Schlumberger Technology Corporation

Services Petroliers Schlumberger

Tong Zhou

Christian Stoller

David Rose

Sicco Beekman

Jeffrey Miles

Richard J Radtke

James Leslie Thornton

Adil Busaidy

Timothy M Quinlan

Bradley Albert Roscoe

James A Grau

Felix K Chen

Darwin V Ellis

Charles R Case

Fabien Haranger

Pascal Millot

A presence of cement may be identified based on a downhole tool that may emit neutrons into a wellbore having at least one cement casing. The neutrons may interact with the particular material via inelastic scattering, inelastic neutron reactions, capture of neutrons and/or neutron activation through one of these reactions and cause a material to emit an energy spectrum of gamma rays, and wherein the downhole tool is configured to detect an energy spectrum of the gamma rays that is specific to at least one of a plurality of elements and associated a region within the wellbore. An amount of elements, such as calcium and silicon, may be determined from the gamma ray spectra that may indicate a present of cement within the wellbore.

Systems and methods for determining the presence of cement behind at least one casing using spectroscopy measurement

Elemental concentrations in subterranean formations may be determined using neutron spectroscopy. For example, neutrons may be emitted by a downhole tool into the formation and produce gamma rays via inelastic scattering of fast neutrons or capture of slow neutrons. The borehole surrounding a downhole tool may introduce artifacts in the neutron spectroscopy measurement. Embodiments of the present disclosure are directed to techniques that reduce artifacts signals in downhole tools that include one or multiple detectors based at least in part on the inelastic and capture measurements.

Compensated spectroscopy measurements

A method for determining a fractional volume of at least one component of a formation includes entering into a computer a number of detected radiation events resulting from imparting neutrons into the formation at an energy level of at least 1 million electron volts (MeV). The detected radiation events correspond to at least one of an energy level of the imparted neutrons and thermal or epithermal energy neutrons. A measurement of at least one additional petrophysical parameter of the formation is made. The at least one additional petrophysical parameter measurement and at least one of a fast neutron cross-section and a thermal neutron cross-section determined from the detected radiation events are used in the computer to determine the fractional volume of the at least one component of the formation. In another embodiment, the fast neutron cross-section and the thermal neutron cross-section may be used on combination to determine the fractional volume.

Method for using neutron interaction cross section to interpret neutron measurements

A method for evaluating a formation includes determining a number of detected gamma rays resulting from imparting neutrons into a formation. The detected gamma rays are each characterized by an energy level thereof. The gamma rays are detected at a first distance from a position of imparting the neutrons into the formation. Those of the detected gamma rays attributable to neutron capture by hydrogen nuclei are removed from the number of detected gamma rays. The number of detected gamma rays having hydrogen neutron capture gamma rays removed therefrom are used to calculate a property of the formation.

Method for evaluating formations using neutron induced gamma ray measurements

A system may include a pulsed neutron generator designed to emit neutrons into a borehole of a geological formation using a pulsing scheme. The system may also include a gamma-ray detector designed to take measurements of capture gamma-rays during a time period during the pulsing scheme. The system may also include data processing circuitry designed to calculate one or more sigma values based at least in part on the measurements of the capture gamma-rays taken during the time period during the pulsing scheme. The data processing circuitry may also calculate a factor of yields to weights value based at least in part on the one or more sigma values and convert a plurality of relative yields of corresponding elements in the geological formation to a plurality of elemental relative weights based at least in part on the factor of yields to weights.

Systems and methods to determine relative elemental concentrations from nuclear spectroscopy measurements

A method for determining formation hydrogen index includes using as input to a computer measurements of numbers of burst gamma rays (gamma rays detected during operation of a pulsed neutron source) and numbers of thermal neutron capture gamma rays made at two different axial spacings from the pulsed neutron source. A ratio of the numbers of burst gamma rays and a ratio of the numbers of thermal neutron capture gamma rays is determined. A corrected ratio of the numbers of thermal neutron capture gamma rays using the ratio of numbers of burst gamma rays is determined. The formation hydrogen index is determined from the corrected ratio.

Borehole fluid effect correction for pulsed neutron porosity measurements

A method for evaluating wellbore conduit condition includes using measurements of at least one of (i) inelastic gamma rays made during emission a burst of neutrons into the conduit from within the conduit at at least one spaced apart location from a position of the emission and (ii) epithermal neutrons or capture gamma rays therefrom detected at at least two spaced apart locations from the position of the emission within a selected time after the emission. The at least one of the measurements of inelastic gamma rays and epithermal neutron or capture gamma ray counts are characterized to estimate an amount of loss of iron in the conduit.

Casing inspection using pulsed neutron measurements

Systems and methods are provided for distinguishing between elements located at different distances from a radiation detector used in neutron-induced gamma-ray spectroscopy using a Doppler effect. A pulsed neutron generator may emit neutrons out of a downhole tool in a geological formation at an energy level high enough to cause inelastic scattering with nuclei of an element to generate gamma-rays. A gamma-ray detector may detect the energy levels of the gamma-rays, in the reference frame of the detector, and data processing circuitry takes the detected spectrum of gamma-rays and distinguishes spectra of gamma-ray energy levels for nuclei of the element located nearer to or farther from the detector based at least in part on the Doppler shift of the energy levels of respective gamma-rays.

Systems and methods to differentiate elements located at different distances using neutron-induced gamma-ray spectroscopy and the doppler effect

A method for determining a petrophysical property of a formation includes detecting radiation events resulting from imparting neutrons into the formation at an energy level of at least 1 MeV. The petrophysical property is determined from an elastic scattering cross section of the formation. The elastic scattering cross section is related to a number of detected radiation events.

A method for determining a petrophysical property of a formation includes detecting gamma rays at two different spaced apart positions from a position of emitting neutrons into the formation at an energy level sufficient to induce inelastic scatting gamma rays. The neutrons are emitted in a plurality of bursts of neutrons into the formation, the bursts each having a first selected duration. Each burst is followed by a wait time having a second selected duration, the gamma rays detected during each of the bursts and each of the wait times. A ratio of numbers of gamma rays detected during the bursts is determined (burst ratio). A ratio of numbers of gamma rays detected during the wait times is determined (capture ratio). The burst ratio is used to correct the capture ratio. The petrophysical property is determined from the corrected capture ratio.

Method for using pulsed neutron induced gamma ray measurements to determine formation properties

A method for determining hydrogen index includes using measurements of gamma rays detected during operation of a pulsed neutron source and numbers of thermal neutron capture gamma rays made at at least two different axial spacings from a pulsed neutron source. Either a first ratio of or a first logarithm of the first ratio of the burst gamma rays is determined for a first and second axial distance from the neutron source. A second logarithm is determined of a second ratio of the thermal neutron capture gamma rays detected at a first axial spacing with respect to the numbers detected at a second axial spacing. A corrected second ratio of the numbers of thermal neutron capture gamma rays is determined using either (i) the first logarithm and (ii) the first ratio, and the second logarithm. The corrected second ratio is used to determine the hydrogen index.

Borehole fluid and/or casing effect correction for pulsed neutron measurements

A method for determining a formation thermal neutron decay rate from measurements of radiation resulting from at least one burst of high energy neutrons into formations surrounding a wellbore includes determining a first apparent neutron decay rate in a time window beginning at a first selected time after an end of the at least one burst, a second apparent decay rate from a time window beginning at a second selected time after the burst and a third apparent decay rate from a third selected time after the burst. The second time is later than the first time. A thermal neutron capture cross section of fluid in the wellbore is determined. A decay rate correction factor is determined based on the first and second apparent decay rates and a parameter indicative of the wellbore capture cross-section. The correction factor is applied to the third apparent decay rate to determine the formation thermal neutron decay rate.

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Sugar Land, TX, United States of America

Tong Zhou