A method for calculating the permeability of tight sandstone based on saturated nuclear magnetic resonance experiment

Abstract

The application belongs to the technical field of oil and gas reservoir evaluation and development research, and discloses a tight sandstone permeability calculation method based on saturated nuclear magnetic resonance experiment. Em ; the tight sandstone sample is subjected to a saturated nuclear magnetic resonance experiment, and a nuclear magnetic resonance T2 spectrum curve is obtained; the nuclear magnetic resonance T2 spectrum and the porosity component are taken as logarithms, and a cross curve is constructed, and the inflection points of the curve are respectively denoted as T 2‑1 and T 2‑2 ; T 2‑1 and T 2‑2 are taken as independent variables, K Em is taken as a dependent variable, a tight sandstone permeability calculation model based on the saturated nuclear magnetic resonance experiment is established; the permeability value calculated from the permeability calculation model is denoted as K Cal , a linear relationship of K Cal -K Em is established, and the effectiveness of the model is determined by the correlation coefficient of the two. The method can realize the accurate calculation of the tight sandstone reservoir permeability by only carrying out the saturated nuclear magnetic resonance experiment, and has the advantages of economy, simplicity, high efficiency and strong practicability.

Description

Technical Field

[0001] The invention belongs to the technical field of oil and gas reservoir evaluation and development research, and particularly relates to a method for calculating the permeability of tight sandstone based on saturation nuclear magnetic resonance experiments. Background Art

[0002] Permeability controls the migration and accumulation of oil and gas in tight sandstone reservoirs and is a key parameter in reservoir evaluation. Accurately determining the permeability of tight sandstone reservoirs is crucial for formulating oil and gas field development plans. However, due to the complex tectonic and diagenetic evolution associated with the formation of tight sandstone reservoirs, the microscopic pore structure of these reservoirs is extremely complex, significantly increasing the difficulty of permeability calculation.

[0003] Domestic and international scholars have conducted extensive research on reservoir permeability calculations. The most commonly used method for calculating permeability is the porosity and permeability correlation analysis method. This method calculates permeability by establishing a mathematical relationship between porosity and permeability. However, this method is primarily applicable to conventional sandstone reservoirs with high porosity and medium-high permeability, and has poor applicability to tight sandstone reservoirs with medium-low porosity and ultra-low permeability.

[0004] Patent application number CN 107917865 B provides a method for predicting the permeability of tight sandstone reservoirs using multiple parameters. This method identifies several key geological factors controlling permeability in tight sandstone reservoirs and establishes a comprehensive multi-parameter permeability prediction model based on seismic and well logging constraints. While this method can accurately predict the permeability of tight sandstone reservoirs at the single-well and planar level, it lacks the ability to precisely calculate permeability at the reservoir microscale.

[0005] Patent application number CN 103279647 B provides a method for calculating the permeability of tight sandstone reservoirs based on pore characteristic parameters. This patent calibrates high-pressure mercury injection capillary pressure curves using nuclear magnetic resonance logging data to obtain pseudo-capillary pressure curves, which are then used to calculate reservoir permeability. However, due to the complex microscopic pore structure and poor connectivity of tight sandstone reservoir samples, it is difficult to accurately determine the conversion coefficient between the nuclear magnetic resonance T2 spectrum curve and the high-pressure mercury injection capillary pressure curve. Therefore, the accuracy of the permeability calculation method provided by this patent is difficult to guarantee.

[0006] Nuclear magnetic resonance (NMR) technology is widely used in the characterization of pore structure in unconventional oil and gas reservoirs due to its rapidity, non-destructiveness, and ease of operation. Reservoir permeability can be calculated using NMR technology, with the SDR model being the most commonly used permeability calculation model, which can be expressed as follows:

[0007] Log(K)=ALog(φ)+B Log(FFI / BVI)+C (1)

[0008] Where K is permeability, mD; φ is porosity, %; FFI / BVI is the ratio of free water to bound water saturation obtained by nuclear magnetic resonance. A and B are dimensionless fitting parameters of the multivariate linear regression equation; and C is a constant.

[0009] Patent No. CN 109932297 B provides a method for calculating the permeability of tight sandstone reservoirs. This patent establishes a permeability logging interpretation model based on the three pore components of the nuclear magnetic resonance (NMR) T2 spectrum. Patent No. CN 109932297 B provides a method for calculating the permeability of tight sandstones based on a double-cutoff NMR spectrum. This patent establishes a permeability model based on a double-cutoff NMR spectrum by studying the relationship between the absolute movable water content and the absolute immovable water content. However, these NMR-based permeability calculation methods all rely on the calculation of the T2 cutoff value, which is currently difficult to accurately determine, resulting in unavoidable errors in permeability calculations. Summary of the Invention

[0010] The purpose of the present invention is to provide a method for calculating the permeability of tight sandstone based on saturation nuclear magnetic resonance experiments, so as to solve the problem of insufficient calculation accuracy of traditional methods.

[0011] To achieve the above object, the technical solution adopted by the present invention is:

[0012] A method for calculating the permeability of tight sandstone based on saturation nuclear magnetic resonance experiments includes the following steps:

[0013] (1) Conduct a permeability test on a dense sandstone sample to obtain the sample permeability value, which is recorded as K Em ;

[0014] (2) Perform saturation NMR experiments on dense sandstone samples to obtain NMR T2 spectrum curves;

[0015] (3) NMR T2 spectrum and porosity component Take the logarithm and construct The intersection curve, the inflection point of the curve is marked as T 2-1 and T 2-2 ;

[0016] (4) T 2-1 and T 2-2 As the independent variable, K Em As the dependent variable, a calculation model for tight sandstone permeability based on saturated NMR experiments was established;

[0017] (5) The permeability value calculated by the permeability calculation model is recorded as K Cal , establish K Cal -K EmThe effectiveness of the model is judged by the size of the correlation coefficient between the two.

[0018] Furthermore, for the same dense sandstone sample, the permeability measurement experiment in step (1) and the saturation nuclear magnetic resonance experiment in step (2) are carried out in sequence.

[0019] Furthermore, in step (1) and step (2), the dense sandstone sample is a core column with a length of 5 cm and a diameter of 2.5 cm.

[0020] Furthermore, in step (1), the dense sandstone sample needs to be pretreated, and the pretreatment items include washing the oil, washing the salt and drying the sample.

[0021] Furthermore, in step (1), the tight sandstone sample permeability determination experiment and the sample pretreatment before the experiment are carried out according to the provisions of the Petroleum and Natural Gas Industry Standard SY / T5336-2006 "Core Analysis Method" of the People's Republic of China.

[0022] Furthermore, in step (2), a saturated nuclear magnetic resonance experiment is performed on the tight sandstone sample that has completed the permeability measurement experiment. The specific operation process is carried out in accordance with the provisions of the Petroleum and Natural Gas Industry Standard SY / T6490-2014 "Laboratory Measurement Specifications for Nuclear Magnetic Resonance Parameters of Rock Samples" of the People's Republic of China.

[0023] Furthermore, in step (3), the inflection point T is determined. 2-1 and T 2-2 The specific operation method is: NMR T2 spectrum and porosity component Take the logarithm and record it as logT2 and Establish logT2 and The linear equation of two variables is expressed by formula (1):

[0024]

[0025] In formula (1), T2 is the saturation NMR T2 spectrum, ms; is the NMR pore volume component, %; A and B are the coefficients of the quadratic equation, dimensionless; C is a constant.

[0026] The T2 value corresponding to the inflection point of the intersection curve is T 2-1 and T 2-2 .

[0027] Furthermore, in step (3), logT2 and The method for determining the inflection point of the intersection curve is: to differentiate the quadratic equation described in formula (1), and the point where the derivative is equal to zero is the inflection point.

[0028] Furthermore, in step (4), the tight sandstone permeability calculation model based on saturation nuclear magnetic resonance experiment can be expressed by formula (2):

[0029] LogK Cal =ALogT 2-1 +BLogT 2-2 +C (2)

[0030] In formula (2), T 2-1 and T 2-2 They are T2 value corresponding to the two inflection points of the intersection curve, ms; K Cal is the permeability value calculated by the tight sandstone permeability calculation model, mD; A and B are dimensionless coefficients; C is a constant.

[0031] Furthermore, in step (5), judging the validity of the model by the size of the correlation coefficient between the two specifically means that only when K Cal With K Em Correlation coefficient R 2 Only when ≥0.90 is the tight sandstone permeability calculation model provided by formula (2) considered to be valid.

[0032] The beneficial effects of the present invention are as follows: compared with the traditional method for calculating the permeability of tight sandstone based on nuclear magnetic resonance experiments, the present method does not require centrifugation experiments on samples, does not require obtaining the T2 cutoff value, and only requires conducting saturation nuclear magnetic resonance experiments to achieve accurate calculation of the permeability of tight sandstone reservoirs. It has the advantages of being economical, simple, efficient, and highly practical. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The present invention will be further described below with reference to the accompanying drawings and specific embodiments:

[0034] Figure 1 :pass The inflection point of the intersection curve determines T 2-1 and T 2-2 ;

[0035] Figure 2 :K Em -T 2-1 Intersection diagram;

[0036] Figure 3 :K Em -T 2-2 Intersection diagram;

[0037] Figure 4 :K Cal -K Em Intersection diagram. DETAILED DESCRIPTION

[0038] In order to make the technical means adopted by the present invention and the objectives achieved by the present invention easier to understand, the present invention is further described below in conjunction with specific implementation methods.

[0039] Unless otherwise specified, the experimental methods used in the following examples are conventional methods.

[0040] Reference Figures 1-4 This specific embodiment adopts the following technical solution: a method for calculating the permeability of dense sandstone based on saturation nuclear magnetic resonance experiment, comprising the following steps:

[0041] (1) Conduct a permeability test on a dense sandstone sample to obtain the sample permeability value, which is recorded as K Em

[0042] The dense sandstone samples used in the present invention are natural cores taken from the Linan 6 well in the Lishu fault depression in the southern Songliao Basin, with a total of 20 samples.

[0043] The cores were drilled using a deep hole drilling machine and the dense sandstone samples were made into core columns;

[0044] The core column was 5 cm in length and 2.5 cm in diameter;

[0045] Use a core cutter to cut the cross section flat;

[0046] Before the permeability measurement experiment, the tight sandstone samples need to be pretreated;

[0047] Pretreatment items include washing away residual oil and residual salt from the sample and drying the sample;

[0048] The specific operation process of the core pretreatment project shall be carried out in accordance with the provisions of the Petroleum and Natural Gas Industry Standard SY / T5336-2006 "Core Analysis Method" of the People's Republic of China.

[0049] The pretreatment before the experiment was carried out as follows:

[0050] First, the core column is placed in a high-temperature and high-pressure oil washer, and the residual oil and mud in the core are washed away with an organic solvent;

[0051] Then put the oil-washed core column into a crucible and repeatedly add distilled water to boil to wash away the residual salt in the core column;

[0052] Finally, the washed core column is placed in an oven and the temperature is raised to 100°C to remove the water in the core column;

[0053] The permeability determination experiment of tight sandstone samples and the sample pretreatment before the experiment were carried out in accordance with the provisions of the Petroleum and Natural Gas Industry Standard SY / T5336-2006 "Core Analysis Method" of the People's Republic of China.

[0054] (2) Perform saturation NMR experiments on dense sandstone samples to obtain NMR T2 spectrum curves

[0055] Saturated nuclear magnetic resonance experiments were conducted on the tight sandstone samples that had completed the permeability measurement experiment. The specific operation procedures were carried out in accordance with the provisions of the Petroleum and Natural Gas Industry Standard SY / T6490-2014 "Laboratory Measurement Specifications for Nuclear Magnetic Resonance Parameters of Rock Samples".

[0056] (3) NMR T2 spectrum and porosity component Take the logarithm and construct The intersection curve, the inflection point of the curve is marked as T 2-1 and T 2-2

[0057] Take the sample No. 6-16 of Li Nan as an example. Figure 1 Shown The intersection curve has obvious segmented characteristics and can be divided into two segments. Each segment has an inflection point, which is recorded as inflection point 1 and inflection point 2.

[0058] Fitting the two curves with quadratic equations can be expressed by formula (3) and formula (4) respectively:

[0059]

[0060]

[0061] Derivatives of formula (3) and formula (4) are:

[0062]

[0063]

[0064] make and is zero, then: logT 2-1 =0.1128,logT 2-2 =1.7468;

[0065] Further: T 2-1 =10 0.1128 =1.30ms, T 2-2 =10 1.7468 =55.82ms

[0066] (4) T 2-1 and T 2-2 As the independent variable, K Em As the dependent variable, a tight sandstone permeability calculation model based on saturated nuclear magnetic resonance experiments was established.

[0067] As shown in Table 1, all 20 tight sandstone samples K Em 、T 2-1 and T 2-2 Statistical results:

[0068] Table 1

[0069]

[0070]

[0071] like Figure 2 and Figure 3 As shown, T 2-1 and T 2-2 Both and K Em Has a good power function relationship, correlation coefficient R 2 They are 0.8976 and 0.8287 respectively, which shows that T 2-1 and T 2-2 Establish a tight sandstone permeability model;

[0072] Therefore, T 2-1 and T 2-2 As the independent variable, K Em As the dependent variable, a calculation model for the permeability of tight sandstone is established, which can be expressed by formula (5):

[0073] LogK Cal =0.9239LogT 2-1 +3.8655LogT 2-2 -7.6350 (5)

[0074] (5) The permeability value calculated by the permeability calculation model is recorded as K Cal , establish K Cal -K Em The effectiveness of the model is judged by the size of the correlation coefficient between the two.

[0075] like Figure 4 As shown, K Cal and K Em Has a good linear relationship, the correlation coefficient R 2 The error analysis statistical results show that the average absolute error is only 6.09%, which proves the effectiveness of the method.

[0076] Although some embodiments of the present invention have been described herein, those skilled in the art will appreciate that modifications may be made to the embodiments herein without departing from the spirit of the present invention. The above embodiments are merely exemplary and should not be used as limitations on the scope of the present invention.

Claims

1. A method for calculating the permeability of tight sandstone based on saturation nuclear magnetic resonance experiments, characterized in that: The following steps are involved: (1) Conduct a permeability test on a tight sandstone sample to obtain the sample permeability value, which is recorded as K Em ; (2) Perform saturation NMR experiments on tight sandstone samples to obtain NMR T2 spectrum curves; (3) Take the logarithm of the NMR T2 spectrum and the NMR pore volume component φ, construct the LogT2-Logφ intersection curve, and differentiate the quadratic equation of formula (1). The point where the derivative is equal to zero is the inflection point. The inflection points of the curve are respectively denoted as T 2-1 and T 2-2 , determine the inflection point T 2-1 and T 2-2 The specific operation method is: take the logarithm of the NMR T2 spectrum and the NMR pore volume component φ, record them as logT2 and logφ respectively, and establish a quadratic equation of logT2 and logφ, which is expressed by formula (1): logφ=A(logT2) 2 +B(logT2)+C (1) In formula (1), T2 is the saturated NMR T2 spectrum, ms; φ is the NMR pore volume component, %; A and B are the coefficients of the quadratic equation, dimensionless; C is a constant; The T2 value corresponding to the inflection point of the logT2-logφ intersection curve is T 2-1 and T 2-2 ; (4) T 2-1 and T 2-2 As the independent variable, K Em As the dependent variable, a calculation model for tight sandstone permeability based on saturated NMR experiments was established; (5) The permeability value calculated by the permeability calculation model is recorded as K Cal , establish K Cal -K Em The linear relationship between the two is established, and the validity of the model is judged by the size of the correlation coefficient. The tight sandstone permeability calculation model based on saturated nuclear magnetic resonance experiments can be expressed by formula (2): LogK Cal =ALogT 2-1 +BLogT 2-2 +C (2) In formula (2), T 2-1 and T 2-2 are the T2 values ​​corresponding to the two inflection points of the logT2-logφ intersection curve, ms; K Cal is the permeability value calculated by the tight sandstone permeability calculation model, mD; A and B are dimensionless coefficients; C is a constant.

2. The method for calculating the permeability of tight sandstone based on saturation nuclear magnetic resonance experiment according to claim 1, characterized in that: For the same dense sandstone sample, the permeability measurement experiment in step (1) and the saturation nuclear magnetic resonance experiment in step (2) are carried out in sequence.

3. The method for calculating the permeability of tight sandstone based on saturation nuclear magnetic resonance experiment according to claim 1, characterized in that: In step (1) and step (2), the dense sandstone sample is a core column with a length of 5 cm and a diameter of 2.5 cm.

4. The method for calculating the permeability of tight sandstone based on saturation nuclear magnetic resonance experiment according to claim 1, characterized in that: In step (1), the dense sandstone sample needs to be pretreated, and the pretreatment includes washing the oil, washing the salt and drying the sample.

5. The method for calculating the permeability of tight sandstone based on saturation nuclear magnetic resonance experiment according to claim 4 is characterized in that: In step (1), the permeability measurement and pretreatment of the tight sandstone sample are carried out in accordance with the provisions of the Petroleum and Natural Gas Industry Standard SY / T5336-2006 "Core Analysis Method" of the People's Republic of China.

6. The method for calculating the permeability of tight sandstone based on saturation nuclear magnetic resonance experiment according to claim 1, characterized in that: In step (2), the specific operation process of the saturation nuclear magnetic resonance experiment is carried out in accordance with the provisions of the Petroleum and Natural Gas Industry Standard SY / T6490-2014 "Laboratory Measurement Specifications for Nuclear Magnetic Resonance Parameters of Rock Samples" of the People's Republic of China.

7. The method for calculating the permeability of tight sandstone based on saturation nuclear magnetic resonance experiment according to claim 1, characterized in that: In step (5), judging the validity of the model by the size of the correlation coefficient between the two specifically means that only when K Cal With K Em Correlation coefficient R 2 Only when ≥0.90 can the tight sandstone permeability calculation model provided by formula (2) be considered valid.

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