Evaluation of formation composition using neutron induced gamma spectroscopy tools
Systems and methods for measuring formation properties in downhole operations are provided. The systems and methods include generating, at a neutron source, neutrons that are emitted into a downhole formation ( 302 ), registering, at a detector, photons generated by chemical constituents of the downhole formation ( 304 ), measuring a response of the photons registered at the detector ( 306 ), transforming, with a computing system, the measured responses of the photons registered at the detector into thermal neutron capture probabilities ( 308 ), and transforming the thermal neutron capture probabilities into weight concentrations of the chemical constituents of the downhole formation ( 312 ).
1. A method for measuring formation properties in downhole operations, the method comprising:
generating, at a neutron source, neutrons that are emitted into a downhole formation;
registering, at a detector, photons generated by chemical constituents of the downhole formation;
measuring a response of the photons registered at the detector, wherein measuring the response of the photons comprises both (i) measuring capture photon energy spectra and (ii) measuring thermal neutron decay time;
transforming, with a computing system, the measured responses of the photons registered at the detector into thermal neutron capture probabilities; and
transforming the thermal neutron capture probabilities into weight concentrations of the chemical constituents of the downhole formation.
2. The method of claim 1 , wherein the neutron source is a pulsed neutron source.
3. The method of claim 1 , further comprising normalizing the thermal neutron capture probabilities to unity prior to obtaining the weight concentrations.
4. The method of claim 1 , wherein the computing system comprises a surface computing element and a downhole computing element.
5. The method of claim 1 , further comprising performing a drilling operation using bottomhole assembly, wherein the neutron source, the detector, and at least a part of the computing system are part of the bottomhole assembly.
6. The method of claim 1 , wherein the thermal neutron capture probabilities are linear functions of elemental yields.
7. The method of claim 6 , wherein the thermal neutron capture probabilities are calculated as
P
j
=
ρ
Y
j
-
B
j
K
j
,
j=1, N, wherein Y j is the yield of the j-th element, ρ is formation density, and K j and B j are tool sensitivity factors.
8. The method of claim 1 , wherein transforming the thermal neutron capture probabilities into weight concentrations comprises representing the thermal neutron capture probabilities as functions of cross-sections.
9. The method of claim 1 , further comprising at least one of measuring a thermal neutron decay time, wherein the weight concentration is calculated as
W
j
=
Σ
·
P
j
·
M
j
N
A
·
ρ
·
σ
j
,
j=1, N, wherein Wj is the weight concentration of the j-th element, Σ is a formation macroscopic cross section of thermal neutron absorption, P j is the thermal neutron capture probability, M j is the molar mass, N A is the Avogadro constant, ρ is formation density, and σ j is a microscopic cross section of thermal neutron absorption.
10. A system for measuring formation properties in downhole operations, the system comprising:
a neutron source configured to emit neutrons into a downhole formation;
a detector configured to register photons that are generated by chemical constituents of the downhole formation; and
a computing system configured to:
measure a response of the photons detected by the detector;
transform the measured response of the photons into thermal neutron capture probabilities;
normalize the thermal neutron capture probabilities to unity prior to obtaining the weight concentrations; and
transform the thermal neutron capture probabilities into weight concentrations of the chemical constituents of the downhole formation.
11. The system of claim 10 , wherein the neutron source is a pulsed neutron source.
12. The system of claim 10 , wherein the computing system comprises a surface computing element and a downhole computing element.
13. The system of claim 10 , further comprising a bottomhole assembly for performing drilling operations, wherein the neutron source, the detector, and at least a part of the computing system are part of the bottomhole assembly.
14. The system of claim 10 , wherein measuring the response of the photons comprises measuring capture photon energy spectra and measuring thermal neutron decay time.
15. The system of claim 10 , wherein the thermal neutron capture probabilities are linear functions of elemental yields.
16. The system of claim 15 , wherein the thermal neutron capture probabilities are calculated as
P
j
=
ρ
Y
j
-
B
j
K
j
,
j=1, N, wherein Y j is the yield of the j-th element, ρ is formation density, and K j and B j are tool sensitivity factors.
17. The system of claim 10 , wherein transforming the thermal neutron capture probabilities into weight concentrations comprises representing the capture probabilities as functions of cross-sections.
18. The system of claim 10 , the computing system further configured to measure a thermal neutron decay time, wherein the weight concentration is calculated as
W
j
=
Σ
·
P
j
·
M
j
N
A
·
ρ
·
σ
j
,
j=1, N, wherein W j is the weight concentration of the j-th element, Σ is a formation macroscopic cross section of thermal neutron absorption, P j is the thermal neutron capture probability, M j is the molar mass, N A is the Avogadro constant, ρ is formation density, and σ j is a microscopic cross section of thermal neutron absorption.
19. A method for measuring formation properties in downhole operations, the method comprising:
generating, at a neutron source, neutrons that are emitted into a downhole formation;
registering, at a detector, photons generated by chemical constituents of the downhole formation;
measuring a response of the photons registered at the detector;
transforming, with a computing system, the measured responses of the photons registered at the detector into thermal neutron capture probabilities, wherein the thermal neutron capture probabilities are linear functions of elemental yields; and
transforming the thermal neutron capture probabilities into weight concentrations of the chemical constituents of the downhole formation.