Method for determining CO2 oil displacement effect and pore change degree after CO2 displacement

Abstract

The invention discloses a method for determining the CO2 oil displacement effect and the pore change degree after CO2 oil displacement. The method for determining the CO2 oil displacement effect comprises the steps that the nuclear magnetic resonance T2 spectrum signal intensity sum S1 of shale is tested while simulated oil displacement is conducted on the shale after oil saturation; cO2 is injected into the shale subjected to simulated oil displacement, and the T2 spectrum signal intensity sum S2 of the shale in the CO2 displacement process is tested; the difference value of S1 and S2 is divided by S1 to obtain the CO2 oil displacement efficiency; the method for determining the pore change degree after CO2 flooding comprises the steps that secondary simulated oil displacement is conducted after CO2 flooding so as to test the T2 spectrum signal intensity sum S3 of shale, and the pore volume change degree after CO2 flooding is obtained by dividing the difference value of S3 and S1 by S1; the problem that certain limitation exists when the change degree of the shale pore structure is represented by a single porosity change method is effectively solved.

Description

Technical Field

[0001] The present invention relates to the field of oil and gas field development, and in particular to a method for simulating shale CO2 flooding effect and pore changes after CO2 flooding based on nuclear magnetic resonance. Background Art

[0002] At present, unconventional resources have become a replacement resource for the oil industry. Due to the extremely low porosity and permeability of shale, conventional water flooding, chemical flooding and other methods have certain limitations in the development of shale oil. The practice of shale oil development at home and abroad has confirmed that CO2 flooding is a very promising method for improving the recovery rate of shale oil. Since CO2 will undergo a series of physical and chemical reactions with the matrix or clay minerals in the shale, it often causes changes in the pore structure after huffing and puffing. Patent CN112304842A proposes to use the helium method to measure the porosity of shale after CO2 flooding and compare it with the initial porosity to obtain the degree of pore change in the core, but the result obtained can only be that the pores of the core increase or decrease. Due to the complex pore structure of shale, the pore change trends of different scales in shale are different. The pores are not a single decrease or increase. The pores of a certain scale may increase, and the pores of another scale may decrease. Therefore, the single method of characterizing the degree of change in the pore structure of shale by porosity change has certain limitations. Summary of the invention

[0003] In view of this, the present disclosure provides a method for determining the degree of pore change after CO2 flooding based on nuclear magnetic resonance, which solves the problem that the single method of characterizing the degree of change in shale pore structure by porosity change has certain limitations.

[0004] In addition, the present disclosure further provides a method for determining the effect of CO2 oil recovery based on nuclear magnetic resonance.

[0005] In a first aspect, the method for determining CO2 flooding efficiency comprises:

[0006] While performing simulated oil displacement on the oil-saturated shale, the total signal intensity S1 of the nuclear magnetic resonance T2 spectrum of the shale is tested;

[0007] Injecting CO2 into the shale after simulated oil displacement, and testing the total signal intensity S2 of the nuclear magnetic resonance T2 spectrum of the shale during the CO2 displacement process;

[0008] The CO2 recovery efficiency is obtained by dividing the difference between the S1 and the S2 by the S1.

[0009] In the present disclosure and possible embodiments, according to the shape of the T2 spectrum, pores with a T2 value less than or equal to 1 ms are classified as small pores, pores with a T2 value greater than 1 ms and less than or equal to 33 ms are classified as medium-large pores, and pores with a T2 value greater than 33 ms are classified as lamellae;

[0010] The signal amplitudes of the small pores, the medium-large pores and the lamina fractures are added together to obtain the sum of the signal amplitudes of the small pores, the medium-large pores and the lamina fractures, which are S 1小孔 , S 1中大孔 , S 1页理缝 , S 2小孔 , S 2中大孔 and S 2页理缝 ;

[0011] The CO2 oil recovery efficiency calculation method was used to obtain the small pore oil recovery efficiency, medium and large pore oil recovery efficiency and lamella fracture oil recovery efficiency respectively.

[0012] In the present disclosure and possible embodiments, during the simulated oil displacement process, the displacement rate is 0.05 ml / min, the confining pressure is always kept greater than the injection pressure by 2 MPa, and the nuclear magnetic resonance T2 spectrum of the shale is tested every 30 minutes until the total change amplitude of the nuclear magnetic resonance T2 spectrum signal intensity does not exceed 0.5%, and the simulated oil displacement is stopped to obtain the S1, S 1小孔 , S 1中大孔 and S 1页理缝 .

[0013] In the present disclosure and possible embodiments, during the CO2 displacement process, the displacement rate is 0.05 ml / min, the back pressure is set to the minimum miscible pressure value of shale oil and CO2, the confining pressure is kept always greater than the injection pressure by 2 MPa, and the nuclear magnetic resonance T2 spectrum of the shale is tested every 30 minutes after oil is seen at the core outlet, and the injection of CO2 is stopped when the total change amplitude of the T2 spectrum signal intensity does not exceed 0.5%, and the core pore pressure and confining pressure are reduced until the pore pressure drops to 0, and the S2, S 2小孔 , S 2中大孔 and S 2页理缝 .

[0014] In a second aspect, the method for determining the degree of pore change after CO2 flooding comprises:

[0015] While performing simulated oil displacement on the oil-saturated shale, the total signal intensity S1 of the nuclear magnetic resonance T2 spectrum of the shale is tested;

[0016] Performing CO2 displacement on the shale after the simulated oil displacement, performing a second simulated oil displacement on the shale after the CO2 displacement, and testing the total signal intensity S3 of the nuclear magnetic resonance T2 spectrum of the shale;

[0017] The difference between S3 and S1 divided by S1 gives the degree of change in pore volume after CO2 flooding.

[0018] In the present disclosure and possible embodiments, according to the shape of the T2 spectrum, pores with a T2 value less than or equal to 1 ms are classified as small pores, pores with a T2 value greater than 1 ms and less than or equal to 33 ms are classified as medium-large pores, and pores with a T2 value greater than 33 ms are classified as lamellae;

[0019] The signal amplitudes of different pores in the small pores, the medium-large pores and the lamellae are added respectively, and the sum of the signal amplitudes in the small pores, the medium-large pores and the lamellae is S 1小孔 , S 1中大孔 , S 1页理缝 , S 3小孔 , S 3中大孔 and S 3页理缝 ;

[0020] The pore volume change degree after CO2 flooding was calculated using the method to obtain the change degree of small pore volume, medium and large pore volume and lamellae fracture pore volume.

[0021] In the present disclosure and possible embodiments, after the shale is vacuumed for 8 hours after the CO2 displacement, a second simulated oil displacement is carried out, with a displacement rate of 0.05 ml / min, and the confining pressure is always kept greater than the injection pressure by 2 MPa. After oil is seen at the outlet, the nuclear magnetic resonance T2 spectrum of the shale is tested every 30 minutes until the total change amplitude of the nuclear magnetic resonance T2 spectrum signal intensity does not exceed 0.5%, and the second simulated oil displacement is stopped to obtain the S3, S 3小孔 , S 3中大孔 and S 3页理缝 .

[0022] The present disclosure has the following beneficial effects:

[0023] The present invention combines the nuclear magnetic resonance experimental technology with the gas injection displacement experimental technology. According to the change of the nuclear magnetic resonance T2 spectrum of the shale before and after the displacement, the CO2 oil displacement efficiency of different pores can be obtained. Moreover, as long as the shale after the displacement is re-saturated with shale oil and the nuclear magnetic resonance T2 spectrum after the saturation is tested, and compared with the nuclear magnetic resonance T2 spectrum of the oil-saturated shale before the displacement, the change of the pore volume of different scales of the shale after the displacement can be obtained according to the calculation formula of the pore change degree of different types of pores, thereby providing an experimental method for the study of the mechanism of improving the recovery rate of shale oil by CO2 injection, and providing basic parameters for the efficient development of shale oil. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The above and other objects, features and advantages of the present disclosure will become more apparent through the following description of the embodiments of the present disclosure with reference to the accompanying drawings, in which:

[0025] Figure 1 is a diagram of a device for simulating the effect of CO2 flooding of shale and the change of pore structure after CO2 flooding of shale in an embodiment of the present disclosure;

[0026] Figure 2 is a nuclear magnetic resonance T2 spectrum of the first simulated oil displacement of the oil-saturated shale of the embodiment of the present disclosure;

[0027] Figure 3 is the nuclear magnetic resonance T2 spectrum of shale after CO2 displacement in the embodiment of the present disclosure;

[0028] Figure 4 This is the nuclear magnetic resonance T2 spectrum of the second simulated oil flooding of shale after CO2 flooding in the embodiment of the present disclosure.

[0029] Figure 1 In the middle: 1 is an ISCO pump, 2 is a piston container filled with simulated oil, 3 is a piston container filled with CO2, 4 is a vacuum system, 5 and 13 are pressure gauges, 7 and 9 are magnets for nuclear magnetic resonance analyzer, 8 is a core holder, 10 is a signal acquisition and processing system for nuclear magnetic resonance analyzer, 11 is a high temperature and high pressure system, 12 is a back pressure valve, 14 is a hand pump, 15 is an oil and gas metering system, 16 is a temperature monitoring system, and V1-V7 are valves. DETAILED DESCRIPTION

[0030] The present disclosure is described below based on embodiments, but it is worth noting that the present disclosure is not limited to these embodiments. In the detailed description of the present disclosure below, some specific details are described in detail. However, for the parts that are not described in detail, those skilled in the art can also fully understand the present disclosure.

[0031] In addition, those skilled in the art should understand that the drawings are provided only to illustrate the purpose, features and advantages of the present disclosure, and the drawings are not actually drawn to scale. At the same time, unless the context clearly requires, the words "include", "comprise" and the like in the entire specification and claims should be interpreted as inclusive rather than exclusive or exhaustive; that is, "including but not limited to" meaning.

[0032] The technical concept of the method of the present invention is based on nuclear magnetic resonance experimental technology. The shale after CO2 displacement is saturated with simulated oil, and the nuclear magnetic resonance T2 spectrum of the shale after saturation with simulated oil is measured. The spectrum is compared with the nuclear magnetic resonance T2 spectrum of the oil-saturated shale before CO2 displacement to determine the degree of change of pores of different scales in the shale. At the same time, it is used to evaluate the CO2 flooding efficiency of pores of different scales in the shale.

[0033] According to the above technical concept, Figure 1 The simulation device shown in the figure is used to explain the method of the present invention in detail through the embodiment of the present disclosure. The specific steps are as follows:

[0034] (1) After drying the washed oil in shale, vacuum saturated shale oil is extracted, and the shale saturated with shale oil is placed in a core holder 8. Temperature and confining pressure are applied to the core holder 8 through a high temperature and high pressure system 11. The temperature is set to the formation temperature where the shale is located, and a temperature detection system 16 is used to monitor the temperature change in the confining pressure fluid in the core holder.

[0035] (2) Open valves V1, V3, V6 and V7, and slowly displace the simulated oil in the shale with the simulated oil in the piston container 2 at a rate of 0.05 ml / min, and keep the confining pressure always greater than the injection pressure by 2 MPa. Test the nuclear magnetic resonance T2 spectrum of the shale every 30 minutes until the total change in the nuclear magnetic resonance T2 spectrum signal intensity does not exceed 0.5%, and then stop the simulated oil displacement. At this time, the total nuclear magnetic resonance T2 spectrum signal intensity of the oil-saturated shale is S1.

[0036] According to the shape of T2 spectrum, pores with T2 value less than or equal to 1 ms are classified as small pores, pores with T2 value greater than 1 ms and less than or equal to 33 ms are classified as medium-large pores, and pores with T2 value greater than 33 ms are classified as lamellae.

[0037] By adding the signal amplitudes of different pores in the saturated shale pores, the total signal amplitude in the saturated shale pores can be obtained as S 1小孔 Similarly, the sum of the signal intensities of shale oil in macropores and lamination fractures in saturated shale is S 1中大孔 , S 1页理缝 ; See the attached NMR T2 spectrum of shale after simulated oil displacement Figure 2 .

[0038] (3) Close valves V1 and V3, open valves V2 and V4, and slowly inject CO2 into the core at a rate of 0.05 ml / min. At the same time, set the back pressure to the minimum miscible pressure value of shale oil and CO2, and always keep the core holder confining pressure greater than the injection pressure by 2 MPa. After oil is seen at the core outlet, test the core's nuclear magnetic resonance T2 spectrum every 30 minutes until the total change in the T2 spectrum signal intensity does not exceed 0.5%. Stop injecting CO2 and slowly reduce the core pore pressure and confining pressure until the pore pressure drops to 0. At this time, the sum of the core nuclear magnetic resonance T2 spectrum signal intensities is S2, and the sum of the signal intensities of shale oil in small pores, medium and large pores, and lamella fractures is S2, respectively. 2小孔 , S 2中大孔 , S 2页理缝 ; The NMR T2 spectrum of shale after CO2 displacement is attached. Figure 3 .

[0039] (4) Close valves V2, V4 and V7, open valve V5, and vacuum the shale for 8 hours, then close valve V5, set the back pressure value to the minimum miscible pressure value of CO2 and shale oil, open valves V1, V3 and V7, and slowly inject the shale oil in the piston container 2 into the core holder 8 at a rate of 0.05 ml / min. After oil is seen at the core outlet, measure the nuclear magnetic resonance T2 spectrum of the core every 30 minutes until the total signal intensity of the T2 spectrum changes by no more than 0.5%, and stop injecting shale oil. At this time, the total signal amplitude of the nuclear magnetic resonance T2 spectrum of the saturated shale is S3, and the total signal intensity of the shale oil in the small pores, medium and large pores and lamella fractures of the shale is S3, respectively. 3小孔 , S 3中大孔 , S 3页理缝 ; The NMR T2 spectrum of shale after the second simulated oil displacement is attached. Figure 4 .

[0040] (5) Calculate the CO2 flooding efficiency. The calculation formula for shale CO2 flooding efficiency is:

[0041] E 页岩 =(S1-S2) / S1*100 / 100 (1)

[0042] The calculation formula of small hole oil displacement efficiency is:

[0043] E 小孔 =(S 1小孔 -S 2小孔 ) / S 1小孔 *100 / 100 (2)

[0044] The calculation formula for medium and large pore oil displacement efficiency is:

[0045] E 中大孔 =(S 1中大孔 -S 2中大孔 ) / S 1中大孔 *100 / 100 (3)

[0046] The calculation formula of the oil displacement efficiency of lamellar fractures is:

[0047] E 页理缝 =(S 1页理缝 -S 2页理缝 ) / S 1页理缝 *100 / 100 (4)

[0048] The sum of signal strengths S1, S2 in formula (1) to (4) is 1小孔 , S 1中大孔 , S 1页理缝 , S2, S 2小孔 , S 2中大孔 , S 2页理缝 Is a dimensionless quantity.

[0049] according to Figure 2-Figure 4 The T2 spectrum changes of shale before and after CO2 displacement are shown. The oil recovery efficiencies of small pores, medium and large pores, lamellae and shale are calculated by equations (1) to (4) to be 17.07%, 78.72%, 87.38% and 50.32%, respectively.

[0050] (6) Calculation of the degree of change in shale pore volume: The degree of change in pore volume is calculated based on the T2 spectrum signal intensity of the shale oil saturated twice before and after CO2 displacement.

[0051] The calculation formula for the change degree of shale pore volume is:

[0052]

[0053] The calculation formula for the change in the pore volume is:

[0054]

[0055] The calculation formula for the change of the pore volume of medium and large pores is:

[0056]

[0057] The calculation formula for the change in the volume of laminar fractures is:

[0058]

[0059] The sum of signal strengths S3 and S in formula (5) to (8) is 3小孔 , S 3中大孔 , S 3页理缝 Is a dimensionless quantity.

[0060] according to Figure 2-Figure 4 The T2 spectrum changes of shale before and after CO2 displacement are shown in Figure 1. According to equations (5) to (8), the changes in the pore volume of small pores, medium and large pores, lamellae and shale are -9.71%, -5.68%, -64.77% and -12.38%, respectively. The change in pore volume is negative, indicating that the pore volume of shale has become smaller after CO2 displacement. The change in lamellae is the largest, followed by small pores.

[0061] The above-described embodiments are only embodiments of the present disclosure, and the descriptions thereof are relatively specific and detailed, but they cannot be construed as limiting the scope of the present disclosure. It should be noted that, for a person of ordinary skill in the art, without departing from the concept of the present disclosure, several variations, equivalent substitutions, improvements, etc. may be made, all of which fall within the scope of protection of the present disclosure. Therefore, the scope of protection of the present disclosure shall be subject to the attached claims.

Claims

1. A method for determining CO2 flooding efficiency, characterized in that: include: While performing simulated oil displacement on the oil-saturated shale, the total signal intensity S1 of the nuclear magnetic resonance T2 spectrum of the shale is tested; Injecting CO2 into the shale after simulated oil displacement, and testing the total signal intensity S2 of the nuclear magnetic resonance T2 spectrum of the shale during the CO2 displacement process; The CO2 recovery efficiency is obtained by dividing the difference between the S1 and the S2 by the S1.

2. The method for determining the CO2 flooding efficiency according to claim 1, characterized in that: According to the shape of the T2 spectrum, pores with a T2 value of less than or equal to 1 ms are classified as small pores, pores with a T2 value of greater than 1 ms and less than or equal to 33 ms are classified as medium-large pores, and pores with a T2 value of greater than 33 ms are classified as lamellae; The signal amplitudes of the small pores, the medium-large pores and the lamina fractures are added together to obtain the sum of the signal amplitudes of the small pores, the medium-large pores and the lamina fractures, which are S 1小孔 , S 1中大孔 , S 1页理缝 , S 2小孔 , S 2中大孔 and S 2页理缝 ; The CO2 oil recovery efficiency calculation method was used to obtain the small pore oil recovery efficiency, medium and large pore oil recovery efficiency and lamella fracture oil recovery efficiency respectively.

3. The method for determining the CO2 flooding efficiency according to claim 1 or 2, characterized in that: During the simulated oil displacement process, the displacement rate is 0.05 ml / min, the confining pressure is always kept greater than the injection pressure by 2 MPa, and the nuclear magnetic resonance T2 spectrum of the shale is tested every 30 minutes until the total change amplitude of the nuclear magnetic resonance T2 spectrum signal intensity does not exceed 0.5%, and the simulated oil displacement is stopped to obtain the S1, S 1小孔 , S 1中大孔 and S 1页理缝 .

4. The method for determining the CO2 flooding efficiency according to claim 3, characterized in that: During the CO2 displacement process, the displacement rate is 0.05 ml / min, the back pressure is set to the minimum miscible pressure value of shale oil and CO2, the confining pressure is always kept greater than the injection pressure by 2 MPa, and the nuclear magnetic resonance T2 spectrum of the shale is tested every 30 minutes after oil is seen at the core outlet, and the injection of CO2 is stopped when the total change amplitude of the T2 spectrum signal intensity does not exceed 0.5%, and the core pore pressure and confining pressure are reduced until the pore pressure drops to 0, and the S2 and S 2小孔 , S 2中大孔 and S 2页理缝 .

5. A method for determining the degree of pore change after CO2 flooding based on the method for determining CO2 flooding efficiency according to any one of claims 1 to 4, characterized in that: include: While performing simulated oil displacement on the oil-saturated shale, the total signal intensity S1 of the nuclear magnetic resonance T2 spectrum of the shale is tested; Performing CO2 displacement on the shale after the simulated oil displacement, performing a second simulated oil displacement on the shale after the CO2 displacement, and testing the total signal intensity S3 of the nuclear magnetic resonance T2 spectrum of the shale; The difference between S3 and S1 divided by S1 gives the degree of change in pore volume after CO2 flooding.

6. The method for determining the degree of pore change after CO2 flooding according to claim 5, characterized in that: According to the shape of the T2 spectrum, pores with a T2 value of less than or equal to 1 ms are classified as small pores, pores with a T2 value of greater than 1 ms and less than or equal to 33 ms are classified as medium-large pores, and pores with a T2 value of greater than 33 ms are classified as lamellae; The signal amplitudes of different pores in the small pores, the medium-large pores and the lamellae are added respectively, and the sum of the signal amplitudes in the small pores, the medium-large pores and the lamellae is S 1小孔 , S 1中大孔 , S 1页理缝 , S 3小孔 , S 3中大孔 and S 3页理缝 ; The pore volume change degree after CO2 flooding was calculated using the method to obtain the change degree of small pore volume, medium and large pore volume and lamellae fracture pore volume.

7. The method for determining the degree of pore change after CO2 flooding according to claim 6, characterized in that: After the shale was vacuumed for 8 hours after the CO2 displacement, a second simulated oil displacement was carried out, with a displacement rate of 0.05 ml / min, and the confining pressure was always kept greater than the injection pressure by 2 MPa. After oil was seen at the outlet, the nuclear magnetic resonance T2 spectrum of the shale was tested every 30 minutes until the total change amplitude of the nuclear magnetic resonance T2 spectrum signal intensity did not exceed 0.5%, and the second simulated oil displacement was stopped to obtain the S3, S 3小孔 , S 3中大孔 and S 3页理 Sew.

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