A calculation method for cementation index of vuggy carbonate rocks based on ball-and-tube model

Abstract

The invention discloses a calculation method for the cementation index of a porous carbonate rock based on a ball-and-tube model, and the steps are as follows: S1, performing nuclear magnetic resonance analysis and porosity testing on carbonate rock to obtain a nuclear magnetic resonance T2 spectrum and gas-measured porosity; S2, taking the relaxation time corresponding to the trough between the double peaks as a dividing line, dividing the nuclear magnetic resonance T2 spectrum into two parts, a short relaxation time and a long relaxation time, wherein the short relaxation time part reflects the pore characteristics, and the long relaxation time part reflects the cave characteristics, and determining the ratio of the pore radius to the cave radius based on the characteristics of the two parts; S3, calculating the equivalent cave radius through an iterative equation; S4, substituting the equivalent cave radius calculated in step S3 into a formula to calculate a formation factor F; S5, calculating the cementation index m according to the calculated formation factor and the gas-measured porosity e The cementation index calculation method of the present invention effectively improves the calculation accuracy of the cementation index of porous carbonate rocks.

Description

Technical Field

[0001] The invention relates to the technical field of oil and gas field exploitation, in particular to a calculation method for a cementation index of a vug-type carbonate rock based on a ball-and-tube model. Background Art

[0002] The cementation index m is an important parameter for calculating the hydrocarbon / water saturation of heterogeneous carbonate reservoirs. It is related to the accuracy of reserve calculation based on the hydrocarbon saturation of the reservoir section of the exploration well. The commonly used method for calculating the cementation index m is the Archie formula method. Draw a cross plot of porosity and formation factors (resistivity of saturated core divided by resistivity of saturated fluid in the pores), use a power exponential function to fit, the regular line is linear under double logarithmic coordinates, and the slope of the trend line is the cementation index. For example Figure 1 The cementation index m obtained according to Archie's formula is 2.39.

[0003] Caves are well developed in vug-type carbonate reservoirs, and the difference between the pores and the equivalent cave radius is large, resulting in complex pore morphology and strong heterogeneity inside the core. The cementation index is no longer a constant, which makes conventional rock-electricity laws such as Archie's formula inapplicable and the rock-electricity relationship of carbonate rocks difficult to characterize. In other words, due to the influence of caves, the core coefficient a and cementation index m obtained by the fitting line are often unreasonable, that is, the results obtained have no physical meaning. Summary of the invention

[0004] Aiming at the problem that the existing Archie formula method is not suitable for calculating the cementation index of porous carbonate rocks, the present invention provides a method for calculating the cementation index of porous carbonate rocks based on a ball-and-tube model.

[0005] The method for calculating the cementation index of vug-type carbonate rock based on the ball-and-tube model provided by the present invention comprises the following steps:

[0006] S1. Carry out nuclear magnetic resonance analysis and porosity test on carbonate rock to obtain nuclear magnetic resonance T2 spectrum and gas measurement porosity φ.

[0007] S2, the relaxation time corresponding to the trough between the double peaks in the NMR T2 spectrum is used as the dividing line to divide the NMR T2 spectrum into two parts: short relaxation time and long relaxation time. The short relaxation time part reflects the pore characteristics, and the long relaxation time part reflects the cave characteristics. The ratio of the peak transverse relaxation time of the NMR spectrum corresponding to the pore and the cave is the pore-cavity radius ratio RR;

[0008] S3. Calculate the equivalent cave radius r by iterative equation s :The iterative equation of the equivalent cave radius is as follows:

[0009]

[0010] in, B=πτRR 2 ,

[0011] Where, φ is the gas-measured porosity;

[0012] RR is the pore-cavity radius ratio.

[0013] L m is the equivalent pore path length; L is the distance between the ends of the equivalent pore path; τ is the ratio of the equivalent pore path length to the distance between the ends of the equivalent pore path, L m / L;

[0014] The iteration termination condition is |r s+1 -r s |<0.001, and finally calculate the equivalent cave radius r s ;

[0015] S4, calculate the formation factor F, and convert the equivalent cave radius r calculated in step S3 into s Substitute it into the following formula to calculate the formation factor F:

[0016]

[0017] S5. Substitute the formation factor F and gas porosity φ calculated in the previous step into the following formula to calculate the cementation index m e :

[0018] m e =-lgF / lgφ.

[0019] S6, cementation index m e Regional experience correction; the specific method is to use the cementation index m calculated in step S5 e The cross plot between the measured cementation index m, the horizontal axis is the cementation index m e , the vertical axis is the measured cementation index m; the fitting formula is the regional empirical correction formula for the cementation index, and the formula is y=2.75x-3.32.

[0020] Preferably, in step S1, multiple pore-type carbonate rock core samples in a certain area are selected, and nuclear magnetic resonance analysis and porosity test are performed on each core sample. The core sample is a cylindrical carbonate rock core made by cutting and polishing. In step S2, the transverse relaxation time of the short relaxation characteristic peak and the long relaxation characteristic peak in the nuclear magnetic resonance T2 spectrum of each carbonate rock sample is analyzed; the transverse relaxation time average x of all short relaxation characteristic peaks is calculated, and the transverse relaxation time average y of all long relaxation characteristic peaks is calculated, and the pore-cavern radius ratio RR=x / y.

[0021] Compared with the prior art, the present invention is beneficial in that:

[0022] The present invention combines nuclear magnetic resonance experimental data and gas porosity data to provide an efficient calculation method for the cementation index of porous carbonate rocks based on the ball-and-tube model. The method overcomes the problem that the conventional Archie formula method is not applicable to porous carbonate reservoirs and the rock-electric relationship of carbonate rocks is difficult to characterize. The method effectively improves the calculation accuracy of the cementation index m of porous carbonate rocks.

[0023] Other advantages, objectives and features of the present invention will be embodied in part through the following description, and in part will be understood by those skilled in the art through study and practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Crossplot of porosity and stratigraphic factors.

[0025] Figure 2 NMR T2 spectrum.

[0026] Figure 3 MQ group NMR T2 spectrum.

[0027] Figure 4 Schematic diagram of the ball-and-tube model.

[0028] Figure 5 Relative error diagram of fitting calculation of cementation index. DETAILED DESCRIPTION

[0029] The preferred embodiments of the present invention are described below in conjunction with the accompanying drawings. It should be understood that the preferred embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

[0030] The calculation method of the cementation index of vug-type carbonate rock based on the ball-and-tube model provided by the present invention mainly comprises the following steps:

[0031] (1) Nuclear magnetic resonance analysis and porosity testing were performed on the core samples of the vug-type carbonate rock to obtain the nuclear magnetic resonance T2 spectrum and gas-measured porosity φ.

[0032] (2) Determine the ratio RR of the core pore radius to the equivalent cave radius. The distribution of the nuclear magnetic resonance T2 spectrum (transverse relaxation time spectrum) directly reflects the pore structure characteristics of the core. The rock pore structure can be obtained by measuring the amplitude of the nuclear magnetic resonance relaxation signal of the hydrogen nuclei in the rock pore fluid. The relationship between the transverse relaxation time T2 and the pore volume is as follows:

[0033]

[0034] Where T2 is the transverse relaxation time, ms; ρ is the surface relaxation rate, μm / s; S is the core surface area, cm 2 ; V is the pore volume, cm 3If the pore structure is simplified to a columnar or spherical shape, the relationship between the transverse relaxation time and the pore radius can be obtained:

[0035]

[0036] In the formula, F s is the pore shape factor; R is the pore radius, cm.

[0037] From formula (2), it can be seen that the ratio of the pore radius to the equivalent cave radius is obtained by the ratio of the nuclear magnetic resonance T2 relaxation time corresponding to the pore and the cave, that is, by the nuclear magnetic resonance T2 spectrum characteristics.

[0038] Since the morphology of NMR T2 spectrum varies greatly in reservoirs with different pore structures, the T2 spectrum is divided into two parts at the first wave point of the main peak to characterize the pore structure, namely pores and caves. Figure 2 ,in accordance with Figure 2 The ratio of the peak transverse relaxation times of the NMR spectra corresponding to the cave and pore is the pore-cavity radius ratio RR.

[0039] Take the carbonate rock in a certain area (MQ Group) as an example. There are 13 core test data in total. The cores are numbered in sequence: T1-1, T1-2, T1-3, T1-4, T1-5, T1-6, T1-7, T1-8, T1-9, T1-10, T1-11, T1-12, T1-13. The pore distribution characteristics of all carbonate rock samples are obtained through nuclear magnetic resonance experiments ( Figure 3 ). First, the nuclear magnetic resonance T2 spectrum of the reservoir core is converted into a pore distribution curve, that is, it is divided into two parts: pores and caves. Based on the nuclear magnetic resonance theory, the ratio of the core pore radius to the equivalent cave radius is determined, that is, the corresponding short relaxation characteristic peak and long relaxation characteristic peak in the nuclear magnetic resonance T2 spectrum of each carbonate rock sample are analyzed; the average value of all short relaxation characteristic peaks is calculated as x=60ms, and the average value of all long relaxation characteristic peaks is calculated as y=1000ms, then the pore-cavity radius ratio RR=x / y=0.06.

[0040] like Figure 4 As shown in Figure 1, according to the ball-and-tube model, the pore space inside the rock can be represented as a combination of curved pores and caves. The different geometric shapes of the pore space can be described by changing the tortuosity τ or the pore-cavity radius ratio RR. The pore-cavity radius ratio RR (pore radius r c and the equivalent cave radius r s The ratio of) and τ can be obtained by the following formulas:

[0041] RR=r c / r s (3)

[0042] τ=L m / L (4)

[0043] Where, L m is the actual length of the equivalent pore path; L is the straight-line distance between the ends of the equivalent pore path. Since the tortuosity parameter is difficult to obtain from experiments, it is taken as 1 in the calculation.

[0044] (3) Calculation of equivalent cave radius r s

[0045] Based on nuclear magnetic resonance experiments, the pore-cavity radius ratio RR is expressed as the ratio of the transverse relaxation time. Combined with rock sample data, assuming that the volume of the rock is unit 1, the porosity is equal to the total volume of the pore space, which can be expressed as the sum of the volume of a sphere and the volume of a curved cylinder. The calculation formula is as follows:

[0046]

[0047] Relative to the equivalent cave radius, the core porosity φ has been obtained through gas logging experiments in step (1), so the porosity needs to be considered as known, and the equivalent cave radius of the core is solved by the iteration method according to equation (5). First, the formula (5) is sorted out and the porosity is written in the form of an equation for the equivalent cave radius:

[0048]

[0049]

[0050] In the formula, B=πτRR 2

[0051] Since it is currently impossible to accurately obtain the equivalent cave radius of the core, the core porosity can be accurately obtained through gas logging experiments relative to the equivalent cave radius. Therefore, the porosity needs to be considered as known, and then according to the Newton iteration method, the iterative equation for solving the equivalent cave radius can be obtained as follows:

[0052]

[0053] In the formula, r s+1 It is the s+1th approximation of the equivalent cave radius;

[0054] The iteration termination condition is |r s+1 -r s |<0.001.

[0055] (4) Calculate the formation factor F. Based on the ball-and-tube model, the calculation formula for the formation factor F is:

[0056]

[0057] The equivalent cave radius r s , tortuosity, and pore-cavity radius ratio are substituted into the formation factor calculation formula (9) to complete the calculation of the formation factor F.

[0058] (5) Substitute the formation factor F and gas porosity φ calculated in the previous step into formula (10) to calculate the cementation index m e :

[0059] m e =-lgF / lgφ (10)

[0060] (6) Cementation index m e Regional experience correction.

[0061] Since the tortuosity parameter in the tube model is difficult to obtain from experiments, this method assumes that it is 1 during calculation, so there is a deviation and regional experience correction is required. The specific method is as follows: Figure 5 As shown in step S5, the cementation index m is calculated. e The cross plot between the measured cementation index m, the horizontal axis is the cementation index m e , the vertical axis is the measured cementation index m; Figure 5 The formula obtained by fitting is the regional empirical correction formula for the cementation index, and the formula is y=2.75x-3.32.

[0062] Table 1 is a comparison of the measured cementation index of each core of the MQ Group reservoir, the cementation index predicted by the Archie formula method, and the patent method. Table 1 shows that for the MQ Group reservoir, the average relative error of the Archie formula method is 10.27%, and the m calculated by the patent method is e The average relative error is 5.66%. The method of the present invention effectively improves the calculation accuracy of the cementation index m of the porous carbonate rock.

[0063] Table 1 Comparison of the measured cementation index of each core of the MQ Formation reservoir and the calculation results of the two existing methods

[0064]

[0065] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. Although the present invention has been disclosed as a preferred embodiment as above, it is not used to limit the present invention. Any technician familiar with this profession can make some changes or modifications to equivalent embodiments of equivalent changes using the technical contents disclosed above without departing from the scope of the technical solution of the present invention. However, any simple modification, equivalent change and modification made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A method for calculating the cementation index of vug-type carbonate rocks based on a ball-and-tube model, characterized in that: Here are the steps: S1. Carry out nuclear magnetic resonance analysis and porosity test on carbonate rocks to obtain nuclear magnetic resonance T 2 Spectrum and gas porosity ; S2. Nuclear Magnetic Resonance T 2 The relaxation time corresponding to the trough between the double peaks is used as the dividing line, and the NMR T 2 The spectrum is divided into two parts: short relaxation time and long relaxation time. The short relaxation time part reflects the pore characteristics, and the long relaxation time part reflects the cave characteristics. The ratio of the peak transverse relaxation time of the NMR spectrum corresponding to the pore and the cave is the pore-cavity radius ratio. RR; S3. Calculate the equivalent cave radius through iterative equation :The iterative equation of the equivalent cave radius is as follows: in, In the formula, is the gas-measured porosity; RR is the pore-cavity radius ratio; is the equivalent cave radius s +1 approximation; L m / L , L m is the equivalent pore path length; L is the distance between the ends of the equivalent pore paths; The iteration termination condition is , and finally calculate the equivalent cave radius ; S4, calculate the formation factor F, and convert the equivalent cave radius calculated in step S3 into Substitute into the following formula to calculate the formation factor : S5. The formation factors calculated in the above steps are Porosity measurement Substitute the following formula to calculate the cementation index m e : 。 2. The method for calculating the cementation index of vug-type carbonate rock based on the ball-and-tube model according to claim 1, characterized in that: Also includes step S6, cementation index m e Regional experience correction; the specific method is to use the cementation index calculated in step S5 m e The measured cementation index m The cross-plot between the horizontal axis and the cementation index m e , the vertical axis is the measured cementation index m ; The fitted formula is the regional empirical correction formula for cementation index.

3. The method for calculating the cementation index of vug-type carbonate rock based on the ball-and-tube model according to claim 1, characterized in that: In step S1, a plurality of vug-type carbonate rock core samples in a certain area are selected, and nuclear magnetic resonance analysis and porosity test are performed on each core sample.

4. The method for calculating the cementation index of vug-type carbonate rock based on the ball-and-tube model according to claim 3, characterized in that: In step S1, the core sample is a cylindrical carbonate core made by cutting and grinding.

5. The method for calculating the cementation index of vug-type carbonate rock based on the ball-and-tube model according to claim 4, characterized in that: In step S2, the NMR of each carbonate rock sample is analyzed. T 2 Transverse relaxation time corresponding to the short relaxation characteristic peak and long relaxation characteristic peak in the spectrum; Calculate the average transverse relaxation time of all short relaxation feature peaks x , calculate the average transverse relaxation time of all long relaxation characteristic peaks y , pore-cavity radius ratio RR = x / y .

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