A method for correcting dean-stark measurement of fluid saturation

Abstract

This invention discloses a correction method for Dean-Stark fluid saturation measurement, comprising: measuring the oil saturation and water saturation of core samples using the Dean-Stark method; collecting core analysis data on porosity, clay content, and overburden properties; correcting the core analysis porosity to formation conditions using the overburden property data to obtain the overburden-corrected fluid saturation; performing clay correction on the core analysis water saturation based on mudstone drying experiment data; and performing depressurization and degassing correction on the core analysis oil and water saturation using minimum error constraints to obtain the true underground fluid saturation. This invention has a wide range of applications, strong effectiveness, and high predictive accuracy. It achieves the same correction effect as physical simulation experiments without relying on depressurization and degassing experiments, shortens experimental time, and saves experimental costs. It has significant guiding significance for tapping remaining oil potential and describing developed oilfield reservoirs.

Description

Technical Field

[0001] This invention belongs to the field of oil and gas exploration and development and rock physics research technology, specifically relating to a correction method for Dean-Stark measurement of fluid saturation. Background Technology

[0002] Oil-water saturation analysis data from closed core wells is crucial for evaluating reservoir oil-bearing potential. If the formation contains montmorillonite, structural water within the montmorillonite precipitates under high-temperature conditions during distillation measurements, leading to an overestimation of water saturation in the Dean-Stark method. Furthermore, during core extraction, the overlying pressure gradually decreases from formation conditions to surface conditions, increasing core porosity and causing degassing of crude oil within the pores. Both of these factors contribute to an underestimation of fluid saturation. Currently, laboratory measurements of the sum of oil and water saturations in closed core wells typically range from 70% to 85%, lower than the theoretical value of 100%. Therefore, laboratory saturation measurements cannot represent the true underground geological conditions and require correction.

[0003] Currently, there are three main methods for saturation correction: statistical regression, experimental simulation, and theoretical modeling. Each method has its drawbacks: statistical regression lacks a theoretical foundation; experimental simulation relies on depressurization degassing experiments, which are time-consuming and expensive; and theoretical modeling relies on the split rate obtained from relative permeability, but cannot demonstrate that the split rate during depressurization degassing is the same as the relative permeability split rate. Furthermore, none of these three correction methods consider the impact of clay dewatering on the measured saturation. Summary of the Invention

[0004] This invention is proposed to solve the problem of mismatch between laboratory fluid saturation measurements and formation fluid saturation, and its purpose is to provide a correction method for Dean-Stark fluid saturation measurements.

[0005] This invention is achieved through the following technical solution:

[0006] A correction method for Dean-Stark measurement of fluid saturation includes the following steps:

[0007] (i) The oil saturation and water saturation of the core were measured using the Dean-Stark method, and data on core porosity, clay content and overburden properties were collected.

[0008] The Dean-Stark method is specifically performed according to the content recorded on page 44 of the national standard GB / T 29172-2012 Core Analysis Methods; the method for determining the porosity and permeability of rocks under overburden is performed according to the industry standard SY / T6385-2016.

[0009] (ii) Using overburden physical property data, the porosity of the core analysis was corrected to the formation conditions to obtain the fluid saturation after overburden correction;

[0010] (iii) Based on the pure mudstone drying experiment, the ratio of the volume of structural water precipitated in clay to the volume of mudstone plunger was measured at different temperatures. The ratio of the volume of clay water precipitated in pure mudstone to the volume of mudstone plunger was determined at the Dean-Stark measurement temperature. The clay saturation of conventional sand and mudstone plungers was corrected using the ratio of the volume of clay water precipitated in pure mudstone to the volume of mudstone plunger.

[0011] (iv) The oil saturation and water saturation of the core analysis were corrected by depressurization and degassing through minimum error constraint to obtain the true fluid saturation in the underground.

[0012] In the above technical solution, the formula for calculating the fluid saturation after pressure correction in step (ii) is:

[0013]

[0014]

[0015] In the formula: S of1 Oil saturation after pressure correction, %;

[0016] S wf1 Water saturation after pressure correction, %;

[0017] S os Oil saturation (%) is the result of core analysis measured by Dean-Stark under surface conditions.

[0018] S ws Water saturation (%) is the water content of the core measured by Dean-Stark under surface conditions.

[0019] φ s Core porosity under surface conditions, %;

[0020] φ f Core porosity under formation conditions, %;

[0021] C o The volume coefficient of crude oil is dimensionless.

[0022] C w is the formation water volume coefficient, dimensionless.

[0023] Where: crude oil volume coefficient C o and formation water volume factor C w Based on experimental results, the specific experimental methods were carried out in accordance with the description on page 20 of the national standard GB / T 26981-2020 Methods for Analysis of Fluid Properties in Oil and Gas Reservoirs.

[0024] In the above technical solution, the specific operation of step (iii) mudstone drying experiment is as follows: Based on the core fluorescence scanning results, a pure mudstone plunger without fluorescence display is selected for the drying experiment. The ratio of the volume of structural water precipitated in the clay to the volume of the mudstone plunger is measured at different temperatures, thereby obtaining the ratio m of the volume of structural water precipitated in the clay to the volume of the mudstone plunger at the Dean-Stark measurement temperature. The purpose of measuring the ratio of the volume of structural water precipitated in the clay to the volume of the mudstone plunger at different temperatures in the mudstone drying experiment is to clarify the degree of water loss in the clay at different temperatures, and to directly interpolate to obtain the volume of water precipitated from the clay at that temperature.

[0025] In the above technical solution, the specific process of the drying experiment is as follows: ① Sample selection: Based on the core fluorescence scanning results, take 4 pure mudstone plungers without fluorescence display; ② Measure the volume of the plungers at room temperature; ③ Weigh: Weigh the plungers at room temperature; ④ Drying: Place the plungers in an oven, set the temperature to 60℃, dry to constant weight, and measure their mass; ⑤ Multiple temperature measurements: Set the temperature to 116℃, 150℃, and 200℃ respectively, and repeat step ④.

[0026] In the above technical solution, the Dean-Stark temperature measurement is 116℃.

[0027] In the above technical solution, the formula for clay correction of the overburden saturation of conventional sandstone and mudstone plungers based on the clay structural water precipitation ratio at the Dean-Stark measurement temperature is as follows:

[0028]

[0029] In the formula: S wf2 The corrected water saturation of clay, in %;

[0030] m is the percentage of the volume of clay water that precipitates from the pure mudstone plunger at the Dean-Stark measurement temperature, representing % of the plunger volume.

[0031] V sh The clay content (obtained in step i) of a conventional sandstone-mudstone plunger is %.

[0032] (iv) The oil saturation and water saturation of the core analysis were corrected by depressurization and degassing through minimum error constraint to obtain the true fluid saturation in the underground.

[0033] In the above technical solution, the specific process of pressure reduction and degassing correction for oil saturation and water saturation in core analysis using the minimum error constraint in step (iv) is as follows:

[0034] ① Based on the depressurization and degassing experiment, the formula for calculating the fluid saturation under formation conditions after the depressurization and degassing experiment is as follows:

[0035] S of2 =k o ·S of1 ........................(4)

[0036] S wf3 =k w ·ln(S wf2 )-b w ........................(5)

[0037] In the formula: S of2 The oil saturation after pressure reduction and degassing correction is expressed as %, %.

[0038] k o This is the dimensionless correction coefficient for oil saturation depressurization and degassing.

[0039] S wf3 The water saturation after depressurization and degassing correction is expressed in %.

[0040] k w The slope of the water saturation degassing correction formula is dimensionless.

[0041] b w The intercept of the correction formula for degassing due to water saturation is dimensionless.

[0042] ② Determine the correction coefficient k based on the minimum error constraint method o k w b w The specific formula is as follows:

[0043]

[0044] In the formula: The cumulative error, in %, is the correction for depressurization and degassing of a certain core sample.

[0045] n is the number of core saturation measurement points for a certain core sample, which is dimensionless;

[0046] ③Based on the degassing experiment, the correction coefficient k was obtained. w and b w The relationship is:

[0047] b w =3.8704·k w -56.385........................(7)

[0048] ④ Solve for k according to equations (4) to (7). o k w and bw Complete the pressure reduction and degassing correction.

[0049] The beneficial effects of this invention are:

[0050] This invention provides a correction method for Dean-Stark measurements of fluid saturation, clarifying the influence of clay water on the measurement results. Based on experimental simulation data, a depressurization degassing model is proposed, and the minimum error constraint method is used to determine the parameters of the depressurization degassing experimental model. The influence of depressurization degassing on fluid saturation is quantitatively characterized. Based on clay correction for saturation, the laboratory-measured saturation is accurately corrected to formation conditions, rapidly obtaining the true underground fluid saturation. This invention has a wide range of applications, strong effectiveness, and high predictive accuracy. It achieves the same correction effect as physical simulation experiments without relying on depressurization degassing experiments, shortens experimental time, and saves experimental costs. It has significant guiding significance for tapping remaining oil potential and describing developed oilfield reservoirs. Attached Figure Description

[0051] Figure 1 This is a flowchart illustrating the present invention;

[0052] Figure 2 This is a diagram showing the volume percentage of structural water precipitated from pure mudstone at different temperatures in the study area of ​​Example 1 of this invention;

[0053] Figure 3 This is a diagram showing the effect of clay saturation correction in the closed core sampling area of ​​the study area in Embodiment 1 of the present invention;

[0054] Figure 4 This is a diagram showing the relationship between correction coefficients in the degassing model for water saturation reduction in Embodiment 1 of the present invention;

[0055] Figure 5 This is a diagram showing the effect of pressure reduction and degassing correction in the closed-loop coring saturation of the study area in Embodiment 1 of the present invention;

[0056] Figure 6 This is a comparison chart of the saturation of the closed core sample taken from the study area and the saturation of the well logging interpretation in Embodiment 1 of the present invention.

[0057] For those skilled in the art, other related figures can be obtained from the above figures without any creative effort. Detailed Implementation

[0058] To enable those skilled in the art to better understand the technical solution of the present invention, the technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0059] Example 1

[0060] A correction method for Dean-Stark measurement of fluid saturation, the process of which is as follows: Figure 1 As shown, the specific steps include:

[0061] (i) The oil saturation and water saturation of the core were measured using the Dean-Stark method, and data on core porosity, clay content and overburden properties were collected.

[0062] (ii) Using overburden physical property data, the porosity of the core analysis was corrected to the formation conditions to obtain the fluid saturation after overburden correction;

[0063] Oil saturation S after pressure correction of1 and water saturation S wf1 for:

[0064]

[0065]

[0066] In the formula: S of1 Oil saturation after pressure correction, %;

[0067] S wf1 Water saturation after pressure correction, %;

[0068] S os Oil saturation (%) is the result of core analysis measured by Dean-Stark under surface conditions.

[0069] S ws Water saturation (%) is the water content of the core measured by Dean-Stark under surface conditions.

[0070] φ s Core porosity under surface conditions, %;

[0071] φ f Core porosity under formation conditions, %;

[0072] C o The volume coefficient of crude oil is dimensionless.

[0073] C w is the formation water volume coefficient, dimensionless;

[0074] After pressure correction, the water saturation of the closed core sample is as follows: Figure 3 As shown (track 8).

[0075] (iii) Clay correction of water saturation in core analysis based on mudstone drying test data

[0076] Based on pure mudstone drying experiments, the ratio m of the volume of clay water precipitated from pure mudstone to the volume of the mudstone plunger was determined at the Dean-Stark measurement temperature. This m was then used to correct the overburden saturation of conventional sandstone-mudstone plungers for clay content.

[0077] Based on the core fluorescence scanning results, a pure mudstone plunger without fluorescence was selected for drying experiments. The ratio of the volume of structural water precipitated in the clay to the volume of the mudstone plunger was measured at different temperatures, and the results were obtained as follows: Figure 2 The curves shown indicate that the selected mudstone samples came from the same oilfield and the same stratigraphic position, sharing the same sedimentary environment. Therefore, the mudstone samples exhibited similar clay types and compaction degrees, and the volume of clay water precipitated from each sample at a given temperature was essentially the same. The Dean-Stark measurement temperature was 116℃. Figure 2 As shown, at this temperature, the volume of structural water precipitated from the clay accounts for 20.1% of the volume of the mudstone plunger. Based on this proportion, the overburden saturation of conventional sandstone-mudstone plungers is corrected for clay content.

[0078]

[0079] In the formula: S wf2 The corrected water saturation of clay, in %;

[0080] m is the percentage of the volume of clay water precipitated from the pure mudstone plunger at the Dean-Stark measurement temperature, which is 20.1% in this example.

[0081] V sh The mud content of a conventional sandstone-mudstone plunger, in %;

[0082] After clay correction, the water saturation of the sealed core sample is as follows: Figure 3 As shown (question 9). In Figure 3 In the unflooded and weakly flooded zones, the water saturation of the core analysis was basically consistent with the saturation of the well logging interpretation, indicating "oil replenishment". In the strongly flooded zones, the water saturation of the core analysis was significantly lower than the saturation of the well logging interpretation, indicating "water replenishment", which is consistent with the understanding that "oil replenishment is performed in oil-bearing zones and water replenishment is performed in water-bearing zones".

[0083] (iv) By applying minimum error constraints to the depressurization and degassing correction of oil saturation and water saturation in core analysis, the true underground fluid saturation is obtained:

[0084] ① Based on the depressurization and degassing experiment, the change in fluid saturation under formation conditions after the depressurization and degassing experiment is as follows:

[0085] S of2 =k o ·S of1 ........................(4)

[0086] S wf3 =k w ·ln(S wf2 )-b w ........................(5)

[0087] In the formula: S of2 The oil saturation after pressure reduction and degassing correction is expressed as %, %.

[0088] k o This is the dimensionless correction coefficient for oil saturation depressurization and degassing.

[0089] S wf3 The water saturation after depressurization and degassing correction is expressed in %.

[0090] k w The slope of the water saturation degassing correction formula is dimensionless.

[0091] b w The intercept of the correction formula for degassing due to water saturation is dimensionless.

[0092] The depressurization and degassing experiment is a standard experimental technique, and detailed information is available in the following literature:

[0093] [1] Liu Guifang. Experimental study on the effect of pressure reduction and degassing on oil-water saturation during tripping [J]. Daqing Petroleum Geology and Development, 1983, 2(2): 41-47.

[0094] [2] Zhang Liang. Physical simulation experiment on core saturation correction in closed core wells [J]. Oil & Gas Geology and Recovery, 2009, 16(2):94-98.

[0095] [3] Hu Xuejun, Yang Shenglai, Li Hui. Effect of closed coring depressurization and degassing on core water saturation [J]. Journal of Xi'an Petroleum University (Natural Science Edition), 2004, 19(6):27-30.

[0096] [4] Liu Li. Correction of oil-water saturation in closed core wells based on physical simulation experiment [J]. Petroleum Drilling and Production Technology, 2009, 31(2):82-85.

[0097] [5] Zhu Yongxian, Yao Shuaiqi, Zhang Yanbin, et al. Method for correcting fluid saturation in closed coring of light oil reservoirs [J]. Xinjiang Petroleum Geology, 2023, 44(3):359-364.

[0098] The specific experimental steps for the depressurization and degassing experiment are as follows:

[0099] a. Using gas-bearing crude oil to displace rock samples 100% saturated with simulated formation water, the formation process of bound water in the oil reservoir is simulated;

[0100] b. Simulate the water-drive development process by using simulated formation water to replace gas-bearing crude oil in rock samples to a certain water saturation level;

[0101] c. Reduce pressure and temperature to allow dissolved gas in gaseous crude oil to escape, simulating the depressurization and degassing process in core drilling;

[0102] d. Measure the water saturation in the degassed rock sample using conventional saturation testing methods, and determine the systematic error caused by the measurement method to the water saturation.

[0103] e. Obtain the corrected oil saturation based on So + Sw = 100%.

[0104] ② Determine the correction coefficient k based on the minimum error constraint method o k w b w The specific method is as follows:

[0105]

[0106] In the formula: The cumulative error, in %, is the correction for depressurization and degassing of a certain core sample.

[0107] n is the number of core saturation measurement points for a certain core sample, which is dimensionless.

[0108] ③ Because the oil saturation and water saturation in core analysis are correlated, the overdetermined equation set determined by equation (6) cannot obtain a unique solution. Based on the depressurization and degassing experiment, the correction coefficient k is obtained. w and b w relation( Figure 4 )for:

[0109] b w =3.8704·k w -56.385........................(7)

[0110] ④ Solve for ko, kw and bw according to equations (4) to (7) to perform pressure reduction and degassing correction.

[0111] The minimum error constraint method is the innovation of this invention. First, based on the experimental data of the aforementioned references [1] to [5], the following table 1 is summarized:

[0112] Table 1. Experimental data from references [1] to [5]

[0113]

[0114] Based on the above literature, the oil saturation and water saturation before and after the depressurization and degassing experiment satisfy formulas (4) and (5). As long as the three coefficients ko, kw, and bw are determined, the core measurement saturation can be corrected to the formation conditions according to the above formulas. Different crude oil properties correspond to different correction coefficients. Since the sum of underground oil and water saturation after correction should be 100%, the crude oil in the same core cylinder should satisfy formula (6).

[0115] If the oil saturation and water saturation measured on the ground are linearly independent, the above three correction coefficients can be directly obtained from the overdetermined equation set determined by equation (3). However, the oil saturation and water saturation measured on the ground are correlated, which makes it impossible to obtain a unique solution for equation (3).

[0116] A systematic analysis of references 1-5 revealed that although the experiments in previous literature targeted different properties of crude oil, the slope kw and intercept bw of equation (5) obtained by them simultaneously satisfied a certain linear relationship (Equation 7) (see attached figure). Figure 4 Substituting the measured values ​​of the saturation at the surface of this oilfield into equations (4) to (7), we can solve for ko, kw, and bw and perform pressure reduction and degassing correction.

[0117] The minimum error constraint method does not depend on specific experiments, shortens the calibration process and costs, but achieves the same calibration effect as the depressurization and degassing experiment.

[0118] After pressure reduction and degassing correction, the water saturation of the sealed core sample is as follows: Figure 5 As shown (question 10). In Figure 5 In the process, after clay correction and depressurization / degassing correction, the well logging interpretation water saturation, closed core water saturation (channel 10), and mercury injection saturation (channel 7) showed good agreement, with the three corroborating each other, indicating that the closed core water saturation correction results were relatively accurate. The well logging interpretation saturation and corrected core analysis saturation sub-layer average data of another closed core well, X, in this oilfield are shown in Table 2. Figure 6 As shown, after correction by the method in this invention, the difference between the two values ​​is within ±5%, which meets the requirements of the reserve specification, confirming the rationality of the core analysis saturation correction results and achieving good application results.

[0119] Table 2 shows the average saturation data of well logging interpretation and core analysis in well X.

[0120]

[0121] It should be noted that, unless otherwise specified, the embodiments and features described in the embodiments of this invention can be combined with each other. Unless otherwise specified, the experimental methods used in the embodiments are conventional methods; and the materials and reagents used in the embodiments are commercially available unless otherwise specified.

[0122] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A correction method for Dean-Stark measurement of fluid saturation, characterized in that: Includes the following steps: (I) The oil saturation and water saturation of the core were measured using the Dean-Stark method, and data on core porosity, clay content and overburden properties were collected. (II) Using overburden pressure property data, the porosity of the core analysis is corrected to the formation conditions to obtain the overburden pressure-corrected fluid saturation; the overburden pressure-corrected fluid saturation includes the overburden pressure-corrected oil saturation. Water saturation after overburden correction ; (III) Based on the pure mudstone drying experiment, the ratio m of the volume of clay water precipitated from pure mudstone to the volume of mudstone plunger at the Dean-Stark measurement temperature was determined, and the water saturation after overburden correction was calculated using m. Perform clay correction; Water saturation after overburden correction using m The formula for clay correction is: In the formula: The corrected water saturation of clay is %; The mudstone content of the core obtained in step (Ⅰ), % The core porosity under formation conditions, % (IV) Oil saturation after core analysis and overburden correction using minimum error constraints Water saturation after clay correction The depressurization and degassing correction yields the true underground fluid saturation. Oil saturation after core analysis and overburden correction using minimum error constraints Water saturation after clay correction The specific process of pressure reduction and degassing correction is as follows: (i) Based on the depressurization and degassing experiment, the formula for calculating the fluid saturation under formation conditions after the depressurization and degassing experiment is as follows: In the formula: The oil saturation after pressure reduction and degassing correction is % %. This is the dimensionless correction coefficient for oil saturation depressurization and degassing. The water saturation after depressurization and degassing correction is % %. The slope of the water saturation degassing correction formula is dimensionless. The intercept of the correction formula for degassing due to water saturation is dimensionless. (ii) Determine the correction coefficients based on the minimum error constraint method , , The specific formula is as follows: In the formula: % represents the cumulative error caused by depressurization and degassing correction of a certain core sample; n is the number of core analysis saturation measurement points, dimensionless. (iii) Based on the degassing experiment, the correction coefficient was obtained. and The relationship is: (iv) Solve according to equations (4) to (7). , and Complete the pressure reduction and degassing correction.

2. The Dean-Stark calibration method for measuring fluid saturation according to claim 1, characterized in that: The formula for calculating the fluid saturation after pressure correction in step (II) is as follows: In the formula: The oil saturation after pressure correction is % %. The water saturation after accretion correction is %; Oil saturation of core samples measured by Dean-Stark under surface conditions, % Water saturation of core samples measured by Dean-Stark under surface conditions, % The core porosity under ground conditions is %; The core porosity under formation conditions, % The volume coefficient of crude oil is dimensionless. is the formation water volume coefficient, dimensionless.

3. The Dean-Stark calibration method for measuring fluid saturation according to claim 1, characterized in that: The pure mudstone drying experiment in step (III) specifically involves: based on the core fluorescence scanning results, selecting a pure mudstone plunger without fluorescence display for the drying experiment, and measuring the ratio of the volume of clay water precipitation to the volume of the mudstone plunger at different temperatures.

4. The correction method for Dean-Stark measurement of fluid saturation according to claim 1, characterized in that: In step (III), Dean-Stark measured the temperature at 116°C.

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