A carbon-oxygen ratio value calculation method for improving sensitivity of carbon-oxygen ratio logging

Abstract

This invention discloses a method for calculating the carbon-oxygen ratio (COR) in well logging to improve its sensitivity. Relating to the field of petroleum development technology, the method includes: establishing a model and obtaining the non-elastic gamma spectrum of a pure lithological framework; interpreting the spectrum and determining the relative proportions of lithological indicator elements and carbon and oxygen yields in the pure lithological framework; obtaining the measured yields of lithological indicator elements, carbon, and oxygen in the formation; deducting a portion of the framework's carbon and oxygen yields from the total measured formation COR; and calculating the remaining COR in the measured formation. This invention utilizes numerical simulation to establish the proportional relationships between silicon and calcium yields and carbon and oxygen yields in pure sandstone and pure limestone frameworks, respectively. Based on the measured formation silicon and calcium yields, and using the proportional relationships between silicon and calcium and carbon and oxygen yields in the pure framework, a portion of the framework's carbon and oxygen yields is deducted from the total measured formation COR, thereby improving the sensitivity of the COR ratio method to formation fluid responses.

Description

Technical Field

[0001] This invention relates to the field of petroleum development technology, and in particular to a method for calculating the carbon-oxygen ratio to improve the sensitivity of carbon-oxygen ratio logging. Background Technology

[0002] Carbon-oxygen ratio logging evaluates the formation oil saturation by recording the characteristic inelastic and captured gamma rays formed by the interaction of high-energy fast neutrons generated by pulsed neutron sources with formation carbon and oxygen elements. It is less affected by formation water salinity and has wide applications in the later stages of oilfield development.

[0003] Currently, carbon-oxygen ratio (COR) logging primarily uses the energy window method and the production rate method to obtain inelastic gamma counts of carbon and oxygen elements for calculating the COR. Compared to the energy window method, the production rate method directly analyzes the total inelastic or total captured gamma spectrum. Using carbon and oxygen production rates instead of carbon-oxygen energy window counts avoids the influence of inaccurate background subtraction within the carbon and oxygen energy windows, thus reflecting formation information more accurately. However, this method is still significantly affected by the carbon and oxygen content in the formation framework, limiting its ability to reflect formation fluid information in the calculated COR. Especially under low porosity conditions, porosity fluid information is suppressed by framework information, greatly reducing the accuracy of COR logging in calculating oil saturation.

[0004] Therefore, there is an urgent need to study a method for calculating the carbon-oxygen ratio to improve the sensitivity of carbon-oxygen ratio logging. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention discloses a carbon-oxygen ratio calculation method to improve the sensitivity of carbon-oxygen ratio logging. The method utilizes numerical simulation to establish the proportional relationship between the non-inelastic gamma-ray yields of Si and Ca elements in the framework of common sandstone and carbonate reservoirs and the non-inelastic gamma-ray yields of carbon and oxygen elements in the framework. Based on the measured non-inelastic gamma-ray yields of Si and Ca, the method uses the proportional relationship of non-inelastic gamma-ray yields of different elements in the reservoir framework to deduct the contribution of non-inelastic gamma-ray yields of framework carbon and oxygen elements from the total carbon and oxygen non-inelastic gamma-ray yield, thereby improving the sensitivity of the carbon-oxygen ratio method to formation fluid responses.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A method for calculating the carbon-oxygen ratio to improve logging sensitivity includes the following steps:

[0008] S1. Establish a carbon-oxygen ratio logging model to obtain the inelastic gamma spectrum of a pure lithological framework;

[0009] S2. Analyze the spectrum and determine the relative proportions of lithological indicator elements and the yields of carbon and oxygen elements in the pure lithological framework;

[0010] S3. Obtain the measured yields of lithological indicator elements, carbon element yields, and oxygen element yields of the formation;

[0011] S4. Subtract a portion of the measured carbon and oxygen production from the total measured carbon and oxygen production of the formation.

[0012] S5. Calculate the carbon-oxygen ratio of the measured formation after deducting part of the skeleton carbon and oxygen element yield.

[0013] Optionally, step S1, which involves establishing a carbon-oxygen ratio logging model and obtaining the inelastic gamma spectrum of a pure lithological framework, specifically includes:

[0014] Based on the measured formation and actual carbon-oxygen ratio energy spectrum logging instrument information, a numerical simulation method was used to establish an instrument-casing well formation simulation model to obtain the inelastic standard spectrum of different elements and the inelastic gamma spectrum information of the pure lithological skeleton.

[0015] Optionally, the numerical simulation method refers to Monte Carlo numerical simulation, and commonly used software includes MCNP or Super Monte Carlo.

[0016] Optionally, the inelastic standard spectrum is an inelastic gamma energy spectrum generated by a single element, obtained through numerical simulation or calibration.

[0017] Optionally, the pure lithological framework refers to a single lithological stratum with a porosity of 0, such as sandstone or limestone, whose pure lithological framework consists of SiO2 and CaCO3.

[0018] Optionally, step S2, which involves interpreting the spectrum and determining the relative proportions of lithological indicator elements and the yields of carbon and oxygen in the pure lithological framework, specifically includes:

[0019] S21. Analyze the inelastic gamma spectrum of the pure lithological framework using the inelastic standard spectrum to obtain the yield of lithological indicator elements, carbon element yield, and oxygen element yield of the pure lithological framework.

[0020] S22. Ratio the carbon and oxygen yields under different lithological pure frameworks with the corresponding lithological indicator element yields to obtain the relative proportions of lithological indicator elements and carbon and oxygen yields in different lithological frameworks.

[0021] Optionally, in step S21, the method for analyzing the inelastic gamma spectrum of a pure lithological framework is the least squares method, the weighted least squares method, or the singular value method.

[0022] Optionally, in step S21, the lithological indicator element refers to an element that exists in large quantities only in the skeleton but is basically absent in the formation fluid and can specifically indicate the lithology of the formation. For example, the lithological indicator element of the sandstone skeleton is silicon, and the lithological indicator element of the limestone skeleton is calcium.

[0023] Optionally, in step S22, the relative proportion of the lithological indicator element to the production of carbon and oxygen elements refers to the non-elastic gamma counts of carbon and oxygen elements produced simultaneously for each non-elastic gamma count of the lithological indicator element produced in the pure lithological framework.

[0024] Optionally, in step S22, in the step of obtaining the relative proportions of lithological indicator elements and the yields of carbon and oxygen elements in different lithological frameworks, if the stratum is sandstone, it is necessary to obtain the relative proportions of the yields of lithological indicator elements silicon and oxygen; if the stratum is limestone, it is necessary to obtain the relative proportions of the yields of lithological indicator elements calcium and oxygen, as well as the relative proportions of the yields of calcium and carbon.

[0025] Optionally, in step S22, the relative proportions of the lithological indicator elements to the yields of carbon and oxygen are expressed as follows:

[0026]

[0027]

[0028] In the formula, a and b are the relative proportions of the yield of lithological indicator elements to the yields of carbon and oxygen elements, respectively, and n c n o n m These represent the yields of carbon, oxygen, and lithological indicator elements within the pure lithological framework.

[0029] Optionally, in step S4, the carbon and oxygen yields of the partial skeleton are determined based on the measured lithological indicator element yields of the strata and the relative proportions of the lithological indicator elements and carbon and oxygen yields of the pure lithological skeleton.

[0030] Optionally, the carbon and oxygen yields of the framework can be subtracted from the total measured carbon and oxygen yield of the formation, as expressed by:

[0031] N Rc =N c -iaN m (3)

[0032] N Ro =N o -ibN m (4)

[0033] In the formula, N c N o N m These are the measured total carbon, oxygen, and lithological indicator element yields of the strata; N Rc N RoThese represent the measured carbon and oxygen yields of the formation after deducting the contribution of some framework carbon and oxygen elements; i represents the proportion of framework elements deducted.

[0034] Optionally, in step S5, the carbon-oxygen yield ratio of the measured formation after deducting a portion of the framework carbon and oxygen yield is calculated, and the expression is:

[0035]

[0036] In the formula, (C / O) R It is the measured carbon-oxygen ratio of the formation after deducting part of the carbon and oxygen content of the framework, i.e., the remaining carbon-oxygen ratio.

[0037] The beneficial effects of this invention are that the method of this invention uses numerical simulation to establish the proportional relationship between the silicon and calcium elements in the skeleton of common pure sandstone and pure limestone and the carbon and oxygen elements in the skeleton; based on the measured silicon and calcium elements in the strata, the method uses the non-elastic gamma ratio relationship between lithological indicator elements and carbon and oxygen elements in the pure strata skeleton to deduct part of the contribution of skeleton carbon and oxygen elements from the total carbon and oxygen elements, thereby improving the response sensitivity of the carbon-oxygen ratio method to formation fluids. Attached Figure Description

[0038] Figure 1 This is a flowchart illustrating a method for calculating the carbon-oxygen ratio to improve logging sensitivity according to the present invention.

[0039] Figure 2 This is a schematic diagram showing the non-elastic gamma spectrum of a limestone skeleton and the ratio of calcium yield to carbon and oxygen yield, according to an embodiment of the present invention.

[0040] Figure 3 This is a comparison diagram of the oil-water layer response between the original carbon-oxygen ratio and the remaining carbon-oxygen ratio, as shown in an embodiment of the present invention. Detailed Implementation

[0041] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0042] A method for calculating the carbon-oxygen ratio to improve logging sensitivity, taking limestone reservoirs as an example, such as... Figure 1 As shown, it includes the following steps:

[0043] S1. Based on the measured formation and actual carbon-oxygen ratio energy spectrum logging instrument information, a Monte Carlo numerical simulation model of the instrument-casing well formation is established to obtain the non-elastic standard spectrum of formation elements such as calcium, carbon, and oxygen, as well as the non-elastic gamma spectrum information of the pure limestone formation skeleton.

[0044] S2. Referring to the non-elastic standard spectrum information of the stratigraphic elements, the non-elastic gamma spectrum of the pure limestone stratigraphic framework is analyzed using the weighted least squares method to obtain the calcium, carbon, and oxygen content of the pure limestone stratigraphic framework.

[0045] The relative proportions of calcium production to carbon and oxygen production in a pure limestone stratigraphic framework are obtained by comparing the carbon and oxygen production. This is expressed as follows:

[0046]

[0047]

[0048] In the formula, a and b are the relative proportions of calcium production to carbon and oxygen production, respectively, and n c n o n Ca These are the yields of carbon, oxygen, and calcium in a pure limestone skeleton, respectively.

[0049] like Figure 2 The non-elastic gamma spectrum of the pure limestone stratigraphic framework is shown. The spectrum is analyzed to obtain the calcium, carbon, and oxygen content. The ratios of the carbon and oxygen content to the calcium content are calculated to obtain the relative proportions of calcium content to carbon and oxygen content, which are 0.185 and 1.231, respectively.

[0050] S3. Analyze the non-elastic gamma spectrum of the measured limestone strata to obtain the calcium, carbon, and oxygen yields of the measured strata.

[0051] S4. Subtract a portion of the carbon and oxygen production of the framework from the total measured carbon and oxygen production of the strata.

[0052] Based on the relative proportions of calcium, carbon, and oxygen production in the pure limestone strata framework, the carbon and oxygen production of the framework is subtracted from the total measured carbon and oxygen production of the strata, as shown below:

[0053] N Rc =N c -iaN Ca (3)

[0054] N Ro =N o -ibN Ca(4)

[0055] In the formula, N c N o N Ca These are the measured total carbon, oxygen, and calcium yields of the limestone strata; N Rc N Ro These are the carbon and oxygen production of the formation after deducting the contribution of some skeleton carbon and oxygen elements; i represents the proportion of skeleton information deducted. When i is 0, it means that the carbon and oxygen production in the skeleton is not deducted; when i is 1, it means that the carbon and oxygen production in the skeleton is completely deducted. In this embodiment, i is 0.5.

[0056] S5. The carbon and oxygen yields after deducting part of the skeleton's carbon and oxygen yields are compared to obtain the measured residual carbon-oxygen ratio of the formation, as follows:

[0057]

[0058] In the formula, (C / O) R It is the measured residual carbon-oxygen ratio in the formation.

[0059] The residual carbon-oxygen ratios of water-saturated and oil-saturated limestone formations were obtained using the above method and compared with the original carbon-oxygen ratios of the water-saturated and oil-saturated formations. Figure 3 As shown, the residual carbon-oxygen ratio of the water layer is lower than that of the original carbon-oxygen ratio, while the residual carbon-oxygen ratio of the oil layer is higher than that of the original carbon-oxygen ratio. The difference between the responses of the oil and water layers is more obvious. The response sensitivity of the oil layer for the two carbon-oxygen ratios is calculated using formula (6), and the results are shown in Table 1:

[0060]

[0061] In the formula, S is the sensitivity, (C / O) o and (C / O) w These are the carbon-oxygen ratios of the oil layer and the water layer, respectively.

[0062] Table 1. Comparison of sensitivity of two carbon-oxygen ratios to oil-water layer response

[0063]

[0064] As can be seen from Table 1, due to the deduction of some carbon and oxygen non-elastic gamma counts generated in the skeleton, the carbon and oxygen information of the formation fluid is more clearly reflected in the measured formation non-elastic gamma energy spectrum. Compared with the original carbon-oxygen ratio, the remaining carbon-oxygen ratio shows higher fluid sensitivity, and the use of the remaining carbon-oxygen ratio for oil-water layer identification and oil saturation calculation is more accurate.

[0065] Of course, the above description is not intended to limit the present invention, and the present invention is not limited to the examples given above. Any changes, modifications, additions or substitutions made by those skilled in the art within the scope of the present invention should also fall within the protection scope of the present invention.

Claims

1. A method for calculating the carbon-oxygen ratio to improve the sensitivity of carbon-oxygen ratio logging, characterized in that, Includes the following steps: S1. Establish a carbon-oxygen ratio logging model to obtain the inelastic gamma spectrum of a pure lithological framework; S2. Analyze the spectrum and determine the relative proportions of lithological indicator elements and the yields of carbon and oxygen elements in the pure lithological framework; S3. Obtain the measured yields of lithological indicator elements, carbon element yields, and oxygen element yields of the formation; S4. Subtract a portion of the carbon and oxygen production of the framework from the total measured carbon and oxygen production of the strata. S5. Calculate the carbon-oxygen ratio of the measured formation after deducting part of the skeleton carbon and oxygen element yield. In step S4, the carbon and oxygen yields of the partial skeleton are determined based on the measured lithological indicator element yields of the strata and the relative proportions of the lithological indicator elements and carbon and oxygen yields of the pure lithological skeleton. In step S4, the carbon and oxygen yields of the framework are subtracted from the total measured carbon and oxygen yield of the formation. The expression is as follows: In the formula, N c N o N m These are the measured total carbon, oxygen, and lithological indicator element yields of the strata; N Rc N Ro These are the measured carbon and oxygen yields of the formation after deducting the contribution of some skeletal carbon and oxygen elements; i represents the proportion after deducting the skeleton; a and b are the relative proportions of the yield of lithological indicator elements and the yield of carbon and oxygen elements, respectively. In step S5, the measured formation carbon-oxygen yield ratio after deducting a portion of the framework carbon and oxygen yield is calculated, and the expression is: In the formula, (C / O) R It is the measured carbon-oxygen ratio of the formation after deducting part of the carbon and oxygen content of the framework, i.e., the remaining carbon-oxygen ratio.

2. The carbon-oxygen ratio calculation method for improving the sensitivity of carbon-oxygen ratio logging as described in claim 1, characterized in that, Step S1, which involves establishing a carbon-oxygen ratio logging model and obtaining the inelastic gamma spectrum of a pure lithological framework, specifically includes: Based on the measured formation and actual carbon-oxygen ratio energy spectrum logging instrument information, a numerical simulation method was used to establish an instrument-casing well formation simulation model to obtain the inelastic standard spectrum of different elements and the inelastic gamma spectrum information of the pure lithological skeleton.

3. The carbon-oxygen ratio calculation method for improving the sensitivity of carbon-oxygen ratio logging as described in claim 2, characterized in that, The numerical simulation method refers to Monte Carlo numerical simulation.

4. The carbon-oxygen ratio calculation method for improving the sensitivity of carbon-oxygen ratio logging as described in claim 1, characterized in that, Step S2, which involves interpreting the spectra and determining the relative proportions of lithological indicator elements and the yields of carbon and oxygen in the pure lithological framework, specifically includes: S21. Analyze the inelastic gamma spectrum of the pure lithological framework using the inelastic standard spectrum to obtain the yield of lithological indicator elements, carbon element yield, and oxygen element yield of the pure lithological framework. S22. Ratio the carbon and oxygen yields under different lithological pure frameworks with the corresponding lithological indicator element yields to obtain the relative proportions of lithological indicator elements and carbon and oxygen yields in different lithological frameworks.

5. The carbon-oxygen ratio calculation method for improving the sensitivity of carbon-oxygen ratio logging as described in claim 4, characterized in that, In step S21, the method for analyzing the inelastic gamma spectrum of the pure lithological framework is the least squares method, the weighted least squares method, or the singular value method.

6. The carbon-oxygen ratio calculation method for improving the sensitivity of carbon-oxygen ratio logging as described in claim 4, characterized in that, In step S22, in the step of obtaining the relative proportions of lithological indicator elements and the yields of carbon and oxygen in different lithological frameworks, if the stratum is sandstone, it is necessary to determine the relative proportions of the yields of lithological indicator elements silicon and oxygen; if the stratum is limestone, it is necessary to obtain the relative proportions of the yields of lithological indicator elements calcium and oxygen, as well as the relative proportions of the yields of calcium and carbon.

7. The carbon-oxygen ratio calculation method for improving the sensitivity of carbon-oxygen ratio logging as described in claim 4, characterized in that, In step S22, the relative proportions of the lithological indicator elements to the yields of carbon and oxygen are expressed as follows: In the formula, n c n o n m These represent the yields of carbon, oxygen, and lithological indicator elements within the pure lithological framework.

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