RU2155975C2 - Process determining content of oxides of magnesium and calcium in magnesite ore - Google Patents

Abstract

FIELD: geophysical examination of boreholes, specifically, nuclear physical methods of investigation of mineral resources. SUBSTANCE: process includes sequential irradiation of rocks by fluxes of gamma- quanta and fast neutrons, registration of flux of scattered gamma radiation at fixed distance from gamma source whose intensity is used to evaluate presence of magnesite strata in section of boreholes. Soft N₁ and hard N₂ components of scattered gamma radiation are recorded, flux of thermal neutrons N₃ is measured, effective atomic number Z_eff is calculated and lifetime N₃ of thermal neutrons in strata is computed by intensity of flux τ and known diameter of borehole. Contents of MgO and CaO in strata are found correspondingly by nomograms P(Mg0)=f(Z_eff,τ) and P(CaO)=f(Z_eff,τ). Z_eff=a-b lg N₁/N₂, where a and b are constant coefficients. τ_i in i-th interval of boreholes is found as τ_i, where τ_refN_3i(r,d)/N_3ref(r,d) where τ_ref is lifetime of thermal neutrons in reference interval; N_3i(r,d) and N_3ref(r,d) are counting rate of thermal neutrons in i-th and reference intervals of boreholes with fixed dimension r of sonde and diameter d of borehole. Nomograms (Z_eff,τ) and (Z_eff,τ) are constructed with known computed values Z_eff, τ and with known contents of MgO and CaO in core samples by plotting of isocurves of equal contents of MgO and CaO by interpolation method in coordinates Z_eff, τ. EFFECT: enhanced accuracy of evaluation of quality of magnesite ores. 3 cl, 4 dwg, 1 tbl

Description

 The invention relates to the field of geophysical research of wells, and more specifically to a group of nuclear-geophysical methods for the study of mineral raw materials, and can be used in geology, geophysics, mining, metallurgical industry and other fields of national economy.

 A known method for determining the content of magnesium and calcium oxides in magnesite formations crossed by wells, based on the selection of core samples during drilling and their subsequent chemical analysis (1st analogue. Borzunov V.M. Non-metallic mineral deposits, their exploration and industrial assessment .- M .: Nedra, 1969.- S. 149, 161). The main disadvantage of this method is the need for the selection of representative cores, which is associated with high labor costs. In addition, chemical analysis is carried out on discrete samples, excluding the assessment of the quality of the desired magnesite ores directly in the natural occurrence.

 The second known method for determining the content of magnesium and calcium oxides in magnesite ores also involves the selection of core samples during drilling and express analysis of crushed and worn samples using modern X-METR spectroscopic equipment. (2nd analogue. Heiki Sipal. X-MET. Portable X-ray analyzer. A / O Outakumpu, Finland, Research Institute of Physics). The disadvantages of the second analogue are the same as for the first. Its advantage is high productivity, which reduces the cost of analytical work with mass definitions.

 The closest in physical essence and technology of work is a method for studying magnesite formations in wells, based on a complex of geophysical logging methods, including the method of magnetic susceptibility (CMW), density gamma-ray logging (GGK-P), gamma-ray logging (GK) and neutron gamma-ray logging (Prototype. Edited by P.V. Vishnevsky, G.S. Vakhromeev, I.L. Shamansky. Geophysical methods for the search and exploration of non-metallic minerals. - M .: Nedra.- 1984.- P. 124-125). This set of methods provides the allocation of strata of magnesite ores at a slightly reduced level of natural radioactivity (GK), an increase in density (GGK-P) and an increase in the integrated flux of gamma radiation from radiation capture (NGK).

 The last two methods are implemented by irradiating the test medium with a flux of gamma quanta and fast neutrons of stationary isotopic sources with registration at fixed distances from sources of secondary scattered and induced gamma radiation of radiation capture. As gamma-ray detectors, scintillation counters of the NaI (Te) type are used, ranging in size from 18x40 to 30x70 mm. Magnetic susceptibility logging is carried out to isolate contacts, including magnesite strata of rocks characterized by increased magnetic permeability. The disadvantage of the prototype is the inability to quantify the main components of magnesite ores - oxides of magnesium and calcium. For this reason, it does not provide a solution to the problem - a quantitative assessment of the industrial significance of magnesite ores.

The solution to the problem of quantitative determination of the content of magnesium and calcium oxides in magnesite strata in conditions of boreholes of known diameter is possible on the basis of sequential irradiation of rocks with gamma-ray and fast neutron fluxes using stationary isotopic sources of cesium-137 and californium-252, respectively, recording the flux of scattered gamma -radiation at a fixed distance from the source of gamma rays, equal to 5-40 cm, the intensity of which is judged on the presence in the section of wells magnesite pl astov. In the proposed method, separate registration of components of scattered gamma radiation with an energy of less than 200-300 KeV N ₁ and with an energy of more than 200-300 KeV N ₂ and additionally at a distance of 30-60 cm measure the thermal neutron flux N ₃ , in relation to the flux N ₁ and N ₂ calculate the effective atomic number Z _eff , and from the flux intensity N ₃ and the known diameter of the wells — the thermal neutron lifetime τ in the formations, while the contents of magnesium and calcium oxides in the formations are determined by two-dimensional nomograms, respectively, P (MgO) = f ( Z _eff , τ) and P (CaO) = f (Z _eff , τ), constructed according to the results of measurements of Z _eff , τ, P (MgO) and P (CaO) in parametric wells.

Calculation of the effective atomic number is carried out according to an experimentally established dependence of the form Z _eff = ab log N ₁ / N ₂ , where a and b are constant coefficients determined when calibrating the gamma-gamma-ray measuring equipment.

The thermal neutron lifetime in the i-th interval of the wells is carried out according to the formula: τ _i = τ _et • N _3i (r, d) / N _{3 et} (r, d), where τ _et is the thermal neutron lifetime in the reference interval, N _3i (r, d) and N _3et (r, d) are the thermal neutron count rates in the ith and reference well intervals for a fixed probe size (r) and well diameter (d).

The method is characterized in that the construction of the nomogram P (MgO) = f (Z _eff , τ) and P (CaO) = f (Z _eff , τ) is carried out separately at known calculated values of Z _eff , τ and core samples known from chemical analysis the content of magnesium and calcium oxides by constructing by interpolation in coordinates Z _eff , τ isolines equal contents of MgO and CaO in increments of 1-2% MgO and CaO.

 These distinctive features are not found in the known technical solutions, therefore, the proposed method is original, allowing to provide an assessment of the quality of magnesite ores at a quantitative level.

 The physical essence of the method can be understood from the analysis of the laws objectively existing in natural conditions in the material composition of magnesites and their petrophysical properties, which are manifested in the presence of strong correlation bonds between them. The nature of the petrophysical bonds and the principles for determining the content of magnesium oxide and calcium can be understood from the analysis of the main dependencies and nomograms presented in FIG. 1, 2, 3 and 4.

In the drawings, the following notation:
Z _eff is the effective atomic number of rocks, rel. units;
τ is the lifetime of thermal neutrons in the formation, μs;
P (MgO), P (CaO) - weight contents of magnesium and calcium oxides in the reservoirs,%.

где P_i, Z_i - относительное весовое содержание и атомный номер i-го элемента в анализируемой пробе.The dependencies indicated in FIG. 1-4, constructed according to the results of processing 286 core samples from the Ismakayevsky magnesite deposit of the Republic of Bashkortostan. For this purpose, a complete chemical analysis was performed on all samples, according to the results of which the neutron-diffusion parameters were calculated using the NERPA program, including thermal neutron lifetime and effective atomic number:

where P _i , Z _i - the relative weight content and atomic number of the i-th element in the analyzed sample.

From the analysis of the dependence Z _eff = f [P (MgO)], presented in figure 1, it follows that as the content of magnesium oxide increases from 0 to 45%, the effective atomic number of ores naturally decreases from 15.4 rel. units (limestone) up to 9.5 rel. units (pure magnesite) and vice versa increases as the content of calcium oxide in ores increases. Due to this, directly depending on Z _{eff, a} quantitative assessment of the content of magnesium oxide with an absolute error of 1.5-2.0% and calcium oxide with an error of ± 1.5% is possible. The presence of such a strong bond is explained by a regular change in the ratio in the carbonate rocks of light (MgO) and heavy (CaO) oxides in the series limestone (CaCO ₃ ) - dolomite [CaMg (CO ₃ ) ₂ ] - magnesite (MgCO ₃ ). The rather high errors in the determination of MgO and CaO contents through Z _eff are explained by the significant influence of the natural dispersion of relatively heavy components on this parameter — iron oxides (FeO, Fe ₂ O ₃ ), as well as aluminosilicates (Al ₂ O ₃ and SiO ₂ ). Since, as follows from formula (1), Z _eff is calculated through Z _i to a cubic degree, even small changes in iron and calcium oxides can lead to significant variations in Z _eff .

In the same way, it does not provide high accuracy in the calculation of MgO and CaO in magnesites and the dependence presented in Fig. 2, approximated by an equation of the form
ln [P (MgO)] = a + b • τ, (2)
where a and b are the coefficients of the regression equation.

где a₀, a₀ ¹, a₁, a₁ ¹, a₂, a₂ ¹, b₁, b₁ ¹, b₂, b₂ ¹ - постоянные коэффициенты, определяемые известными способами для уравнений множественной корреляции.The correlation coefficient for dependence (2) is 0.76. The presence of such a strong connection between P (MgO) and the thermal neutron lifetime in magnesites is explained by the anomalously low absorption capacity of magnesium, oxygen, and carbon nuclei, which is respectively 0.00171 cm ² / g, 7.57 • 10 ^-6 cm ² / g and 0.000187 cm ² / g, whereas for calcium the absorption macro section is 0.00661 cm ² / g. It follows that as the magnesium oxide content in ores increases, the macroscopic cross section for absorption of thermal neutrons (Σ ₃ ) decreases, and the lifetime τ = 1 / V • Σ ₃ naturally increases, where V is the thermal neutron velocity equal to 2200 m / s. The actual absolute root-mean-square error of determining the content of magnesium oxide from the correlation with the lifetime of thermal neutrons is 2-2.5% MgO, which is also insufficient to assess the quality of magnesites with a confidence equal to the chemical analysis of core samples. Feedback can be traced between τ and the content of calcium oxide. However, at relatively low CaO contents in magnesites, this bond is less stable. A satisfactory solution to the problem can be provided by multidimensional dependencies of the form:

where a ₀ , a ₀ ¹ , a ₁ , a ₁ ¹ , a ₂ , a ₂ ¹ , b ₁ , b ₁ ¹ , b ₂ , b ₂ ¹ are constant coefficients determined by known methods for the multiple correlation equations.

In the general case, equations (3) are nonlinear. Therefore, for practical use, it is more convenient to construct 2-dimensional nomograms separately for magnesium and calcium oxides of the type: P (MgO) = f (Z _eff , τ) and P (CaO) = f (Z _eff , τ). For this purpose, at least 100 magnesite ore samples are taken from the parametric wells, which are subjected to a complete chemical analysis with the determination of all rock components under the condition:
ΣP _i = 1, (4)
where P _i is the weight content of oxides in the rock.

Having the data of complete chemical analyzes, they are further calculated for each sample Z _eff by the formula (1) and τ by the analytical formulas given, for example, in the book of D. Kozhevnikov. Neutron characteristics of rocks and their use in oilfield geophysics. - M .: Subsoil. - 1974.

In coordinates Z _eff and τ, points with different contents of P (MgO) and P (CaO) are taken out. Next, the interpolation method in the content fields are isoconcentrates separately for magnesium oxide and calcium in increments of 1-2% (figure 3, 4). The inverse problem of the quantitative assessment of the content of MgO and CaO in the i-th intervals is _solved by the values of Z _eff and τ _i with interpolation of the position of the desired point between adjacent isoconcentrates.

For the practical implementation of the proposed method in well conditions, known technologies for measuring the thermal neutron lifetime (τ) and effective atomic number (Z _eff ) can be used, for example, described in the monograph by Davydov Yu.B., Kuzina V.F. Theoretical background of fission neutron logging. - Novosibirsk .: VO "Science". - 1994. - S. 49-76. and book: Gamma-gamma methods in ore geology. / Edited by A.P. Ochkur. - M.: bowels. - 1975. In particular, the technology for determining Z _eff can be based on irradiating the test medium with a gamma-ray flux with an energy of 0.6-1.0 MeV and detecting two scattered radiation components N ₁ and N ₂ at a distance of 5.0-40 cm from source of gamma rays such as Cs-137, Co-60, etc.). The first component is recorded in the energy region of less than 200-300 keV, the second more than 200-300 keV. The intensity ratio N ₁ / N ₂ is a function of the effective atomic number. The real algorithm for calculating Z _{eff is} as follows:
Z _eff -a-blnN ₁ / N ₂ ,
where a and b are constant coefficients determined when calibrating a gamma-gamma-ray logging tool (GHG) on certified samples of formations of an effective atomic number.

When using a gamma source with a low-energy spectrum of primary gamma radiation, for example, cobalt-57 (123 KeV), selenium-75 (138 and 268 KeV), integral measurements are performed, and the ratio N _i / N _{et is} used as an analytical parameter , where N _i , N _et - integral flows in an unknown and reference medium. The calculation algorithm Z _eff in this case also corresponds to equation (5).

 Determination of the lifetime of thermal neutrons of rocks crossed by wells is carried out according to neutron-neutron logging. The method is implemented by irradiating the test medium with a neutron flux of a stationary isotope source, for example, California-252, and recording a thermal neutron flux at a distance of 30-60 cm.

The calculation of the lifetime for the i-th formation is possible according to the algorithm described, for example, in the book of Davydov Yu.B., Kuzin V.F.
τ _i = τ _fl • N _i / N _fl , (6)
where τ _i and τ _et - the lifetime of thermal neutrons in the i-th and reference medium;
N _i , N _et - recorded flows in the i-th and reference environment.

Algorithm (6) provides a satisfactory calculation accuracy with a fixed diameter of wells. Therefore, in practice, the values of τ _et and N _{et are} determined for a typical series of well diameters in the same parametric wells, the data for which were used to construct 2-dimensional nomograms P (MgO) = f (Z _eff , τ) and P (CaO) = f (Z _eff , τ). For practical implementation of the NW method for thermal neutrons, any serial equipment can be used, for example, SRK-2, MARK-1, etc.

 Measuring the effective atomic number in wells is possible with GGK-PS-36, GGK-PS-42 devices, which are produced in small batches by the pilot production of NPP VNIIGIS (Oktyabrsky).

окиси кальция - 9,3-10,0%, где δ -суммарные среднеквадратичные относительные расхождения. Значительно более высокая относительная погрешность расчета содержаний окиси кальция обусловлена его низким средним содержанием в магнезитах, составляющем 3,6%, тогда как среднее содержание окиси магния в магнезитах составляет порядка 38,5%. Абсолютные погрешности количественной оценки содержаний окислов магния и кальция в магнезитовых рудах, соответственно равны ±0,98% и ±0,65%, что соизмеримо с результатами химического анализа кернового материала.Currently, the proposed method has been widely tested in the wells of the Ismakayevsky magnesite deposit. The main results of statistical processing to determine the content of magnesium and calcium oxides in comparison with the data of chemical analysis are presented in the table. As can be seen from the table, the proposed method provides a convergence with chemical analysis data within ± 0.98% MgO and ± 0.65% CaO, there are no systematic errors, the relative mean square random errors on average for three wells are ± 2.5% for oxide magnesium and ± 12-15% for calcium oxide. Moreover, these errors include errors of this method, as well as errors in sampling, sample preparation, and chemical analysis proper. Experience shows that these errors are of approximately the same order as for nuclear-geophysical testing. Given the above, the intrinsic relative error of the method for magnesium oxide is

calcium oxides - 9.3-10.0%, where δ are the total rms relative differences. A significantly higher relative error in the calculation of the content of calcium oxide is due to its low average content in magnesites of 3.6%, while the average content of magnesium oxide in magnesites is about 38.5%. The absolute errors of the quantitative assessment of the contents of magnesium and calcium oxides in magnesite ores are respectively ± 0.98% and ± 0.65%, which is comparable with the results of chemical analysis of core material.

 According to the prototype, a similar problem is at best solved with a relative error of ± 50-100%. High accuracy and reliability of the quality assessment of magnesite ores suggest the use of the proposed method for mass determination of magnesium and calcium oxides directly in the natural occurrence of magnesite strata. Thanks to this, it seems possible to substantially refine the geological section and ensure a significant economic effect.

 The implementation of the method can be carried out using domestic equipment of selective gamma-gamma and neutron neutron logging.

Claims (4)

1. The method for determining the content of magnesium oxides P (MgO) and calcium P (CaO) in magnesite ores in well-known diameter wells, which consists in sequential irradiation of rocks with gamma-ray and neutron fluxes using, respectively, stationary isotopic sources Cs-137 and California 252, registration of the scattered gamma radiation flux at a fixed distance from the gamma-ray source, the intensity of which judges the presence of magnesite strata in the well sections, characterized in that they carry out separate EGISTRATION scattered radiation component with energies less than 200 - 300 KeB N ₁ and with an energy of 200 - 300 KeB N ₂ and further at a distance r = 30 - 60 cm measured thermal neutron flux N _3, based on N ₁ and N ₂ flows calculated effective atomic number (Z _eff ), and according to the intensity of the stream N ₃ and the known diameter of the wells, the lifetime of thermal neutrons in the formations (τ), while the content of magnesium and calcium oxides in the formations is carried out according to two-dimensional nomograms P (MgO) = f (Z _eff , , τ) and P (CaO) = f (Z _eff, τ), constructed from measurements of Z _eff, τ, P (MgO) and P (CaO) in a parametrically wells. 2. The method according to claim 1, characterized in that the calculation of the effective atomic number is carried out according to an experimentally established dependence of the form
Z _eff = ab InN ₁ / N ₂ ,
where a and b are constant coefficients determined during the graduation of gamma-gamma-ray measuring equipment. 3. The method according to claim 1, characterized in that the calculation of the lifetime of thermal neutrons in the rock formations (τ _i ), crossed by wells of known diameter, is carried out in relation
τ _i = τ _et N _3i (r, d) / N _{3 et} (r, d),
where τ _et is the lifetime of thermal neutrons in the reference interval;
N _3i (r, d) and N _{3 et} (r, d) are the thermal neutron count rates in the i-th and reference interval of wells, with a fixed probe length (r) and well diameter (d). 4. The method according to p. 1, characterized in that the construction of nomograms P (MgO) = f (Z _eff , τ) and P (CaO) = f (Z _eff , τ) are carried out separately for known calculated values of Z _eff , τ and known contents of magnesium and calcium oxides according to chemical analysis of core samples by constructing equal contents of MgO and CaO by interpolation in the coordinates Z _eff and τ of isolines in increments of 1 - 2% MgO and CaO.

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