Method for calculating dynamic T2 cutoff value of deep clastic rock reservoir

Abstract

A method for calculating a dynamic T2 cutoff value of a deep clastic rock reservoir comprises the steps that firstly, a rock sample nuclear magnetic resonance experiment is carried out on a certain deep clastic rock reservoir, and the T2 cutoff value is determined; secondly, calculating a difference quotient value of saturation porosity components of the nuclear magnetic resonance experiment of the rock sample, and reading a second T2 value intersected with the relaxation time axis from left to right; thirdly, establishing a conversion relational expression between a T2 cutoff value determined by a rock sample nuclear magnetic resonance experiment and a T2 value calculated by difference quotient; and finally, performing difference quotient processing on the T2 value of the actual nuclear magnetic resonance logging to obtain a second T2 value intersected with the relaxation time axis, and substituting the T2 value into a conversion relation between the constructed experimental T2 cutoff value and the difference quotient calculation T2 value, so that the dynamic T2 cutoff value of the deep clastic rock reservoir can be calculated point by point. According to the method, the defects of a method for calculating the variable T2 cut-off value of the deep clastic rock reservoir in nuclear magnetic resonance logging are overcome, and technical support is provided for accurate evaluation of the movable fluid of the reservoir and accurate calculation of the permeability in nuclear magnetic resonance logging.

Description

Technical Field

[0001] The invention relates to the technical field of oil and gas exploration, and in particular to a method for calculating a dynamic T2 cutoff value of a deep clastic rock reservoir. Background Art

[0002] With the continuous deepening of oil and gas exploration, deep clastic reservoirs have become the focus of oil and gas resource exploration and reserve and production increase in my country. However, they are generally characterized by low porosity and low permeability, and have strong heterogeneity. The NMR T2 cutoff value is not accurately determined, resulting in low calculation accuracy of reservoir permeability, which brings great difficulties to the large-scale and effective development of gas reservoirs. If there is no core NMR experimental data, the T2 cutoff value is a fixed value. The cutoff value of conventional clastic reservoir sections is generally 33ms. However, for deep clastic rocks, the T2 cutoff value is less than 33ms due to the influence of complex pore structure. Therefore, it is difficult to meet the accuracy requirements when using a fixed cutoff value to calculate the permeability. Based on this, it is particularly important to propose a targeted dynamic T2 cutoff value calculation method for deep clastic reservoirs.

[0003] Most of the existing T2 cutoff calculation methods are based on nuclear magnetic resonance experiments. For example, the invention with patent number CN202410047952.2 and titled "A method for calculating a variable T2 cutoff value for deep clastic reservoirs" discloses a method for determining the nuclear magnetic T2 time corresponding to different pore throat boundaries based on the measurement of the nuclear magnetic T2 attenuation signal of saturated water and centrifugal state, calculating the corresponding porosity component percentage, the T2 mean corresponding to different types of pore throats and the comprehensive parameters of different types of pore throats, and establishing a calculation model between the T2 cutoff value and different porosity components. This invention is mainly aimed at nuclear magnetic resonance indoor experiments. The invention with patent number CN201811203489.7 and titled "A method for calculating the T2 cutoff value of nuclear magnetic resonance logging point by point" discloses a method for calculating the pseudo cutoff time geometric mean of the nuclear magnetic resonance T2 spectrum from the pseudo T2 cutoff value, determining the fitting relationship between the T2 cutoff value and the pseudo cutoff time geometric mean by a fitting method, and then realizing the point-by-point calculation of the T2 cutoff value of nuclear magnetic resonance logging. This invention effectively improves the calculation accuracy of parameters such as irreducible water saturation and effective porosity, but the calculation process is relatively complicated.

[0004] Practice has found that the saturated porosity component of the nuclear magnetic resonance experiment effectively maps the different pores of deep clastic reservoirs and can reflect the characteristics of the bound fluid in the micropores of deep clastic reservoirs. Based on this, it is necessary to perform difference quotient processing on the saturated porosity component of the nuclear magnetic resonance experiment for deep clastic reservoirs, and obtain the T2 spectrum value calculated by the difference quotient that is closely related to the bound fluid in the micropores, so as to construct the conversion relationship of the T2 cutoff value and then form a T2 cutoff value calculation method. From the existing methods, there is no difference quotient processing of the saturated porosity component of the nuclear magnetic resonance experiment to construct a calculation method for the T2 cutoff value, which brings inconvenience to the evaluation of deep clastic reservoirs. Summary of the invention

[0005] In order to overcome the shortcomings of the above-mentioned prior art, the purpose of the present invention is to provide a method for calculating the dynamic T2 cutoff value of deep clastic reservoirs. Based on the core nuclear magnetic resonance experiment in the study area, the dynamic T2 cutoff value of the deep clastic reservoir is determined, and a conversion relationship between the T2 cutoff value determined by the nuclear magnetic resonance experiment and the T2 value calculated by the difference quotient is constructed, and then according to the conversion relationship, the point-by-point calculation of the dynamic T2 cutoff value of the deep clastic reservoir is realized; the present invention makes up for the shortcomings of the variable T2 cutoff value calculation method of deep clastic reservoirs in nuclear magnetic resonance logging, and provides technical support for the accurate evaluation of movable fluid in reservoirs and the precise calculation of permeability in nuclear magnetic resonance logging.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is:

[0007] A method for calculating the dynamic T2 cutoff value of a deep clastic reservoir comprises the following steps:

[0008] Step 1: Conduct rock sample nuclear magnetic resonance experiment: Drill deep clastic rock standard rock samples, pre-treat the rock samples, measure basic parameters such as porosity and permeability, saturate the rock samples according to the actual reservoir formation water mineralization, and conduct rock sample nuclear magnetic resonance experiments in accordance with the petroleum industry standard "Rock Sample Nuclear Magnetic Resonance Laboratory Measurement Specification SY / T 6490-2023". After measuring the T2 value under the saturated water state, centrifuge the rock samples and conduct nuclear magnetic resonance experiments to obtain the bound water saturation of deep clastic rocks and centrifuged nuclear magnetic T2 spectrum;

[0009] Step 2: Determine the T2 cutoff value of the rock sample NMR: draw a straight line parallel to the relaxation time axis for the cumulative curve after centrifugation, and intersect it with the saturated water cumulative curve at point N. Draw a straight line perpendicular to the relaxation time axis through point N. The T2 relaxation time at the intersection of the straight line and the relaxation time axis is the T2 cutoff value of the saturated porosity component.

[0010] Step 3: Calculate the difference quotient of the saturated porosity component of the rock sample NMR experiment: Perform forward difference calculations on the saturated porosity component and the T2 values ​​under different saturated porosity components, and finally obtain the saturated porosity component difference quotient DQφ i .

[0011] The specific calculation of step three is as follows:

[0012]

[0013] ΔT 2(i) =T 2(i+1) -T 2(i) (2)

[0014] Where: is the difference value of the saturated porosity component at the i-th point, %;

[0015] are the saturated porosity component values ​​at the i+1th and ith points, %;

[0016] ΔT 2(i) is the T2 difference value corresponding to the saturated porosity component at the i-th point, ms;

[0017] T 2(i+1) , T 2(i) are the T2 values ​​of the i+1th and ith points, ms;

[0018] i is the current point of the difference calculation;

[0019] Based on the above saturated porosity component difference value and T2 difference value, the quotient is calculated

[0020]

[0021] Where: DQφ i is the saturated porosity component difference quotient of the i-th point, %;

[0022] Step 4: Read the T2 value calculated by the difference quotient: Based on the saturated porosity component difference quotient value DQφ i The T2 values ​​under different saturated porosity components are used as the horizontal coordinates, and the saturated porosity component difference quotient DQφ is used as the horizontal coordinates. i As the ordinate, a cross-plot is made; in the cross-plot, the second point where the difference quotient T2 spectrum intersects with the relaxation time axis from left to right is close to the T2 cutoff value determined by the rock sample nuclear magnetic resonance experiment, and accordingly, the T2 value corresponding to the second point intersecting with the relaxation time axis is read;

[0023] Step 5: Establish the conversion relationship between the T2 cutoff value determined by the rock sample NMR experiment and the T2 value calculated by the difference quotient.

[0024] The conversion relationship of step 5 is as follows:

[0025] y=0.9967x 0.6763 (4)

[0026] Where: y is the T2 cutoff value determined by the rock sample NMR experiment, ms;

[0027] x is the T2 value calculated by the difference quotient, ms;

[0028] Step 6: Calculate the dynamic T2 cutoff value of nuclear magnetic resonance logging: Perform difference quotient processing on the T2 value of the actual nuclear magnetic resonance logging to obtain the second T2 value intersecting with the relaxation time axis. Substitute this T2 value into the conversion relationship between the experimental T2 cutoff value constructed in step 5 and the T2 value calculated by difference quotient, and the dynamic T2 cutoff value of the deep clastic reservoir can be determined.

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] (1) Steps 1 to 3 of the present invention are based on core nuclear magnetic resonance experiments in the study area to determine the dynamic T2 cutoff value of the deep clastic reservoir, effectively mining the T2 information of bound and movable fluids contained in the nuclear magnetic saturation porosity component and T2 value.

[0031] (2) In step 4 of the present invention, the second T2 value intersecting the relaxation time axis obtained by difference quotient calculation has a good correlation with the T2 cutoff value determined by the nuclear magnetic resonance experiment of the rock sample. Based on this, a conversion relationship between the T2 cutoff value determined by the nuclear magnetic resonance experiment and the T2 value calculated by difference quotient is constructed. Then, step 5 realizes the point-by-point calculation of the dynamic T2 cutoff value of the deep clastic reservoir according to the conversion relationship, providing nuclear magnetic resonance logging technology support for the accurate evaluation of movable fluids in deep clastic reservoirs and the precise calculation of permeability. The method is simple, practical and has good application prospects.

[0032] In summary, the present invention firstly conducts a rock sample nuclear magnetic resonance experiment on a certain deep clastic reservoir and determines the T2 cutoff value; secondly, calculates the difference quotient value of the saturated porosity component of the rock sample nuclear magnetic resonance experiment, and reads the second T2 value intersecting the relaxation time axis from left to right; again establishes a conversion relationship between the T2 cutoff value determined by the rock sample nuclear magnetic resonance experiment and the T2 value calculated by the difference quotient; finally, performs difference quotient processing on the T2 value of the actual nuclear magnetic resonance logging to obtain the second T2 value intersecting the relaxation time axis, and substitutes the T2 value into the conversion relationship between the constructed experimental T2 cutoff value and the T2 value calculated by the difference quotient, so as to calculate the dynamic T2 cutoff value of the deep clastic reservoir point by point, which makes up for the deficiency of the calculation method of the variable T2 cutoff value of the deep clastic reservoir in nuclear magnetic resonance logging, and provides nuclear magnetic resonance logging technology support for the accurate evaluation of movable fluids and the precise calculation of permeability in deep clastic reservoirs. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is a flow chart of the method for calculating the dynamic T2 cutoff value of deep clastic reservoirs of the present invention.

[0034] Figure 2 The cross-plot of the T2 cutoff value and the saturated porosity component difference quotient value of the present invention.

[0035] Figure 3 This is a conversion chart of the difference quotient T2 value of the present invention and the T2 cutoff value determined by the rock sample nuclear magnetic resonance experiment.

[0036] Figure 4 This is a diagram showing the calculation results of the dynamic T2 cutoff value of the deep clastic reservoir of the present invention.

[0037] Figure 5This is an error analysis diagram of the T2 cutoff value calculated by the present invention and the T2 cutoff value determined by the core nuclear magnetic resonance experiment. DETAILED DESCRIPTION

[0038] The present invention is further described in detail below with reference to the embodiments.

[0039] Reference Figure 1 A method for calculating the dynamic T2 cutoff value of a deep clastic reservoir comprises the following steps:

[0040] Step 1: Conduct rock sample nuclear magnetic resonance experiments. Drill deep clastic rock standard rock samples, pre-treat the core, measure basic parameters such as porosity and permeability, saturate the rock samples according to the actual reservoir formation water mineralization, and conduct rock sample nuclear magnetic resonance experiments in accordance with the petroleum industry standard "Rock Sample Nuclear Magnetic Resonance Laboratory Measurement Specification (SY / T 6490-2023)". After measuring the T2 value under the saturated water state, centrifuge the core with a centrifugal force of 450psi and conduct nuclear magnetic resonance experiments to obtain the bound water saturation of deep clastic rocks and the centrifugal nuclear magnetic T2 spectrum.

[0041] Step 2: Determine the T2 cutoff value of the rock sample nuclear magnetic resonance: draw a straight line parallel to the relaxation time axis for the cumulative curve after centrifugation, and intersect it with the saturated water cumulative curve at point N. Draw a straight line perpendicular to the relaxation time axis through point N. The T2 relaxation time at the intersection of the straight line and the relaxation time axis is the T2 cutoff value of the saturated porosity component.

[0042] Step 3: Calculate the difference quotient of the saturated porosity component of the rock sample NMR experiment. Perform forward difference calculations on the saturated porosity component and the T2 values ​​under different saturated porosity components:

[0043]

[0044] ΔT 2(i) =T 2(i+1) -T 2(i) (2)

[0045] Where: is the difference value of the saturated porosity component at the i-th point, %;

[0046] are the saturated porosity component values ​​at the i+1th and ith points, %;

[0047] ΔT 2(i) is the T2 difference value corresponding to the saturated porosity component at the i-th point, ms;

[0048] T 2(i+1) , T 2(i) is the T2 value of the i+1th and ith points, ms;

[0049] i is the current point of the difference calculation.

[0050] Based on the above saturated porosity component difference value and T2 difference value, the quotient is calculated

[0051]

[0052] Where: DQφ i is the saturated porosity component difference quotient of the i-th point, %.

[0053] Step 4: Read the T2 value calculated by the difference quotient: Based on the saturated porosity component difference quotient value DQφ i The T2 values ​​under different saturated porosity components are used as the horizontal coordinates, and the saturated porosity component difference quotient is used as the vertical coordinates to make the Figure 2 The cross-plot shown in FIG. In the cross-plot, the second point where the difference quotient T2 spectrum intersects with the relaxation time axis from left to right is close to the T2 cutoff value determined by the core NMR experiment. Based on this, the T2 value corresponding to the second point intersecting with the relaxation time axis is read.

[0054] Step 5: Establish a conversion relationship between the T2 cutoff value determined by the rock sample NMR experiment and the T2 value calculated by the difference quotient. Use the T2 value calculated by the difference quotient as the independent variable and the T2 cutoff value determined by the NMR experiment as the dependent variable to construct Figure 3 The functional relationship shown in the figure shows that the square correlation coefficient of the fitting function is as high as 0.8199, indicating that the T2 value determined by the difference quotient method can accurately calculate the T2 cutoff value. The conversion relationship is as follows:

[0055] y=0.9967x 0.6763 (4)

[0056] Where: y is T2 cutoff value, ms;

[0057] x is the T2 value calculated by the difference quotient, in ms.

[0058] Step 6: Determine the dynamic T2 cutoff value of nuclear magnetic resonance logging: Perform difference quotient processing on the T2 value of the actual nuclear magnetic resonance logging to obtain the second T2 value intersecting the relaxation time axis, and substitute this T2 value into the conversion relationship between the experimental T2 cutoff value constructed in step 5 and the difference quotient calculated T2 value, and the dynamic T2 cutoff value of the deep clastic reservoir can be determined. In actual production, the difference quotient calculation of the nuclear magnetic T2 spectrum can be performed using the functions provided in the logging data processing and interpretation software, or the program module can be compiled and attached to the logging data processing and interpretation software platform, so that the T2 spectrum difference quotient calculation can be quickly performed, and then according to the conversion relationship between the experimental T2 cutoff value and the difference quotient calculated T2 value, the dynamic T2 cutoff value of the deep clastic reservoir can be calculated point by point.

[0059] The above method was programmed and attached to the CIFLog logging data processing and comprehensive interpretation software platform to realize the point-by-point calculation of the T2 cutoff value of the deep clastic reservoir in the offshore X area. Figure 4 This is the result of determining the dynamic T2 cutoff value for the deep clastic reservoir in Well BD-X. Track 5 in the figure is a comparison of the T2 spectrum of the NMR logging, the T2 cutoff value calculated by this method, and the T2 cutoff value determined by the NMR experiment. It can be seen from the figure that the T2 cutoff value calculated by this method is basically consistent with the T2 cutoff value determined by the NMR experiment, and the data points are basically overlapped. Figure 5 From the error analysis diagram of the calculated T2 cutoff value and the T2 cutoff value determined by the core nuclear magnetic resonance experiment, it can be seen that the T2 cutoff value calculated by this method basically falls on the 45° line, with a relative error ranging from 0.54% to 16.89% and an average relative error of 7.27%, achieving good application results and meeting the requirements for the accuracy of the T2 cutoff value in the permeability evaluation of deep clastic reservoirs in this area.

[0060] This fully demonstrates that the calculation results of the dynamic T2 cutoff value of the deep clastic reservoir in this study are consistent with the T2 cutoff value of the nuclear magnetic resonance experiment. It also further shows that the use of the dynamic T2 cutoff value calculation method for deep clastic reservoirs described in the present invention can provide nuclear magnetic resonance logging technical support for the accurate evaluation of reservoir movable fluid and precise calculation of permeability.

[0061] Those skilled in the art should understand that, due to the possibility of mud intrusion in deep clastic reservoirs, and the detection depth of nuclear magnetic resonance logging is relatively shallow, the nuclear magnetic resonance signal may be distorted due to mud intrusion, and the dynamic T2 cutoff value determination directly using nuclear magnetic resonance logging may have low accuracy. In order to ensure the feasibility of the dynamic T2 cutoff value calculation method for deep clastic reservoirs, it is necessary to correct the influence of mud intrusion on its nuclear magnetic resonance logging data. In addition, the pore structure of the reservoir in each block is quite different. When using this method, it is necessary to carry out nuclear magnetic resonance experiments on rock samples and establish a conversion relationship between the T2 cutoff value determined by the nuclear magnetic resonance experiment of rock samples applicable to the study area and the T2 value calculated by the difference quotient, so that the T2 cutoff value can be calculated more accurately.

Claims

1. A method for calculating the dynamic T2 cutoff value of deep clastic reservoirs, characterized in that: The following steps are involved: Step 1: Conduct rock sample nuclear magnetic resonance experiment: Drill deep clastic rock standard rock samples, pre-treat the rock samples, measure basic parameters such as porosity and permeability, saturate the rock samples according to the actual reservoir formation water mineralization, and conduct rock sample nuclear magnetic resonance experiments in accordance with the petroleum industry standard "Rock Sample Nuclear Magnetic Resonance Laboratory Measurement Specification SY / T 6490-2023". After measuring the T2 value under the saturated water state, centrifuge the rock samples and conduct nuclear magnetic resonance experiments to obtain the bound water saturation of deep clastic rocks and centrifuged nuclear magnetic T2 spectrum; Step 2: Determine the T2 cutoff value of the rock sample NMR: draw a straight line parallel to the relaxation time axis for the cumulative curve after centrifugation, and intersect it with the saturated water cumulative curve at point N. Draw a straight line perpendicular to the relaxation time axis through point N. The T2 relaxation time at the intersection of the straight line and the relaxation time axis is the T2 cutoff value of the saturated porosity component. Step 3: Calculate the difference quotient of the saturated porosity component of the rock sample NMR experiment: Perform forward difference calculations on the saturated porosity component and the T2 values ​​under different saturated porosity components, and finally obtain the saturated porosity component difference quotient DQφ i Step 4: Read the T2 value calculated by the difference quotient: Based on the saturated porosity component difference quotient value DQφ i The T2 values ​​under different saturated porosity components are used as the horizontal coordinates, and the saturated porosity component difference quotient DQφ is used as the horizontal coordinates. i As the ordinate, a cross-plot is made; in the cross-plot, the second point where the difference quotient T2 spectrum intersects with the relaxation time axis from left to right is close to the T2 cutoff value determined by the rock sample nuclear magnetic resonance experiment, and accordingly, the T2 value corresponding to the second point intersecting with the relaxation time axis is read; Step 5: Establish a conversion relationship between the T2 cutoff value determined by the rock sample NMR experiment and the T2 value calculated by the difference quotient; Step 6: Calculate the dynamic T2 cutoff value of nuclear magnetic resonance logging: Perform difference quotient processing on the T2 value of the actual nuclear magnetic resonance logging to obtain the second T2 value intersecting with the relaxation time axis. Substitute this T2 value into the conversion relationship between the experimental T2 cutoff value constructed in step 5 and the T2 value calculated by difference quotient, and the dynamic T2 cutoff value of the deep clastic reservoir can be determined.

2. A method for calculating the dynamic T2 cutoff value of a deep clastic reservoir according to claim 1, characterized in that: The specific calculation of step three is as follows: ΔT 2(i) =T 2(i+1) -T 2(i) (2) Where: is the difference value of the saturated porosity component at the i-th point, %; are the saturated porosity component values ​​at the i+1th and ith points, %; ΔT 2(i) is the T2 difference value corresponding to the saturated porosity component at the i-th point, ms; T 2(i+1) , T 2(i) are the T2 values ​​of the i+1th and ith points, ms; i is the current point of the difference calculation; Based on the above saturated porosity component difference value and T2 difference value, the quotient is calculated Where: DQφ i is the saturated porosity component difference quotient of the i-th point, %.

3. A method for calculating the dynamic T2 cutoff value of a deep clastic reservoir according to claim 1, characterized in that: The conversion relationship of step 5 is as follows: <h2 style=";text-align:left;direction:ltr">y = 0.9967x<h2 style=";text-align:left;direction:ltr"> 0.6763 <h2 style=";text-align:left;direction:ltr"> (4) Where: y is the T2 cutoff value determined by the rock sample NMR experiment, ms; x is the T2 value calculated by the difference quotient, in ms.

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