Method for judging rationality of nuclear magnetic resonance pore throat radius distribution

Abstract

The invention relates to the technical field of oil and gas field development, in particular to a method for judging nuclear magnetic resonance pore throat radius distribution reasonability. The invention discloses a method for judging the rationality of nuclear magnetic resonance pore throat radius distribution, and the method comprises the steps: taking a high-pressure mercury injection pore throat radius cumulative volume ratio distribution curve as a left boundary line, and taking a converted nuclear magnetic resonance pore throat radius cumulative volume ratio distribution curve as a right boundary line; if the target nuclear magnetic resonance pore throat radius cumulative volume proportion distribution curve obtained by other methods is located between the left boundary line and the right boundary line, the target nuclear magnetic resonance pore throat radius cumulative volume proportion distribution curve is reasonable, otherwise, the target nuclear magnetic resonance pore throat radius cumulative volume proportion distribution curve is not reasonable; the method has an important value for oil and gas reservoir evaluation.

Description

Technical Field

[0001] The present invention relates to the technical field of oil and gas field development, and particularly to a method for judging the rationality of nuclear magnetic resonance pore throat radius distribution. Background Art

[0002] Nuclear magnetic resonance experiments have advantages such as not damaging the pore throat structure, high measurement accuracy, and many measurement parameters, and are widely used in aspects such as porosity measurement, permeability measurement, fluid identification, and pore throat structure analysis, and are an important technical support for oil and gas exploration and development. The transverse relaxation time measured by nuclear magnetic resonance experiments is not directly equal to the pore throat radius. Therefore, a large number of studies have been carried out at home and abroad, combining nuclear magnetic resonance with experiments such as high-pressure mercury injection, constant-rate mercury injection, and nitrogen adsorption to realize the conversion of the transverse relaxation time distribution measured by nuclear magnetic resonance into the nuclear magnetic resonance pore throat radius distribution. At the same time, there are also some studies that directly use methods such as the empirical method or the analogy method to quickly establish the nuclear magnetic resonance pore throat radius distribution curve.

[0003] In practical applications, it is found that there are significant differences between the pore throat radius distributions obtained by different methods, but there is a lack of a method for judging the rationality of different nuclear magnetic resonance pore throat radius distributions. Therefore, establishing a method for judging the rationality of nuclear magnetic resonance pore throat radius distribution is of great significance for oil and gas reservoir evaluation. Summary of the Invention

[0004] The present invention aims to address the above problems and proposes a method for judging the rationality of nuclear magnetic resonance pore throat radius distribution.

[0005] The technical solution of the present invention lies in: The transverse relaxation process of the fluid in the pore throat is affected by the combined action of three mechanisms: free relaxation, diffusion relaxation, and surface relaxation. Its transverse relaxation time is expressed as: 1 / T 2 =1 / T 2B +1 / T 2D +1 / T 2S (1) In the formula: T 2 is the transverse relaxation time, ms; T 2B is the transverse free relaxation time, ms; T 2D is the transverse diffusion relaxation time, ms; T 2S is the transverse surface relaxation time, ms.

[0006] When the fluid in the pore throat is the wetting phase, its transverse free relaxation time T2B Much greater than the transverse relaxation time T 2 , the 1 / T 2B term in Equation (1) can be neglected; when the magnetic field gradient is small and the echo spacing is short enough, the transverse diffusion relaxation time T 2D is usually long, and the 1 / T 2D term in Equation (1) can be neglected. For nuclear magnetic resonance experiments, the fluid used in the experiment is usually simulated formation water, the magnetic field is weak and the echo spacing is short, so Equation (1) can be approximately written as: 1 / T 2 ≈1 / T 2S = ρ 2 S / V (2) In the formula: ρ 2 is the transverse surface relaxation intensity, μm / ms; S is the surface area of the pore throat, μm 2 ; V is the pore throat volume, μm 3 ; Let S / V = F S / r , substitute it into Equation (2), and get: r ≈ ρ 2 F S T 2 (3) In the formula: F S is the pore throat shape factor, dimensionless; r is the pore throat radius, μm.

[0007] Since the influence of 1 / T 2B and 1 / T 2D on 1 / T 2 is neglected, so both Equation (2) and Equation (3) are approximate equalities. For the convenience of solution, assume that the equality relationship between the pore throat radius r and the transverse relaxation time T 2 is: r= (ρ 2 F S + δ ) T 2 (4) In the formula: δ is the correction coefficient of the transverse free relaxation time and the transverse diffusion relaxation time, μm / ms; Introduce δ in formula (4) to make formula (3) an equation.

[0008] Since ρ 2 F S and δ both have unknown variation laws and are difficult to accurately obtain, resulting in the pore throat radius r and the transverse relaxation time T 2 having an unknown correlation.

[0009] If we let C = ρ 2 F S + δ , and substitute it into formula (4), we get: r = CT 2 (5) In the formula: C is the conversion coefficient, μm / ms.

[0010] Formula (5) is the linear correlation formula between the pore throat radius r and the transverse relaxation time T 2 . The transverse relaxation time T 2 is not equal to the pore throat radius r . There is a conversion coefficient T 2 between the transverse relaxation time r and the pore throat radius C .

[0011] Compared with constant-rate mercury injection and nitrogen adsorption, the high-pressure mercury injection pore throat radius r (pc) measured by high-pressure mercury injection has a wider distribution range and lower experimental cost. Therefore, a method for judging the rationality of the nuclear magnetic resonance pore throat radius distribution can be established by combining nuclear magnetic resonance and high-pressure mercury injection. Due to the difference in experimental principles and the limitation of the mercury injection pressure, the maximum mercury injection saturation of high-pressure mercury injection is generally less than 100%, resulting in the high-pressure mercury injection pore throat radius r(pc) The distribution curve cannot represent the complete pore throat radius r distribution, while the simulated formation water used in nuclear magnetic resonance can saturate almost all pore throats, and the T 2 distribution measured by it is converted into the nuclear magnetic resonance pore throat radius r T2 distribution, then the nuclear magnetic resonance pore throat radius r T2 distribution represents an almost complete pore throat radius r distribution, so there exists the fact that the nuclear magnetic resonance pore throat radius r T2 distribution contains the high-pressure mercury injection pore throat radius r (pc) distribution. For this reason, two constraint conditions can be derived: ① The minimum nuclear magnetic resonance pore throat radius < the minimum high-pressure mercury injection pore throat radius; ② For any pore throat radius interval, the cumulative volume proportion of nuclear magnetic resonance ≥ the cumulative volume proportion of high-pressure mercury injection.

[0012] For the convenience of analysis, let: r (pc) be the high-pressure mercury injection pore throat radius, μm; r T2 be the nuclear magnetic resonance pore throat radius, μm; r min (pc) be the minimum high-pressure mercury injection pore throat radius, μm; T 2 ( r min (pc) ) is the transverse relaxation time corresponding to the minimum high-pressure mercury injection pore throat radius, T 2min is the minimum transverse relaxation time, r ( T 2min ) is the minimum nuclear magnetic resonance pore throat radius.

[0013] According to constraint condition ①, it can be known that: r ( T 2min ) < r min (pc) . Since r ( T 2min ) = CT 2min 、 r min (pc) = CT 2 ( r min (pc)), so there exists T 2min < T 2 ( r min (pc) ).

[0014] Since the measurement results of high-pressure mercury injection and nuclear magnetic resonance are both discrete data points, the volume fraction corresponding to any discrete data point represents the volume fraction of a certain pore throat radius interval, that is, from T 2min to T 2 ( r min (pc) ) the cumulative volume fraction represents the pore throat radius interval r ( T 2min ) ≤ r ≤ r min (pc) ) the cumulative volume fraction. According to constraint condition ②, it can be known that: from T 2min to T 2 ( r min (pc) ) the cumulative volume fraction ≥ r min (pc) the corresponding volume fraction.

[0015] In the process of obtaining the complete pore throat radius r distribution by combining nuclear magnetic resonance and high-pressure mercury injection, r min (pc) and T 2min are known parameters, T 2 ( r min (pc) ) are unknown parameters and their accurate values are difficult to obtain. Since both high-pressure mercury injection and nuclear magnetic resonance measure discrete data points, and it is necessary to simultaneously satisfy " T 2min < T 2 ( r min (pc) )" and "from T 2min to T 2 ( r min (pc)) cumulative volume ratio ≥ r min (pc) corresponding volume ratio”, so T 2 ( r min (pc) ) has a lower limit T 2min * , satisfying T 2 ( r min (pc) ) ≥ T 2min * .

[0016] In equation (5), r and T 2 are variables. When T 2 in equation (5) takes the value T 2 ( r min (pc) ), then r in equation (5) is equal to r min (pc) , so there exists T 2 ( r min (pc) ) = r min (pc) / C . Since T 2 ( r min (pc) ) is unknown, but its corresponding r min (pc) is known. Substitute T 2 ( r min (pc) ) = r min (pc) / C into T 2 ( r min (pc) ) ≥ T 2min *, it can be deduced that C ≤ r min (pc) / T 2min * , indicating that the conversion coefficient C has an upper limit of r min (pc) / T 2min * .

[0017] After obtaining T 2min * , adopt r T2 =( r min (pc) / T 2min * )× T 2 Convert T 2 to the nuclear magnetic resonance pore throat radius r T2 , and then draw the cumulative volume fraction distribution curve of the nuclear magnetic resonance pore throat radius, which represents the right boundary line.

[0018] Constraint condition ② For any pore throat radius interval, the cumulative volume fraction of nuclear magnetic resonance ≥ the cumulative volume fraction of high-pressure mercury injection. The r (pc) cumulative volume fraction distribution curve can be used as the left boundary line.

[0019] Draw the right boundary line and the left boundary line on the same graph, and then the rationality of the target cumulative volume fraction distribution curve of the nuclear magnetic resonance pore throat radius obtained by other methods can be judged: If the target cumulative volume fraction distribution curve of the nuclear magnetic resonance pore throat radius obtained by other methods is located between the left boundary line and the right boundary line, it indicates that the target cumulative volume fraction distribution curve of the nuclear magnetic resonance pore throat radius is reasonable; otherwise, it is unreasonable.

[0020] The technical effect of the present invention lies in: The present invention uses the cumulative volume fraction distribution curve of the high-pressure mercury injection pore throat radius obtained from the high-pressure mercury injection experiment as the left boundary line, and uses the r T2 =( r min (pc) / T 2min * )× T2 The cumulative volume fraction distribution curve of the converted nuclear magnetic resonance pore throat radius is the right boundary line, and a rationality judgment chart is established to judge whether the cumulative volume fraction distribution curve of the target nuclear magnetic resonance pore throat radius obtained by other methods is reasonable, which has important value for oil and gas reservoir evaluation. Description of the Drawings

[0021] Figure 1 is the left boundary line.

[0022] Figure 2 is the right boundary line.

[0023] Figure 3 is the rationality judgment chart.

[0024] Figure 4 is a schematic diagram for judgment according to the rationality judgment chart. Detailed Implementation Manner

[0025] A method for judging the rationality of nuclear magnetic resonance pore throat radius distribution, the method is as follows: Step 1: Draw the left boundary line: Determine the distribution data of the high-pressure mercury injection pore throat radius through high-pressure mercury injection experiments, and obtain the minimum value of the high-pressure mercury injection pore throat radius r (pc) and r min (pc) and r min (pc) of the volume fraction; draw r (pc) cumulative volume fraction distribution curve; Step 2: Determine T 2 distribution data through nuclear magnetic resonance experiments to obtain the minimum transverse relaxation time T 2min ; Step 3: Draw the right boundary line; the specific process is as follows: (1) Define the intermediate parameter T 2min M ; (2) Construct the judgment condition: T 2min to T 2min M cumulative volume fraction ≥ r min (pc) volume fraction; T 2min M The value of T 2minStart by determining whether the judgment condition is met; if the judgment condition is met, use the current T 2min M value as T 2min * ; if not, increase the T 2min M value and re-judge whether the judgment condition is met until the judgment condition is met to obtain T 2min * ; (3) Use r T2 =( r min (pc) / T 2min * )× T 2 , and convert T 2 to the nuclear magnetic resonance pore throat radius r T2 , to obtain the cumulative volume fraction distribution curve of the nuclear magnetic resonance pore throat radius, and use it as the right boundary line; Step 4: Judgment: If the target cumulative volume fraction distribution curve of the nuclear magnetic resonance pore throat radius is between the left boundary line and the right boundary line, it indicates that the target cumulative volume fraction distribution curve of the nuclear magnetic resonance pore throat radius is reasonable, otherwise it is unreasonable.

[0026] Specific experimental cases The distribution data of the high-pressure mercury injection pore throat radius measured by the high-pressure mercury injection experiment r (pc) are shown in Table 1; the distribution data of T 2 measured by the nuclear magnetic resonance experiment are shown in Table 2; Table 1 Distribution data of the high-pressure mercury injection pore throat radius r (pc) ; ; Table 2 T 2 ;

[0027] A method for judging the rationality of the nuclear magnetic resonance pore throat radius distribution is as follows: Step 1: According to the data in Table 1, read the minimum value of the high-pressure mercury injection pore throat radius r min (pc) = 0.0036 μm,r min (pc) The volume percentage = 1.04%; With the high-pressure mercury intrusion pore throat radius r (pc) as the abscissa, and with r (pc) the corresponding cumulative volume percentage as the ordinate, draw the left boundary line, as Figure 1 .

[0028] Step 2: According to the data in Table 2, read the minimum transverse relaxation time T 2min = 0.08 ms; Step 3: Draw the right boundary line; The specific process is as follows: (1) Define the intermediate parameter T 2min M ; (2) Let T 2min M = T 2min = 0.08 ms. At this time, T 2min from T 2min M to T 2min M the cumulative volume percentage = 0.002%; The judgment condition is not satisfied; According to Table 2, increase T 2min from T 2min M to T 2min M and take the value of 0.09 ms. At this time, T 2min M from T 2min to T 2min M the cumulative volume percentage = 0.005%, and the judgment condition is still not satisfied; Continue to increase T 2min M and take the value. When T 2min * takes the value of 1.08 ms, at this time, T 2min * = 1.08 ms.

[0029] Step 3: Using r T2 =( r min (pc) / T 2min * )× T 2 = 0.003333× T 2 , convert T 2 to nuclear magnetic resonance pore throat radius r T2 , the distribution data of the nuclear magnetic resonance pore throat radius r T2 is shown in Table 3; Table 3 Distribution data of nuclear magnetic resonance pore throat radius r T2 of the distribution data ; According to the data in Table 3, taking the nuclear magnetic resonance pore throat radius r T2 as the abscissa and the cumulative volume fraction corresponding to the nuclear magnetic resonance pore throat radius r T2 as the ordinate, draw the right boundary line, as shown in Figure 2 .

[0030] Step 4: Draw the left boundary line and the right boundary line on the same graph to obtain the rationality judgment chart, as shown in Figure 3 ; the distribution data of the target nuclear magnetic resonance pore throat radius obtained by other methods is shown in Table 4; Table 4 Distribution data of the target nuclear magnetic resonance pore throat radius ; According to the data in Table 4, draw it into the rationality judgment chart, as shown in Figure 4 .

[0031] Among them, the cumulative volume fraction distribution curve of the target nuclear magnetic resonance pore throat radius obtained by Method 1 is located on the left side of the left boundary line, so Method 1 is unreasonable; the cumulative volume fraction distribution curve of the target nuclear magnetic resonance pore throat radius obtained by Method 2 is not entirely located between the left boundary line and the right boundary line, so Method 2 is unreasonable; the cumulative volume fraction distribution curve of the target nuclear magnetic resonance pore throat radius obtained by Method 3 is entirely located between the left boundary line and the right boundary line, so Method 3 is reasonable; the cumulative volume fraction distribution curve of the target nuclear magnetic resonance pore throat radius obtained by Method 4 is located on the right side of the right boundary line, so Method 4 is unreasonable.

Claims

1. A method for judging the rationality of nuclear magnetic resonance pore throat radius distribution, characterized in that: Here’s how: by r (pc) The cumulative volume share distribution curve is the left boundary line, passing through r T2 =( r min (pc) / T 2min * )× T 2 The cumulative volume share distribution curve of the NMR pore throat radius obtained by conversion is the right boundary line; if the target NMR pore throat radius cumulative volume share distribution curve obtained by other methods is between the left boundary line and the right boundary line, it indicates that the target NMR pore throat radius cumulative volume share distribution curve is reasonable, otherwise it is unreasonable; in, r (pc) is the pore throat radius of high-pressure mercury injection, μm; r T2 is the NMR pore throat radius, μm; T 2 is the transverse relaxation time, ms; r min (pc) is the minimum pore throat radius of high-pressure mercury injection, μm; T 2min * is the lower limit of the transverse relaxation time corresponding to the minimum pore throat radius of high-pressure mercury injection, ms.

2. The method for judging the rationality of the nuclear magnetic resonance pore throat radius distribution according to claim 1, characterized in that: The specific process of obtaining the right boundary line is as follows: (1) Definition T 2min M ; (2) Construct judgment conditions: T 2min arrive T 2min M The cumulative volume share of ≥ r min (pc) Volume proportion; T 2min M The value is T 2min First, determine whether the judgment condition is met; If the judgment condition is met, the current T 2min M Take the value as T 2min * ; If not satisfied, increase T 2min M The value is re-judged to determine whether the judgment condition is met until the judgment condition is met. T 2min * ; (3) Utilization r T2 =( r min (pc) / T 2min * )× T 2. T 2Conversion to NMR pore throat radius r T2 , and then obtain the NMR pore throat radius cumulative volume distribution curve, which is used as the right boundary line; in, T 2min M is the intermediate parameter, ms; T 2min is the minimum transverse relaxation time, ms.

3. The method for judging the rationality of the nuclear magnetic resonance pore throat radius distribution according to claim 2, characterized in that: The specific process of obtaining the left boundary line is as follows: r (pc) Cumulative volume fraction distribution curve based on high pressure mercury injection pore throat radius r (pc) is the horizontal axis, r (pc) The corresponding cumulative volume percentage is the vertical axis.

4. The method for judging the rationality of the nuclear magnetic resonance pore throat radius distribution according to claim 3, characterized in that: Said r (pc) , r min (pc) and r min (pc) The volume fraction of is obtained through high-pressure mercury injection experiment.

5. The method for judging the rationality of the nuclear magnetic resonance pore throat radius distribution according to claim 4, characterized in that: Said T 2. T 2min Obtained through nuclear magnetic resonance experiments.

6. The method for judging the rationality of the distribution of nuclear magnetic resonance pore throat radius according to claim 5, characterized in that: The horizontal coordinate of the NMR pore throat radius cumulative volume share distribution curve is the NMR pore throat radius r T2 , with the NMR pore throat radius r T2 The corresponding cumulative volume percentage is the vertical axis.

7. The method for judging the rationality of the distribution of nuclear magnetic resonance pore throat radius according to claim 6, characterized in that: The increase T 2min M The value is obtained from the nuclear magnetic resonance experiment T 2 The distribution increases accordingly.

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