Characterization method for the sealing performance of mudstone caprock considering sandstone interlayer

Abstract

The present invention relates to a method for characterizing the sealing performance of mudstone caprock considering sandstone interlayers, and the method is as follows: Step 1: Identify all sandstone interlayers in the caprock from the logging curves of regional production wells, and number the mudstone caprock sections and sandstone interlayer sections separately from bottom to top; Step 2: Read the average acoustic travel time and thickness h<subgt;ij< / subgt; corresponding to each sandstone interlayer section, and calculate the displacement pressure P<subgt;ij< / subgt> of each sandstone interlayer section respectively; Step 3: Segmentally fit the acoustic travel time curve of each mudstone caprock section, calculate the average acoustic travel time and thickness h<subgt;ck< / subgt> of each mudstone caprock section, and calculate the displacement pressure P<subgt;ck< / subgt> of each mudstone caprock section respectively; Step 4: Calculate the overall displacement pressure P of the caprock; In the process of evaluating the sealing property of the mudstone caprock, the present invention takes into account the actual geological conditions of the sandstone interlayers developed in the mudstone caprock. On the other hand, it is simpler and more convenient to characterize the sealing performance of the mudstone caprock considering sandstone interlayers by the present invention.

Description

Technical Field

[0001] The present invention belongs to the technical field of oil exploitation, and relates to a method for quantitatively characterizing the sealing performance of a mudstone caprock with a sandstone interlayer above an oil reservoir formation. Background Art

[0002] During the process of CO2 flooding and sequestration, along with the lateral migration of CO2 within the reservoir, the formation pressure increases accordingly. At the same time, CO2 migrates upward and gradually accumulates at the bottom of the caprock. Therefore, the overlying caprock is an important barrier to prevent CO2 from escaping upward and ensure the safety of CO2 sequestration. Thus, it is particularly important to quantitatively evaluate the sealing property of the overlying caprock.

[0003] The oil reservoirs in the Ordos Basin are in a continental sedimentary environment, and the formation characteristics of interbedded mud and sand are prominent. During the process of CO2 flooding and sequestration, if there is a sandstone interlayer in the overlying mudstone caprock, after a large amount of CO2 is injected into the reservoir, CO2 will enter the sand body interlayer and migrate laterally after breaking through the first caprock upward, resulting in that CO2 will not rapidly accumulate at the bottom of the second caprock, which has a certain "pressure relief" effect on the breakthrough of CO2 through the caprock. This effect actually increases the pressure for CO2 to break through the subsequent caprock and improves the sealing ability of the overall overlying caprock. If there are more sandstone interlayers in the overall overlying caprock, its overall sealing ability will be stronger.

[0004] Currently, the quantitative characterization of caprock sealing property mainly uses the displacement pressure as the evaluation index, which is mostly obtained through experimental methods such as core displacement and mercury injection tests or theoretical formula calculations. Since the core scale used in experimental tests is very limited, it is difficult to accurately simulate the formation conditions with sandstone interlayers in the actual mudstone caprock. The displacement pressure of the caprock obtained by this type of method actually only represents the displacement pressure of the local mudstone caprock and is difficult to reflect the true sealing ability of the caprock. The displacement pressure of the caprock obtained by the theoretical formula method regards the caprock as an integral mudstone and does not carefully consider the situation of interbedded mud and sand in the caprock. Summary of the Invention

[0005] Aiming at the above problems, the present invention proposes a method for characterizing the sealing performance of a mudstone caprock considering sandstone interlayers. First, the displacement pressures of the mudstone part and the sandstone interlayers in the overlying caprock are evaluated section by section, and then an overall displacement pressure parameter is established to characterize the sealing ability of the overlying caprock.

[0006] The technical solution of the present invention is as follows:

[0007] A method for characterizing the sealing performance of a mudstone caprock considering sandstone interlayers is as follows:

[0008] Step 1: Identify all sandstone interlayers in the caprock from the logging curves of regional production wells, and number the mudstone caprock section and the sandstone interlayer section separately from bottom to top;

[0009] Step 2: Read the average acoustic time difference corresponding to each sandstone interlayer section and the thickness h ij , and calculate the displacement pressure P of each sandstone interlayer section ij ;

[0010] Step 3: Segmentally fit the acoustic time difference curve of the caprock of each mudstone section, and calculate the average acoustic time difference of the caprock of each mudstone section and the thickness h ck , and calculate the displacement pressure P of the caprock of each mudstone section ck ;

[0011] Step 4: Calculate the overall displacement pressure P of the caprock;

[0012] If the displacement pressure of the caprock of a certain mudstone section obtained by calculation is higher than the sum of the displacement pressures of the sandstone interlayer section below it and the caprock of the adjacent mudstone section below the sandstone interlayer section, then the displacement pressure of the caprock composed of all the strata below the caprock of this mudstone section is the displacement pressure of the caprock of this mudstone section;

[0013] Conversely, the displacement pressure of the caprock composed of all the strata below the caprock of this mudstone section is the sum of the displacement pressures of the sandstone interlayer section below the caprock of this mudstone section and the caprock of the adjacent mudstone section below it.

[0014] Among them, the specific calculation process of the said Step 2 is:

[0015] (3)

[0016] (2)

[0017] In the formula: f ijmax (x) is the maximum value of the acoustic time difference function of a certain sandstone interlayer section, μs / m;

[0018] f ijmin (x) is the minimum value of the acoustic time difference function of this sandstone interlayer section, μs / m;

[0019] h ij is the thickness of this sandstone interlayer, m;

[0020] j is the number of this sandstone interlayer section, dimensionless;

[0021] is the average acoustic time difference of this sandstone interlayer section, μs / m;

[0022] P ij is the displacement pressure of this sandstone interlayer section, MPa.

[0023] Among them, the specific calculation process of the said Step 3 is:

[0024] (6)

[0025] (5)

[0026] (4)

[0027] In the formula: k is the serial number of the caprock in a certain mudstone section, dimensionless;

[0028] m is the serial number of the acoustic travel time function corresponding to a certain section in the caprock of this mudstone section, dimensionless;

[0029] Δh ckm is the mudstone thickness of the caprock of this mudstone section, m;

[0030] h ck is the thickness of the entire mudstone caprock between two sandstone interlayers, m;

[0031] x ck is the formation depth value of the caprock of this mudstone section, m;

[0032] f ckm (x) is the acoustic travel time function of the caprock of this mudstone section, μs / m;

[0033] is the average acoustic travel time of the caprock of this mudstone section, μs / m;

[0034] P ck is the displacement pressure of the caprock of this mudstone section, MPa.

[0035] Among them, the specific calculation process of step 4 is as follows:

[0036] (1)

[0037] In the formula: P is the overall displacement pressure, MPa.

[0038] The technical effect of the present invention is that:

[0039] In the process of evaluating the sealing property of the mudstone caprock, the present invention takes into account the actual geological conditions of the sandstone interlayers developed in the mudstone caprock. On the other hand, it is simpler and more convenient to characterize the sealing performance of the mudstone caprock considering the sandstone interlayers through the present invention. Description of the Drawings

[0040] Figure 1 It is the logging curve of the Chang 4+5 caprock in the Ordos Basin in the specific embodiment of the present invention.

[0041] Figure 2 and Figure 3 are both Figure 1 local enlarged views of. Specific Embodiment

[0042] A method for characterizing the sealing performance of mudstone caprock considering sandstone interlayers is as follows.

[0043] Step 1: Identify the mudstone caprock and all its sandstone interlayers on the logging curves of regional production wells, and number each mudstone caprock section and sandstone interlayer section from bottom to top as k = 1, 2, 3, 4 and j = 1, 2, 3 respectively.

[0044] Step 2: Sequentially read the thickness, the maximum value of the acoustic travel time function, and the minimum value of the acoustic travel time function corresponding to the 3 sandstone interlayer sections, and calculate the displacement pressures corresponding to the 3 sandstone interlayer sections according to formula (2) and formula (3) respectively;

[0045] Taking the sandstone interlayer sections numbered 1 and 2 as an example;

[0046] Sandstone interlayer section 1: The thickness is 1.5 m, the maximum and minimum values of the acoustic travel time are 281.5 μs / m and 228.9 μs / m respectively, and the displacement pressure P i1 is 1.51 MPa;

[0047] Sandstone interlayer section 2: The thickness is 1.5 m, the maximum and minimum values of the acoustic travel time are 317.4 μs / m and 237.2 μs / m respectively, and the displacement pressure P i2 is 1.46 MPa.

[0048] Step 3: Piecewise fit the acoustic travel time function of each mudstone caprock section into the function f ckm (x), m = 1, 2, …… 5; Calculate the acoustic travel time of each mudstone caprock section according to formula (5) and formula (6) respectively;

[0049] Taking the mudstone caprock sections numbered 1 and 2 as an example;

[0050] Mudstone caprock section 1: Divided into 3 small sections of mudstone; the thicknesses of each small section of mudstone are 4.25 m, 1.75 m and 2.25 m respectively, and the corresponding acoustic travel time functions are as follows:

[0051] ;

[0052] ;

[0053] ;

[0054] The calculated average acoustic travel time is 230.8 μs / m;

[0055] Mudstone section caprock 2: It is divided into 5 small sections of mudstone; the thicknesses of each small section of mudstone are 2.75 m, 1 m, 1.75 m, 2.5 m, and 1.75 m respectively, and the corresponding acoustic time difference functions are as follows:

[0056] ;

[0057] ;

[0058] ;

[0059] ;

[0060] ;

[0061] The average acoustic time difference is calculated to be 223.2 μs / m;

[0062] According to formula (4), the displacement pressures of each small section of mudstone are calculated to be P c1 = 2.82 MPa, P c2 = 2.71 MPa, P c3 = 2.91 MPa, P c4 = 3.07 MPa.

[0063] Step 4: Compare the displacement pressure values of each mudstone section caprock with the sandstone interlayer section below it and the adjacent mudstone section caprock below the sandstone interlayer section in sequence from bottom to top according to formula (1), and the displacement pressure of the Chang 4+5 caprock in the Ordos Basin is calculated to be 7.96 MPa;

[0064] Taking the mudstone caprocks numbered 1 and 2 and the sandstone interlayer section between them as an example, since P c1 + P i1 > P c2 , the displacement pressure below the mudstone section caprock numbered 2 is the sum of the displacement pressures of the mudstone section caprock numbered 1 and the sandstone interlayer section numbered 1, which is 4.33 MPa, and the displacement pressure of the caprock below the mudstone section numbered 3 is P c1 + P i1 + P i2 , which is 5.79 MPa.

Claims

1. A method for characterizing the sealing performance of a mudstone caprock considering a sandstone interlayer, characterized in that: The method is as follows: Step 1: Identify all sandstone interlayers in the caprock from the well logging curves of the regional production wells, and number the mudstone caprock and the sandstone interlayer sections separately from bottom to top; Step 2: Read the average acoustic travel time corresponding to each sandstone interbed section and the thickness h ij , and calculate the displacement pressure P of each sandstone interbed section respectively ij ; Step 3: Segmentally fit the acoustic time difference curves of each mudstone section caprock, and calculate the average acoustic time difference of each mudstone section caprock and the thickness h ck , and calculate the displacement pressure P of each mudstone section caprock respectively ck ; Step 4: Calculate the overall displacement pressure P of the caprock; If the displacement pressure of a certain mudstone caprock section calculated is higher than the sum of the displacement pressures of the sandstone interlayer section below it and the mudstone caprock section adjacent to the sandstone interlayer section below, then the displacement pressure of the caprock composed of all the strata below this mudstone caprock section is the displacement pressure of this mudstone caprock section; Otherwise, the displacement pressure of the caprock composed of all the strata below this mudstone caprock section is the sum of the displacement pressures of the sandstone interlayer section below this mudstone caprock section and the mudstone caprock section adjacent to it below.

2. The method for characterizing the sealing performance of mudstone caprock considering sandstone interlayers according to claim 1, wherein: The specific calculation process of Step 2 is as follows: （3） （2） where: f ijmax (x) is the maximum value of the acoustic travel-time difference function for a certain sandstone interbed section, μs / m; f ijmin (x) is the minimum value of the acoustic time difference function of this sandstone interbed section, μs / m; h ij is the thickness of the sandstone interbed, m; j is the number of this sandstone interlayer section, dimensionless; is the average acoustic transit time of this sandstone interbed section, μs / m; P ij is the displacement pressure of this sandstone interbed section, MPa.

3. The method for characterizing the sealing performance of mudstone caprock considering sandstone interlayers according to claim 2, characterized in that: The specific calculation process of Step 3 is as follows: （6） （5） （4） In the formula: k is the number of a certain mudstone caprock section, dimensionless; m is the number of a certain acoustic travel time function corresponding to this mudstone caprock section, dimensionless; Δh ckm is the shale thickness of the caprock in this shale section, m; h ck It is the thickness of the entire shale caprock between two sandstone interbeds, in m; x ck is the formation depth value of the caprock in this mudstone section, m; f ckm (x) is the acoustic time difference function of the caprock in this mudstone section, μs / m; is the average acoustic travel time of the caprock in this mudstone section, μs / m; P ck is the displacement pressure of the caprock in this mudstone section, MPa.

4. The method for characterizing the sealing performance of mudstone caprock considering sandstone interlayers according to claim 3, characterized in that: The specific calculation process of Step 4 is as follows: （1） In the formula: P is the overall displacement pressure, MPa.

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