Low-resistance hydrocarbon reservoir conductivity evaluation method based on XRF element logging technology

Abstract

The invention discloses a low-resistance hydrocarbon reservoir conductivity evaluation method based on an XRF element logging technology. The method comprises the following steps: determining a main internal cause causing low resistivity; determining main minerals influencing the low resistivity of the sedimentary rock; setting an element conductivity index; dividing the element into a low-conductivity region, a middle-conductivity region and a high-conductivity region according to the interval size of the element conductivity index, and determining the type of the reservoir according to the three regions. According to the method, on the basis of element analysis of rock mineral components of a regional target low-resistance hydrocarbon reservoir, the correlation between the mineral components and rock conductivity is obtained through inversion, optimal mineral evaluation parameters are selected, element conductivity indexes are established, and the reservoir type is determined according to the element conductivity indexes. The method is simple and convenient, has the characteristics of real-time evaluation, high accuracy and the like, and provides favorable data and technical support for exploration and development of a construction party and improvement of the productivity construction effect of a target area.

Description

Technical Field

[0001] The invention belongs to the technical field of unconventional oil and gas exploration and development, and particularly relates to a method for evaluating the conductivity of low-resistance oil and gas layers based on XRF element logging technology. Background Art

[0002] According to the approximate theory of equivalent resistivity, the resistivity of rock is affected by multiple factors such as its structure, fluid content, and formation pressure. Therefore, the resistivity of rocks with different structures is anisotropic. In the actual exploration process, high-resistivity formations rich in oil are usually easy to identify, while low-resistivity formations rich in oil are easy to be lost. Such formations are called low-resistivity oil and gas layers. With the increasing maturity of domestic exploration and development, unconventional oil and gas reservoirs and remaining oil reservoirs have become the focus of exploration and development of major oil fields. Low-resistivity oil and gas reservoirs are one of the most important and complex and concealed oil and gas reservoirs, and are also a type of oil and gas layer that makes it difficult to interpret oil and water layers.

[0003] Some blocks in some oilfield areas have low-resistivity oil and gas layer characteristics, and these oil and gas layers have high resource and economic value. The construction party has conducted many reviews of old wells and plans to increase development efforts. In the process of exploration and development, logging professionals usually rely on gas logging data and some engineering parameters to identify and evaluate reservoirs. The gas logging values ​​and engineering parameters of oil-rich high-resistivity formations have obvious trends, while the oil-rich low-resistivity formations are affected by factors such as low reservoir resistivity, unclear logging characteristics, and a large degree of on-site lithological fragmentation. The trend of change is unclear and cannot be distinguished and judged during the drilling process. Therefore, it is difficult to meet the requirements for rapid and accurate identification of such reservoirs. At the same time, there are many objective constraints on on-site engineering logging. In the later logging interpretation process, due to the low resistivity and unclear characteristics, it is usually interpreted as a water layer or an oil-water layer, and it is impossible to provide an accurate interpretation conclusion, which directly affects the development effect of the construction party. Therefore, based on the production needs of the construction party, using element logging technology as a means, a new method for reservoir resistivity evaluation is formed through research on formation lithology, mineral conductivity, etc., to accurately distinguish and identify high and low resistivity oil and gas layers during logging while drilling, to make up for the technical gap caused by oil layer leakage due to unclear rock characteristics and inability to interpret logging, and to provide strong data support for later oil layer interpretation and evaluation, which is of great practical significance for helping the construction party to maximize the potential of oil layers, accelerate the development and utilization of peripheral oilfield reserves, and increase reserves and production. However, there is still a lack of corresponding methods. Summary of the invention

[0004] In order to solve the above problems, the purpose of the present invention is to provide a method for evaluating the conductivity of low-resistance oil and gas layers based on XRF element logging technology.

[0005] In order to achieve the above object, the method for evaluating conductivity of low-resistance oil and gas layers based on XRF element logging technology provided by the present invention comprises the following steps performed in sequence:

[0006] 1) Analyze the internal and external factors of low resistivity of the formation and determine that the main internal factor causing low resistivity is the minerals in the rock;

[0007] 2) Analyze the correlation between the composition of the three-terminal minerals in the sedimentary rocks that make up the low-resistivity oil and gas layers and the resistivity of the sedimentary rocks, and determine that the main minerals that affect the low resistivity of the sedimentary rocks are good conductors including clay minerals and metals;

[0008] 3) Analyze the chemical composition of three-terminal minerals in sedimentary rocks, select representative elements of clay minerals, and then set the element conductivity index based on the above representative elements;

[0009] 4) According to the interval size of the element conductivity index, it is divided into three areas: low conductivity area, medium conductivity area and high conductivity area, and the type of reservoir is determined accordingly.

[0010] In step 2), the low-resistance oil and gas layer is mainly composed of sedimentary rocks, which are mainly composed of three-terminal minerals, namely clay minerals, felsic minerals and carbonate minerals: carbonate minerals in sedimentary rocks are mainly precipitated in a pure chemical way, mainly in an electronic conductive way, with small particle size and almost no pores between particles, so their resistivity is usually very high; felsic minerals are mainly quartz and feldspar, which are non-conductors or poor conductors with poor conductivity, and present a high resistivity in electrical properties; the conductive way of clay minerals is mainly ionic conduction, and the resistivity is relatively stable; metals in sedimentary rocks can also cause low resistance, so the main minerals affecting the low resistivity of sedimentary rocks are good conductors including clay minerals and metals.

[0011] In step 3), the element conductivity index is the ratio of the content of representative elements Al and Fe in clay minerals to the content of representative elements Na, Mg, K, Al, Si, Ca, and Fe in sedimentary rocks, and the unit is dimensionless, and the formula is as follows:

[0012] Ec i=Content(Al+Fe) / Content(Na+Mg+K+Al+Si+Ca+Fe)

[0013] In step 4), the method of dividing the element conductivity index into three regions, namely, a low conductivity region, a medium conductivity region and a high conductivity region, according to the interval size of the element conductivity index, and determining the type of the reservoir accordingly is:

[0014] The element conductivity index Eci of the low conductivity zone is 0.146-0.201, the element conductivity index Eci of the medium conductivity zone is 0.201-0.246, and the element conductivity index Eci of the high conductivity zone is 0.246-0.265;

[0015] The reservoir types corresponding to the low-conductivity zone are water layers and high-resistivity oil and gas layers; the reservoir types corresponding to the medium-conductivity zone are low-resistivity oil and gas layers and dry layers; and the reservoir type corresponding to the high-conductivity zone is shale layers.

[0016] The conductivity evaluation method of low-resistivity oil and gas layers based on XRF element logging technology provided by the present invention has the following beneficial effects: based on the elemental analysis of the rock mineral components of the regional target low-resistivity oil and gas layers, the correlation between the mineral composition and the rock conductivity is inverted, the optimal evaluation parameters of the minerals are selected, the element conductivity index is established, and the reservoir type is determined accordingly. The method is simple and convenient, has the characteristics of real-time evaluation and high accuracy, and provides favorable data and technical support for the construction party to explore and develop and improve the production capacity construction effect of the target area. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a schematic diagram of the causes of low formation resistivity in the present invention.

[0018] Figure 2 This is a comparison chart of the correlation between Si and Al elements and conductivity in the present invention. DETAILED DESCRIPTION

[0019] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0020] The method for evaluating conductivity of low-resistance oil and gas layers based on XRF element logging technology provided by the present invention comprises the following steps performed in sequence:

[0021] 1) Analyze the internal and external factors of low resistivity of the formation and determine that the main internal factor causing low resistivity is the minerals in the rock;

[0022] Low-resistivity oil and gas layer (low-resistivity oil and gas layer) refers to a type of oil and gas layer whose resistivity is almost the same as that of the adjacent water layer, or even lower than that of the water layer, thus causing difficulties in logging interpretation of the oil and water layer. Figure 1 This is a schematic diagram of the causes of low stratum resistivity. Figure 1 As shown in the figure, from a geological perspective and based on the composition of rocks, the causes of low formation resistivity can be summarized as internal and external factors. The internal cause is the low resistivity value caused by minerals or elements in the rock. In addition to being affected by external factors such as surrounding rock, electrical measurement resolution, formation temperature and pressure, the external cause is mainly controlled by formation fluids in the rock pores, such as oil and gas or highly mineralized formation water. The invasion of highly mineralized drilling fluid can also lead to low formation resistivity.

[0023] 2) Analyze the correlation between the composition of the three-terminal minerals in the sedimentary rocks that make up the low-resistivity oil and gas layers and the resistivity of the sedimentary rocks, and determine that the main minerals that affect the low resistivity of the sedimentary rocks are good conductors including clay minerals and metals;

[0024] Rock resistivity is related to the resistivity of the minerals that make up the rock, the content of the minerals, and the distribution of the minerals. The main components of low-resistivity oil and gas layers are sedimentary rocks, which are mainly composed of three-terminal minerals, namely clay minerals, felsic minerals, and carbonate minerals. The composition of sedimentary rocks in a certain stratum is shown in Table 1. As can be seen from Table 1, the sum of the three-terminal minerals accounts for an average of more than 70% in sedimentary rocks, which is one of the main factors affecting the resistivity of sedimentary rocks. The orientation of sedimentary rocks is relatively complex, and mainly takes the following forms:

[0025] Carbonate minerals in sedimentary rocks are mainly precipitated in a purely chemical way, with electronic conductivity as the main mode. The particle size is small and there are almost no pores between the particles. Therefore, their resistivity is usually very high, up to 5x103-104o.m.

[0026] Felsic minerals are mainly quartz and feldspar, which are non-conductors or poor conductors with poor conductivity. They are electrically in the form of high resistivity. Their electrical resistance is generally lower than that of carbonate minerals, mainly because of the presence of pores and mud in cements and fillings.

[0027] Determined by the crystal structure and chemical properties, clay minerals have ion exchangeability. Certain anions and cations are adsorbed in the crystal lattice and maintained in an exchange state. Generally, exchangeable cations are Ca2+, Mg2+, H+, K+, (NH4)+, and Na+. Common exchangeable anions are (SO4)2-, Cl-, (PO4)3-, and (NO3). These ions are the main cause of low formation resistance. The main conduction mode is ionic conduction. Since the surface charge is basically the same, the resistivity of clay minerals is relatively stable.

[0028] In addition, some good conductors in sedimentary rocks, such as metals, graphite, anthracite, etc., can also cause low resistance. From the previous analysis, it can be seen that the main minerals that affect the low resistivity of sedimentary rocks are good conductors including clay minerals and metals.

[0029] Table 1. Sedimentary rock analysis data table of a certain stratum

[0030]

[0031]

[0032] 3) Analyze the chemical composition of three-terminal minerals in sedimentary rocks, select representative elements of clay minerals, and then set the element conductivity index based on the above representative elements;

[0033] As mentioned above, the resistivity of sedimentary rocks is determined by the content of clay minerals and metals. The representative elements of felsic minerals in the three-end member minerals are Si, K, and Na, the representative elements of carbonate minerals are Ca and Mg, and the main components of clay minerals are: kaolinite, illite, chlorite, and montmorillonite. From the chemical composition of the above three-end member minerals, the representative elements include Na, Mg, K, Al, Si, Ca, and Fe, among which Si and Al are the main elements. However, Figure 2 As shown in the figure, the correlation between the resistivity of sedimentary rocks and the elements Si and Al in clay minerals is poor, and the regional correlation R = 0.04. The main reason is that the element Si is the representative element of felsic minerals. Therefore, when studying the correlation between elements and resistivity, the representative elements should exclude common elements, and take into account the regional conductive minerals including pyrite and siderite, and select the elements Al and Fe as the representative elements of clay minerals.

[0034] The ratio of the content of representative elements Al and Fe in clay minerals to the content of representative elements Na, Mg, K, Al, Si, Ca, and Fe in sedimentary rocks is set as the element conductivity index Eci, which is dimensionless and has the following formula:

[0035] Eci=Content(Al+Fe) / Content(Na+Mg+K+Al+Si+Ca+Fe)

[0036] In order to verify whether the setting of the element conductivity index Eci is reasonable, the inventors analyzed and compared the correlation between the element conductivity index Eci and the resistivity in a certain area. The results are shown in Table 2.

[0037] Table 2. Comparison of the correlation between element conductivity index Eci and resistivity

[0038]

[0039]

[0040]

[0041]

[0042] The above data were further analyzed. The results are shown in Table 3, where the dense layer is mainly shale layer.

[0043] Table 3. Comparison table of element conductivity index and resistivity

[0044]

[0045] By comparing the data in this area, it can be seen that the resistivity is significantly negatively correlated with the element conductivity index Eci, and the regional correlation R = -0.71, which proves that the element conductivity index Eci is instructive and practical.

[0046] Main manifestations and characteristics:

[0047] High resistance, low element conductivity index Eci or low resistance, high element conductivity index Eci.

[0048] The resistivity of low-resistivity oil and gas layers and shale layers and the element conductivity index Eci vary slightly, indicating that their resistivity is mainly controlled by muddy or metallic conductive minerals.

[0049] The element conductivity index Eci of the high-resistivity oil and gas layer is close to that of the water layer, but the resistivity varies greatly, indicating that its resistivity is mainly controlled by the formation fluid.

[0050] The element conductivity index Eci of the shale layer is significantly higher than that of other reservoirs and its value range is relatively stable. The element conductivity index Eci of the dry layer fluctuates greatly, which is mainly affected by its mud content.

[0051] The element conductivity index Eci of water layer and low-resistance oil and gas layer is relatively stable, and the element conductivity index Eci of high-resistance oil and gas layer is relatively stable, close to the water layer. The large fluctuations are mainly caused by the high resistivity of the rock and the amount of oil and gas content.

[0052] 4) According to the interval size of the element conductivity index, it is divided into three areas: low conductivity area, medium conductivity area and high conductivity area, and the type of reservoir is determined accordingly.

[0053] The element conductivity index Ec i in the low conductivity zone is 0.146-0.201, the element conductivity index Ec i in the medium conductivity zone is 0.201-0.246, and the element conductivity index Ec i in the high conductivity zone is 0.246-0.265;

[0054] The reservoir types corresponding to the low-conductivity zone are water layers and high-resistivity oil and gas layers; the reservoir types corresponding to the medium-conductivity zone are low-resistivity oil and gas layers and dry layers; and the reservoir type corresponding to the high-conductivity zone is shale layers.

Claims

1. A method for evaluating conductivity of low-resistance oil and gas layers based on XRF element logging technology, characterized in that: The method comprises the following steps performed in sequence: 1) Analyze the internal and external factors of low resistivity of the formation and determine that the main internal factor causing low resistivity is the minerals in the rock; 2) Analyze the correlation between the composition of the three-terminal minerals in the sedimentary rocks that make up the low-resistivity oil and gas layers and the resistivity of the sedimentary rocks, and determine that the main minerals that affect the low resistivity of the sedimentary rocks are good conductors including clay minerals and metals; 3) Analyze the chemical composition of three-terminal minerals in sedimentary rocks, select representative elements of clay minerals, and then set the element conductivity index based on the above representative elements; 4) According to the interval size of the element conductivity index, it is divided into three areas: low conductivity area, medium conductivity area and high conductivity area, and the type of reservoir is determined accordingly.

2. The method for evaluating conductivity of low-resistance oil and gas layers based on XRF element logging technology according to claim 1 is characterized in that: In step 2), the low-resistance oil and gas layer is mainly composed of sedimentary rocks, which are mainly composed of three-terminal minerals, namely clay minerals, felsic minerals and carbonate minerals: carbonate minerals in sedimentary rocks are mainly precipitated in a pure chemical way, mainly in an electronic conductive way, with small particle size and almost no pores between particles, so their resistivity is usually very high; felsic minerals are mainly quartz and feldspar, which are non-conductors or poor conductors with poor conductivity, and present a high resistivity in electrical properties; the conductive way of clay minerals is mainly ionic conduction, and the resistivity is relatively stable; metals in sedimentary rocks can also cause low resistance, so the main minerals affecting the low resistivity of sedimentary rocks are good conductors including clay minerals and metals.

3. The method for evaluating conductivity of low-resistance oil and gas layers based on XRF element logging technology according to claim 1, characterized in that: In step 3), the element conductivity index is the ratio of the content of representative elements Al and Fe in clay minerals to the content of representative elements Na, Mg, K, Al, Si, Ca, and Fe in sedimentary rocks, and the unit is dimensionless, and the formula is as follows: Eci=Content(Al+Fe) / Content(Na+Mg+K+Al+Si+Ca+Fe) 4. The method for evaluating conductivity of low-resistance oil and gas layers based on XRF element logging technology according to claim 1, characterized in that: In step 4), the method of dividing the element conductivity index into three regions, namely, a low conductivity region, a medium conductivity region and a high conductivity region, according to the interval size of the element conductivity index, and determining the type of the reservoir accordingly is: The element conductivity index Eci of the low conductivity zone is 0.146-0.201, the element conductivity index Eci of the medium conductivity zone is 0.201-0.246, and the element conductivity index Eci of the high conductivity zone is 0.246-0.265; The reservoir types corresponding to the low-conductivity zone are water layers and high-resistivity oil and gas layers; the reservoir types corresponding to the medium-conductivity zone are low-resistivity oil and gas layers and dry layers; and the reservoir type corresponding to the high-conductivity zone is shale layers.

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