Low-resistance reservoir deep lateral resistivity argillaceous correction method

Abstract

A low-resistivity reservoir deep lateral resistivity argillaceous correction method belongs to the technical field of sedimentary reservoir evaluation, and comprises the following steps: firstly, obtaining a core sample of a research area, observing a low-resistivity reservoir drilling coring rock sample of the research area, determining the thickness of the shale sandstone reservoir according to the measurement principle of lateral logging, and determining the thickness of the shale sandstone reservoir according to the thickness of the shale sandstone reservoir, the thickness of the shale sandstone reservoir and the thickness of the shale sandstone reservoir; a main current emitted by the main electrode horizontally and radially flows into the formation. According to the method, an improved electrical chart is established by utilizing the formation resistivity subjected to argillaceous correction, and after argillaceous correction, the resistivity of an oil layer and the resistivity of the same oil-water layer are obviously increased, and the resistivity of a water layer is only slightly increased. Therefore, on the improved electrical chart, the oil layer and the oil-water same layer area are separated from the water layer area, which is beneficial to the identification of the oil-water layer.

Description

Technical Field

[0001] The invention belongs to the technical field of sedimentary reservoir evaluation, and in particular relates to a method for correcting the deep lateral resistivity mud of a low-resistance reservoir. Background Art

[0002] In the field of oil and gas development technology, identification of low-resistance reservoir fluids is one of the important means of evaluating reservoir reserves. The effective use of logging data to accurately determine the deep lateral resistivity of the oil layer is particularly important.

[0003] The identification of low-resistivity production layers through cable logging analysis has a history of more than 50 years. During this period, low-resistivity production layers have been found in many parts of the world, such as Daqing, Jilin, Liaohe, Dagang, Tarim and other eastern and western oil fields in China. The so-called low-resistivity oil and gas layers on the sandstone and mudstone profiles have the common characteristics of being not much different from the adjacent water layer in resistivity (sometimes even lower than the water layer), having a small difference in resistivity with the upper and lower mudstones, and a resistivity increase rate (ratio of oil layer resistivity to water layer resistivity) less than or equal to 3. These oil and gas layers are not easy to identify from the resistivity logging curve. Summary of the invention

[0004] In order to solve the above-mentioned problems, the present invention proposes: a method for deep lateral resistivity mud correction of low-resistance reservoir, comprising the following steps: first, obtaining core samples in the study area, observing the core samples taken from drilling in the low-resistance reservoir in the study area, the mudstone sandstone reservoir is composed of sandstone and mudstone laminae, and according to the measurement principle of lateral logging, the main current emitted by the main electrode flows horizontally and radially into the formation.

[0005] Assume that there are M sand layers and N mudstone laminae in a unit thickness of mudstone, and the resistivities of sandstone and mudstone laminae are R t , R sh ; The cross-sectional area of ​​the i-th sandstone layer is (i=1,2,...,M); the cross-sectional area of ​​the jth sandstone layer is and That is, the reservoir cross section is per unit area; the resistivity of muddy sandstone is R t ', without considering mud invasion, the resistivity R of deep lateral measurement LLD For R t ', R t ' is the equivalent resistance of the above sandstone and mudstone laminae per unit length connected in parallel, that is:

[0006]

[0007]

[0008]

[0009] Substituting equations (2) and (3) into equation (1), we obtain:

[0010]

[0011] The mud content is obtained from the given conditions The above formula can be written as:

[0012]

[0013] Right now:

[0014]

[0015] The shale content is calculated by natural gamma logging, and an improved electrical map is established:

[0016]

[0017]

[0018] R sh Take the average resistivity logging value of mudstone and calculate the formation resistivity R after mud correction using equations (5) and (6): t , establish an improved electrical diagram.

[0019] The beneficial effect of the present invention is as follows: the present invention uses the formation resistivity corrected by mud to establish an improved electrical property map. After the mud correction, the resistivity of the oil layer and the oil-water layer increases significantly, while the resistivity of the water layer increases only slightly. Therefore, on the improved electrical property map, the oil layer and the oil-water layer area are more clearly distinguished from the water layer, which is beneficial to the identification of the oil-water layer. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 An improved electrical property plate for the low-resistance reservoir of the present invention;

[0021] Figure 2 This is the low-resistance reservoir electrical property plate of the present invention. DETAILED DESCRIPTION

[0022] In order to make the technical means and objectives of the present invention easy to understand, the present invention is further described below in combination with a specific implementation method. A method for shale correction of deep lateral resistivity of low-resistance reservoirs specifically includes the following steps: first, obtain core samples from the study area. By observing a large number of core samples from low-resistance reservoirs in the study area, we found that most shale sandstone reservoirs are composed of many sandstone and mudstone laminae. According to the measurement principle of lateral logging, the main electrode A 0 The main current I 0 Flowing horizontally and radially into the stratum. Assume that there are M sand layers and N mudstone layers in a unit thickness of mudstone, and the resistivity of sandstone and mudstone layers are Rt , R sh ; The cross-sectional area of ​​the i-th sandstone layer is The cross-sectional area of ​​the jth sandstone layer is and That is, the reservoir cross section is per unit area; the resistivity of muddy sandstone is R t ', without considering mud invasion, the resistivity R of deep lateral measurement LLD For R t '. R t ' can be regarded as the equivalent resistance of the above-mentioned sandstone and mudstone laminae in parallel per unit length. That is:

[0023]

[0024] again

[0025]

[0026]

[0027] Substituting equations 2 and 3 into equation 1, we get:

[0028]

[0029] The mud content can be obtained from the given conditions The above formula can be written as:

[0030]

[0031] Right now:

[0032]

[0033] For the low-resistivity reservoirs in the study area, although they are located in different layers, their lithology is basically the same, all of which are siltstone, and their pore structure and fluid distribution are roughly similar, and the formation water salinity does not change much. Therefore, in addition to formation water saturation, mud content is the main factor affecting formation resistivity. The mud content is calculated by natural gamma logging to establish an improved electrical property chart:

[0034]

[0035]

[0036] R sh Take the average resistivity logging value of mudstone and calculate the formation resistivity R after mud correction using equations 5 and 6. t , establish an improved electrical diagram.

[0037] Using the formation resistivity corrected for mud, an improved electrical map is created, such as Figure 1 Compare Figure 2 (unmodified electrical diagram) and Figure 1 After the mud correction, the resistivity of the oil layer and the oil-water layer increased significantly, while the resistivity of the water layer increased only slightly. Therefore, on the improved electrical map, the oil layer and the oil-water layer area are more separated from the water layer, which is conducive to the identification of the oil and water layers.

[0038] For different research areas, due to the differences in their geological backgrounds, the formation of various types of low-resistivity oil and gas reservoirs is related to their geological backgrounds. They are mainly controlled by reservoir properties (oil-wet, water-wet, pores, permeability, saturation, pore structure, etc.), formation water properties (mineralization, ion activation, density, etc.), reservoir structure amplitude, oil column height and driving mode, etc. The mud content in the reservoir is different and varies greatly. In practice, the corresponding mud correction formula should be selected according to the different mud contents for research.

[0039] First, in a region, for strata with basically the same lithology, similar pore structure, and little change in formation water mineralization, in addition to the formation water saturation factor, the shale content is the main factor affecting the formation resistivity. The resistivity of the shale sandstone is corrected by "shale" to become the resistivity of the "pure sandstone" formation. Then, the resistivity of the "pure sandstone" formation is used instead of the deep detection resistivity logging value to make an electrical chart to identify the properties of the pore fluid. The actual results show that this method is simple to operate and has good results.

[0040] According to statistics (Zhou Xiaozhou et al., 2000), the resistance increase rate of known low-resistance layers in the study area is generally ≤3 (Table 1). The deep lateral and deep induction resistivity of low-resistance oil and gas layers are 35-50Ω·m (28 wells / 45 layers), while the deep lateral and deep induction resistivity of water layers are 30-40Ω·m (10 wells / 11 layers), and the resistance increase rate is <1.3;

[0041] Table 1 Comparison of low resistance layer resistance increase rate

[0042]

[0043]

[0044] Regardless of the type of low-resistivity oil and gas reservoir, its formation is related to its geological background. It is mainly controlled by the reservoir properties. When the mud content in the reservoir is high, the positive ions adsorbed on the surface of clay mineral particles with illite / montmorillonite and illite as the main components move along the surface under the action of the external electric field, thus increasing the conductivity of the formation and reducing the resistivity of the reservoir.

[0045] Since clay minerals such as montmorillonite, illite, and illite often carry this unsaturated structural negative charge, according to the principle of electrical neutrality, an equal amount of cations must be adsorbed on the surface of the clay mineral to achieve electrical balance. The cations adsorbed on the surface of the clay mineral can exchange with the free cations between the layers, that is, cation exchange adsorption. The cation exchange adsorption capacity of different types of clay minerals varies greatly (Table 2). The free cations between the layers are exchanged and adsorbed on the surface of the clay minerals to form an electric current, thus forming a good ion conductive network, resulting in a decrease in the resistivity of the entire sandstone layer. The additional conductivity of montmorillonite, illite, and illite is much stronger than that of kaolinite and chlorite.

[0046] Table 2 Cation exchange adsorption capacity

[0047]

[0048] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical solutions and concepts of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A method for shale correction of deep lateral resistivity of low-resistance reservoirs, characterized in that: The method includes the following steps: firstly, obtaining core samples from the study area, observing the core samples taken from the low-resistance reservoir in the study area, and the mudstone sandstone reservoir is composed of sandstone and mudstone laminae. According to the measurement principle of lateral logging, the main current emitted by the main electrode flows horizontally and radially into the formation.

2. The method for shale correction of deep lateral resistivity of low-resistance reservoir according to claim 1, characterized in that There are M sand layers and N mudstone layers in a unit thickness of mudstone. The resistivities of sandstone and mudstone layers are R t , R sh ; The cross-sectional area of ​​the i-th sandstone layer is The cross-sectional area of ​​the jth sandstone layer is and That is, the reservoir cross section is per unit area; the resistivity of muddy sandstone is R t ', without considering mud invasion, the resistivity R of deep lateral measurement LLD For R t ', R t ' is the equivalent resistance of the above sandstone and mudstone laminae per unit length connected in parallel, that is:

3. The method for shale correction of deep lateral resistivity of low-resistance reservoir according to claim 3, characterized in that:

4. The method for shale correction of deep lateral resistivity of low-resistance reservoir according to claim 3, characterized in that: Substituting equations (2) and (3) into equation (1), we obtain:

5. The method for shale correction of deep lateral resistivity of low-resistance reservoir according to claim 4, characterized in that: The mud content is obtained from the given conditions The above formula can be written as:

6. The method for shale correction of deep lateral resistivity of low-resistance reservoir according to claim 5, characterized in that: Right now:

7. The method for shale correction of deep lateral resistivity of low-resistance reservoir according to claim 6, characterized in that: The shale content is calculated by natural gamma logging, and an improved electrical map is established:

8. The method for shale correction of deep lateral resistivity of low-resistance reservoir according to claim 7, characterized in that: R sh Take the average resistivity logging value of mudstone and calculate the formation resistivity R after mud correction using equations (5) and (6): t , establish an improved electrical diagram.

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