T2 cut-off value calculation method based on saturated nuclear magnetic resonance experiment

Abstract

The invention belongs to the technical field of evaluation, development and research of oil and gas reservoirs, and discloses a T2 cutoff value calculation method based on a saturated nuclear magnetic resonance experiment. The method comprises the following steps: acquiring a saturated / bound water nuclear magnetic resonance T2 spectrum curve of a compact sandstone sample; determining a T2 cut-off value based on a saturated / bound water nuclear magnetic resonance accumulative porosity curve, and recording the T2 cut-off value as T2-Cuff; acquiring a logT2-logphi intersection curve of the saturated nuclear magnetic resonance T2 spectrum, and recording a T2 value corresponding to an inflection point of the curve as T2-Log; and determining a mathematical relationship between the T2-Log and the T2-Cuff, and establishing a T2 cut-off value calculation model based on the saturated nuclear magnetic resonance experiment. According to the method, the T2 cutoff value can be calculated only by performing the nuclear magnetic resonance experiment on the saturated water sample and performing mathematical treatment on the T2 spectrum, and the method has the advantages of being economical, simple, efficient and high in 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 T2 cutoff value calculation method based on a saturation nuclear magnetic resonance experiment. Background Art

[0002] Nuclear magnetic resonance technology is widely used in the characterization of pore structure of unconventional oil and gas reservoirs, including shale, tight sandstone, carbonate rock and coal, due to its rapid, non-destructive and easy-to-operate characteristics. By processing the nuclear magnetic resonance T2 spectrum, key parameters such as porosity, permeability, bound water saturation, movable fluid saturation, movable fluid porosity, etc. can be obtained.

[0003] However, the premise for obtaining the above key parameters is to determine the T2 cutoff value. The T2 cutoff value is a parameter for dividing movable fluid and bound fluid in the reservoir. When the well is not cored and the NMR experiment cannot be performed, geologists usually use empirical values ​​as the T2 cutoff value, which is 33ms for sandstone and 92ms for carbonate rock. However, a large number of studies have shown that the T2 cutoff value is not a fixed value. Therefore, if the empirical value is used to process the NMR data, errors are bound to occur.

[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 the saturated water / immobilized water state, draws a line parallel to the X-axis from the maximum point of the T2 spectrum cumulative porosity curve after centrifugation (i.e., the immobilized water volume) to intersect the saturated water cumulative porosity curve, and draws a vertical line from the intersection point to the X-axis, and its corresponding value is the T2 cutoff value; the invention patent with the authorization announcement number CN 105866160 B provides "A method for calculating the nuclear magnetic resonance T2 cutoff value based on longitudinal wave constraint", which establishes a T2 cutoff value calculation model based on longitudinal wave velocity and nuclear magnetic resonance porosity; the invention patent with the authorization announcement number CN 104237957 B provides "A method for determining the T2 cutoff value of nuclear magnetic resonance logging", which establishes a mathematical model of the immobilized water volume under uniform pores, constructs a comparison chart of T2 spectrum and porosity, and calculates the immobilized water volume corresponding to each T2 time point, and finally realizes the calculation of the T2 cutoff value; the invention patent with the authorization announcement number CN 105866160 B provides "A method for determining the T2 cutoff value of nuclear magnetic resonance logging", which establishes a mathematical model of the immobilized water volume under uniform pores, constructs a comparison chart of T2 spectrum and porosity, and calculates the immobilized water volume corresponding to each T2 time point, and finally realizes the calculation of the T2 cutoff value; the invention patent with the authorization announcement number CN The invention patent 109580689B provides a point-by-point calculation method for the T2 cutoff value of nuclear magnetic resonance logging. 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] The above methods for obtaining T2 cutoff values ​​all require centrifugation experiments on samples, which makes the experimental process cumbersome and time-consuming, and difficult to meet on-site needs. At the same time, the above methods reference many parameters when establishing the T2 cutoff value calculation model, and the acquisition of these parameters depends on experiments other than nuclear magnetic resonance, which is 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 purpose, 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 saturated / bound water NMR T2 spectrum curves of tight sandstone samples;

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

[0012] (3) Obtaining saturated NMR T2 spectrum 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 between the two was used 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 sample pretreatment, and the pretreatment items include washing away residual oil and residual 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 a certain 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, and 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".

[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 / immobile water NMR cumulative porosity curves respectively, draw a horizontal straight line parallel to the X-axis along the immobile 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 NMR T2 spectrum curve and the cumulative porosity curve, and record them as logT2 and logP2, 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: taking the derivative of 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 needs to perform 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 in conjunction with 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 and objectives adopted by the present invention easy 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 implementation 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 saturated / bound water NMR T2 spectrum curves of tight sandstone samples;

[0035] The dense sandstone samples used in the examples of 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 length is 5 cm and the diameter is 2.5 cm;

[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 in 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, repeatedly add distilled water and boil it 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 carried out 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 Carry out nuclear magnetic resonance experiments on water-saturated dense sandstone samples to obtain saturated water nuclear magnetic resonance T2 spectra;

[0050] ③ The dense sandstone samples that have completed the saturated NMR experiment are centrifuged. The experimental instrument is a high-speed refrigerated centrifuge with a centrifugal force set to 400 psi (2.76 MPa) to keep the dense sandstone samples in a bound water state;

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

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

[0053] As attached Figure 1 As shown: Taking Li Nan 6-10 samples as an example, the cumulative porosity curves of the saturated state / 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, and the corresponding T2 value is T 2-Cutoff ;

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

[0055] The logarithm of the saturated water NMR T2 spectrum curve and the cumulative porosity curve is 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 between the two was used 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 No. <![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 of 2-Cutoff .

[0070] The T2 cutoff value calculated using formula (4) is denoted 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 given herein, those skilled in the art should understand that the embodiments of the present invention may be modified without departing from the spirit of the present invention. The above embodiments are merely exemplary and should not be used as a limitation on the scope of the present invention.

Claims

1. A method for calculating T2 cutoff value based on saturation nuclear magnetic resonance experiment, characterized in that: The following steps are involved: (1) Obtaining saturated / bound water NMR T2 spectrum curves of tight sandstone samples; (2) Based on the saturated / bound water NMR cumulative porosity curve, the T2 cutoff value is determined and recorded as T 2-Cutoff ; (3) Obtain the logT2-logφ intersection curve of the saturated NMR T2 spectrum, and the T2 value corresponding to the inflection point of the curve is recorded as T 2-Log ; (4) Determine T 2-Log and T 2-Cutoff A mathematical relationship between the two was used 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, characterized in that: 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, characterized in that: In step (1), the sample is washed with oil, washed with salt and dried in the order of priority. 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.

5. The method for calculating 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 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.

7. The method for calculating T2 cutoff value based on saturation nuclear magnetic resonance experiment according to claim 1, characterized in that: In step (2), determine the T2 cutoff value T 2-Cutoff The specific operation method is: obtain the saturated / immobile water NMR cumulative porosity curves respectively, draw a horizontal straight line parallel to the X-axis along the immobile 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 .

8. The method for calculating T2 cutoff value based on saturation nuclear magnetic resonance experiment according to claim 1, characterized in that: In step (3), determine T 2-Log The specific operation method is: take the logarithm of the saturated water NMR T2 spectrum curve and the cumulative porosity curve, record them as logT2 and logφ respectively, and establish the mathematical relationship of the binary linear 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; and C is a constant. The T2 value corresponding to the inflection point of the logT2-logφ intersection curve is T 2-Log .

9. The method for calculating T2 cutoff value based on saturation nuclear magnetic resonance experiment according to claim 1, characterized in that: In step (3), the method for determining the inflection point of the intersection curve of logT2 and logφ is: taking the derivative of the quadratic equation shown in formula (1), and the point where the derivative is equal to zero is the inflection point.

Images