Patent Number: 052375943
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to the detection and quantification of certain earth formation elements surrounding a borehole, and especially elements sensitive to the nuclear activation reaction. 2. Description of the related art The detection and determination of certain elements present in an earth formation or in the well itself is of great importance in the oil production business, since the presence and amount of these elements provide useful information. Among the logging techniques used for such determination are the nuclear logging techniques, and particularly the nuclear activation method wherein a sonde comprising a high energy neutron source and a gamma ray detector is lowered in the well to investigate. Certain atoms interact with neutrons in being activated to an unstable state which decays back exponentially in time to a stable state, while emitting gamma rays of given energy representative of the activated atom. Count rates in the gamma ray detector are proportional to the total amount of the element of interest present around the sonde. By way of example, oxygen, silicon, aluminum, magnesium, or gold atoms may be activated. Oxygen atoms are representative of water. For example, a well which has been determined to be promising for oil production, is provided with a metallic casing, and cement is injected between the earth formation and the casing. Perforations are then made through the casing/cement and in the oil productive formation, so as to allow oil to flow up to the surface through a tubing beforehand arranged in the well coaxially to the casing. Unwanted vertical flow of water can occur in the cement, between the casing and the formation. This phenomenon, usually called "water channeling", causes undesirable paths between formations located at different depths, i.e. at different pressures, e.g. by allowing water from a first formation layer to mix with oil coming from a second formation layer. This phenomenon disturbs substantially the oil production. It is of great importance to identify fluid points of entrance to or exit from the borehole, as well as to determine the mechanical integrity of the cement annulus. The invention may also be used, besides the water flow detection hereabove referred to, to qualify gravel packing which is usually disposed in the annulus between the productive formation and the screened tubing, for preventing sand invasion in the tubing. As a matter of fact, aluminum and silicon are found in abundance in gravel-pack material. Aluminum e.g. is found in sintered bauxite and silicon in standard gravel pack. The use of the activation method for the detection of aluminum has been depicted in the article "The Aluminum Activation Log" from H. D. Scott and M. P. Smith, in The Log Analyst, Sep.-Oct. 1973, pages 3-12. Magnesium is another element of interest which is sensitive to the activation reaction. Magnesium is indicative of the presence of shales. Moreover, gold atoms, when bombarded with high energy neutrons, become activated. It is known to inject gold as a tracer in oil wells. Activated gold follows the fluid paths in the formation and thus, presence of gold is thus representative of fractures in the formations. Accordingly, there is a need for a better knowledge relating to the above mentioned elements in the earth formation. However, the known methods and apparatus, although satisfactory, show limitations. First, they do not offer a straightforward and simple way to distinguish the different activated elements one from the other. As a matter of fact, the gamma rays received by the detector may come from different types of activated atoms. Furthermore, the known methods do not generally provide any quick and reliable information about the radial distance between the activated elements and the borehole. According to the above, there is a need for a reliable method for obtaining quantitative and qualitative information related to given earth formation elements able to be activated by neutrons. SUMMARY OF THE INVENTION An object of the invention is a logging method and apparatus providing reliable quantitative and qualitative information on elements able to be activated present in earth formation surrounding a borehole. Another object of the invention is to identify the activated element(s). A further object of the invention is to provide information on the radial distance between the activated atoms of the element of interest and the borehole. An even further object of the invention is to provide a straightforward and simple method providing easily interpretable results to the user. The foregoing and other objects are attained in accordance with the invention by a nuclear logging method for obtaining qualitative and quantitative information related to elements in earth formation surrounding a borehole, comprising the steps of: (1) irradiating the formation with neutrons from a neutron source, the neutrons being of sufficient energy to interact with at least one element according to the activation reaction; PA1 (2) detecting and counting, at at least two locations spaced from the source, the gamma rays resulting from the activation of the element; PA1 (3) determining at each depth number of gamma ray counts detected during a time period starting when the neutron source passes that depth and ending when the detector passes that depth, this determination being made for each detector at each depth; and establishing a relationship between the instant times and the counts for all of the detecting locations; and PA1 (4) deriving from the relationship, for each depth, at least one characteristic of said element. PA1 (1) neutron source means for irradiating the earth formation with neutrons of sufficient energy to interact with atoms of at least a given element according to the activation reaction; PA1 (2) means for detecting and counting at at least two locations spaced from the source, the gamma rays emitted during the activation reaction; PA1 (3) means for determining, at each depth, the number of gamma ray counts detected during a time period starting when the source passes that depth and ending when the detector passes that depth, this determination being made for each detector at each depth; PA1 (4) means for establishing a relationship, for each depth, between the counts from the respective detectors for that depth and the corresponding times when the corresponding detector reaches that depth; and PA1 (5) means for deriving from the relationship at least one characteristic of said element. More particularly, the relationship is expressed in the form of a plot of counts versus time; the plot is approximately a straight line the slope of which is representative of the element of interest. Moreover, the number of counts is representative of the quantity of the activated element at the depth at stake and of the radial distance between the activated element and the borehole. In a preferred embodiment, the method includes moving in the borehole an elongated sonde comprising four detectors longitudinally spaced from the neutron source. The present invention also contemplates a nuclear apparatus for obtaining qualitative and quantitative information related to an element of earth formation surrounding a borehole, comprising: The characteristics and advantages of the invention will appear better from the description to follow, given by way of a nonlimiting example, with reference to the appended drawing in which: