A single-well analysis method for fracture activity

Abstract

The present invention provides a single-well analysis method for fracture activity, which is applicable to the field of oil and gas geology. Collect imaging logging data and pore pressure data of the target well; collect experimental samples of drilling cores, and through CT scanning, select samples with consistent internal pore structures, and obtain the friction coefficient, maximum horizontal principal stress, minimum horizontal principal stress and vertical principal stress based on friction sliding experiments and differential strain in-situ stress tests; use the imaging logging data to pick up fracture attitude parameters, borehole wall collapse and drilling-induced fracture information, and determine the current in-situ stress direction near the well according to the relationship between the borehole wall collapse, induced fractures and the current in-situ stress; based on the spatial state between the fracture attitude and the current in-situ stress direction and magnitude, combined with the pore pressure data, calculate the shear stress and effective normal stress on the fracture surface; construct a fracture activity index to quantitatively analyze the single-well fracture activity under the current in-situ stress state. The method has simple principle, strong operability and high result credibility.

Description

Technical Field

[0001] The present invention relates to a single - well analysis method for fracture activity, which is particularly applicable to the field of oil and gas geology. Background Art

[0002] Fractures are crucial for oil and gas exploration and development. They are not only an important type of reservoir space for oil and gas but also the main channels for seepage. Therefore, it is of great significance to predict the development and distribution laws of fractures and their activity levels.

[0003] Jiang Tongwen et al. (2021) published a paper in "China Petroleum Exploration" and proposed a fracture activity prediction technology. The ratio of shear stress to normal stress on the fracture surface is defined as an index reflecting the activity of the fracture surface and is also an important index reflecting the fracture permeability and fluid. This method does not consider the influence of cohesion and friction coefficient. The invention patent with the application publication number CN107844614A proposes a method and device for predicting the potential mechanical activity of faults. The method includes: obtaining the three - dimensional stress field and three - dimensional spatial data model of the fault, where the three - dimensional stress field and three - dimensional spatial data model have the same three - dimensional coordinate system, obtaining the current stress field parameters of each point on the fault plane according to the three - dimensional stress field, obtaining the attitude information of each point on the fault plane according to the three - dimensional spatial data model, and determining the potential activity index of each point on the fault plane according to the three - dimensional coordinates, current stress field parameters, and attitude information of each point on the fault plane. The potential activity index is used to quantitatively represent the potential mechanical activity of each point on the fault plane. However, this method mainly focuses on faults and also does not consider the influence of cohesion and friction coefficient. Summary of the Invention

[0004] In order to solve the problems existing in the prior art, the present invention provides a single - well analysis method for fracture activity with simple steps and high reliability. On the basis of obtaining the friction coefficient, the magnitude and direction of the current in - situ stress, and the fracture attitude, a fracture activity index is constructed.

[0005] To achieve the above objectives, the present invention proposes a single-well analysis method for crack activity, which collects imaging logging data and pore pressure data of the target well; collects multiple drilling core experimental samples at any depth in the target interval, uses CT to scan each drilling core experimental sample, and selects the drilling core experimental samples with consistent internal pore structures; conducts friction sliding experiments and differential strain in-situ stress tests on the selected drilling core experimental samples to obtain the friction coefficients, maximum horizontal principal stress, minimum horizontal principal stress, and vertical principal stress of the drilling core experimental samples; uses the imaging logging data to pick up the crack occurrence parameters, borehole wall collapse, and drilling-induced fracture information in the target interval, and determines the current in-situ stress direction near the well according to the relationship between the borehole wall collapse, induced fractures, and the current in-situ stress; based on the spatial state between the crack occurrence and the current in-situ stress direction and magnitude, combined with the pore pressure data, calculates the shear stress and effective normal stress on the crack surface, and constructs a crack activity index by fully considering the influence of the friction coefficient and cohesion, and quantitatively analyzes the single-well crack activity under the current in-situ stress state.

[0006] The specific steps are as follows:

[0007] Step 1: Collect imaging logging data of the target well and pore pressure data of the target interval;

[0008] Step 2: Collect drilling core samples, record the sampling depth, prepare plunger experimental samples with a diameter of 20 mm and a height of 40 mm, machine the end faces of the plunger experimental samples with a grinding machine or lathe to keep the upper and lower end faces parallel, cut along the direction with an angle of 35° to the axis as the sliding surrounding rock blocks, and prepare at least 10 plunger experimental samples cut along the 35° direction;

[0009] Step 3: Collect full-size drilling cores, record the depth, prepare sector experimental samples with a radius of 50 mm, a height of 50 mm, and a central angle of 90°, machine the upper and lower end faces of the sector experimental samples with a grinding machine or lathe to keep them parallel, and then use a grinding machine to trim the end faces and the periphery smoothly, and prepare at least 10 sector experimental samples;

[0010] Step 4: Use CT to scan the plunger samples obtained in Step 2 and the sector experimental samples obtained in Step 3, and select 5 plunger test samples and 5 sector test samples with consistent internal pore structures;

[0011] Step 5: Conduct friction sliding experiments on the selected plunger test samples, load the pore pressure collected in Step 1 during the experiment, obtain the friction coefficients of 5 groups of plunger test samples, and take the average of the 5 groups of friction coefficients:

[0012]

[0013]

[0014] where: μ is the friction coefficient, dimensionless; is the average friction coefficient, dimensionless; τ is the shear stress, MPa; σ ne represents the effective normal stress, MPa; P o is the pore pressure, MPa;

[0015] Step 6: Perform differential strain in-situ stress testing on the fan-shaped test samples optimized in Step 4 to obtain the horizontal maximum principal stress, horizontal minimum principal stress, and vertical principal stress of 5 groups of fan-shaped test samples, and take the average of the 5 groups of stress data:

[0016]

[0017] where: S Hmax is the horizontal maximum principal stress, S hmin is the horizontal minimum principal stress, S v is the vertical principal stress, with the unit of MPa; is the horizontal maximum principal stress, is the horizontal minimum principal stress, is the average value of the vertical principal stress, with the unit of MPa;

[0018] Step 7: Use the imaging logging data collected in Step 1 to interpret the fractures, borehole wall sloughing, and drilling-induced fractures in the target interval, pick up the attitude parameters of the fractures, the borehole wall sloughing azimuth, and the drilling-induced fracture azimuth, and record the borehole wall sloughing azimuth ±90° as β j , record the drilling-induced fracture azimuth as β k , and convert β j and β k into values within the range of [0, 180°) through ±180°, and calculate the current in-situ stress direction near the wellbore using the relationship between borehole wall sloughing, induced fractures, and current in-situ stress:

[0019]

[0020] where: c and d represent the number of borehole wall sloughing and drilling-induced fractures respectively.

[0021] Step 8: Use the pore pressure data collected in Step 1, the current in-situ stress magnitude obtained in Step 6, the current in-situ stress direction obtained in Step 7, and the fracture attitude parameters to calculate the shear stress and effective normal stress on the fracture surface. The formula is as follows:

[0022] τ×A 11 A 12 σ1+A 12 A 22 σ2+A 13 A 23 σ3

[0023]

[0024] Where: A pq is the direction cosine:

[0025]

[0026] In the formula: σ1, σ2, and σ3 are the maximum principal stress, intermediate principal stress, and minimum principal stress respectively, in MPa. Determine the maximum, intermediate, and minimum principal stresses according to the magnitude relationship of the horizontal maximum principal stress, horizontal minimum principal stress, and vertical principal stress obtained in Step 6; γ is the angle between the normal of the fracture plane and the minimum principal stress; ω is the angle between the projection of the fracture strike in the σ1-σ2 plane and the maximum principal stress; τ is the shear stress, in MPa; σ ne is the effective normal stress, in MPa; P o is the pore pressure, in MPa;

[0027] Step 9: Using the friction coefficient obtained in Step 5 and the shear stress and effective normal stress on the fracture surface obtained in Step 8, construct a more comprehensive and accurate fracture activity index V to quantitatively analyze the single-well fracture activity under the current in-situ stress state;

[0028] The formula is:

[0029]

[0030] In the formula: V is the fracture activity index, dimensionless; μ is the friction coefficient, dimensionless; C is the cohesion, in MPa, assigned according to the fracture filling degree. For fully filled fractures, C = 15 - 20 MPa; for semi-filled fractures, C = 5 - 8 MPa; for unfilled fractures, C = 0 - 2 MPa; τ is the shear stress on the fracture surface, in MPa; σ ne is the effective normal stress on the fracture surface, in MPa.

[0031] Furthermore, the size of the core samples collected in Step 2 should be at least a block of 25 mm × 25 mm × 50 mm, and the diameter of the full-size drilling core in Step 3 is 100 mm.

[0032] Furthermore, the fracture occurrence parameters picked up in Step 7 include fracture strike, fracture dip, and fracture dip angle.

[0033] Furthermore, the fracture activity index V calculated in Step 9 ranges from 0 to 1, and the larger the value, the stronger the fracture activity.

[0034] Beneficial effects:

[0035] This method collects target drilling imaging logging data and pore pressure data; collects drilling core experimental samples, selects samples with consistent internal structures through CT scanning, and obtains the friction coefficient, maximum horizontal principal stress, minimum horizontal principal stress, and vertical principal stress based on friction sliding experiments and differential strain in-situ stress tests; uses the imaging logging data to pick up fracture occurrence parameters, borehole wall collapse, and drilling-induced fracture information, and determines the current in-situ stress direction near the wellbore according to the relationship between borehole wall collapse, induced fractures, and the current in-situ stress; based on the spatial state between fracture occurrence and the current in-situ stress direction and magnitude, combined with pore pressure data, calculates the shear stress and effective normal stress on the fracture surface, constructs a fracture activity index, and quantitatively analyzes the single-well fracture activity under the current in-situ stress state. Description of the Drawings

[0036] Figure 1 It is a schematic flow chart of a single-well analysis method for fracture activity of the present invention. Detailed Embodiment

[0037] The following further describes the embodiments of the present invention with reference to the drawings:

[0038] As Figure 1 shown, a single-well analysis method for fracture activity collects target drilling imaging logging data and pore pressure data; collects multiple drilling core experimental samples at any depth in the target interval, uses CT to scan each drilling core experimental sample, and selects the drilling core experimental samples with consistent internal pore structures; conducts friction sliding experiments and differential strain in-situ stress tests on the selected drilling core experimental samples to obtain the friction coefficient, maximum horizontal principal stress, minimum horizontal principal stress, and vertical principal stress of the drilling core experimental samples; uses the imaging logging data to pick up the occurrence parameters of fractures, borehole wall collapse, and drilling-induced fracture information in the target interval, and determines the current in-situ stress direction near the wellbore according to the relationship between borehole wall collapse, induced fractures, and the current in-situ stress; based on the spatial state between fracture occurrence and the current in-situ stress direction and magnitude, combined with pore pressure data, calculates the shear stress and effective normal stress on the fracture surface, fully considers the influence of the friction coefficient and cohesion, constructs a fracture activity index, and quantitatively analyzes the single-well fracture activity under the current in-situ stress state.

[0039] The specific steps are as follows:

[0040] Step 1, collect target drilling imaging logging data and pore pressure data in the target interval;

[0041] Step 2: Collect drilling core samples. The size of the core samples should be at least a block of 25mm×25mm×50mm. Record the sampling depth, and prepare plunger test samples with a diameter of 20mm and a height of 40mm. Use a grinding machine or a lathe to process the end faces of the plunger test samples to make the upper and lower end faces parallel. Cut them along the direction with an angle of 35° to the axial direction as sliding surrounding rock blocks. Prepare at least 10 plunger test samples cut along the 35° direction;

[0042] Step 3: Collect full-size drilling cores, record the depth, and prepare sector-shaped test samples with a radius of 50mm, a height of 50mm, and a central angle of 90°. The diameter of the full-size drilling core is 100mm. Use a grinding machine or a lathe to process the upper and lower end faces of the sector-shaped test samples to be parallel, and then use a grinding machine to trim the end faces and the peripheries smoothly. Prepare at least 10 sector-shaped test samples;

[0043] Step 4: Use CT to scan the plunger samples obtained in Step 2 and the sector-shaped test samples obtained in Step 3, and select 5 plunger test samples and 5 sector-shaped test samples with consistent internal pore structures;

[0044] Step 5: Conduct friction sliding experiments on the selected plunger test samples. During the experiment, load the pore pressure collected in Step 1 to obtain the friction coefficients of 5 groups of plunger test samples, and take the average value of the 5 groups of friction coefficients:

[0045]

[0046]

[0047] In the formula: μ is the friction coefficient, dimensionless; is the average friction coefficient, dimensionless; τ is the shear stress, MPa; σ ne represents the effective normal stress, MPa; P o is the pore pressure, MPa;

[0048] Step 6: Use the sector-shaped test samples selected in Step 4 to conduct differential strain geostress tests, so as to obtain the horizontal maximum principal stress, horizontal minimum principal stress, and vertical principal stress of 5 groups of sector-shaped test samples, and take the average value of the 5 groups of stress data:

[0049]

[0050] In the formula: S Hmax is the horizontal maximum principal stress, S hmin is the horizontal minimum principal stress, S v is the vertical principal stress, with the unit of MPa; is the horizontal maximum principal stress, is the horizontal minimum principal stress, is the average value of the vertical principal stress, with the unit of MPa;

[0051] Step 7: Using the imaging logging data collected in Step 1, interpret the fractures, borehole wall collapses, and drilling-induced fractures in the target interval, pick up the attitude parameters of the fractures, the borehole wall collapse azimuth, and the drilling-induced fracture azimuth. The picked-up fracture attitude parameters include fracture strike, fracture dip, and fracture dip angle. Denote the borehole wall collapse azimuth ±90° as β j , and denote the drilling-induced fracture azimuth as β k , and for β j and β k , convert all of them into values within the range of [0, 180°) through ±180°, and calculate the current in-situ stress direction near the wellbore using the relationship between borehole wall collapse, induced fractures, and current in-situ stress:

[0052]

[0053] In the formula: c and d represent the number of borehole wall collapses and drilling-induced fractures respectively.

[0054] Step 8: Using the pore pressure data collected in Step 1, the current in-situ stress magnitude obtained in Step 6, the current in-situ stress direction obtained in Step 7, and the fracture attitude parameters, calculate and obtain the shear stress and effective normal stress on the fracture surface. The formula is as follows:

[0055] τ = A 11 A 12 σ1 + A 12 A 22 σ2 + A 13 A 23 σ3

[0056]

[0057] Among them: A pq is the direction cosine:

[0058]

[0059] In the formula: σ1, σ2, and σ3 are the maximum principal stress, intermediate principal stress, and minimum principal stress respectively, in MPa. Determine the maximum, intermediate, and minimum principal stresses according to the magnitude relationship of the horizontal maximum principal stress, horizontal minimum principal stress, and vertical principal stress obtained in Step 6; γ is the angle between the fracture surface normal and the minimum principal stress; ω is the angle between the projection of the fracture strike in the σ1 - σ2 plane and the maximum principal stress; τ is the shear stress, in MPa; σ ne is the effective normal stress, in MPa; P o is the pore pressure, in MPa;

[0060] Step 9: Using the friction coefficient obtained in Step 5 and the shear stress and effective normal stress on the fracture surface obtained in Step 8, construct a more comprehensive and accurate fracture activity index V to quantitatively analyze the fracture activity of a single well under the current in-situ stress state;

[0061] The formula is:

[0062]

[0063] In the formula: V is the fracture activity index, dimensionless; μ is the friction coefficient, dimensionless; C is the cohesion, MPa, assigned according to the fracture filling degree. For fully filled fractures, C = 15 - 20 MPa; for semi-filled fractures, C = 5 - 8 MPa; for unfilled fractures, C = 0 - 2 MPa; τ is the shear stress on the fracture surface, MPa; σ ne is the effective normal stress on the fracture surface, MPa.

[0064] Furthermore, the size of the core samples collected in Step 2 should be at least a block of 25 mm × 25 mm × 50 mm, and the diameter of the full-size drilling core in Step 3 is 100 mm.

[0065] Furthermore, the fracture attitude parameters picked up in Step 7 include fracture strike, fracture dip direction, and fracture dip angle.

[0066] Furthermore, the fracture activity index V calculated in Step 9 ranges from 0 to 1, and the larger the value, the stronger the fracture activity.

Claims

1. A single-well analysis method for fracture activity, characterized in that Collect the target drilling imaging logging data and pore pressure data; collect multiple drilling core experimental samples at any depth in the target interval, use CT to scan each drilling core experimental sample, and select the drilling core experimental samples with consistent internal pore structures; conduct friction sliding experiments and differential strain in-situ stress tests on the preferentially selected drilling core experimental samples to obtain the friction coefficient, maximum horizontal principal stress, minimum horizontal principal stress, and vertical principal stress of the drilling core experimental samples; use the imaging logging data to pick up the fracture occurrence parameters, borehole wall breakouts, and drilling-induced fracture information in the target interval, and determine the current in-situ stress direction near the wellbore based on the relationship between the borehole wall breakouts, induced fractures, and the current in-situ stress; based on the spatial state between the fracture occurrence and the current in-situ stress direction and magnitude, combined with the pore pressure data, calculate the shear stress and effective normal stress on the fracture surface, fully consider the influence of the friction coefficient and cohesion to construct a fracture activity index, and quantitatively analyze the single-well fracture activity under the current in-situ stress state. The formula is: In the formula: V is the fracture activity index, dimensionless. The fracture activity index V ranges from 0 to 1, and the larger the value, the stronger the fracture activity; μ is the friction coefficient, dimensionless; C is the cohesion, in MPa, assigned according to the fracture filling degree. For fully filled fractures, C = 15 - 20 MPa; for semi-filled fractures, C = 5 - 8 MPa; for unfilled fractures, C = 0 - 2 MPa; τ is the shear stress on the fracture surface, in MPa; σ ne is the effective normal stress on the fracture surface, MPa.

2. The single-well analysis method for crack activity according to claim 1, characterized in that The specific steps are as follows: Step 1: Collect the target drilling imaging logging data and the pore pressure data of the target interval; Step 2: Collect drilling core samples, record the sampling depth, prepare plunger experimental samples with a diameter of 20 mm and a height of 40 mm, machine the end faces of the plunger experimental samples with a grinder or lathe to keep the upper and lower end faces parallel, cut along the direction with an angle of 35° to the axis as the sliding surrounding rock blocks, and prepare at least 10 plunger experimental samples cut along the 35° direction; Step 3: Collect full-size drilling cores, record the depth, prepare sector experimental samples with a radius of 50 mm, a height of 50 mm, and a central angle of 90°, machine the upper and lower end faces of the sector experimental samples with a grinder or lathe to keep them parallel, and then use a grinding machine to trim the end faces and the periphery smooth. Prepare at least 10 sector experimental samples; Step 4: Use CT to scan the plunger samples obtained in Step 2 and the sector experimental samples obtained in Step 3, and select 5 plunger test samples and 5 sector test samples with consistent internal pore structures; Step 5: Conduct friction sliding experiments on the selected plunger test samples, load the pore pressure collected in Step 1 during the experiment, obtain the friction coefficients of 5 groups of plunger test samples, and take the average of the 5 groups of friction coefficients: In the formula: μ is the friction coefficient, dimensionless; `μ is the average friction coefficient, dimensionless; τ is the shear stress, MPa; σ ne represents the effective normal stress, MPa; P o is the pore pressure, MPa; Step 6: Use the sector test samples selected in Step 4 to conduct differential strain in-situ stress tests to obtain the maximum horizontal principal stress, minimum horizontal principal stress, and vertical principal stress of 5 groups of sector test samples, and take the average of the 5 groups of stress data: Where: S Hmax is the maximum horizontal principal stress, S hmin is the minimum horizontal principal stress, S v is the vertical principal stress, with the unit of MPa; is the average value of the maximum horizontal principal stress, is the average value of the minimum horizontal principal stress, is the average value of the vertical principal stress, with the unit of MPa; Step 7: Using the imaging logging data collected in Step 1, interpret the fractures, borehole wall collapses, and drilling-induced fractures in the target interval, pick up the attitude parameters of the fractures, the borehole wall collapse azimuth, and the drilling-induced fracture azimuth, and record the borehole wall collapse azimuth ±90° as β j , record the drilling-induced fracture azimuth as β k , and convert all of β j and β k into values within the range of [0, 180°) through ±180°, and calculate the current in-situ stress direction near the well using the relationships between borehole wall collapse, induced fractures, and current in-situ stress: In the formula: c and d represent the number of borehole wall breakouts and drilling-induced fractures respectively; Step 8: Using the pore pressure data collected in Step 1, the current in-situ stress magnitude obtained in Step 6, the current in-situ stress direction obtained in Step 7, and the fracture occurrence parameters, calculate the shear stress and effective normal stress on the fracture surface. The formula is as follows: Where: A pq is the direction cosine: Where: σ1, σ2, and σ3 are the maximum principal stress, intermediate principal stress, and minimum principal stress, respectively, in MPa. Determine the maximum, intermediate, and minimum principal stresses based on the magnitude relationship among the horizontal maximum principal stress, horizontal minimum principal stress, and vertical principal stress obtained in Step 6; γ is the angle between the normal of the fracture plane and the minimum principal stress; ω is the angle between the projection of the fracture strike in the σ1-σ2 plane and the maximum principal stress; τ is the shear stress, in MPa; σ ne is the effective normal stress, in MPa; P o is the pore pressure, in MPa; Step 9: Using the friction coefficient obtained in Step 5, the shear stress on the fracture surface, and the effective normal stress obtained in Step 8, construct a more comprehensive and accurate fracture activity index V to quantitatively analyze the single-well fracture activity under the current in-situ stress state; The formula is: Where: V is the crack activity index, dimensionless, and the crack activity index V ranges from 0 to 1. The larger the value, the stronger the crack activity; μ is the friction coefficient, dimensionless; C is the cohesion, MPa, which is assigned according to the crack filling degree. For fully filled cracks, C = 15 - 20 MPa, for semi-filled cracks, C = 5 - 8 MPa, and for unfilled cracks, C = 0 - 2 MPa; τ is the shear stress on the crack surface, MPa; σ ne is the effective normal stress on the crack surface, MPa.

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