Method for determining lower limit of effective reservoir of metamorphic buried hill

Abstract

The application provides a method for determining the lower limit of effective reservoirs of metamorphic buried hill, and relates to the technical field of development and research of metamorphic buried hill reservoirs. Specifically, the method comprises the following steps: firstly, based on the field wall core and cutting data, the lithology type, element composition and wall core length at the same depth are analyzed; according to the analysis, the lithology index (LT), chemical weathering index (CIA) and rock fragmentation degree (RFD) are determined; the tectonic fragmentation index (TFI) and weathering fragmentation index (WFI) are calculated; a two-dimensional fan chart composed of TFI and WFI is created, and the metamorphic rock reservoir quality index (MRQI) is calculated; the relationship between MRQI and single well oil and gas production is analyzed, and the lower limit of effective reservoirs of metamorphic buried hill is determined; the application is based on the characteristics of metamorphic rock as a fractured reservoir, and comprehensively considers the development factors of the reservoir, and a new method for determining the lower limit of effective reservoirs is provided. The overall method is simple, fast and effective, and is suitable for the evaluation of metamorphic buried hill reservoirs.

Description

Technical Field

[0001] This invention belongs to the technical field of metamorphic rock buried hill reservoir development research, specifically involving a method for determining the lower limit of effective reservoirs in metamorphic rock buried hills. Background Technology

[0002] "Buried hills" refer to ancient topographical highlands covered by relatively recent sediments. For a long time, buried hills have been an important area for oil and gas exploration, characterized by their large reservoir thickness and substantial economic reserves. Currently, hundreds of buried hill oil and gas reservoirs with industrial oil and gas flows have been discovered worldwide, with lithologies including sedimentary rocks, volcanic rocks, and metamorphic rocks. Of the buried hill oil and gas fields discovered globally, metamorphic rock buried hills account for 40% of the total number and 75% of the total reserves. This demonstrates the enormous exploration and development potential of metamorphic rock buried hill reservoirs.

[0003] Unlike conventional sedimentary reservoirs, metamorphic rocks are inherently dense and lack primary reservoir space. After undergoing multiple phases of tectonic stress alteration and long-term weathering, a series of natural fractures are formed, which serve as the primary reservoir space and seepage channels for oil and gas. However, these natural fractures are highly heterogeneous, posing a series of challenges to reservoir evaluation. Therefore, determining the lower limit of effective reservoirs is extremely important.

[0004] Currently, the methods for determining the effective reservoir lower limit both domestically and internationally are mainly based on core data, oil testing data, and well logging data.

[0005] Core-based methods mainly include the porosity-permeability cross-method, the bound water saturation method, and the minimum flow throat radius method. Methods combining core data with oil testing data mainly include testing methods, oil testing methods, and oil-bearing occurrence methods. However, porosity and permeability are important evaluation criteria in these methods, primarily applicable to porous reservoirs. Since metamorphic buried hills are fractured reservoirs, the measured porosity and permeability cannot represent the actual reservoir conditions; therefore, these methods are not applicable to metamorphic reservoirs.

[0006] Methods based on well logging data primarily determine the lower limit of effective reservoirs by plotting cross-plots between various well logging parameters, such as resistivity-neutron cross-plots. However, these methods require extensive core data for calibration, making their application quite complex.

[0007] In summary, how to provide a simple and efficient method for determining the lower limit of effective reservoirs in metamorphic buried hills is a problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0008] The purpose of this invention is to provide a method for determining the lower limit of effective reservoirs in metamorphic buried hills. The method is simple and effective, and is applicable to the determination of the lower limit of effective reservoirs in the exploration and development of metamorphic buried hills.

[0009] To achieve the above objectives, the present invention provides a method for determining the lower limit of effective reservoirs in metamorphic buried hills, specifically including the following steps:

[0010] Step 1: Collect multiple wall cores and rock fragments samples;

[0011] Step 2: Observe and analyze the core and rock fragment samples obtained in Step 1 respectively to determine the lithological type, core length and elemental composition of the rock fragment samples;

[0012] Step 3: Determine the lithology index (LT) based on lithology type, calculate the rock fragmentation degree (RFD) based on core length, and calculate the chemical weathering index (CIA) based on element abundance;

[0013] Step 4: Calculate the Tectonic Fracture Index (TFI) and Weathering Fracture Index (WFI) based on the indices obtained in Step 3;

[0014] Step 5: Create a two-dimensional sector plot consisting of TFI and WFI to calculate the Metamorphic Reservoir Quality Index (MRQI);

[0015] Step 6: Analyze the relationship between MRQI and single-well oil and gas production to determine the lower limit of effective reservoir.

[0016] In a preferred embodiment, in step 3, the specific method for determining the lithology index (LT) based on the lithology type is as follows:

[0017] The lithology with the highest fracture linear density is considered the most favorable lithology for reservoir formation, and its LT value is assigned to 1. The LT values ​​of other lithologies are calculated based on the ratio of the fracture linear density of that lithology to the fracture linear density of the most favorable lithology.

[0018] In a preferred embodiment, in step 3, the rock fragmentation degree (RFD) is calculated based on the core length, as shown in equation (1):

[0019]

[0020] In formula (1): L max L represents the maximum length of the core sample, in cm; L represents the length of a single core sample, in cm.

[0021] In a preferred embodiment, step 3, the specific method for calculating the Chemical Weathering Index (CIA) based on elemental abundance, includes:

[0022] The elemental composition of the obtained rock fragments was converted into oxides, and the CIA was obtained according to equation (2):

[0023] CIA = [Al₂O₃ / (Al₂O₃+CaO)] * +Na₂O+K₂O)]×100 (2)

[0024] In equation (2), all oxides are expressed as mole fractions, where CaO * It is the CaO content in silicate minerals after apatite correction.

[0025] In a preferred embodiment, the correction method for the apatite specifically includes:

[0026] The molar fractions of P2O5 and CaO in the rock cuttings sample were determined, and the CaO content was obtained according to equation (3). ap :

[0027] CaO ap =CaO-(10 / 3×P2O5) (3)

[0028] Determine CaO ap The relationship with Na2O is obtained from equations (4) and (5) to obtain CaO. * Values:

[0029] If CaO ap If the number of moles is less than the number of moles of Na₂O, then take CaO. ap The value of is given by the following formula (4):

[0030] CaO * =CaO ap (4)

[0031] If CaO ap The number of moles is greater than the number of moles of Na₂O, CaO * Then take the value of Na2O, as shown in equation (5):

[0032] CaO * =Na2O (5)

[0033] In a preferred embodiment, in step 4, the structural fragmentation index (TFI) is obtained according to equation (6):

[0034] TFI = LT × RFD (6)

[0035] In a preferred embodiment, in step 4, the weathering fragmentation index (WFI) is obtained according to equation (7):

[0036] WFI = LT × CIA (7)

[0037] In a preferred embodiment, in step 4, both the TFI and the WFI are standardized to a value between 0 and 100, and are dimensionless.

[0038] In a preferred embodiment, in step 5, the calculation of the metamorphic reservoir quality index (MRQI) is as shown in equation (8):

[0039]

[0040] Compared with the prior art, the technical solution of the present invention has the following advantages:

[0041] This invention addresses the characteristics of fractured reservoirs in metamorphic buried hills by comprehensively analyzing and quantifying the main controlling factors of reservoir formation. A comprehensive parameter for reservoir quality evaluation is established to determine the lower limit of effective reservoirs. First, this method considers the lithology, structure, and weathering processes involved in the formation of metamorphic buried hill reservoirs, providing a solid theoretical foundation. Second, the method, combined with test data, has strong practical significance. Furthermore, the method requires no additional correction and is simple and effective overall. Attached Figure Description

[0042] These and / or other aspects and advantages of the present invention will become clearer and more readily understood from the following detailed description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein:

[0043] Figure 1 This is a flowchart illustrating the method for determining the lower limit of effective reservoirs in metamorphic buried hills according to the present invention.

[0044] Figure 2 This is a chart illustrating the calculation of the Metamorphic Reservoir Quality Index (MRQI) in this invention.

[0045] Figure 3 This diagram illustrates how the effective reservoir lower limit is determined by the relationship between test data and MRQI in this invention. Detailed Implementation

[0046] To facilitate understanding of the present invention, a more complete description will be given below with reference to the accompanying drawings. Preferred embodiments of the invention are shown in the drawings. However, the invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of the invention.

[0047] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

[0048] This invention provides a method for determining the effective lower limit of metamorphic rock buried hill reservoirs, solving the problems of poor applicability and complexity in existing methods for determining the lower limit of metamorphic rock reservoirs.

[0049] The technical solution in this invention aims to solve the above problems, and the overall approach is as follows:

[0050] See Figure 1 As shown, this invention provides a method for determining the lower limit of effective reservoirs in metamorphic buried hills. Lithology is the main internal factor affecting reservoir development. Among several lithological types, natural fractures are most developed in granulite, followed by breccia and gneiss, while mixed gneiss and mixed granite have the worst fracture development. Tectonics and weathering are key external factors affecting reservoir development; the stronger the tectonic and weathering processes, the more developed the reservoir. Based on these main controlling factors, they are quantified, and a comprehensive Metamorphic Buried Hill Reservoir Quality Index (MRQI) is established. Combined with actual production testing, this is used to determine the lower limit of effective reservoirs in metamorphic buried hills.

[0051] This invention provides a method for determining the lower limit of effective reservoirs in metamorphic buried hills, specifically including the following steps:

[0052] Step 1: Collect multiple wall cores and rock fragments samples;

[0053] Step 2: Observe and analyze the core and rock fragment samples obtained in Step 1 respectively to determine the lithological type, core length and elemental composition of the rock fragment samples;

[0054] Step 3: Determine the lithology index (LT) based on lithology type, calculate the rock fragmentation degree (RFD) based on core length, and calculate the chemical weathering index (CIA) based on element abundance;

[0055] Step 4: Calculate the Tectonic Fracture Index (TFI) and Weathering Fracture Index (WFI) based on the indices obtained in Step 3;

[0056] Step 5: Create a two-dimensional sector plot consisting of TFI and WFI to calculate the Metamorphic Reservoir Quality Index (MRQI);

[0057] Step 6: Analyze the relationship between MRQI and single-well oil and gas production to determine the lower limit of effective reservoir.

[0058] In a preferred embodiment, in step 2, core and rock fragment samples at the same depth are observed and analyzed respectively.

[0059] In a preferred embodiment, in step 2, the method for determining the lithological type and core length of the core sample is a conventional method known to those skilled in the art and is not limited here; the method for determining the elemental composition of the rock cutting sample can also be a conventional method known to those skilled in the art, such as performing X-ray fluorescence analysis (XDF) on the rock cutting sample to determine its elemental composition.

[0060] In a preferred embodiment, in step 3, the specific method for determining the lithology index (LT) based on the lithology type is as follows:

[0061] The lithology index (LT) is determined based on the linear density of natural fractures in different metamorphic rock types.

[0062] The lithology with the highest fracture linear density is considered the most favorable lithology for reservoir formation, and its LT value is assigned to 1.

[0063] The LT value for other lithologies is calculated based on the ratio of the fracture linear density of that lithology to the fracture linear density of the most favorable lithology.

[0064] In a preferred embodiment, in step 3, the rock fragmentation degree (RFD) is calculated based on the core length, as shown in equation (1):

[0065]

[0066] In formula (1): L max is the maximum length of the core sample in the sample group, in cm; L is the length of a single core sample, in cm. Since the core sample is a cylinder, the lengths measured in the formula are all cylinder lengths.

[0067] In a preferred embodiment, step 3, the specific method for calculating the Chemical Weathering Index (CIA) based on elemental abundance, includes:

[0068] The elemental composition of the obtained rock fragments was converted into oxides, and the CIA was obtained according to equation (2):

[0069] CIA = [Al₂O₃ / (Al₂O₃+CaO)] * +Na₂O+K₂O)]×100 (2)

[0070] In equation (2), all oxides are expressed as mole fractions, where CaO * It is the CaO content in silicates after apatite correction.

[0071] In a preferred embodiment, the correction method for the apatite specifically includes:

[0072] Determine the molar fractions of P2O5 and CaO in the rock cuttings sample, and obtain the CaO content according to equation (3).ap :

[0073] CaO ap =CaO-(10 / 3×P2O5) (3)

[0074] Determine CaO ap The relationship with Na2O is obtained from equations (4) and (5) to obtain CaO. * Values:

[0075] If CaO ap If the number of moles is less than the number of moles of Na₂O, then take CaO. ap The value of is given by the following formula (4):

[0076] CaO * =CaO ap (4)

[0077] If CaO ap The number of moles is greater than the number of moles of Na₂O, CaO * Then take the value of Na2O, as shown in equation (5):

[0078] CaO * =Na2O (5)

[0079] In a preferred embodiment, in step 4, the tectonic fragmentation index (TFI) is calculated based on the lithology index (LT) and the degree of rock fragmentation (RFD), as shown in equation (6):

[0080] TFI = LT × RFD (6)

[0081] In a preferred embodiment, in step 4, the weathering fragmentation index (WFI) is calculated based on the lithology index (LT) and the chemical weathering index (CIA), as shown in equation (7):

[0082] WFI = LT × CIA (7)

[0083] In a preferred embodiment, in step 4, both the TFI and the WFI are standardized to a value between 0 and 100, and are dimensionless.

[0084] In a preferred embodiment, step 5, the specific method for creating a two-dimensional sector pattern composed of TFI and WFI, includes:

[0085] With TFI as the horizontal axis and WFI as the vertical axis, the length of the diagonal of the sector plot is MRQI, which is set between 0 and 100.

[0086] In a preferred embodiment, in step 5, the calculation of the metamorphic reservoir quality index (MRQI) is as shown in equation (8):

[0087]

[0088] In a preferred embodiment, step 6, the specific method for analyzing the relationship between MRQI and single-well oil and gas production to determine the effective reservoir lower limit includes:

[0089] Plot the relationship curve between single-well productivity and MRQI, analyze its changing trend, and determine the inflection point, which is the lower limit of the effective reservoir of metamorphic buried hill.

[0090] The technical solution of this application will be described in detail below through specific embodiments:

[0091] Example

[0092] The technical solution provided by this invention was applied in the Bozhong 19-6 gas field in the Bohai Bay Basin of China. The steps and results are as follows:

[0093] Step 1: Survey 8 wells in the gas field, and collect multiple core and cuttings samples from each well;

[0094] Step 2: Observe and analyze the core and cuttings samples at the same depth to determine the lithological type and core length of the core samples. Perform X-ray fluorescence analysis (XDF) on the cuttings samples to determine their elemental composition. Take the arithmetic mean of the sample data from each well and label them as BZ-B, BZ-I, BZ-K, BZ-L, BZ-M, BZ-N, BZ-O, and BZ-Q groups according to the well number. Record the data as shown in Table 1.

[0095] Step 3: Determine the lithology index (LT) based on the lithology type, calculate the rock fragmentation degree (RFD) based on the core length, and calculate the chemical weathering index (CIA) based on the element abundance. Take the arithmetic mean of each group of samples and record the results as shown in Table 1.

[0096] Step 31: Determine the lithology index (LT) based on lithology type: The lithology type with the highest fracture linear density is considered the most favorable lithology for reservoir formation, and its LT value is assigned as 1. The LT values ​​of other lithologies are calculated based on the ratio of the fracture linear density of the lithology to that of the most favorable lithology. That is, the LT value of granulite is 1. The LT value of each sample group is calculated by the ratio of the fracture linear density of the sample group to that of the granulite.

[0097] Step 32: Calculate the rock fragmentation degree (RFD) based on the core length, as shown in Equation (1):

[0098]

[0099] In formula (1): L maxL represents the maximum length of the core sample in the sample group, in cm; L represents the length of a single core sample, in cm.

[0100] Step 33: Convert the elemental abundance into oxide molar ratios, and obtain the Chemical Weathering Index (CIA) according to equation (2):

[0101] CIA = [Al₂O₃ / (Al₂O₃+CaO)] * +Na₂O+K₂O)]×100 (2)

[0102] In equation (2), all oxides are expressed as mole fractions, where CaO * It is the CaO content in silicates after apatite correction.

[0103] The specific correction method for the apatite includes:

[0104] The molar fraction of P2O5 in apatite and the molar fraction of CaO in silicate were determined, and CaO was obtained according to equation (3). ap :

[0105] CaO ap =CaO-(10 / 3×P2O5) (3)

[0106] Determine CaO ap The relationship with Na2O is obtained from equations (4) and (5) to obtain CaO. * Values:

[0107] If CaO ap If the number of moles is less than the number of moles of Na₂O, then take CaO. ap The value of is given by the following formula (4):

[0108] CaO * =CaO ap (4)

[0109] If CaO ap The number of moles is greater than the number of moles of Na₂O, CaO * Then take the value of Na2O, as shown in equation (5):

[0110] CaO * =Na2O (5)

[0111] Step 4: Calculate the tectonic fragmentation index (TFI) and weathering fragmentation index (WFI) for each group based on the LT, RFD and CIA obtained in Step 3, take the arithmetic mean of each group of samples, and record the results as shown in Table 1.

[0112] Step 41: Obtain the Tectonic Fracture Index (TFI) according to Equation (6):

[0113] TFI = LT × RFD (6)

[0114] Step 42: In step 4, the weathering fragmentation index (WFI) is obtained according to equation (7):

[0115] WFI = LT × CIA (7)

[0116] The obtained TFI and WFI were both dimensionless and standardized to between 0 and 100.

[0117] Step 5: Create a two-dimensional sector plot composed of TFI and WFI, and calculate the metamorphic reservoir quality index (MRQI) based on it. Take the arithmetic mean of each group of samples and record the results as shown in Table 1.

[0118] Step 51: As Figure 2 As shown, to create a sector chart: use TFI as the x-axis and WFI as the y-axis. The length of the diagonal of the sector chart is MRQI, which is set between 0 and 100.

[0119] Step 52: The calculation of the metamorphic reservoir quality index (MRQI) is shown in equation (8):

[0120]

[0121] Step 6: As Figure 3 As shown, a graph depicting the relationship between MRQI and single-well productivity was plotted. Based on its changing pattern, an inflection point was determined, which was then used as the lower limit of the effective reservoir. If the MRQI value is greater than this lower limit, it is considered an effective reservoir; otherwise, it is considered an ineffective reservoir. The graph shows that in this example, the lower limit of the effective reservoir in the metamorphic buried hill is MRQI = 65, meaning BZ-B and BZ-K are effective reservoirs.

[0122] Table 1

[0123]

[0124]

[0125] This invention provides a method for determining the lower limit of effective reservoirs in metamorphic buried hills. This method, considering the characteristic of fractured reservoirs in metamorphic buried hills, comprehensively considers and quantifies the main controlling factors of reservoir development, establishing a new comprehensive reservoir quality assessment parameter (MRQI). This parameter, combined with actual test results, is used to determine the lower limit of effective reservoirs in metamorphic buried hills. This method is applicable to fractured metamorphic reservoirs and requires no additional correction, making it a simple and effective reservoir evaluation method.

[0126] The foregoing description of specific exemplary embodiments of the invention is for illustrative and explanatory purposes. These descriptions are not intended to limit the invention to the precise forms disclosed, and it will be apparent that many changes and variations can be made in accordance with the foregoing teachings. The exemplary embodiments were chosen and described in order to explain the specific principles of the invention and its practical application, thereby enabling those skilled in the art to implement and utilize various different exemplary embodiments of the invention, as well as various different choices and variations. The scope of the invention is intended to be defined by the claims and their equivalents.

Claims

1. A method for determining the lower limit of effective reservoirs in metamorphic buried hills, characterized in that, Includes the following steps: Step 1: Collect multiple wall cores and rock fragments samples; Step 2: Observe and analyze the core and rock fragment samples obtained in Step 1 respectively to determine the lithological type, core length and elemental composition of the rock fragment samples; Step 3: Determine the lithology index (LT) based on lithology type, calculate the rock fragmentation degree (RFD) based on core length, and calculate the chemical weathering index (CIA) based on element abundance; Step 4: Calculate the structural fragmentation index (TFI) and weathering fragmentation index (WFI) based on the indices obtained in Step 3. Step 5: Create a two-dimensional sector plot consisting of TFI and WFI to calculate the Metamorphic Reservoir Quality Index (MRQI). Step 6: Analyze the relationship between MRQI and single-well oil and gas production to determine the lower limit of effective reservoirs; In step 3, the specific method for determining the lithology index (LT) based on lithology type is as follows: The lithology with the highest fracture linear density is considered the most favorable lithology for reservoir formation, and its LT value is assigned to 1. The LT values ​​of other lithologies are calculated based on the ratio of the fracture linear density of that lithology to the fracture linear density of the most favorable lithology. In step 3, the rock fragmentation degree (RFD) is calculated based on the core length, as shown in equation (1): In formula (1): L max L represents the maximum length of the core sample, in cm; L represents the length of a single core sample, in cm. In step 4, the structural fragmentation index (TFI) is obtained according to equation (6): In step 4, the weathering fragmentation index (WFI) is obtained according to equation (7): In step 5, the calculation of the metamorphic reservoir quality index (MRQI) is as shown in equation (8): 。 2. The method for determining the lower limit of effective reservoirs in metamorphic buried hills as described in claim 1, characterized in that, Step 3, the specific method for calculating the Chemical Weathering Index (CIA) based on elemental abundance includes: The elemental composition of the obtained rock fragments was converted into oxides, and the CIA was obtained according to equation (2): In equation (2), all oxides are expressed as mole fractions, where CaO * It is the CaO content in silicate minerals after apatite correction.

3. The method for determining the lower limit of effective reservoirs in metamorphic buried hills as described in claim 2, characterized in that, The specific correction method for the apatite includes: Determine the molar fractions of P2O5 and CaO in the rock cuttings sample, and obtain the CaO content according to equation (3). ap : Determine CaO ap The relationship with Na2O is obtained from equations (4) and (5) to obtain CaO. * Values: If CaO ap If the number of moles is less than the number of moles of Na₂O, then take CaO. ap The value is given by the following formula (4): If CaO ap The number of moles is greater than the number of moles of Na₂O, CaO * Then take the value of Na2O, as shown in equation (5): 。 4. The method for determining the lower limit of effective reservoirs in metamorphic buried hills as described in claim 1, characterized in that, In step 4, both the TFI and the WFI are standardized to a value between 0 and 100, which is dimensionless.

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