Patent Number: 050948083
Section: description

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT The challenge of oxygen activation is to measure activation counts which are moving in the channel behind the casing before those counts diminish to an energy and population level where they cannot be detected. With various injection rates and channel volumes, water will move within a channel at varying fluid velocities. In channel constrictions, water velocity will increase given a constant flow rate. Conversely, in a washout or large channel, the fluid velocity will decrease. It can be seen that the distance the counts will travel before decaying to zero will be changing while logging, although the distance the detectors are spaced from the neutron source will remain fixed. Whenever the fluid velocity does not coincide with the spacing of the detectors, the count rates received by those detectors will be near zero. To solve this detection problem, according to the invention, several detectors are spaced so that for various fluid velocities, one can acquire gamma ray counts before they decay below a given threshold percentage of their original population, ranging from 3% to 10%, and preferably approximating 6.25%. To maintain high count rates, a tool T, with three, and preferably four, gamma detectors 10, 12, 14 and 16, at the proposed spacings shown below, is adapted to be suspended in and moved through the casing 20 by an armored cable. The gamma ray detectors are known per se, and may be, e.g., of the NaI T1 or BGO type. By placing four detectors at these preferred longitudinal spacings from a neutron source 18, the number of counts received under a wide range of fluid velocities can be maximized. The source 18 is, e.g., an electronic generator of the type which generates discrete pulses of fast neutrons (14 MeV), and may be of the type described in U.S. Pat. Nos. 2,991,364 or 3,596,572, which are herein incorporated by reference. A chart of the theoretical count-rate response of these detector spacings is included as FIG. 2. The detector spacings for a prior art tool are also shown. ______________________________________ DETECTOR SPACINGS PRIOR ART PREFERRED EMBODIMENT DETECTOR SPACINGS ______________________________________ D.sub.1 - 14.75" D.sub.1 - 27" D.sub.2 - 24.6" D.sub.2 - 42.75" D.sub.3 - 44" D.sub.4 - 56.5" ______________________________________ The preferred spacings can vary by about .+-.15 percent, without departing from the spirit of the invention. When measuring these count rates, it must be ensured that the count rates that are acquired are strictly from the effect of oxygen activation and not from other sources of gamma ray emission. Other sources of gamma ray emission include steel activation, NaI detector activation, as well as formation or background radiation. Steel activation occurs in the well while logging and emits 1.8 MeV gamma rays while decaying with a 2.5-hour half-life. NaI detector activation occurs in the detectors, and this reaction gives off a 2.2 MeV Beta decay with a 25-minute half-life. Detector activation is more of a problem in detectors which are spaced close to the neutron source. A discriminator is used at the output of the detectors to discriminate all counts of gamma rays which have an energy less than 2.5-3 MeV. This eliminates all counts from the three sources mentioned above. This procedure eliminates a large portion of the counts which were emitted through oxygen activation, but have low energy levels when they reach the detector due to the collisions with other formation elements prior to reaching the detector. This factor puts added emphasis on ensuring that count rates are maximized in every other way to offset this necessary discrimination procedure. The oxygen activation counts looked for must also be distinguished from gamma rays of capture within the formation. To perform this function, a time delay circuit is engaged to avoid any readings until a period in time from 3000 microseconds to 4500 microseconds after the burst ceases. This should ensure that any capture gamma ray activity is not measured. Due to the long half-life of the oxygen activation counts compared to the half-life of the formation's capture gamma ray activity, the readings obtained should contain only formation background counts and oxygen activation counts. The formation background counts should be relatively small because most background radiation counts will be discriminated out by the 3 MeV discriminator. With this configuration, oxygen activation counts will be measured about 30% of the time that the neutron source is being cycled. The prior art measures oxygen activation counts only 17.9% of the time. The above-mentioned prior art tool is configured so that it predominantly measures formation effects. It operates on approximately 28 short cycles of one millisecond each, followed by a long seven-millisecond cycle. During the short cycles, the effects of oxygen activation and background are not measured. The background and oxygen activation levels are measured during the seven-millisecond cycle. The net result is that only about 17 percent of the time are there any measurements of oxygen activation or background in the above-mentioned tool. The apparatus and method of the present invention employ cycles which are longer than the prior art cycles; e.g., several millisecond cycles, and, more particularly, five-millisecond cycles. Due to these longer cycles, the background is more frequently and efficiently measured. The use of the four detectors aids in maximizing total count rates of oxygen activation and the background, thereby statistically diminishing the effectiveness of the background as a percentage of the total measurement. Additionally, the longer cycles allow the background itself to be more accurately measured. When the four detectors are combined with the use of gates, statistical variations in the background itself are made less meaningful in view of the fact that the total number of counts measured is enhanced and the background measurement is also improved due to the longer cycle time. Typically, readings from the background occur due to the presence of uranium salts or thorium or potassium found in shales. Readings from the formation typically come from the presence of chlorides found in salt water. The advantage of this design is the increased count rates obtained due to the multiple detectors that are employed. The prior art devices' count rates are very low and yield very high statistics under many conditions. With higher count rates and lower statistics, interpretation of the data will become much more precise. The four-detector arrangement gives answers over a wider span of fluid velocities than the prior art. This would be especially advantageous when evaluating producing wells. The advantage of the apparatus of the present invention is seen in that it provides a greater span of distances between detectors and the neutron source, which makes it more adept at measuring higher water velocities than the embodiments known in the prior art. Additionally, this same feature enables the apparatus of the present invention to better measure background conditions before any neutron sources are activated, with smaller statistical errors than with the prior art devices. There are various variables that could affect the accuracy of background statistics. These include cement voids and cement thickness variations, as well as how well the cement has bonded to the formation. The apparatus of the present invention, by providing a very short spacing as well as a long spacing, provides flexibility to measure at the shorter detector spacing, which is more sensitive at low flow velocities. The use of the additional detectors in the apparatus of the present invention allows higher velocity water intrusions to be seen, where with prior art devices there would have been an indication of no water flow at all. This occurred due to high velocity of water sweeping activated oxygen beyond the detectors too quickly for the prior art devices to obtain a measurement. The apparatus and method of the present invention seek to optimize the accuracy of oxygen activation measurements. Two important variables in making accurate oxygen activation measurements are the spacing of the detectors and the frequency of measurement. In the preferred embodiment, these variables are optimized by using the four detectors in the approximate spacing hereinabove indicated. The use of the detectors in this spacing allows for an apparatus and method which can accurately detect water flow rates somewhere in the range of about 11/2 to 50 ft/min. The tool further optimizes the oxygen activation measurements by employing time gates included in the electronic circuitry 22 for processing the counts, and by cycling the neutron source 18. The rest of the circuitry 22 is known per se and may be of the type described in U.S. Pat. No. 3,603,795. The time gates are designated time delays which control the length of time the detectors can measure gamma radiation within the wellbore. The above-mentioned prior art tool is set up for a series of approximately twenty-eight short cycles of about one millisecond each, followed by one long cycle of about seven millisecond. It is only during the long cycle that meaningful background measurements can be obtained. The reason for this is that short-measurement durations do not allow for accurate measurement of the total gamma rays in the well due to background as well as oxygen activation. The background gamma ray level can vary somewhat in different portions of the well. It becomes important to measure as many counts as possible, especially in low-velocity waterflow situations where fewer counts are generated. The fewer the counts generated due to oxygen activation, the more statistically significant are the background levels of counts. The apparatus of the present invention seeks to reduce the significance of the effect of the statistical variation of the background counts by longer measurement times which promote higher count rates by optimizing detector spacing to diminish the effect of the background level of gamma rays found in the wellbore. The level of background radiation can vary within a statistical field. For example, background gamma radiation could result in a measurement of four or five counts. In low-velocity waterflow situations where the number of total counts measured is, in itself, low, the effect of the background level of radiation and its own variability become much more statistically significant. However, if the number of counts measured is increased due to the use of four detectors and long measurement cycles, raising the total counts measured decreases the statistical importance of the background level of radiation. Longer measurement cycles yield more repeatable results and their use with four detectors improves the ability to measure more counts, thereby obtaining more statistically meaningful results in low-velocity waterflow situations a greater percentage of the time than prior art tools. The apparatus and method of the present invention employs longer cycles of detector measurement (5 millisecond) through the use of gates so that, in each time cycle of source activation, oxygen activation readings are obtained. The detectors continue to sense gamma rays in an effort to more continuously determine the background level of gamma radiation in the wellbore. The combination of the four detectors, along with judicious use of the gates, permits the apparatus and method of the present invention to obtain meaningful statistical data about the background level of gamma radiation approximately 30% of the time, as opposed to only about 18% of the time for the tool of the prior art. This is particularly important since there can occur changes in the background level of gamma radiation along the depth of the well. The foregoing disclosure and description of the invention are illustrative and explanatory thereof, and various changes in the size, shape and materials, as well as in the details of the illustrated construction, may be made without departing from the spirit of the invention.