above the previous station or preferably to surface if the fluid velocity allows sufficient time.
10. Continue the procedure until reaching the next to last station. It is desirable to have at least 3 stations above the base of the casing (casing shoe), or injection perforations.
11. Drop to the last station a few feet above the base of the casing or perforations (it has been suggested by EPA Region V that the distance should be 20 feet above the casing shoe or perforations, but their reasoning is unknown). With the recorder switched to time drive, record the tracer slug(s) as they pass. The tool should be left at this last station with the recorder in the time drive mode for a sufficient period of time to allow the slug to pass below the casing shoe or completely enter the perforations (this time period may be up to 30 minutes, but if upward migration is present, it will normally be seen on the log within the first few minutes of the stationary reading).
12. Switch the recorder to depth drive and make at least two additional logging runs from the casing shoe or perforations to surface as a final check for leakage. The last run(s) can be made at normal sensitivity instead of reduced sensitivity if well conditions permit.
13. The casing shoe or perforations (or packer in the case of the Variant RATS) investigation can be repeated with a bigger slug if necessary. This can be accomplished by lubricating the tracer slug in at surface or, if the time to displace the slug is excessive, an injector (ejector) tool or breaker sub may be utilized.
Interpretation of the log generated by the foregoing procedure is relatively simple and straightforward. The following is an outline of the methodology:|
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A. Using the "Self" method, calculate the size of all tracer slugs logged and compare the results. Loss of tracer material as it progresses down the pipe indicates leakage. (See Note 1)
B. If the dual tracer slug technique was used, compare the distance between slug peaks for each logging run or pass. Any decrease in distance between peaks as the tracer slugs progress down the pipe indicates leakage. This analysis complements the "Self" method. (See Note 2)
C. Any secondary peaks observed on the depth drive curves indicate leakage and can be analyzed to determine the disposition of any leaking injection fluid. Again, this analysis complements the previous calculations.
D. Examine the time drive curve from the last station (just above the casing shoe) after the slug(s) pass for any increase in count rate. An increase indicates leakage on the outside of the pipe most probably resulting from a defect in the primary cementation.
E. Any secondary peaks observed on the final depth drive curves indicate leakage and can be analyzed to determine the disposition of any leaking injection fluid that is bypassing the casing shoe.
Note 1: It has been suggested that the "Self" method is not consistent with a tracer material balance, though the method provides empirical results that have compared favorably with quantitative velocity measurements. It may be more appropriate to calculate the area of the tracer concentration profiles or to use the product rather than the sum of the base and height of the "Self" triangles.
Note 2: The patented dual slug or "Two Pulse Method" is used with the generous permission of Dr. A.D. Hill and is limited to non-commercial experimental use by AnaLog Services, Inc. Any commercial use by AnaLog or others will require a licensing agreement.