Method and apparatus for using pulsed neutron generators and dual-function detectors to obtain formation mineralogy
A method for obtaining elemental concentration and determining the formation mineralogy uses a tool having multiple dual-function detectors that can detect both neutrons and gamma rays simultaneously. The method includes emitting neutrons into the formation, detecting neutron and gamma ray signals from the formation and discriminating the neutron signal and the gamma ray signal, obtaining the space, time and energy dependent fluence rates for fast neutrons and thermal neutrons, obtaining gamma ray energy spectra from inelastic scattering and neutron capture reactions from one or more detectors, obtaining the energy spectrum of inelastic gamma rays and/or capture gamma rays from a single element, calculating concentration of the element in the formation using its energy spectrum of inelastic gamma rays or capture gamma rays from one or more detectors and the space, time and energy dependent fluence rate of fast neutrons or thermal neutrons, and determining the formation mineralogy.
1. A method for evaluating a formation mineralogy of a subterranean formation, comprising:
S 1 : deploying a nuclear logging tool into the subterranean formation, wherein the nuclear logging tool comprises one or more neutron generators and two or more dual-function detectors configured to simultaneously detect neutrons and gamma rays
S 2 : causing the one or more neutron generators to emit neutrons into the subterranean formation;
S 3 : detecting neutrons and gamma rays from the subterranean formation at the three or more detectors to form electric signals;
S 4 : separating electric signals from neutrons and gamma rays for each of the three or more detectors;
S 5 : obtaining detector count rates of total neutrons, fast neutrons and thermal neutrons; obtaining space, time and energy dependent neutron fluence rates of fast neutrons and thermal neutrons;
S 6 : obtaining detector count rates of inelastic gamma rays and capture gamma rays, and the energy spectrum of inelastic gamma rays and the energy spectrum of capture gamma rays;
S 7 : calculating elemental concentrations of a plurality of elements in the formation using the energy spectrum of inelastic gamma rays and/or the energy spectrum of capture gamma rays, and the space, time and energy dependent neutron fluence rate of fast neutrons and/or the space, time and energy dependent neutron fluence rate of thermal neutrons; and
S 8 : determining the formation mineralogy based on the elemental concentration of the plurality of elements in the formation.
2. The method of claim 1 , wherein the neutron generators is utilized to generate neutrons in either pulse mode or continuous mode.
3. The method of claim 1 , wherein the plurality of elements is selected from Mg, Fe, S, C, Al, Si, Ca, O, Ti, K, Gd, Cl, and H.
4. The method of claim 3 , wherein single element gamma ray energy spectrum for each of a plurality of elements is obtained.
5. The method of claim 4 , further comprising obtaining space, time and energy dependent neutron fluence rate using neutrons detected at each dual-function detector.
6. The method of claim 5 , wherein the elemental concentration is calculated using a gamma ray count rate and a space, time and energy dependent neutron fluence rate obtained at one or more of the pluralities of dual-function detectors.
7. The method of claim 6 , wherein the neutron fluence rate is a fast neutron fluence rate or a thermal neutron fluence rate.
8. The method of claim 6 , wherein the gamma ray count ray is a count rate of gamma rays generated by fast neutron inelastic scattering or by thermal neutron capture reactions.
9. The method of claim 6 , wherein the elemental concentration is calculated according to
n
i
(
t
)
=
C
i
(
t
,
E
γ
)
S
d
ϵ
(
E
γ
)
∫
0
V
∫
0
14
e
-
∑
L
4
π
L
2
Φ
(
r
⇀
,
t
,
E
n
)
σ
i
(
E
n
)
dE
n
d
Ω
wherein:
C i (t, E γ )E γ S d is the detector count rate (1/s) of gamma rays at energy from element i;
C i (t, E γ )E γ S d is detector's effective area (cm 2 );
C i (t, E γ )E γ S d Σ is macroscopic absorption cross section (1/cm) of formation to gamma rays from element i;
C i (t, E γ )E γ S d L is the distance from the detector to a small volume Ω;
C i (t, E γ )E γ S d is the distance vector from the neutron source to a small volume Ω;
Φ( , t, E n ) is the neutron fluence rate at time t, at energy En, at position , per unit energy bin, per unit time;
ni (t) is atomic density of the target element i at time t, which changes with time as the logging tool passing through different depth intervals;
σ i (E n ) is cross section of fast neutron inelastic scattering or thermal neutron capture reaction with the target element i when the neutron energy in En;
ϵ(E γ ) is relative detection efficiency of detector to gamma rays at energy E γ (unitless); and
V is the total volume of near wellbore space (mainly wellbore and formation) where neutron-induced gamma rays can get to the detector.
10. The method of claim 1 , wherein the nuclear logging tool has at least one neutron generator and at least two dual-function detectors at different locations configured to simultaneously detect both neutrons and gamma rays.
11. The method of claim 1 , wherein the neutron source can be a D-T neutron generator or a D-D neutron generator.
12. The method of claim 1 , wherein the inelastic gamma rays and the capture gamma rays can be recorded separately or together in the total energy spectrum without further discrimination.