A method for evaluating buried mountain helium using an array pulsed neutron instrument

Abstract

The invention discloses a method for evaluating buried hill helium by array pulsed neutron instrument, which belongs to the field of nuclear logging technology. Specifically, the invention constructs three gas layer identification models of IGCR, CGCR and FNES, performs Monte Carlo simulation according to the porosity, lithology and density of different layers of the formation to be measured, calculates the weight coefficients of the three gas layer identification models under different porosity formation segment types, and constructs the optimal gas calculation model FIA; calculates the FIA ​​of different layers in the formation to be measured, obtains the measured curve, and compares it with the water-saturated curve, helium-saturated curve and methane-saturated curve corresponding to the FIA. If the measured curve is between the helium-saturated curve and the methane-saturated curve, and is lower than the water-saturated curve, it is considered that there is helium in the layer. The invention combines the advantages of the three gas identification models, realizes adaptive helium identification and quantification in buried hill reservoirs with different lithology and different porosity, solves the problem that helium in formation reservoirs cannot be effectively identified in conventional logging at present, and has important application value for helium exploration in my country in the future.

Description

Technical Field

[0001] The invention belongs to the technical field of nuclear well logging, and in particular relates to a method for evaluating buried hill helium by using an array type pulsed neutron instrument. Background Art

[0002] As a colorless and odorless inert gas, helium has unique physical and chemical properties and plays an irreplaceable role in aerospace, medical, cryogenic superconductivity, deep diving, electronics and other fields. Helium is a non-renewable natural resource, and currently the only way to produce helium industrially is to extract it from natural gas reservoirs.

[0003] The buried hill is an ancient mountain range buried under the strata. Exploration has shown that it has the conditions for helium accumulation, and it is urgent to explore and develop helium. The geological structure of the buried hill is usually complex, containing ancient rock layers and relatively new sediments, which makes the lithology of the buried hill reservoir more complex than that of conventional strata. It is mainly composed of Archean metamorphic rocks and Lower Paleozoic carbonate rocks. Among them, metamorphic rocks are mainly composed of minerals such as quartz, feldspar, and biotite, and carbonate rocks are mainly limestone and dolomite. In addition, there are mud minerals such as chlorite, illite, and kaolinite. This complexity affects the generation and distribution of helium. The helium in the buried hill mainly comes from the uranium-thorium decay process. The uranium and thorium elements in the rock produce helium in the long-term radioactive decay, and this helium may accumulate in the pores of the rock. The gas storage conditions of the buried hill are greatly affected by its geological structure. Helium may accumulate in rock cracks, pores, and faults to form reservoirs. The accumulation of helium is also affected by the permeability and porosity of the rock and the formation pressure. Due to the complexity of the geological structure of the buried hill and the trace presence of helium, the detection of helium often requires high-precision instruments and technologies. Traditional geological exploration methods may be difficult to accurately assess the distribution and reserves of helium, so there is an urgent need to provide a method for identifying helium in buried hill reservoirs. Summary of the invention

[0004] In view of the problem that traditional geological exploration methods may be difficult to accurately evaluate the distribution and reserves of helium, the present invention provides a method for evaluating buried hill helium by using an array pulsed neutron instrument to achieve the identification and quantitative calculation of helium in the formation.

[0005] In order to achieve the above purpose, the technical method adopted by the present invention is as follows:

[0006] A method for evaluating buried mountain helium by an array pulsed neutron instrument comprises the following steps:

[0007] Step 1: Obtain the porosity of the formation to be measured Lithology and density, and divide the stratum to be measured into sections according to the lithology composition and its content;

[0008] Step 2: Based on the obtained porosity The layers are divided into three categories: low-porosity layers, medium-porosity layers and high-porosity layers.

[0009] Step 3, constructing a gas identification model IGCR based on the inelastic gamma count ratio method, a gas identification model CGCR based on the captured gamma count ratio method, and a gas identification model FNES based on the fast neutron elastic cross section method;

[0010] Step 4: Monte Carlo simulation is performed on different layers of the formation to be tested to obtain the helium-saturated curves and methane-saturated curves corresponding to the three gas layer identification models, and then the weight coefficients of the three gas layer identification models under the low porosity formation section, the medium porosity formation section and the high porosity formation section are calculated, that is:

[0011]

[0012] In the formula, and They are the weight coefficients of the three gas layer identification models of IGCR, CGCR and FNES under the low porosity formation section; and They are the weight coefficients of the three gas layer identification models IGCR, CGCR and FNES under the medium porosity formation section; and They are the weight coefficients of the three gas layer identification models of IGCR, CGCR and FNES under the high porosity formation section; is the critical porosity between the low-porosity formation section and the medium-porosity formation section; is the critical porosity value between the medium porosity formation section and the high porosity formation section; and They are the accuracy coefficients of the three gas layer identification models IGCR, CGCR and FNES under the low porosity formation section; and They are the accuracy coefficients of the three gas layer identification models IGCR, CGCR and FNES under the medium porosity formation section; and They are the accuracy coefficients of the three gas layer identification models IGCR, CGCR and FNES under the high porosity formation section; and They are the sensitivities of the three gas identification models, IGCR, CGCR and FNES, in the low-porosity formation section; and They are the sensitivities of the three gas identification models, IGCR, CGCR and FNES, in the medium porosity formation section; and are the sensitivities of the three gas identification models IGCR, CGCR and FNES in the high porosity formation segment; j = 1, 2, 3 represent low porosity formation, medium porosity formation and high porosity formation respectively;

[0013] in, The calculation formula is:

[0014]

[0015] In the formula, i=1, 2, 3 represent the three gas identification models of IGCR, CGCR and FNES respectively; and They represent the values ​​of helium and methane in the same porosity formation segment j on the gas identification model i, respectively, obtained according to the helium-saturated curve and methane-saturated curve of the gas identification model i; σ1 is the statistical error of the counting rate corresponding to helium; σ2 is the statistical error of the counting rate corresponding to methane;

[0016] Step 5: According to the porosity formation segment types of different layers of the formation to be measured, the optimal gas calculation model FIA is constructed:

[0017]

[0018] Then, Monte Carlo simulation is performed on different layers of the formation to be tested to obtain the water-saturated curve, helium-saturated curve and methane-saturated curve corresponding to the optimal gas calculation model FIA;

[0019] Step 6: Use the array pulsed neutron logging instrument to detect each layer of the formation to be tested. According to the detection results, calculate the values ​​of the three gas identification models IGCR, CGCR, and FNES in different layers, and bring them into the porosity of the layer. The corresponding optimal gas calculation model FIA calculates the measured curves of different layers of the formation to be measured;

[0020] Step 7: Compare the measured curve with the water-saturated curve, helium-saturated curve and methane-saturated curve obtained in step 5. For each layer section of the formation to be measured, if the measured curve is between the helium-saturated curve and the methane-saturated curve and lower than the water-saturated curve, it is considered that helium exists in the layer section.

[0021] Furthermore, in the layer with helium, the helium content S He The calculation formula is:

[0022]

[0023] Where FIA ​​is the value of the optimal gas calculation model FIA for this layer in the measured curve; is the value of the optimal gas calculation model FIA for this layer in the helium saturation curve obtained in step 5; FIA CH4 The value of the optimal gas calculation model FIA for this layer in the saturated methane curve obtained in step 5.

[0024] Furthermore, the array-type pulsed neutron logging instrument includes an electronic device component, and a pulsed neutron generator, a near-source gamma-ray detector, a first 3 He tube neutron detector, distant source gamma ray detector, second 3 He tube neutron detector and ultra-distant source gamma ray detector; wherein, the electronic equipment component is used to convert the gamma rays detected downhole into electrical signals and communicate with the instrument on the ground; the pulse neutron generator is used to emit high-energy fast neutrons to the formation to be measured with a fixed pulse width and repetition period T; the near-source gamma ray detector, the far-source gamma ray detector and the ultra-distant source gamma ray detector are used to detect the prompt gamma ray energy spectrum and its corresponding time spectrum dt generated when neutrons interact with various elemental substances in the formation to be measured; the first 3 He tube neutron detector and the second 3 He tube neutron detector is used to detect the number of thermal neutrons that reach the formation after being slowed down by fast neutrons.

[0025] Furthermore, the gas identification model FNES is constructed based on the mapping relationship between the inelastic gamma counts obtained by Monte Carlo simulation of the known formation and the calculated comprehensive FNES value of the known formation. The comprehensive FNES value of the formation to be tested can be obtained by acquiring the inelastic gamma counts of the formation to be tested; wherein the calculation process of the comprehensive FNES value of the known formation is: first calculate the elastic scattering cross section of each mineral and fluid in the known formation, and then obtain it according to the FNES volume model.

[0026] Furthermore, the elastic scattering cross section of each mineral and fluid in the known formation is calculated by first calculating the product of the elastic scattering cross section of each atom in the mineral or fluid under 14 MeV fast neutrons and the number of corresponding atoms, then summing the products corresponding to all atoms, and finally multiplying the sum by 10 -28 get.

[0027] Furthermore, the gas identification model IGCR utilizes the inelastic gamma ray count rate ratio between the near-source gamma ray detector and the far-source gamma ray detector / ultra-distant-source gamma ray detector during the pulsed neutron burst period combined with the statistical error of the count rate to perform gas identification;

[0028] The formula for calculating the IGCR value using the near-source gamma-ray detector and the far-source gamma-ray detector is:

[0029]

[0030] The formula for calculating the IGCR value using a near-source gamma-ray detector and an ultra-distant-source gamma-ray detector is:

[0031]

[0032] Where, t Burst represents the pulse neutron burst period; N SS (t), N LS (t) and N XLS (t) represent the gamma ray counts received by the near-source gamma ray detector, the far-source gamma ray detector and the ultra-far-source gamma ray detector at time t respectively; and They represent the statistical errors of gamma-ray counts received by the near-source gamma-ray detector, the far-source gamma-ray detector and the ultra-distant-source gamma-ray detector respectively.

[0033] Furthermore, the gas identification model CGCR uses the ratio of captured gamma ray count rates between the near-source gamma ray detector and the far-source gamma ray detector / ultra-far-source gamma ray detector within a measurement period T combined with the statistical error of the count rates to perform gas identification;

[0034] The formula for calculating the CGCR value using the near-source gamma-ray detector and the far-source gamma-ray detector is:

[0035]

[0036] The formula for calculating the CGCR value using a near-source gamma-ray detector and an ultra-distant-source gamma-ray detector is:

[0037]

[0038] Further,

[0039] Further,

[0040] Compared with the prior art, the present invention has the following beneficial effects:

[0041] The present invention proposes a method for evaluating buried hill helium by using an array pulsed neutron instrument, and uses gas identification models IGCR, CGCR and FNES with different gas identification sensitivities under different conditions to construct an optimal gas calculation model FIA. By combining the advantages of the three gas identification models, the method realizes adaptive helium identification and quantification in buried hill reservoirs with different lithologies and porosities, greatly reduces the influence of porosity and lithology on helium identification, and always maintains a high identification sensitivity under any circumstances, thereby solving the problem that helium in formation reservoirs cannot be effectively identified in current conventional logging, and has important application value for my country's future helium exploration. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0043] Figure 1 is a schematic diagram of the structure of an array pulsed neutron logging instrument proposed in Example 1;

[0044] Figure 2 is a flowchart of a method for evaluating buried mountain helium by using an array pulsed neutron instrument proposed in Example 1;

[0045] Figure 3 is a graph showing the change of the gas identification model IGCR based on the inelastic gamma count ratio method with the porosity in Example 1;

[0046] Figure 4 is a graph showing the change of the gas identification model CGCR based on the captured gamma count ratio method with the porosity in Example 1;

[0047] Figure 5 is a graph showing the change of the gas identification model FNES based on the fast neutron elastic cross section method with the net inelastic gamma count in Example 1;

[0048] Figure 6 It is a schematic diagram of the use effect after downhole data collection in Example 1.

[0049] The reference numerals are as follows:

[0050] 1. Pulsed neutron generator; 2. Near-source gamma-ray detector; 3. First 3 He tube neutron detector; 4. Distant source gamma ray detector; 5. Second 3 He tube neutron detector; 6. Ultra-distant source gamma-ray detector; 7. Fluid. DETAILED DESCRIPTION

[0051] In order to further understand the present invention, preferred embodiments of the present invention are described below in conjunction with examples, but it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, rather than limiting the claims of the invention.

[0052] Example 1

[0053] This embodiment proposes a method for evaluating helium in a buried hill by using an array pulsed neutron logging instrument, which involves using an array pulsed neutron logging instrument to detect a formation to be tested, wherein the formation contains various minerals and fluids 7.

[0054] The structure of the array pulse neutron logging instrument is as follows: Figure 1 As shown, it includes an electronic device component, and a pulsed neutron generator 1, a near-source gamma-ray detector 2, a first 3 He tube neutron detector 3, distant source gamma ray detector 4, second 3 He tube neutron detector 5 and ultra-distant source gamma ray detector 6; wherein the electronic equipment component is used to convert the gamma rays detected underground into electrical signals and communicate with the instrument on the ground; the pulse neutron generator 1 is equipped with a DT pulse neutron source, which is a controllable neutron source, used to emit high-energy fast neutrons to the formation to be measured with a fixed pulse width of 60s and a repetition period of T=1000μs. When the power switch is turned on, the DT pulse neutron source in the pulse neutron generator 1 can produce fast neutrons with a particle energy of 14.1MeV when DT fusion occurs; the near-source gamma ray detector 2, the far-source gamma ray detector 4 and the ultra-distant source gamma ray detector 6 are used to detect the prompt gamma ray energy spectrum and its corresponding time spectrum dt generated when neutrons interact with various elemental substances in the formation to be measured; the first 3 He tube neutron detector 3 and the second 3 He tube neutron detector 5 is used to detect fast neutrons that are slowed down by the formation to be tested and arrive at the corresponding 3 The number of thermal neutrons in the He tube neutron detector.

[0055] For the gas identification model IGCR based on the inelastic gamma count ratio method, the gas identification model CGCR based on the captured gamma count ratio method, and the gas identification model FNES based on the fast neutron elastic cross section method, since the above three gas identification models have low identification sensitivity under certain circumstances, the gas identification models IGCR, CGCR and FNES with different gas identification sensitivities under different circumstances can be used to construct the optimal gas calculation model FIA, combining the advantages of the three gas identification models to realize the identification and quantification of helium in buried hill reservoirs.

[0056] Gas identification model IGCR based on inelastic gamma count ratio method: gas identification is performed by using the inelastic gamma ray count rate ratio between the near-source gamma ray detector 2 and the far-source gamma ray detector 4 / ultra-distant-source gamma ray detector 6 during the pulse neutron burst period combined with the statistical error of the count rate;

[0057] The formula for calculating the IGCR value using the near-source gamma-ray detector 2 and the far-source gamma-ray detector 4 is:

[0058]

[0059] The formula for calculating the IGCR value using the near-source gamma-ray detector 2 and the ultra-distant-source gamma-ray detector 6 is:

[0060]

[0061] In the formula, t Burst represents the pulse neutron burst period; N SS (t), N LS (t) and N XLS (t) respectively represent the gamma ray counts received by the near-source gamma ray detector 2, the far-source gamma ray detector 4 and the ultra-far-source gamma ray detector 6 at time t; and They respectively represent the statistical errors of gamma ray counts received by the near-source gamma ray detector 2 , the far-source gamma ray detector 4 and the ultra-distant-source gamma ray detector 6 .

[0062] IGCR is sensitive to fluid density and hydrogen index. When the reservoir is filled with different fluids, due to the differences in density and hydrogen index, under the same porosity, as the fluid density and hydrogen index decrease, the near-source gamma ray detector 2 and the far-source gamma ray detector 4 / ultra-far-source gamma ray detector 6 receive more inelastic gamma ray counts, among which the far-source gamma ray detector 4 / ultra-far-source gamma ray detector 6 receives an increasing rate of inelastic gamma rays faster than the near-source gamma ray detector 2, and thus the IGCR is lower. Figure 3 The curve of IGCR of the gas identification model based on the inelastic gamma counting ratio method with the change of porosity shows that helium has the lowest density and hydrogen index, so the IGCR curve of helium is the lowest. This principle can be used to effectively identify helium.

[0063] Gas identification model CGCR based on captured gamma ray count ratio method: Gas identification is performed by using the ratio of captured gamma ray count rates between the near-source gamma ray detector 2 and the far-source gamma ray detector 4 / ultra-far-source gamma ray detector 6 within the measurement period T combined with the statistical error of the count rate;

[0064] The formula for calculating the CGCR value using the near-source gamma-ray detector 2 and the far-source gamma-ray detector 4 is:

[0065]

[0066] The formula for calculating the CGCR value using the near-source gamma-ray detector 2 and the ultra-distant-source gamma-ray detector 6 is:

[0067]

[0068] As the porosity of the formation increases, there are more hydrogen atoms in the formation, and more elastic collisions occur. Therefore, the faster the fast neutron energy decays, the shorter the propagation distance of the fast neutrons, and the fewer capture collision reactions occur in the area far away from the neutron source. The number of captured gamma rays received by the near-source gamma-ray detector 2 and the far-source gamma-ray detector 4 / ultra-far-source gamma-ray detector 6 is smaller, and the number of captured gamma rays received by the far-source gamma-ray detector 4 / ultra-far-source gamma-ray detector 6 is reduced relatively more, and the CGCR is higher. According to Figure 4 From the graph of the gas identification model CGCR based on the captured gamma count ratio method versus porosity, it can be seen that helium has the lowest hydrogen index, and therefore the CGCR curve of helium is the lowest.

[0069] Gas identification model FNES based on fast neutron elastic cross section method: It is an indicator to measure the interaction ability between formation and fast neutrons. Like density and resistivity, it is an inherent physical property of formation.

[0070]

[0071] In the formula, elastic represents elasticity.

[0072] It is very sensitive to gas, and there are large differences in FNES of different gases. There is a conversion relationship between FNES and inelastic gamma-ray count rate. The formation FNES obtained by using the conversion relationship of the inelastic gamma-ray count rate measured by the instrument can effectively identify different fluids in the formation.

[0073] The method is constructed based on the mapping relationship between the inelastic gamma counts obtained by Monte Carlo simulation of known formations and the calculated comprehensive FNES values ​​of the known formations. The comprehensive FNES value of the formation to be measured can be obtained by obtaining the inelastic gamma counts of the formation to be measured. The calculation process of the comprehensive FNES value of the known formation is as follows: first, the elastic scattering cross sections of various minerals and fluids in the known formation are calculated, and then the values ​​are obtained based on the FNES volume model.

[0074] The calculation process of the elastic scattering cross section of each mineral and fluid in the known formation is: first calculate the product of the elastic scattering cross section of each atom in the mineral or fluid under 14 MeV fast neutrons and the number of corresponding atoms, then sum the products corresponding to all atoms, and finally multiply the sum by 10 -28 get.

[0075] Example: Assuming the molecular formula of a mineral is AB2, then the unit volume (1m 3 ) The number of AB2 molecules (N) in the equation is:

[0076]

[0077] Where N A is Avogadro's constant, ρ is the density of AB2 (g / cm 3 ), M is the molar mass of AB2.

[0078] Then the number of atoms of A per unit volume is N, and the number of atoms of B is 2N, so the FNES of AB2 is

[0079]

[0080] In the formula, ∑ A and∑ B are the elastic scattering cross sections of atoms A and B under 14MeV fast neutrons.

[0081] like Figure 5 As shown, due to the different fast neutron elastic scattering cross sections of helium-containing formations and methane-containing formations, different gases in the formation fluid can be separated.

[0082] Furthermore, the process of the method for evaluating buried mountain helium by array pulsed neutron instrument described in this embodiment is as follows: Figure 2 As shown, the specific steps include:

[0083] Step 1: Obtain the porosity of the formation to be measured Lithology and density, and divide the stratum to be measured into sections according to the lithology composition and its content; Step 2, according to the obtained porosity The layers are divided into three categories: low porosity, medium porosity and high porosity. The critical porosity between low porosity and medium porosity is 10, and the critical porosity between medium porosity and high porosity is 20.

[0084] Step 3, constructing a gas identification model IGCR based on the inelastic gamma count ratio method, a gas identification model CGCR based on the captured gamma count ratio method, and a gas identification model FNES based on the fast neutron elastic cross section method;

[0085] Step 4: Monte Carlo simulation is performed on different layers of the formation to be tested to obtain the helium-saturated curves and methane-saturated curves corresponding to the three gas layer identification models, and then the weight coefficients of the three gas layer identification models under the low porosity formation section, the medium porosity formation section and the high porosity formation section are calculated, that is:

[0086]

[0087] In the formula, and They are the weight coefficients of the three gas layer identification models of IGCR, CGCR and FNES under the low porosity formation section; and They are the weight coefficients of the three gas layer identification models IGCR, CGCR and FNES under the medium porosity formation section; and They are the weight coefficients of the three gas layer identification models of IGCR, CGCR and FNES under the high porosity formation section; and They are the sensitivities of the three gas identification models, IGCR, CGCR and FNES, in the low-porosity formation section; and They are the sensitivities of the three gas identification models, IGCR, CGCR and FNES, in the medium porosity formation section; and are the sensitivities of the three gas identification models IGCR, CGCR and FNES in the high porosity formation segment; j = 1, 2, 3 represent low porosity formation, medium porosity formation and high porosity formation respectively;

[0088] in, The calculation formula is:

[0089]

[0090] In the formula, i=1, 2, 3 represent the three gas identification models of IGCR, CGCR and FNES respectively; and They represent the values ​​of helium and methane in the same porosity formation segment j on the gas identification model i, respectively, obtained according to the helium-saturated curve and methane-saturated curve of the gas identification model i; σ1 is the statistical error of the counting rate corresponding to helium; σ2 is the statistical error of the counting rate corresponding to methane;

[0091] Step 5: According to the porosity formation segment types of different layers of the formation to be measured, the optimal gas calculation model FIA is constructed by combining the three gas identification models of IGCR, CGCR and FNES:

[0092]

[0093] Then, Monte Carlo simulation is performed on different layers of the formation to be tested to obtain the water-saturated curve, helium-saturated curve and methane-saturated curve corresponding to the optimal gas calculation model FIA;

[0094] Step 6: Use the array pulsed neutron logging instrument to detect each layer of the formation to be tested. According to the detection results, calculate the values ​​of the three gas identification models IGCR, CGCR, and FNES in different layers, and bring them into the porosity of the layer. The corresponding optimal gas calculation model FIA calculates the measured curves of different layers of the formation to be measured;

[0095] Step 7: Compare the measured curve with the water-saturated curve, helium-saturated curve and methane-saturated curve obtained in step 5. For each layer section of the formation to be measured, if the measured curve is between the helium-saturated curve and the methane-saturated curve and lower than the water-saturated curve, it is considered that helium exists in the layer section.

[0096] Figure 6 As shown in the schematic diagram of the use effect after downhole data acquisition, the measured curve after FIA model processing is compared with the water-saturated curve, helium-saturated curve and methane-saturated curve obtained in step 5. In the 3650-3700m, 3720-3740m and 3780-3790m layers, the measured curve is between the helium-saturated curve and the methane-saturated curve and is lower than the water-saturated curve, so helium exists in these three sections. At the same time, the quantitative calculation of helium is performed using the following saturation formula.

[0097]

[0098] Among them, S He is the helium saturation, FIA i , These are the results of the measured curve, helium-saturated curve and methane-saturated curve at the same depth layer after being processed by the FIA ​​model.

[0099] The above embodiments are intended to provide a better understanding of the present invention, but are not limited to the optimal implementation mode and do not constitute a limitation on the content and protection scope of the present invention. Any product identical or similar to the present invention obtained by anyone under the inspiration of the present invention or by combining the present invention with the features of other prior arts shall be within the protection scope of the present invention.

Claims

1. A method for evaluating buried mountain helium by array pulsed neutron instrument, characterized in that: The following steps are involved: Step 1: Obtain the porosity of the formation to be measured Lithology and density, and divide the stratum to be measured into sections according to the lithology composition and its content; Step 2: Based on the obtained porosity The layers are divided into three categories: low-porosity layers, medium-porosity layers and high-porosity layers. Step 3, constructing a gas identification model IGCR based on the inelastic gamma count ratio method, a gas identification model CGCR based on the captured gamma count ratio method, and a gas identification model FNES based on the fast neutron elastic cross section method; Step 4: Monte Carlo simulation is performed on different layers of the formation to be tested to obtain the helium-saturated curves and methane-saturated curves corresponding to the three gas layer identification models, and then the weight coefficients of the three gas layer identification models under the low porosity formation section, the medium porosity formation section and the high porosity formation section are calculated, that is: In the formula, and They are the weight coefficients of the three gas layer identification models of IGCR, CGCR and FNES under the low porosity formation section; and They are the weight coefficients of the three gas layer identification models IGCR, CGCR and FNES under the medium porosity formation section; and They are the weight coefficients of the three gas layer identification models of IGCR, CGCR and FNES under the high porosity formation section; is the critical porosity between the low-porosity formation section and the medium-porosity formation section; is the critical porosity value between the medium porosity formation section and the high porosity formation section; and They are the accuracy coefficients of the three gas layer identification models IGCR, CGCR and FNES under the low porosity formation section; and They are the accuracy coefficients of the three gas layer identification models IGCR, CGCR and FNES under the medium porosity formation section; and They are the accuracy coefficients of the three gas layer identification models IGCR, CGCR and FNES under the high porosity formation section; and They are the sensitivities of the three gas identification models, IGCR, CGCR and FNES, in the low-porosity formation section; and They are the sensitivities of the three gas identification models, IGCR, CGCR and FNES, in the medium porosity formation section; and are the sensitivities of the three gas identification models IGCR, CGCR and FNES in the high porosity formation section; j=1, 2, 3 represent low porosity formation, medium porosity formation and high porosity formation respectively; in, The calculation formula is: In the formula, j = 1, 2, 3 represent the three gas identification models of IGCR, CGCR and FNES respectively; and They represent the values ​​of helium and methane in the same porosity formation segment j on the gas identification model i, respectively, obtained according to the helium-saturated curve and methane-saturated curve of the gas identification model i; σ1 is the statistical error of the counting rate corresponding to helium; σ2 is the statistical error of the counting rate corresponding to methane; Step 5: According to the porosity formation type of different layers, the optimal gas calculation model FIA is constructed: Then, Monte Carlo simulation is performed on different layers to obtain the water-saturated curve, helium-saturated curve and methane-saturated curve corresponding to the optimal gas calculation model FIA; Step 6: Use the array pulsed neutron logging instrument to detect each layer of the formation to be tested. According to the detection results, calculate the values ​​of the three gas identification models IGCR, CGCR, and FNES in different layers, and bring them into the porosity of the layer. The corresponding optimal gas calculation model FIA calculates the measured curves of different layers of the formation to be measured; Step 7: Compare the measured curve with the water-saturated curve, helium-saturated curve and methane-saturated curve obtained in step 5. For each layer section of the formation to be measured, if the measured curve is between the helium-saturated curve and the methane-saturated curve and lower than the water-saturated curve, it is considered that helium exists in the layer section.

2. The method for evaluating buried mountain helium by array pulsed neutron instrument according to claim 1, characterized in that: In the layer with helium, the helium content S He The calculation formula is: Where FIA ​​is the value of the optimal gas calculation model FIA for this layer in the measured curve; is the value of the optimal gas calculation model FIA for this layer in the helium saturation curve obtained in step 5; FIA CH4 The value of the optimal gas calculation model FIA for this layer in the saturated methane curve obtained in step 5.

3. The method for evaluating buried mountain helium by array pulsed neutron instrument according to claim 1, characterized in that: The array-type pulsed neutron logging instrument comprises an electronic device assembly, and a pulsed neutron generator, a near-source gamma-ray detector, a first 3 He tube neutron detector, distant source gamma ray detector, second 3 He tube neutron detector and ultra-distant source gamma ray detector; wherein, the electronic equipment component is used to convert the gamma rays detected downhole into electrical signals and communicate with the instrument on the ground; the pulse neutron generator is used to emit high-energy fast neutrons to the formation to be measured with a fixed pulse width and repetition period T; the near-source gamma ray detector, the far-source gamma ray detector and the ultra-distant source gamma ray detector are used to detect the prompt gamma ray energy spectrum and its corresponding time spectrum dt generated when neutrons interact with various elemental substances in the formation to be measured; the first 3 He tube neutron detector and the second 3 He tube neutron detector is used to detect the number of thermal neutrons that reach the formation after being slowed down by fast neutrons.

4. The method for evaluating buried mountain helium by array pulsed neutron instrument according to claim 1, characterized in that: The gas identification model FNES is constructed based on the mapping relationship between the inelastic gamma counts obtained by Monte Carlo simulation of known formations and the calculated comprehensive FNES values ​​of the known formations. The comprehensive FNES value of the formation to be measured can be obtained by obtaining the inelastic gamma counts of the formation to be measured; wherein the calculation process of the comprehensive FNES value of the known formation is: firstly, the elastic scattering cross sections of each mineral and fluid in the known formation are calculated, and then obtained according to the FNES volume model.

5. The method for evaluating buried mountain helium by array pulsed neutron instrument according to claim 4, characterized in that: The calculation process of the elastic scattering cross section of each mineral and fluid in the known formation is: first calculate the product of the elastic scattering cross section of each atom in the mineral or fluid under 14MeV fast neutrons and the number of corresponding atoms, then sum the products corresponding to all atoms, and finally multiply the sum by 10 -28 get.

6. The method for evaluating buried mountain helium by array pulsed neutron instrument according to claim 1, characterized in that: The gas identification model IGCR uses the inelastic gamma ray count rate ratio between the near-source gamma ray detector and the far-source gamma ray detector / ultra-distant-source gamma ray detector during the pulse neutron burst period combined with the statistical error of the count rate to perform gas identification; The formula for calculating the IGCR value using the near-source gamma-ray detector and the far-source gamma-ray detector is: The formula for calculating the IGCR value using a near-source gamma-ray detector and an ultra-distant-source gamma-ray detector is: Where, t Burst represents the pulse neutron burst period; N SS (t), N LS (t) and N XLS (t) represent the gamma ray counts received by the near-source gamma ray detector, the far-source gamma ray detector and the ultra-far-source gamma ray detector at time t respectively; and They represent the statistical errors of gamma-ray counts received by the near-source gamma-ray detector, the far-source gamma-ray detector and the ultra-distant-source gamma-ray detector respectively.

7. The method for evaluating buried mountain helium by array pulsed neutron instrument according to claim 6, characterized in that: The gas identification model CGCR uses the ratio of captured gamma ray count rates between the near-source gamma ray detector and the far-source gamma ray detector / ultra-far-source gamma ray detector within a measurement period T combined with the statistical error of the count rates to perform gas identification; The formula for calculating the CGCR value using the near-source gamma-ray detector and the far-source gamma-ray detector is: The formula for calculating the CGCR value using a near-source gamma-ray detector and an ultra-distant-source gamma-ray detector is:

8. The method for evaluating buried mountain helium by array pulsed neutron instrument according to claim 1, characterized in that:

9. The method for evaluating buried mountain helium by array pulsed neutron instrument according to claim 1, characterized in that:

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