The radioactive tracer survey (RTS or as we prefer RATS) was developed in the late thirties. Birdwell acquired the first patent on radioactive tracer surveys, and held the first license for the industrial use of radioactivity. Some newer "high tech" methods like oxygen activation (OA) logging are essentially radioactive tracer logs where the tracer is manufactured downhole with high energy neutrons. This technology is well suited to the investigation of flow behind pipe, but OA logging and similar surveys are beyond the scope of this effort. All manner of RATS methodologies have been devised; what follows is a brief and very incomplete look at RATS technology. It is important to note that while the RATS produces graphically appealing results that are usually simple to interpret, analysts have made serious mistakes by jumping to conclusions without considering alternate explanations for the observed behavior. Further, the RATS has the ability to resolve amazingly tiny flows; misuse can lead to costly attempts to repair non-existent problems.Profiling
The RATS log can be used for injection (or more rarely production) profiling, with the radioactive (RA) tracer material introduced at the surface, from a "breaker sub" (exploding test tube), or from an injector (ejector) tool (in effect a motorized downhole syringe). The analytical RATS is used to investigate various mechanical integrity (MI) issues such as casing leaks, and uses the same basic concepts employed in profiling. Yet another type of RATS involves locating radioactive tagged materials. Top of cement and squeezed cement can be located when the cement is RA tagged. Gravel pack quality can be assessed by RA tagging the gravel. RA tagged acid placement can be investigated as well as the placement of RA tagged frac fluid or proppant. RA tags are even placed downhole with shaped charges, and radioactive depth markers are sometimes used. Multiple RA tracers, emitting gamma rays of differing energy levels, can be identified with spectral gamma ray logging techniques to evaluate staged well stimulation efforts. One of the biggest players in the RA tagging business is ProTechnics.
Tracer tools are often incorporated in production logging tool stacks, but usually are run with at least a temperature tool. RA tracer can be made water, oil, or mutually soluble, and gaseous RA tracers are available. Liquid or gaseous RA tracer material can be introduced into a well from the surface with some type of lubricator. The breaker sub is an early downhole RA tracer material delivery device; tracer is contained in a test tube and released at the appropriate depth with a small electric blasting cap or squib. The injector (a/k/a ejector) tool is a motorized device capable of ejecting controlled quantities of RA tracer when activated from the surface. One, two, or rarely three gamma ray tool sections are configured with a breaker sub or injector tool above, below, or between said sections, and a casing collar locator (CCL) is normally included. See The Gamma Ray Log for an overview of gamma ray logging.
Profiling of injection wells is an extremely common use of the RATS. The RATS tool has the best low flow rate resolution of any flowmeter, and it is the only absolute flowmeter available. However, its depth of investigation is ordinarily less than that of the thermometer (90% of gammas registered by a tracer tool originate within one foot of the detector). Many different radioactive isotopes have been used for profiling work, but currently Iodine -131 (I -131) with a half-life of 8.04 days is the only isotope readily available at reasonable cost (other isotopes can be obtained through specialty or medical suppliers at higher cost). Probably the oldest profiling technique involves the release of a relatively large slug of RA tracer into the injection stream, usually at the surface. The tracer is monitored with a number of logging passes as it is pumped into the formation; it is assumed that the RA tracer plates out at the formation face with the "hottest" spots having taken the most injectant. To assure the desired plating effect, service companies have used RA flakes or beads that ultimately dissolve. Another technique involves the detection of the stagnation plane when injecting through both casing and tubing; the annular space is dosed with RA tracer to make the measurements possible.
Two main types of injection profiling RATS procedures are in common use today, slug tracking (a/k/a tracer loss) and velocity shot methods. Slug tracking methods include several different schemes including the "Self" method (so named for Charles Self, paper presented 1967), a number of other slug activity calculation schemes (Self's method works decently, but many have proposed other ways to measure and calculate the area under the curve), with both stationary tool and moving tool variations, and Dr. A.D. Hill's Two Pulse Method (not a tracer loss RATS, though tracer loss measurements can also be made), to name a few. All tracer loss slug tracking methods involve tracking RA tracer slugs as they move down the well; a reduction in area of the slug curve on the resultant chart indicates a loss of RA tracer concentration, and that loss is taken to be proportional to fluid loss. The tracer loss slug tracking RATS procedures are insensitive to changes in wellbore diameter, and are thus well suited for use in openhole or shothole completions. The Two Pulse Method involves measurements of the distance between slugs of RA tracer; a reduction in said distance is interpreted as a loss of injectant (this method is sensitive to changes in well bore diameter). Single detector gamma ray tools are commonly used for slug tracking RATS logs.
The velocity shot method of RATS injection profiling is generally regarded as the most accurate profiling log. In the velocity shot log, a small quantity (shot) of RA tracer is ejected and the time required to travel between detection points is measured. These time measurements are converted to fluid velocities, then to an injection profile. The preferable methodology is to have two gamma ray detectors, with the injector (ejector) section located either above (usually) or below said detectors, depending on the direction of fluid flow; the time of RA tracer travel between detectors is then easily measured on a time drive chart. Poor boy loggers have done velocity shot logging with one detector, measuring the time between ejection and arrival at the single gamma ray detector. Measurements are taken from the "eyeballed" first arrival time, the first arrival based on a tangent to the inflection point, the eyeballed peak, the shaped peak based on the intersection of the tangents to both inflection points, the so called half peak arrival time, and other spots, each method having its ardent supporters and detractors. The half peak height method has been suggested as best for laminar flow conditions or anytime the slugs become "strung out". Velocity shot measurements are made with the tool stationary; improved resolution can be achieved with the interval method, where overlapping readings are taken. In the "Ford" method a/k/a timed slug analysis (so named for W.O. Ford, published 1962), a single slug is followed down the hole with its time of arrival measured at various depths (tracer loss measurements can also be made). The Ford method is included here because it involves timing slug arrival, but like Hill's Two Pulse Method, it can also be regarded as a slug tracking methodology where tracer loss calculations are optional.
Properly used, the RATS log is the most sensitive method available for detection of flow behind pipe lost from the injection zone. Both slug tracking and velocity shot methods can be used to log for flow behind pipe, though "channel checks" are probably better done with slug tracking methodology. Holes in casing or tubing, packer integrity, or just about any other element of well integrity can be investigated with the RATS. When performed to investigate MI issues, surveys are sometimes called analytical RATS logs. Our shallow well RATS procedure illustrates some of the analytical RATS techniques; it was designed as a mechanical integrity test (MIT) under the underground injection control (UIC) program administered by the US EPA here in Kentucky (many states have primacy and their UIC program is administered by a state agency). Care must be exercised when selecting RATS sensitivity settings or a mere whiff of RA tracer, an amount that can be squeezed behind pipe in many wells, may be mistaken for a real problem.Some Pointers
The RATS received US EPA final approval as an alternative MIT effective December 10, 1987 (MITs not included in the original UIC rule making can become approved alternative MITs through an arduous process set out in the UIC regulations). The RATS is the only MIT authorized nationally that can demonstrate both internal and external MI, suggesting EPA recognized the power of this valuable technology (certain restrictions apply to the use of the RATS for demonstrations of external MI). At this time, US EPA specifically prohibits the use of the velocity shot methodology as an MIT under the UIC program, though ironically, US EPA Region IV has required a velocity shot RATS under at least one UIC administrative order (AO).
The RATS can be used to check crossflow in shut in wells. If a well has positive wellhead pressure when shut in, then the same crossflow check methodology is the most sensitive technique available for detecting tubing or casing leaks. With the well shut in, a slug of RA tracer is placed above the topmost perforations or in the casing shoe area. If the pressured injection zone(s) backflow to casing or tubing leaks, the RA tracer slug will accordingly move up the wellbore. With proper procedure, incredibly small leaks can be detected in this fashion (resolution on the order of ounces per day is possible). This use of the RATS is "lost art", technology not now familiar to the industry, but well known in the early days of RATS logging.
Always run baseline gamma ray logs at normal (high) sensitivity and at RATS (low) sensitivity. Tracer tools should be centralized when practicable. When logging up through any RA tracer slug, always log a reasonable distance above said slug as a check for secondary peaks; movement of secondary peaks indicates some sort of problem. When doing tracer loss surveys, record the times the slugs are logged so a timed slug analysis can also be performed. A RATS should include a well diagram or description, a tool sketch or description, a description of surface well conditions, a tabulation of chart time drive speeds, etc., providing all information necessary for an independent log analysis. Anomalous results should never be "smoothed". Hire Dr. R.M. "Mac" McKinley to tell you what the heck your RATS means when you get done...|
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Thanks to Dr. R.M. "Mac" McKinley and Dr. A.D. Hill; without them, this work would be even more incomprehensible.
The foregoing is an oversimplified discussion of RATS logging; most theoretical background and interpretative information has been omitted.
See also the handy Radioactive Tracer Survey Tables and the Radioactive Cheat Sheet.