EPITHERMAL NEUTRON DETECTOR AND LOGGING METHOD USING IT

Abstract: A borehole logging system includes a pulsed source of fast neutrons and an epithermal neutron detector. The detec­ tor comprises a neutron-sensitive counter and a neutron filter encasing the counter; the filter has a thickness to permit the longest possible delay time following each burst of fast neut­ rons before epithermal neutrons from the irradiated forma­ tion are detected by the neutron counter while at the same time minimizing thermal neutron effects.

Description: EPITHERAL NEUTRON DETECTOR ZD LOGGING METHOD USING IT This invention relates to radioactive well logging and more particularly to a well logging system for irradiating subterranean formations under investigation with bursts of fast neutrons and cnaracterizing the formation on the basis of the decay of the epithermal neutron population produced. Various neutron logging techniques may be employed in order to characterize subterranean formations with regard to tneir fluid or mineral content, lithologic cnaracteristics or porosity, or to provide for stratigrapnic correlation. The neutron source may be a steady-state source or a pulsed source. For example, neutron porosity logging may De carried out using a steady-state neutron source in order to bombara tne formation with fast neutrons; the porosity of tne formation may be determined by measuring thermal neutrons employing two detectors at different spacings from the source or by measuring epithermal neutrons with a single detector. In pulsed neutron logging procedures, the formations are irradiated with repetitive bursts of fast neutrons, normally neutrons exhibiting an energy greater than 1 Mev. When the fast neutrons enter the formation, they are moderated, or slowed down, oy nuclei within the formation to form lower energy neutron populations. The fast neutrons are moderated to lower energy levels by the nuclear collision processes of elastic and inelastic scattering. In elastic scattering the neutron loses a portion of its energy in a collision that is perfectly elastic, i.e., the energy lost by the neutron is acquired as Kinetic energy by the nucleus with which it collides. In inelastic scattering only some of the energy lost by the neutron is acquired as kinetic energy by the nucleus with which it collides. The remaining energy loss generally takes the form of a gamma ray emitted from the collision nucleus. In the course of moderation, tne neutrons reacn tne epithermal range and tnence are furtner moderatea until tney reacn the tnermal range. Thermal neutrons are neutrons which are in thermal equilibrium with their environment. The distrioution in speed of thermal neutrons follows the so-called Maxwellian distrizution law. Tne energy corresponding to the most probable speed for a temperature of 200C is 0.025 electron volt. Epithermal neutrons are tnose neutrons which exhibit energies within tne range from immediately above the thermal neutron region to about 100 electron volts. While the boundary between thermal and epithermal neutrons is, of necessity, somewhat arbitrary, it is normally placed in the range of 0.1-1 electron volt. The populations of neutrons at the various energy levels decay with time following primary irradiation and thus offer means of characterizing the formation. For example, in the case of elastic scattering, which predominates for energies between a few Mev and about 1 Mev, the number of collisions required for a neutron to moderate from one energy level to a second lower energy level varies more or less directly with the atomic weignt of the nuclei available for collision. In subterranean formations, collisions with hydrogen nuclei present in hydrogenous materials such as oil, water and gas tend to predominate in the slowing down process. Thus1 the rate of decay of the epithermal neutron population gives a qualitative indication of the amount of hydrogenous material present wflich in turn may be indicative of tne porosity of the formation. For example, U.S. Patent 4,097,737 descriDes a method and system for epithermal neutron die-away logging utilizing a 14 Mev pulsed neutron source and a neutron detector that is sensitive to epithermal neutrons ana highly discriminatory against thermal neutrons. The detector is relatively insensitive to the nigh energy neutrons and has a filter that renders it sharply insensitive to tnermal neutrons. In accordance with tne present invention, there is provided an epither;nal neutron detector, comprising: (a) a neutron-sensitive counter, and (b) a neutron filter encasing the counter, having an optimum thickness whicn provides for a counting rate delay time corresponding to a selected lower limit for statistical precision of epithermal die-away measurements, tne delay time being equal to a counting rate delay time corresponding to a selected upper limit for thermal neutron effects. The invention also provides a system for epitnermal neutron die-away logging of subterranean formations surrounding a borehole comprising: (a) a source of fast neutrons for irradiating the formations surrounding the borehole, (b) a neutron counter for counting epithermal neutrons returning to the borehole from the irradiated formations, ana (c) a neutron filter surrounding the neutron counter having a thickness so as to permit the longest possible delay time following each burst of fast neutrons from the source before the epithermal neutrons from tne irradiated formations are detected by tne neutron counter while optimizing the epithermal neutron counting rates by minimizing thermal neutron effects. According to the invention, a source of fast neutrons irradiates the formations surrounding the borehole. A neutron counter detects primarily epithermal neutrons returning to the borehole from the irradiated formations. A neutron filter surrounds the neutron counter and has an optimum filter thickness so as to permit the longest possiDle delay time following each burst of fast neutrons from tne neutron source before the epithermal neutrons from the irradiated formations are detected by the neutron counter, wnile at the same time optimizing the epithermal neutron counting rates with minimal thermal neutron effects. The invention is oescribed below in greater detail by way of example only with reference to tne accompanying drawings, in nnicn FIG. 1 illustrates the neutron detector of the invention; FIG. 2 illustrates counting rate curves for various neutron filter thicknesses; FIGS. 3 and 7 illustrate counting rate delay time curves for use in determining optimum neutron filter thickness; and FIGS. 4-6 illustrate auxiliary curves derived from counting rate curves and used in determining optimum neutron filter thickness. Referring to FIG. 1 of tne drawings, an epithermal neutron detector includes a neutron-sensitive counter 10, preferably a proportional counter filled with hel1us3 gas, surrounded by a neutron filter 1l, preferably a thin layer of gadolinium. Such a detector can be employed in a system such as that described in U.S. Patent 4,097,737 for epithermal die-away porosity logging. In accordance with the invention, the neutron filter 11 surrounding the neutron counter 10 is of optiraum thickness to permit the longest possible delay time following each neutron burst before the epithermal neutrons are detected Dy the neutron counter 10, while at the same time maintaining (i) thermal neutron effects within an acceptable level and (ii) acceptaole epithermal neutron counting rates. This can be explained by referring to FIG. 2 which shows die-away curves obtained for a range of neutron filter thicknesses surrounding a given neutron counter when the formation being logged has a given porosity. In FIG. 2, the solid curves show the total observed response, which is the sum of the desired epithermal neutron response and the unwanted thermal neutron response. The broken curves show only an epithermal response. Curve 1 is obtained with a thick filter surrounding the neutron counter, curve 2 is obtained with a medium filter, and curve 3 is obtained with a thin filter. Of course, the terms thick, medium and thin are relative to one another and are material dependent. The important criterion is that if the filter material is tnicK enougn to produce a pure epitnermal neutron decay, unwanted thermal neutrons are eliminated from the measurement and the curve 1 results. However, in this event the measurement time is undesiraoly short, maKing an accurate epithermal die-away measurement dlfficult to achieve. By utilizing a thinner filter, the measurement is spread over a longer tine period. However, if tne filter is too tnin, tne unwanted contribution of tnermal neutrons becomes too large and tne desired epithermal die-away measurement is not achieved, as shown in curve 3. The invention provides for an optimum compromise Detween tne two conditions shown in curves 1 and 3 so as to provide as nearly a pure epithermal die-away over as long a time period as is possible before the thermal neutron contribution becomes objectionably large. Sucn a compromise is shown in curve 2. In the design of such an optimum filter thickness, two conditions need to be considered: [T(t) - bpi(t)3/epi(t) < n (1) and Counting rate (t)/Counting rate (0) > p (2) where: T(t) = observed neutron lifetime at time delay t, Tepi(t) = epithermal neutron lifetime at time delay t, Counting rate (t) = counting rate at time delay t, Counting rate (0) = counting rate at zero time delay, n = a dimensionless parameter chosen to permit a given admixture of thermal neutron response, and P = a dimensionless parameter chosen to permit a given relative statistical precision. The physical significance of tne parameter TI may oe explained as follows. The observed neutron lifetime T is a function of both epithermal and thermal neutron properties of eartn formations. It is desired to maintain the dependence of T on thermal neutron effects as low as is practical. Tne parameter TI is a quantitative assure of the degree to which thermal neutron effects will influence the observed neutron lifetime. For thick filters, # # # epi and # will De very small. For thin filters, # > # epi and # will be relatively large. In determining tne optimum filter thickness, a value of TI is chosen tnat will be acceptable for the purpose of the measurement being made. In some cases a relatively large value of TI will be acceptable; in other cases a small value will be necessary. The physical significance of the parameter o is based on the fact that counting rates in at least two distinct time intervals nust be measured In order to determine a lifetime. Assuming the time Intervals to be equal, the statistical precision within which T can be determined depends primarily on the counting rate in the time interval that occurs latest in time following the burst frol the neutron source. If t is the delay time corresponding to the beginning of the latest interval, then the ratio: Counting rate (t)/Counting rate (0) may be taken as a measure of the statistical precision of #. The above ratio is a function of t and depends on the detector/filter combination; its primary dependence is on filter characteristics. As with the parameter n, an operational value for p is chosen Dased on observed die-away curves ana on a knowledge of how "far down" a decay curve measurements can be made that are statistically useful. The equality conditions of equations (1) and (2) are satisfied for given values of TI and o at counting rate delay times TI and tp. Filter thickness optimization can then be expressed as follows: for a given filter type, that filter thickness is chosen wnicn maximizes the smaller of the delay times tn and tp . Tnis optimization criterion may oe expressed matnematically as: Maximize tmin=Sm(tn, tp) (3) where: tmin is tne smaller of tn and to. The values of tn and tp will depend primarily on the filter type and thickness, Xf, and the values chosen for # ando. Thus, for a given filter type tn = f(Xf,fl) (4) tp = g(Xf,p) (5) where f and g represent functional dependence. For given values of n and , FIG. 3 shows the qualitative oehavior of tn and tp as a function of Xf. The maximum of Sm(te, tp) occurs when tn equals tp and this condition results in a unique optimum value of filter thickness (Xf)OpT. An example of tnis method for determining an optimum filter tnickness can be illustrated for a helium-3 detector that is 6.35 mm in diameter, 25.4 mm in length and filled with nelium-3 gas to a pressure of 10 bar. The type of filter material selected for this example is gadolinium. Calculations can be made to determine the time-dependent epithermal and total (epitnermal plus thermal) relative counting rates for a range of gadolinium thickness Xf Die-away plots such as those of FIG. 2 can be obtained for each value of Xf. From these plots, values of eT(t) = [T(t) - # epi(t)]/#epi (t) (6) and R(t) = Counting rate (t)/Counting rate (0) (7) can be determined and plotted as a function of tine, t. Note tnat ET(t) and R(t) are functions of t, in contrast to n and p wnicn are parameters wnose values are selected. Thus, for each value of Xf a pair of curves brand R is obtained. For selected values of n and P the plots of ## and R are used to determine tn and tp that satisfy tne conditions: dTn(t ) < n (8) and R(tp) > p (9) Plots of dT(t) and R(t) are shown in FIGS. 4-6 for a typical case of a nign-porosity sandstone formation. FIG. 4 is for a gadolinium thickness that is considerably thinner than optimum, FIG. 5 is for gadolinium that is considerably thicker than optilL-, and FIG. 6 is for gadolinium of near optimum thickness. In this example, n = 0.05 and p = 0.125. The values trj and tp that are implied by equations (8) and (9) are indicated in the arawings. FIG. 7 shows plots of tri (solid line) and tp (DroKen line) as a function of gadolinium thickness for tnree sets of values of n and P: Set No. n D (Xf)OpT, C) 1 0.10 0.25 0.075 2 0.05 0.125 0.15 3 0.02 0.05 0.3 The values obtained for the optimum gadolinium thickness in each set are given above. Sets 1 and 3 are considered to be the extreme values of n and p for satisfactory epithermal die-away measurements. Set 2 is considered to be the best compromise for 10 and p. Thus, for gadolinium, (Xf)gpT is in the range of 0.075 to 0.3 mm with a preferred value of 0.15 mm. It is to be understood that this procedure descriDes the determination of the optimum filter tnickness of gadolinium as one example. Other filter materials may be successfully utilized as well as an infinite variety of combinations of filter materials.

Claims: CLAIMS: 1. An epitnermal neutron detector, comprIsing: (a) a neutron-sensitive counter, and (b) a neutron filter encasing the counter, having an optimum thickness which provides for a counting rate delay time corresponding to a selected lower limit for statistical precision of epithermal die-away measurements, the delay time being equal to a counting rate delay time corresponding to a selected upper limit for thermal neutron effects. 2. An epithermal neutron detector according to claim 1, wherein the neutron filter comprises gadolinium. 3. An epitnermal neutron detector according to claim 2, wherein the thickness of the gadolinium neutron filter is at least 0.075-i. 4. An epitnermal neutron detector according to claim 2, wherein the thickness of tne gadolinium neutron filter is no greater than 0.3 mm. 5. A system for epitnermal neutron die-away logging of subterranean formations surrounding a Dorenole comprising: (a) a source of fast neutrons for irradiating the formations surrounding the borehole, (b) a neutron counter for counting epithermal neutrons returning to tne borehole from the irradiated formations, and (c) a neutron filter surrounding the neutron counter having a thickness so as to permit the longest possible delay time following each burst of fast neutrons from the source before the epithermal neutrons from the irradiated formations are detected by the neutron counter wnile optimizing tne epitnermal neutron counting rates by minimizing tnermal neutron effects. 6. An epithermal neutron die-away logging system according to claim 5, wherein the thickness for the neutron filter is such that the following conditions are satisfied: LT(t) - T ep1(t) )/T1(t) j TI and Counting rate (t)/Counting rate (0) > p where: T(t) = observed neutron lifetime at time delay t, Tepi(t) = epithermal neutron lifetime at delay time t, epi Counting rate (t) = counting rate at time delay t, Counting rate (0) = counting rate at zero time delay, n = a dimensionless parameter cnosen to permit a given admixture of thermal neutron response, and P = a dimensionless parameter chosen to permit a given relative statistical precision. 7. An epitnermal neutron ale-away logging system according to claim 5 or claim 6, wnerein optimization of tne thickness of tne neutron filter is satisfied oy: Maximize t;nin = Sm (tn, to) where: tmin is the smaller of times trl tn and ## = delay tine for which (# - #epi)/Tepi = #, and tp = delay time for wnicn Counting rate (t)/Counting rate (0) = p 8. An epithermal neutron die-away logging system according to claim 7, wherein n is in the range of 0.02 to 0.10 and p is in the range of 0.05 to 0.25. 9. An epitnermal neutron die-away logging system according to any one of claims 5 to 8, wherein the neutron filter comprises gadolinium having a thickness of 0.075 to 0.3 mm. 10. A method for measuring the epithermal neutron ale-away cnaracteristics of a subsurface formation surrounding a Dorenole, comprising tne steps of: (a) irradiating tne subsurface formation surrounding tne borehole with bursts of fast neutrons, and (b) filtering the neutron radiation retuning to the borehole and counting tne epithermal neutrons in that radiation usiny an epitnermal neutron detector according to any one of claims 1 to 4.

Inventors: ALLEN, LINUS SCOTT, MILLS, WILLIAM RAYMOND, JR.
Applicants: MOBIL OIL CORPORATION
Application year: 1984
Main classification: G01V5
IPC codes: G01V5