A T2 cutoff value calculation method based on saturation nuclear magnetic resonance experiment

Abstract

The present invention belongs to the technical field of oil and gas reservoir evaluation and development research, and discloses a method for calculating the T2 cutoff value based on saturation nuclear magnetic resonance experiment: obtaining the saturation / imposed water nuclear magnetic resonance T2 spectrum curve of a tight sandstone sample; determining the T2 cutoff value based on the saturation / imposed water nuclear magnetic resonance cumulative porosity curve, which is recorded as T 2‑Cutoff ; Obtain the saturated NMR T2 spectrum logT2-logφ intersection curve, and the T2 value corresponding to the inflection point of the curve is recorded as T 2‑Log ; Determine T 2‑Log and T 2‑Cutoff The mathematical relationship between the two is used to establish a T2 cutoff value calculation model based on saturated nuclear magnetic resonance experiments. The present invention only requires performing a nuclear magnetic resonance experiment on a saturated water sample and calculating the T2 cutoff value by mathematically processing the T2 spectrum. This method has the advantages of being economical, simple, efficient, and highly practical.

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 T2 cutoff value calculation method based on saturation nuclear magnetic resonance experiments. Background Art

[0002] Nuclear magnetic resonance (NMR) technology, due to its rapid, non-destructive nature and ease of use, is widely used to characterize the pore structure of unconventional oil and gas reservoirs, including shale, tight sandstone, carbonate rock, and coal. By processing NMR T2 spectra, key parameters such as porosity, permeability, irreducible water saturation, movable fluid saturation, and movable fluid porosity can be obtained.

[0003] However, obtaining these key parameters presupposes a precise T2 cutoff value. This value delineates between movable and bound fluids in a reservoir. When core sampling is unavailable, making NMR experiments impossible, geologists often use empirical values ​​for T2 cutoffs: 33ms for sandstone and 92ms for carbonates. However, numerous studies have shown that T2 cutoffs are not fixed values. Therefore, using these empirical values ​​to process NMR data is bound to introduce errors.

[0004] Literature research shows that many scholars at home and abroad have given methods for calculating the T2 cutoff value.

[0005] The porosity accumulation method compares the cumulative porosity curves of the nuclear magnetic resonance T2 spectrum under saturated water / imposed water conditions, draws a line parallel to the X-axis from the maximum point of the T2 spectrum cumulative porosity curve after centrifugation (i.e., the irreducible water volume) to intersect with the saturated water cumulative porosity curve, and draws a vertical line from the intersection to the X-axis, the corresponding value of which is the T2 cutoff value; the invention patent with the authorization announcement number CN 105866160 B provides a "Nuclear Magnetic Resonance T2 Cutoff Value Calculation Method Based on P-Wave Constraint", which establishes a T2 cutoff value calculation model based on P-wave velocity and nuclear magnetic resonance porosity; the invention patent with the authorization announcement number CN 104237957 B provides a "Nuclear Magnetic Resonance Logging T2 Cutoff Value Determination Method", which establishes a mathematical model of the irreducible water volume under uniform pores, constructs a comparison chart of T2 spectrum and porosity, and calculates the irreducible water volume corresponding to each T2 time point, ultimately realizing the calculation of the T2 cutoff value; the invention patent with the authorization announcement number CN 105866160 B provides a "Nuclear Magnetic Resonance Logging T2 Cutoff Value Determination Method", which establishes a mathematical model of the irreducible water volume under uniform pores, constructs a comparison chart of T2 spectrum and porosity, and calculates the irreducible water volume corresponding to each T2 time point, ultimately realizing the calculation of the T2 cutoff value; the invention patent with the authorization announcement number CN The invention patent 109580689B provides a method for calculating the T2 cutoff value of nuclear magnetic resonance logging point by point. 2cutoff and the pseudo cutoff time geometric mean G' c The fitting relationship between them is used to realize the point-by-point calculation of the T2 cutoff value of nuclear magnetic resonance logging.

[0006] All of the above methods for obtaining T2 cutoff values ​​require sample centrifugation, making the experimental process cumbersome and time-consuming, making it difficult to meet field requirements. Furthermore, these methods use numerous parameters when establishing the T2 cutoff calculation model. These parameters rely on experiments other than nuclear magnetic resonance (NMR) and are therefore difficult to obtain. Summary of the Invention

[0007] The purpose of the present invention is to provide a method for calculating the T2 cutoff value based on saturation nuclear magnetic resonance experiments to solve the problem of insufficient calculation accuracy of traditional methods.

[0008] To achieve the above object, the technical solution adopted by the present invention is:

[0009] A method for calculating a T2 cutoff value based on a saturation nuclear magnetic resonance experiment comprises the following steps:

[0010] (1) Obtaining the saturated / bound water NMR T2 spectrum curve of tight sandstone samples;

[0011] (2) The T2 cutoff value is determined based on the saturated / bound water NMR cumulative porosity curve, denoted as T 2-Cutoff ;

[0012] (3) Obtaining saturation NMR T2 spectrum The intersection curve, the T2 value corresponding to the inflection point of the curve is recorded as T 2-Log ;

[0013] (4) Determine T 2-Log and T 2-Cutoff A mathematical relationship was established to establish a T2 cutoff value calculation model based on saturation NMR experiments.

[0014] Furthermore, in step (1), the dense sandstone sample is a core column with a length of 5 cm and a diameter of 2.5 cm.

[0015] Furthermore, in step (1), nuclear magnetic resonance requires pretreatment of the sample, and the pretreatment items include washing away the remaining oil and salt in the sample and drying the sample.

[0016] Furthermore, in step (1), the sample is subjected to oil washing, salt washing and drying in this order, and the specific operation process is 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.

[0017] Furthermore, in step (1), a nuclear magnetic resonance experiment is performed on the sample after oil washing, salt washing and drying. 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.

[0018] Furthermore, in step (1), the rock sample is dehydrated using a centrifugal device, and the centrifugal force is selected to be 400 psi (i.e., 2.76 MPa).

[0019] Furthermore, in step (2), the T2 cutoff value T is determined. 2-Cutoff The specific operation method is: obtain the saturated / imposed water NMR cumulative porosity curves respectively, draw a horizontal straight line parallel to the X-axis along the imposed water cumulative porosity curve, and the T2 value corresponding to the intersection of the horizontal straight line and the saturated water cumulative porosity curve is T 2-Cutoff .

[0020] Furthermore, in step (3), determine T 2-Log The specific operation method is: take the logarithm of the saturated water nuclear magnetic resonance T2 spectrum curve and the cumulative porosity curve, and record them as logT2 and logT2 respectively. Establish logT2 and The mathematical relationship of the two-variable linear equation is expressed by formula (1):

[0021]

[0022] In formula (1), T2 is the value of the nuclear magnetic resonance T2 spectrum of the saturated water sample, ms; is the cumulative pore volume of NMR, %; A and B are the coefficients of the quadratic equation, dimensionless; C is a constant.

[0023] The T2 value corresponding to the inflection point of the intersection curve is T 2-Log .

[0024] Furthermore, in step (3), logT2 and The method for determining the inflection point of the intersection curve is: to differentiate the quadratic equation shown in formula (1), and the point where the derivative is equal to zero is the inflection point.

[0025] The beneficial effects of the present invention are:

[0026] The present invention only requires performing a nuclear magnetic resonance experiment on a saturated water sample, and the T2 cutoff value can be calculated by mathematically processing the T2 spectrum, which has the advantages of being economical, simple, efficient, and highly practical. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The present invention will be further described below with reference to the accompanying drawings and specific embodiments:

[0028] Figure 1 : Determination of T by NMR cumulative porosity curves before and after centrifugation 2-Cutoff Schematic diagram;

[0029] Figure 2 :pass The inflection point of the intersection curve determines T 2-Log Schematic diagram;

[0030] Figure 3 :T 2-Cutoff With T 2-Log Intersection diagram. DETAILED DESCRIPTION

[0031] 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.

[0032] Unless otherwise specified, the experimental methods used in the following examples are conventional methods.

[0033] Reference Figure 1-Figure 3 , this specific embodiment adopts the following technical solution: a method for calculating the T2 cutoff value based on saturation nuclear magnetic resonance experiment, comprising the following steps:

[0034] (1) Obtaining the saturated / bound water NMR T2 spectrum curve of tight sandstone samples;

[0035] 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 21 samples.

[0036] The cores were drilled using a deep hole drilling machine and the dense sandstone samples were made into core columns;

[0037] The core column was 5 cm in length and 2.5 cm in diameter;

[0038] Use a core cutter to cut the cross section flat;

[0039] Before the NMR experiment, the dense sandstone samples need to be pretreated;

[0040] Pretreatment items include washing away residual oil and residual salt from the sample and drying the sample;

[0041] 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.

[0042] The pretreatment before the experiment was carried out as follows:

[0043] 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;

[0044] 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;

[0045] 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;

[0046] Nuclear magnetic resonance experiments were performed on the samples after oil washing, salt washing and drying. 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".

[0047] The NMR experiment is carried out as follows:

[0048] ① Saturate the sample with water. Place the dense sandstone sample and distilled water in a saturator and saturate for more than 6 hours under constant temperature;

[0049] ② Conduct nuclear magnetic resonance experiments on water-saturated tight sandstone samples to obtain saturated water nuclear magnetic resonance T2 spectra;

[0050] ③ The dense sandstone sample that has undergone saturation NMR experiment is centrifuged using a high-speed refrigerated centrifuge with a centrifugal force set to 400 psi (2.76 MPa) to keep the dense sandstone sample in a bound water state;

[0051] ④ Perform nuclear magnetic resonance experiments on bound water tight sandstone samples to obtain bound water nuclear magnetic resonance T2 spectra.

[0052] (2) The T2 cutoff value is determined based on the saturated / bound water NMR cumulative porosity curve, denoted as T 2-Cutoff ;

[0053] As attached Figure 1 As shown: Taking Linan No. 6-10 samples as an example, the cumulative porosity curves of the saturated state and centrifuged state NMR T2 spectra are compared. A line parallel to the X axis is drawn from the maximum point of the centrifuged state cumulative porosity curve to intersect the saturated state cumulative porosity curve. A vertical line is drawn from the intersection point to the X axis. The corresponding T2 value is T 2-Cutoff ;

[0054] (3) Obtaining saturation NMR T2 spectrum The intersection curve, the T2 value corresponding to the inflection point of the curve is recorded as T 2-Log ;

[0055] The logarithms of the saturated water NMR T2 spectrum curve and the cumulative porosity curve are recorded as logT2 and log

[0056] like Figure 2 As shown: logT2 and The mathematical relationship of a linear equation with two variables can be expressed by formula (3):

[0057]

[0058] Derivative of formula (3)

[0059] make

[0060] Then LogT2=0.83

[0061] T 2-Log =10 0.83 =6.83ms

[0062] Similarly, the remaining samples T can be calculated 2-Log value.

[0063] (4) Determine T 2-Log and T 2-Cutoff A mathematical relationship was established to establish a T2 cutoff value calculation model based on saturation NMR experiments.

[0064] 21 dense sandstone samples T 2-Cutoff and T 2-Log The calculation results are shown in Table 1.

[0065] Table 1

[0066] Sample number <![CDATA[T 2-Cutoff (ms)]]> <![CDATA[T 2-Log (ms)]]> <![CDATA[T 2-Cal (ms)]]> Relative error (%) Linan 6-1 20.88 5.97 20.72 -0.79 Linan 6-2 17.19 5.01 16.89 -1.74 Linan 6-3 7.58 2.77 7.97 5.10 Linan 6-4 5.73 2.11 5.34 -6.85 Linan 6-5 5.81 2.29 6.05 4.21 Linan 6-6 4.77 1.87 4.38 -8.15 Linan 6-7 7.11 2.45 6.69 -5.88 Linan 6-8 7.88 2.59 7.25 -8.00 Linan 6-9 9.03 3.18 9.60 6.31 Linan 6-10 24.58 6.83 24.14 -1.79 Linan 6-11 16.61 5.26 17.89 7.69 Linan 6-12 19.23 5.53 18.96 -1.39 Linan 6-13 9.72 3.35 10.28 5.73 Linan 6-14 6.33 2.31 6.13 -3.10 Linan 6-15 12.21 3.91 12.51 2.44 Linan 6-16 10.08 3.27 9.96 -1.20 Linan 6-17 25.51 7.02 24.90 -2.40 Linan 6-18 20.08 5.58 19.16 -4.57 Linan 6-19 11.87 3.70 11.67 -1.67 Linan 6-20 23.39 6.66 23.46 0.32 Linan 6-21 15.93 5.18 17.57 10.28

[0067] like Figure 3 As shown, T 2-Cutoff As the vertical axis, T 2-Log As the horizontal axis, the mathematical relationship between the two can be expressed by formula (4):

[0068] T 2-Cutoff =3.9839T 2-Log -3.0688 (4)

[0069] The mathematical model established by formula (4) is given by 2-Log The T2 cutoff value T of the sample can be obtained under the condition 2-Cutoff .

[0070] The T2 cutoff value calculated using formula (4) is recorded as T 2-cal , error analysis shows that T 2-cal With T 2-Cutoff The average relative error is 4.27% and the average absolute error is 0.26%, which proves the effectiveness of the method.

[0071] 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 T2 cutoff value based on saturation nuclear magnetic resonance experiments, characterized in that: The following steps are involved: (1) Obtaining the saturated water and bound water NMR T2 spectrum curves of tight sandstone samples; (2) Based on the cumulative porosity curves of saturated water and bound water NMR, the T2 cutoff value is determined and recorded as T 2-Cutoff The specific operation method is: obtain the saturated water and bound water NMR cumulative porosity curves respectively, draw a horizontal straight line parallel to the X-axis along the bound water cumulative porosity curve, and the T2 value corresponding to the intersection of the horizontal straight line and the saturated water cumulative porosity curve is T 2-Cutoff ; (3) Obtain the logT2-logφ intersection curve of the saturated NMR T2 spectrum and differentiate the quadratic equation shown in formula (1). The point where the derivative is equal to zero is the inflection point of the logT2 and logφ intersection curve. The T2 value corresponding to the inflection point of the curve is recorded as T 2-Log ; Determine T 2-Log The specific operation method is: take the logarithm of the saturated water NMR T2 spectrum curve and the saturated water cumulative porosity curve, record them as logT2 and logφ respectively, and establish the mathematical relationship of the 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 value of the nuclear magnetic resonance T2 spectrum of the saturated water sample, ms; φ is the cumulative pore volume of the nuclear magnetic resonance, %; A and B are the coefficients of the quadratic equation, dimensionless; C is a constant; (4) Determine T 2-Log and T 2-Cutoff A mathematical relationship was established to establish a T2 cutoff value calculation model based on saturation NMR experiments.

2. The method for calculating the T2 cutoff value based on saturation nuclear magnetic resonance experiment according to claim 1, wherein: In step (1), the dense sandstone sample is a core column with a length of 5 cm and a diameter of 2.5 cm.

3. The method for calculating the T2 cutoff value based on saturation nuclear magnetic resonance experiment according to claim 1, characterized in that: In step (1), the sample is pretreated by nuclear magnetic resonance, and the pretreatment includes washing away the remaining oil and salt in the sample and drying the sample.

4. The method for calculating the T2 cutoff value based on saturation nuclear magnetic resonance experiment according to claim 1, wherein: In step (1), the sample is washed with oil, washed with salt and dried in the order of the oil and gas industry standard SY / T5336-2006 of the People's Republic of China, "Core Analysis Method".

5. The method for calculating the T2 cutoff value based on saturation nuclear magnetic resonance experiment according to claim 1, characterized in that: In step (1), a nuclear magnetic resonance experiment is performed on the sample after oil washing, salt washing and drying. 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.

6. The method for calculating the T2 cutoff value based on saturation nuclear magnetic resonance experiment according to claim 1, characterized in that: In step (1), the rock sample is dehydrated using a centrifugal device, and the centrifugal force is selected to be 400 psi.

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