CN115267930A - High-sensitivity neutron porosity measurement method based on D-T pulse neutron source - Google Patents
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Abstract
The invention discloses a high-sensitivity neutron porosity measuring method based on a D-T pulse neutron source. The method adopts a four-detector pulse neutron logging instrument provided with a D-T pulse neutron source, two neutron detectors and two gamma detectors to carry out measurement, obtains the non-elastic gamma count ratio of the near gamma detector and the far gamma detector and the thermal neutron count ratio of the near gamma detector and the far gamma detector, calculates the neutron porosity evaluation parameter of the stratum, establishes the relation between the neutron porosity evaluation parameter and the porosity by a Monte Carlo numerical value calculation method, and accurately determines the porosity of the stratum. According to the method, the thermal neutron count ratio of the double-neutron detector is combined with the non-elastic gamma count ratio of the double-gamma detector, the formation porosity is represented by utilizing the neutron porosity evaluation parameter, the influence of the formation density on the measurement of the hydrogen-containing index is eliminated, the formation porosity is accurately obtained, and meanwhile the porosity sensitivity of the neutron porosity logging instrument in the high-porosity formation is effectively improved.
Description
Technical Field
The invention belongs to the technical field of geophysical logging of a mine field, and particularly relates to a high-sensitivity neutron porosity measuring method based on a D-T pulse neutron source.
Background
The formation porosity is one of the most important geological parameters in the petroleum exploration and development process, and has important significance in the aspects of calculating the oil production capacity of the formation, identifying a gas layer and the like. As the most commonly used formation porosity measurement method, the adopted Am-Be neutron source has a series of problems in the aspects of safety, health, environment and the like, and in order to adapt to the requirements of a new logging method, the Am-Be neutron source is gradually replaced by a D-T pulse neutron source along with the development of a neutron source. The average energy of neutrons emitted by the D-T pulse neutron source is 14MeV, which is obviously higher than the average energy of neutrons generated by the Am-Be source, but the porosity sensitivity of the neutron porosity logging instrument is reduced when the D-T pulse neutron source is adopted to measure the porosity of the stratum due to the obvious influence of the density of the stratum on the hydrogen index.
In the conventional research, the sensitivity of formation porosity measurement can be improved by performing density correction by using the thermal neutron count ratio of a detector, and the influence of formation density on the sensitivity of the porosity measurement can be eliminated by establishing the relationship among the thermal neutron count ratio, the formation density and the hydrogen index. Therefore, it is urgently needed to provide a high-sensitivity neutron porosity measurement method of a D-T pulse neutron source, which eliminates the influence of formation density on hydrogen-containing index measurement and improves the porosity sensitivity of a neutron porosity logging instrument.
Disclosure of Invention
Aiming at the defects, the invention provides a high-sensitivity neutron porosity measuring method based on a D-T pulse neutron source, and based on a measuring system of the D-T pulse neutron source, a double-neutron detector and a double-gamma detector, the formation porosity is represented by utilizing the thermal neutron counting ratio of a near neutron detector and a far neutron detector and the non-elastic gamma counting ratio of the near gamma detector and the far gamma detector, so that the influence of the formation density on the measurement of the hydrogen index of the formation is effectively eliminated, the accurate acquisition of the formation porosity is realized, and the porosity sensitivity of a neutron porosity logging instrument in a high-porosity formation is improved.
In order to achieve the purpose, the invention adopts the following technical scheme:
a high-sensitivity neutron porosity measurement method based on a D-T pulse neutron source adopts a four-detector pulse neutron logging instrument for measurement, a measurement system consisting of the D-T pulse neutron source, two neutron detectors and two gamma detectors is arranged in the four-detector pulse neutron logging instrument, and the method specifically comprises the following steps:
placing a four-detector pulse neutron logging instrument in a well to be attached to a well wall for measurement, emitting fast neutrons by a D-T pulse neutron source in a pulse mode, and recording non-elastic gamma counting of a near gamma detector, non-elastic gamma counting of a far gamma detector, thermal neutron counting of the near neutron detector and thermal neutron counting of the far neutron detector;
determining a neutron porosity evaluation parameter R of the stratum according to the thermal neutron count ratio of the near neutron detector and the far neutron detector and by combining the non-elastic gamma count ratio of the near gamma detector and the far gamma detectortiComprises the following steps:
in the formula, RthIs the thermal neutron count ratio of a near neutron detector and a far neutron detector, RinThe non-elastic gamma count ratio of the near gamma detector to the far gamma detector is obtained;
step three, based on a Monte Carlo numerical calculation method, establishing an MCNP numerical calculation model according to a four-detector pulse neutron logging instrument, changing the formation porosity of the MCNP numerical calculation model, simulating to obtain the non-elastic gamma counting of a near gamma detector, the non-elastic gamma counting of a far gamma detector, the thermal neutron counting of the near neutron detector and the thermal neutron counting of the far neutron detector under different formation porosities, calculating to obtain neutron porosity evaluation parameters corresponding to different formation porosities, and establishing a relation between the neutron porosity evaluation parameters and the formation porosity as follows:
φ＝ARti 2+BRti+C (2)
where phi is the formation porosity, RtiA, B and C are scale coefficients for neutron porosity evaluation parameters;
and fourthly, obtaining accurate formation porosity by utilizing the neutron porosity evaluation parameter of the formation based on the relation between the neutron porosity evaluation parameter and the formation porosity.
Preferably, the instrument diameter of the four-detector pulsed neutron logging instrument is set to be 89mm, the source distances of the near gamma detector and the far gamma detector are set to be 31.25cm and 70cm respectively, and the source distances of the near neutron detector and the far neutron detector are set to be 31.25cm and 57.5cm respectively.
Preferably, a tungsten-nickel-iron shield is arranged between the D-T pulse neutron source and the near neutron detector.
Preferably, in the third step, the hole diameter in the MCNP numerical calculation model is set to 200mm, the lithology of the formation is set to limestone, and the variation range of the porosity of the formation in the MCNP numerical calculation model is 0 to 100%.
The invention has the following beneficial technical effects:
the invention is based on a four-detector pulse neutron logging instrument, adopts a measuring system consisting of a D-T pulse neutron source, two neutron detectors and two gamma detectors to simultaneously obtain the thermal neutron counts of a near neutron detector and a far neutron detector and the non-elastic gamma counts of the near gamma detector and the far gamma detector, calculates the neutron porosity evaluation parameters of a stratum by utilizing the thermal neutron count ratio of the near neutron detector and the far neutron detector and the non-elastic gamma count ratio of the near gamma detector and the far gamma detector to determine the porosity of the stratum, and realizes the accurate measurement of the porosity of the stratum.
According to the method, the formation porosity is determined by combining the thermal neutron count ratio and the non-elastic gamma count ratio of the four-detector pulse neutron logging instrument, the influence of the formation density on the measurement result of the formation hydrogen-containing index can be eliminated without depending on the density information of the formation, the porosity sensitivity of the neutron porosity logging instrument in the high-porosity formation is effectively improved, and a foundation is laid for the exploration and development of oil and gas resources.
Drawings
FIG. 1 is a schematic diagram of a four-detector pulsed neutron logging tool. In the figure, 1, a far gamma detector, 2, a far neutron detector, 3, a near neutron detector, 4, a near gamma detector, 5, a W-Ni-Fe shield, 6, a D-T pulse neutron source, 7, a borehole, 8 and a stratum.
FIG. 2 is a response rule of thermal neutron count ratio of a four-detector pulse neutron logging instrument under different formation conditions. In the figure, the change of the fixed formation density in the hydrogen index curve corresponds to a formation condition in which the fixed formation density is 2.19g/cm3Changing the hydrogen index of the stratum, wherein the change range of the hydrogen index is 0-1; the stratum conditions corresponding to the curve of changing the stratum density by fixing the hydrogen index are that the hydrogen index is 0.3, the stratum density is changed, and the change range of the stratum density is 1-2.708 g/cm3(ii) a Simultaneously changing the stratum conditions corresponding to the stratum density and the hydrogen index curve into the stratum density range of 1-2.708 g/cm3Correspondingly changing the hydrogen index of the stratum, wherein the change range of the hydrogen index is 0-1.
FIG. 3 is a response rule of non-gamma-ray count ratio of a four-detector pulse neutron logging instrument under different formation conditions. In the figure, the change of the fixed formation density in the hydrogen index curve corresponds to a formation condition in which the fixed formation density is 2.19g/cm3Changing the hydrogen index of the stratum, wherein the change range of the hydrogen index is 0-1; the stratum conditions corresponding to the curve of changing the stratum density by fixing the hydrogen index are that the hydrogen index is 0.3, the stratum density is changed, and the change range of the stratum density is 1-2.708 g/cm3(ii) a Simultaneously changing the stratum conditions corresponding to the stratum density and the hydrogen index curve into the stratum density range of 1-2.708 g/cm3Correspondingly changing the hydrogen index of the stratum, wherein the change range of the hydrogen index is 0-1.
FIG. 4 is a response rule of thermal neutron count ratio and neutron porosity evaluation parameters under different formation porosity conditions.
FIG. 5 is a graph of relative sensitivity when porosity is characterized using thermal neutron count ratio and neutron porosity evaluation parameters under different formation porosity conditions.
Detailed Description
The invention is described in further detail below with reference to the following figures and detailed description:
after entering the stratum, fast neutrons emitted by the D-T pulse neutron source react with the nuclei of stratum materials through inelastic scattering, elastic scattering, radiation capture and the like, and the energy of the neutrons is reduced to become epithermal neutrons and thermal neutrons. The distribution of thermal neutron flux in the formation is dependent on the distance r traveled by the neutrons and the length of deceleration L of the formation mediumSThe formation density has a non-negligible effect on the distribution of thermal neutrons, and therefore, the distribution of thermal neutrons in the formation is affected by both the hydrogen index and the formation density. When the porosity of the stratum is smaller, the distribution range of thermal neutrons in the stratum is wide, but with the increase of the porosity of the stratum, the hydrogen index of the stratum is increased, the deceleration capacity of fast neutrons is enhanced, and the distribution range of the thermal neutrons in the stratum is gradually reduced. The thermal neutron count rate recorded by the detectors of the neutron porosity logging instrument decreases, with the thermal neutron count rate of the far detector decreasing faster than that of the near detector, thus showing an increase in the thermal neutron count ratio of the near and far detectors as the porosity of the formation increases. Meanwhile, the formation density is reduced along with the increase of the porosity, and the deceleration and absorption effects of the formation framework on neutrons are gradually reduced, so that the speed of the count rate reduction of the detector is slowed down, and the sensitivity of the thermal neutron count ratio on the porosity is reduced. In order to improve the porosity sensitivity of the neutron porosity logging instrument, the effect of the formation density in neutron transport needs to be considered, density compensation is carried out on thermal neutron distribution, and the influence of the formation density on the porosity representation is eliminated.
According to the theory of two-component diffusion, in an infinite homogeneous medium, the source distance is r1And r2Two neutron detectors of (2) recording a thermal neutron count ratio RthCan be described approximately as:
in the formula (I), the compound is shown in the specification,
is the thermal neutron count rate of the near neutron detector,
is the thermal neutron count rate, r, of a far neutron detector1Is the source distance, r, of the near neutron detector2Source distance, L, for far neutron detectorsSIs the deceleration length of the formation medium.
For an Am-Be neutron source, the hydrogen index of a stratum is a main reason for determining the distribution of thermal neutrons in the stratum, so that the relationship between the thermal neutron count ratio of a detector and the hydrogen index of the stratum can Be directly utilized when the Am-Be neutron source is adopted for measurement, but when the D-T pulse neutron source is adopted for measurement, the thermal neutron count ratio of the detector is utilized for representing the influence of the density of the stratum on the hydrogen index, so that the thermal neutron count ratio is only utilized for representing the hydrogen index of the stratum is inappropriate, and therefore, when the D-T pulse neutron source is adopted for measurement in neutron porosity measurement, the thermal neutron count ratio and the density of the stratum are combined to determine the hydrogen index of the stratum.
According to the neutron-gamma coupling field theory, the inelastic scattering gamma ray flux Φ (r) at the neutron travel distance r is:
where, ΣinIs a macroscopic inelastic scattering cross section of the stratum S0Is a strong source of neutron, lambdasIs the fast neutron scattering free path, rho is the formation density, mumAnd i is the number of gamma photons released by inelastic scattering of a single fast neutron and stratum elements, wherein the mass attenuation coefficient is I.
And (3) transforming the formula (4) according to the Lagrange median theorem to obtain:
where ξ is in (ρ μ [ ])m,1/λs) In the range shown in equation (6):
ξ＝(1-α)(1/λs)+αρμm α∈(0,1) (6)
then near gamma detector (source distance r)3) And far gamma detector (source distance r)4) Non-ballistic gamma ratio RinExpressed as:
therefore, when the source distance is fixed, the non-elastic gamma count ratio R of the near gamma detector and the far gamma detectorinThe formation density can be directly characterized by using a non-ballistic gamma count ratio, which is related only to the formation density. Therefore, in order to improve the sensitivity of the instrument to the porosity while eliminating the influence of the formation density on the formation porosity measurement, the thermal neutron count ratio and the non-elastic gamma count ratio are combined to obtain a new porosity characteristic parameter, namely a neutron porosity evaluation parameter Rti。
Based on the principle, the invention provides a high-sensitivity neutron porosity measuring method based on a D-T pulse neutron source, which adopts a four-detector pulse neutron logging instrument to measure, wherein a measuring system consisting of the D-T pulse neutron source, two neutron detectors and two gamma detectors is arranged in the four-detector pulse neutron logging instrument, and the method specifically comprises the following steps:
placing a four-detector pulse neutron logging instrument in a borehole 7, and measuring the instrument by sticking a borehole wall in the borehole, wherein the diameter of the four-detector pulse neutron logging instrument is set to be 89mm, and a measuring system consisting of a D-T pulse neutron source 6, two neutron detectors and two gamma detectors is arranged in the instrument, as shown in fig. 1, the neutron detectors and the gamma detectors are both in a cylindrical structure, the diameter of a near neutron detector 3 is 4cm, the length of the near neutron detector is 8cm, the diameter of a far neutron detector 2 is 6cm, the length of the far neutron detector is 9cm, the diameter of the near gamma detector 4 is 4cm, the length of the near gamma detector is 5cm, the diameter of the far gamma detector 1 is 6cm, the length of the far gamma detector is 9cm, the source distances of the near gamma detector 4 and the near neutron detector 3 are both set to be 31.25cm, the source distance of the far gamma detector 1 is set to be 70cm, and the source distance of the far neutron detector 2 is set to be 57.5cm; a tungsten nickel iron shielding body 5 in a cylindrical structure is arranged between the D-T pulse neutron source 6 and the near neutron detector 3, and the tungsten nickel iron shielding body 5 is 89mm in diameter and 10cm in length.
And (3) emitting fast neutrons into the stratum 8 in a pulse mode by using a D-T pulse neutron source of the four-detector pulse neutron logging instrument, and recording the non-elastic gamma count of the near gamma detector, the non-elastic gamma count of the far gamma detector, the thermal neutron count of the near neutron detector and the thermal neutron count of the far neutron detector.
Determining a thermal neutron count ratio according to the thermal neutron count of the near neutron detector and the thermal neutron count of the far neutron detector, determining a non-elastic gamma count ratio by utilizing the non-elastic gamma count of the near gamma detector and the non-elastic gamma count of the far gamma detector, combining the thermal neutron count ratio with the non-elastic gamma count ratio, and determining a neutron porosity evaluation parameter R of the stratumtiComprises the following steps:
in the formula, RthIs the thermal neutron count ratio of a near neutron detector and a far neutron detector, RinThe non-elastic gamma count ratio of the near gamma detector to the far gamma detector is obtained.
And thirdly, based on a Monte Carlo numerical calculation method, establishing an MCNP numerical calculation model according to an instrument structure of a four-detector pulse neutron logging instrument, changing the formation porosity of the MCNP numerical calculation model, setting the variation range of the formation porosity to be 0-100%, increasing the formation porosity by 10% each time, simulating to obtain the non-bomb gamma counting of a near gamma detector, the non-bomb gamma counting of a far gamma detector, the thermal neutron counting of the near neutron detector and the thermal neutron counting of the far neutron detector under different formation porosities, and calculating to obtain neutron porosity evaluation parameters corresponding to different formation porosities.
According to the porosity of different stratums and corresponding neutron porosity evaluation parameters thereof, fitting the porosity of the stratums and the neutron porosity evaluation parameters to establish a relation between the neutron porosity evaluation parameters and the porosity of the stratums as follows:
φ＝-0.01385Rti 2+0.62636Rti-0.20035 (8)
where φ is the formation porosity, RtiIs a neutron porosity evaluation parameter.
And fourthly, obtaining accurate formation porosity by utilizing the neutron porosity evaluation parameter of the formation based on the relation between the neutron porosity evaluation parameter and the formation porosity.
For better verification of the improvement of the porosity sensitivity of the neutron porosity logging instrument with the D-T pulse neutron source, the response rule of the thermal neutron count ratio and the non-elastic gamma count ratio of the four-detector pulse neutron logging instrument under the single different formation conditions simulated by the MCNP numerical calculation model is utilized, as shown in the figure 2 and the figure 3, the response rule in the figure 2 and the figure 3 is analyzed to find that the thermal neutron count ratio and the non-elastic gamma count ratio are both influenced by the formation hydrogen content index and the formation density, and the neutron porosity evaluation parameter R is caused by the change of the hydrogen content index along with the increase of the formation porosityti(i.e., the ratio of thermal to non-elastic gamma-counts) and the change in formation density results in a neutron porosity evaluation parameter RtiAnd decreases. However, when the porosity of the formation is larger, the thermal neutron count ratio is more influenced by the formation density, so that the sensitivity of the thermal neutron count ratio to the porosity of the formation is reduced, and the non-bomb-gamma count ratio mainly reflects the change of the formation density and can be used for eliminating the influence of the formation density on the thermal neutron count ratio of the four-detector pulse neutron logging instrument.
Comparing the response rules of the thermal neutron count ratio and the neutron porosity evaluation parameter (the ratio of the thermal neutron count ratio to the non-elastic gamma count ratio) under different formation porosity conditions, as shown in fig. 4, respectively calculating the relative sensitivity of the neutron porosity logging instrument under different formation porosity conditions when the thermal neutron count ratio and the neutron porosity evaluation parameter are adopted to characterize the porosity, as shown in fig. 5, wherein the relative sensitivity calculation formula is shown as formula (9):
where S is the relative sensitivity of the logging instrument to porosity.
From analysis of fig. 4 and 5, it can be seen that both the thermal neutron count ratio and the neutron porosity evaluation parameters of the four-detector pulsed neutron logging instrument decrease with increasing formation porosity. When the formation porosity is below 12%, the sensitivity of the thermal neutron gauge ratio to porosity is higher than the sensitivity of the neutron porosity evaluation parameter to porosity, and when the formation porosity is above 12%, the porosity sensitivity of the neutron porosity evaluation parameter is significantly higher than the porosity sensitivity of the thermal neutron gauge ratio. When the porosity of the stratum is 35%, the porosity sensitivity of the neutron porosity evaluation parameter is 1.98 times of the thermal neutron counting ratio porosity sensitivity, and the higher the porosity of the stratum is, the more remarkable the porosity sensitivity of the neutron porosity evaluation parameter is improved, thereby proving that the sensitivity of the neutron porosity logging instrument in the high-porosity stratum can be effectively improved by adopting the method provided by the invention.
It is to be understood that the above description is not intended to limit the present invention, and the present invention is not limited to the above examples, and those skilled in the art may make various changes, modifications, additions and substitutions within the spirit and scope of the present invention.
Claims (4)
1. A high-sensitivity neutron porosity measurement method based on a D-T pulse neutron source is characterized in that a four-detector pulse neutron logging instrument is adopted for measurement, a measurement system consisting of a D-T pulse neutron source, two neutron detectors and two gamma detectors is arranged in the four-detector pulse neutron logging instrument, and the method specifically comprises the following steps:
placing a four-detector pulse neutron logging instrument in a well to be attached to a well wall for measurement, emitting fast neutrons by a D-T pulse neutron source in a pulse mode, and recording the non-ballistic gamma counting of a near gamma detector, the non-ballistic gamma counting of a far gamma detector, the thermal neutron counting of the near neutron detector and the thermal neutron counting of the far neutron detector;
determining a neutron porosity evaluation parameter R of the stratum according to the thermal neutron count ratio of the near neutron detector and the far neutron detector and by combining the non-elastic gamma count ratio of the near gamma detector and the far gamma detectortiComprises the following steps:
in the formula, RthIs the thermal neutron count ratio of a near neutron detector and a far neutron detector, RinThe non-elastic gamma count ratio of the near gamma detector and the far gamma detector is obtained;
step three, based on a Monte Carlo numerical calculation method, establishing an MCNP numerical calculation model according to a four-detector pulse neutron logging instrument, changing the formation porosity of the MCNP numerical calculation model, simulating to obtain the non-elastic gamma counting of a near gamma detector, the non-elastic gamma counting of a far gamma detector, the thermal neutron counting of the near neutron detector and the thermal neutron counting of the far neutron detector under different formation porosities, calculating to obtain neutron porosity evaluation parameters corresponding to different formation porosities, and establishing a relation between the neutron porosity evaluation parameters and the formation porosity as follows:
φ＝ARti 2+BRti+C (2)
where φ is the formation porosity, RtiA, B and C are scale coefficients for neutron porosity evaluation parameters;
and fourthly, obtaining accurate formation porosity by utilizing the neutron porosity evaluation parameter of the formation based on the relation between the neutron porosity evaluation parameter and the formation porosity.
2. The method for measuring the porosity of the neutrons with high sensitivity based on the D-T pulse neutron source is characterized in that the diameter of a four-detector pulse neutron logging instrument is set to be 89mm, the source distances of a near gamma detector and a far gamma detector are respectively set to be 31.25cm and 70cm, and the source distances of the near neutron detector and the far neutron detector are respectively set to be 31.25cm and 57.5cm.
3. The method for measuring the porosity of the neutrons with high sensitivity based on the D-T pulse neutron source according to the claim 2, characterized in that a tungsten-nickel-iron shield is arranged between the D-T pulse neutron source and the near neutron detector.
4. The method for measuring the porosity of the neutrons with high sensitivity based on the D-T pulse neutron source of the claim 1 is characterized in that in the third step, the diameter of the well in the MCNP numerical calculation model is set to be 200mm, the lithology of the stratum is set to be limestone, and the variation range of the porosity of the stratum in the MCNP numerical calculation model is 0-100%.
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN116500694A (en) * | 2023-06-28 | 2023-07-28 | 中海油田服务股份有限公司 | Post-sleeve physical quantity inversion method, post-sleeve physical quantity inversion device, computing equipment and storage medium |
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Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN116500694A (en) * | 2023-06-28 | 2023-07-28 | 中海油田服务股份有限公司 | Post-sleeve physical quantity inversion method, post-sleeve physical quantity inversion device, computing equipment and storage medium |
| CN116500694B (en) * | 2023-06-28 | 2023-09-01 | 中海油田服务股份有限公司 | Post-sleeve physical quantity inversion method, post-sleeve physical quantity inversion device, computing equipment and storage medium |
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