Method for evaluating shale water-based drilling fluid plugging and wall strengthening capacity

Abstract

The present application relates to a kind of shale water-based drilling fluid plugging reinforced well wall capacity evaluation method, comprising: obtaining shale fracture length and height;Preparation different bedding-fracture angle's shale herringbone slot disc, determine the fracture toughness of different bedding-fracture angle;Develop water-based drilling fluid and shale soaking experiment, clear water-based drilling fluid weakening effect on the fracture toughness of shale;Based on core fracture plugging test, clear the position of the sealing layer inside the shale fracture, and consider the hindering effect of sealing layer, determine the influence of water-based drilling fluid on the fracture toughness of the tip position of the fracture;Comprehensive bedding angle, fracture morphology and extension direction, sealing layer position and drilling fluid effect influence, build shale wellbore-fracture-sealing layer model, clear the change rule of fracture average cracking pressure in the process of water-based drilling fluid action.The present application considers the influence of the synergistic effect of shale bedding and hydration on the well wall strengthening effect of drilling fluid, which can more accurately evaluate the ability of drilling fluid to plug and strengthen well wall.

Description

Technical Field

[0001] This invention relates to the field of shale formation drilling and completion engineering technology, and in particular to an evaluation method for the ability of shale water-based drilling fluid to seal and strengthen the wellbore. Background Technology

[0002] my country's high dependence on imported natural gas necessitates increased development efforts. Currently, shale gas is the main growth driver of my country's natural gas supply, and its efficient development is crucial for ensuring national energy security. Wellbore instability is a major technical bottleneck limiting efficient shale gas development. Especially when drilling into shale fractures, these high-permeability surfaces easily lead to drilling fluid intrusion, inducing hydration and reducing wellbore stability. Therefore, much research currently focuses on the plugging properties of water-based drilling fluids, aiming to strengthen the wellbore and ensure stability through fluid plugging. Thus, to more effectively guide the development of water-based drilling fluids with strong plugging properties, a method is needed to accurately evaluate the wellbore-strengthening capabilities of water-based drilling fluids.

[0003] Existing evaluation methods for wellbore reinforcement primarily consider factors such as in-situ stress, fracture morphology, and plugging layer morphology, and are applicable to formations like sandstone and carbonate rocks. However, for shale formations, which possess well-developed bedding and strong hydration capabilities, the bedding angle and the hydration induced by drilling fluid contact with shale can alter the fracture initiation characteristics of shale, thus affecting the wellbore reinforcement effect of the drilling fluid. Currently, a quantitative characterization method for wellbore reinforcement that considers both shale bedding and hydration is lacking and urgently needs to be developed. Summary of the Invention

[0004] This application provides a method for evaluating the wellbore sealing and strengthening capabilities of shale water-based drilling fluids in order to solve the aforementioned technical problems.

[0005] This application is achieved through the following technical solution:

[0006] A method for evaluating the wellbore sealing and strengthening capabilities of shale water-based drilling fluids includes the following steps:

[0007] Based on downhole shale samples, the length and height of shale fractures were extracted.

[0008] Using downhole shale samples, shale herringbone groove disks with different bedding-fracture angles were prepared. Fracture toughness experiments were carried out on the shale herringbone groove disks to obtain the correlation between the bedding-fracture angle and the fracture toughness of shale, and to clarify the fracture toughness under different bedding-fracture angles.

[0009] Based on a shale herringbone groove disc, we conducted immersion experiments of water-based drilling fluid and shale to obtain the fracture toughness of shale under different treatment times of water-based drilling fluid. By measuring the decrease in fracture toughness, we clarified the weakening effect of water-based drilling fluid on the fracture toughness of shale.

[0010] Based on the extracted shale fracture length and height, a shale-fracture system was constructed using flat plate fracturing. Core fracture plugging tests were conducted to obtain the location and length of the shale fracture plugging layer under wellbore pressure conditions. By analyzing the changes in leakage before and after plugging, the resistance coefficient of the plugging layer was established to determine the influence of water-based drilling fluid on the fracture toughness of shale at the fracture tip.

[0011] Based on the geological data of the target strata, the strike of shale bedding and the direction of extension of natural fractures and their distribution probability are determined, thereby determining the fracture-bedding angles.

[0012] Based on bedding angle, fracture morphology and extension direction, plugging layer location and drilling fluid effect, a shale wellbore-fracture-plugging layer model is constructed to clarify the fracture initiation pressure variation law during the action of water-based drilling fluid.

[0013] Based on the distribution probability of the natural fracture extension direction, the weight coefficient of the main fracture extension direction is determined. Based on the fracture initiation pressure variation law and the weight coefficient, the average fracture initiation pressure variation law during the water-based drilling fluid process is calculated.

[0014] Alternatively, the correlation equation between the bedding-crack angle and fracture toughness is:

[0015]

[0016] In the above formula, The angle between bedding and crack. The fracture toughness of shale under different bedding-fracture angles.

[0017] Optionally, the equation for solving the weakening effect coefficient of the water-based drilling fluid on the fracture toughness of shale is:

[0018]

[0019] In the above formula, This represents the coefficient of weakening effect of water-based drilling fluid on the fracture toughness of shale. This represents the fracture toughness at the moment when the contact time between the water-based drilling fluid and the shale is 0. The fracture toughness of shale under different time exposures with water-based drilling fluid. The angle between bedding and crack.

[0020] Optionally, the solution equation for the bedding-crack angle is as follows:

[0021]

[0022] In the above formula, The angle between bedding and crack. From the perspective of shale bedding, This indicates the direction in which the natural crack extends.

[0023] Optionally, the expression for the effect of the water-based drilling fluid on the fracture toughness of shale at the fracture tip is:

[0024]

[0025] In the above formula, This refers to the fracture toughness of the shale at the fracture tip after contact with drilling fluid, under the obstruction of the sealing layer. The impedance coefficient of the sealing layer. These represent the flow rates before the blockage. This refers to the flow rate after the blockade.

[0026] Optionally, the equation for calculating the crack initiation pressure is as follows:

[0027]

[0028] In the above formula, The fracturing initiation pressure under the action of water-based drilling fluid. For the maximum horizontal ground stress, For the minimum horizontal ground stress, Pore ​​pressure, The angle at which the crack extends;

[0029] The expressions for T1, T2, T3, and T4 are as follows:

[0030]

[0031] In the above formula, α is the fracture length, R is the wellbore radius, L is the distance from the center of the wellbore to the end of the fracture, L is equal to the sum of the wellbore radius R and the fracture length α, and D is the distance from the center of the wellbore to the sealing layer, which is equal to the sum of the wellbore radius R and the position S of the sealing layer inside the fracture.

[0032] The formula for calculating the weighting coefficient is as follows:

[0033]

[0034] In the above formula, This is the weighting coefficient for any crack direction among all crack directions with a probability greater than a specified value, where n represents the number of crack directions with a probability greater than the specified value. This represents the probability distribution of any crack direction among all crack directions with a probability distribution greater than the specified value.

[0035] Optionally, the specified value is 15%.

[0036] Optionally, the equation for calculating the average fracture initiation pressure during the action of water-based drilling fluid is as follows:

[0037]

[0038] In the above formula, The final average crack initiation pressure, These are the weighting coefficients. The crack initiation pressure at different crack extension angles.

[0039] Compared with the prior art, this application has the following beneficial effects:

[0040] This application considers the synergistic effect of shale bedding and hydration on the wellbore reinforcement effect of drilling fluid, which can more accurately evaluate the ability of drilling fluid to plug and reinforce the wellbore, and is more conducive to the design and optimization of drilling fluids for shale formations. Attached Figure Description

[0041] The accompanying drawings, which are included to provide a further understanding of the embodiments of this application and form part of this application, do not constitute a limitation on the embodiments of the present invention.

[0042] Figure 1 This is a flowchart of the evaluation method for the sealing and strengthening of the wellbore by shale water-based drilling fluid in the embodiments;

[0043] Figure 2 This is a schematic diagram of the fracture toughness test based on the herringbone groove disk in the embodiment;

[0044] Figure 3 This represents the decrease in fracture toughness at different times of drilling fluid application in the embodiments.

[0045] Figure 4 This is a schematic diagram of the core fracture sealing test in the embodiment;

[0046] Figure 5 This is a schematic diagram of the shale-fracture-sealing layer model in the embodiment;

[0047] Figure 6 This is a diagram showing the change in fracturing pressure under the action of different drilling fluid systems in the embodiments. Detailed Implementation

[0048] To make the objectives, technical solutions, and advantages of this application clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention.

[0049] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0050] It should be noted that, unless otherwise specified, the embodiments and features described in this invention can be combined with each other. It should also be noted that the various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments; similar or identical parts between embodiments can be referred to interchangeably.

[0051] Example 1

[0052] In this embodiment, the wellbore sealing and strengthening capabilities of two different water-based drilling fluid systems were evaluated for the Longmaxi Shale formation in the Sichuan Basin. Based on existing information, the shale formation has a depth of 3240m and a maximum in-situ stress of [missing information]. and minimum ground stress They were 2.02 g / cm³. 3 and 1.94 g / cm 3 Formation pore pressure P p It is 1.60 g / cm 3 The wellbore radius R is 107.95 mm. The design wellbore pressure P w It is 1.65 g / cm 3 .

[0053] like Figure 1 As shown in this embodiment, the evaluation method for the wellbore sealing and strengthening ability of shale water-based drilling fluid includes the following steps:

[0054] S1, based on downhole shale samples, identifies and extracts the length and height of shale fractures.

[0055] In this embodiment, the average length of the shale fracture is 14.5 mm, and the average height of the shale fracture is 2.2 mm.

[0056] S2, using downhole shale samples, shale herringbone disks with different bedding-fracture angles were prepared, and fracture toughness tests were conducted, such as... Figure 2 As shown, the correlation equation between the bedding-crack angle and fracture toughness is obtained, and the fracture toughness K under different bedding-crack angles β is clarified. I (β).

[0057] In this embodiment, the bedding-crack angle is selected as 0°, 30°, 60°, and 90°. The correlation equation between the bedding-crack angle and fracture toughness is as follows:

[0058]

[0059] In the above formula, To determine the fracture toughness of shale under different bedding-fracture angles, The angle between bedding and crack.

[0060] S3. Based on a herringbone-groove disc, immersion experiments were conducted on shale using water-based drilling fluid to determine the fracture toughness of shale under different exposure times with water-based drilling fluid. Furthermore, the weakening effect coefficient of water-based drilling fluid on shale fracture toughness was determined by the decrease in fracture toughness, as shown below:

[0061]

[0062] In the above formula, This represents the coefficient of weakening effect of water-based drilling fluid on the fracture toughness of shale. This represents the fracture toughness at the moment when the contact time between the water-based drilling fluid and the shale is 0. Shale fracture toughness equal to the angle between different bedding and fractures under original conditions ; The fracture toughness of shale under different time exposures with water-based drilling fluid. The angle between bedding and crack.

[0063] Based on the above formula, the decrease in shale fracture toughness under the action of the two water-based drilling fluids in this embodiment is as follows: Figure 3 As shown.

[0064] Based on the fracture morphology (i.e., fracture length and height) in S4, a wedge-shaped flat plate was used to create fractures, constructing a shale-fracture system. Using this as a basis, core fracture sealing tests were conducted to determine the location of the shale fracture sealing layer under wellbore pressure conditions.

[0065] like Figure 4 As shown, during the core fracture plugging test, a wellbore pressure P was applied at the fracture inlet. w (As the driving force for drilling fluid invasion), flow rate changes are monitored at the fracture outlet to determine the formation of the sealing layer. During the gradual formation of the sealing layer, the flow rate gradually decreases; when it reaches a stable value, the sealing layer is considered to have formed. After the sealing layer forms, the sample is removed after the experiment is depressurized, and the position S of the sealing layer inside the fracture is observed and measured with a ruler.

[0066] It is worth noting that position S refers to the distance between the middle of the plugging layer and the fracture entrance. In this embodiment, the plugging layer positions S under the action of drilling fluid system 1 and system 2 are 5.2 mm and 7.7 mm, respectively.

[0067] S5. Considering the hindering effect of the plugging layer on the water-based drilling fluid, the hindering coefficient of the plugging layer is established by measuring the change in leakage before and after plugging. This allows us to determine the influence of the water-based drilling fluid on the fracture toughness of the shale at the fracture tip, as shown in the following expression:

[0068]

[0069] In the above formula, The impedance coefficient of the sealing layer. These represent the flow rates before the blockage. The flow rate after the blockade; This represents the fracture toughness of the shale at the fracture tip after contact with drilling fluid under the obstruction of the sealing layer, and signifies the fracture toughness at the fracture tip corresponding to different bedding-fracture angles.

[0070] Based on the test results, the blocking coefficients of the plugging layer under drilling fluid system 1 and system 2 are 0.26 and 0.42, respectively.

[0071] S6. Based on geological data, the strike of shale bedding and the direction and probability of natural fracture extension are determined, thereby determining the angle between different bedding and fractures.

[0072] Stratification-crack angle The solution equation is as follows:

[0073]

[0074] In the above formula, From the perspective of shale bedding, This indicates the direction in which the natural crack extends.

[0075] It is worth noting that for a shale formation within a block, the bedding orientation is usually fixed, but the direction of natural fractures has a certain degree of dispersion, thus possessing a certain distribution probability. In this embodiment, a distribution probability of 15% is used as the critical value, and fracture directions with a distribution probability greater than 15% are selected for subsequent analysis.

[0076] In this embodiment, the shale bedding angle is determined. The angle is 26°. The main extension directions of natural cracks are 72.3°, 55.7°, and 16.5°, with corresponding distribution probabilities of 31.6%, 19.5%, and 18.2%. Based on the calculation results, the crack-bedding angle is mainly 46.3°, 29.7°, and 9.5°.

[0077] S7, based on bedding angles and the direction of natural fracture extension, construct a shale wellbore-fracture-sealing layer model, such as... Figure 5 As shown, this clarifies the law of fracture initiation pressure variation during the action of water-based drilling fluid.

[0078] Based on the wellbore-fracture-sealing layer model, the equation for calculating the fracture initiation pressure is as follows:

[0079]

[0080] In the above formula, The fracturing pressure under the action of water-based drilling fluid. For the maximum horizontal ground stress, For the minimum horizontal ground stress, Pore ​​pressure, The expressions for T1, T2, T3, and T4, representing the crack propagation angles, are as follows:

[0081]

[0082] In the above formula, α is the fracture length, R is the wellbore radius, L is the distance from the center of the wellbore to the end of the fracture, L is equal to the sum of the wellbore radius R and the fracture length α, and D is the distance from the center of the wellbore to the sealing layer, which is equal to the sum of the wellbore radius R and the position S of the sealing layer inside the fracture.

[0083] S8. Based on the distribution probability of the natural crack extension direction, determine the weighting coefficient of the crack extension direction.

[0084] The formula for calculating the weighting coefficient is as follows:

[0085]

[0086] In the above formula, This is the weighting coefficient for any crack direction among all crack directions with a probability greater than the specified value, where n represents the number of crack directions with a probability greater than the specified value. This represents the probability distribution of any crack direction among all crack directions with a probability distribution greater than the specified value.

[0087] Based on the weighting coefficients and the fracture initiation pressure at different fracture extension angles in S7, the variation law of the average fracture initiation pressure during the action of water-based drilling fluid is clarified, thereby realizing the evaluation of the wellbore strengthening ability of plugging water-based drilling fluid. The equation is as follows:

[0088]

[0089] In this implementation case, there are three crack directions with a probability greater than 15%, and the weighting coefficients are... , , The values ​​are 0.46, 0.28, and 0.26 respectively. Substituting these values ​​into the above calculation equation, the final average crack initiation pressure can be obtained. .

[0090] Based on the calculation results, the average fracturing pressure distribution under the action of water-based drilling fluid system 1 and system 2 in this embodiment is as follows: Figure 6 As shown, it can be observed that during the drilling fluid process, the initiation pressure of System 1 decreases less and at a slower rate, indicating that System 1 makes it more difficult for fractures to initiate in the wellbore, resulting in better wellbore stability and a more significant wellbore strengthening effect. Therefore, System 1 has a better ability to seal and strengthen the wellbore.

[0091] The above specific embodiments further illustrate the purpose, technical solution, and beneficial effects of this application. It should be understood that the above are merely specific embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for evaluating the wellbore sealing and strengthening capabilities of shale water-based drilling fluids, characterized in that: Includes the following steps: Based on downhole shale samples, the length and height of shale fractures were extracted. Using downhole shale samples, shale herringbone groove disks with different bedding-fracture angles were prepared. Fracture toughness experiments were carried out on the shale herringbone groove disks to obtain the correlation between the bedding-fracture angle and the fracture toughness of shale, and to determine the fracture toughness of shale under different bedding-fracture angles. Based on the shale herringbone groove disc, immersion experiments were conducted on water-based drilling fluid and shale to obtain the fracture toughness of shale under different treatment times of water-based drilling fluid. By measuring the decrease in fracture toughness, the weakening effect coefficient of water-based drilling fluid on the fracture toughness of shale was determined. Based on the extracted shale fracture length and height, a shale-fracture system was constructed using flat plate fracturing. Core fracture plugging tests were conducted to obtain the location and length of the shale fracture plugging layer under wellbore pressure conditions. By analyzing the changes in leakage before and after plugging, the resistance coefficient of the plugging layer was established to determine the influence of water-based drilling fluid on the fracture toughness of shale at the fracture tip. Based on the geological data of the target strata, the strike of shale bedding and the direction of extension of natural fractures and their distribution probability are determined, thereby determining the fracture-bedding angles. Based on bedding angle, fracture morphology and extension direction, plugging layer location and drilling fluid effect, a shale wellbore-fracture-plugging layer model is constructed to clarify the fracture initiation pressure variation law during the action of water-based drilling fluid. Based on the distribution probability of the natural fracture extension direction, the weight coefficient of the main fracture extension direction is determined. Based on the fracture initiation pressure variation law and the weight coefficient, the average fracture initiation pressure variation law during the water-based drilling fluid process is calculated. The equation for solving the weakening effect coefficient of the water-based drilling fluid on the fracture toughness of shale is: ； In the above formula, This represents the coefficient of weakening effect of water-based drilling fluid on the fracture toughness of shale. This represents the fracture toughness at the moment when the contact time between the water-based drilling fluid and the shale is 0. The fracture toughness of shale under different time exposures with water-based drilling fluid. The angle between bedding and crack; The expression for the effect of the water-based drilling fluid on the fracture toughness of shale at the fracture tip is as follows: ； In the above formula, This refers to the fracture toughness of the shale at the fracture tip after contact with drilling fluid, under the obstruction of the sealing layer. The impedance coefficient of the sealing layer. These represent the flow rates before the blockage. The flow rate after the blockade; The equation for calculating the average fracture initiation pressure during the action of the water-based drilling fluid is as follows: ； In the above formula, The final average crack initiation pressure, These are the weighting coefficients. The crack initiation pressure is given at different crack extension angles, where n represents the number of crack directions with a probability greater than a specified value.

2. The evaluation method for the wellbore sealing and strengthening ability of shale water-based drilling fluid according to claim 1, characterized in that: The correlation equation between the bedding-crack angle and fracture toughness is: ； In the above formula, The angle between bedding and crack. The fracture toughness of shale under different bedding-fracture angles.

3. The evaluation method for the wellbore sealing and strengthening ability of shale water-based drilling fluid according to claim 1, characterized in that: The equation for solving the bedding-crack angle is as follows: ； In the above formula, The angle between bedding and crack. From the perspective of shale bedding, This indicates the direction in which the natural crack extends.

4. The evaluation method for the wellbore sealing and strengthening ability of shale water-based drilling fluid according to claim 1, characterized in that: The equation for calculating the crack initiation pressure is as follows: ； In the above formula, The fracturing initiation pressure under the action of water-based drilling fluid. For the maximum horizontal ground stress, For the minimum horizontal ground stress, Pore ​​pressure, The angle at which the crack extends; The expressions for T1, T2, T3, and T4 are as follows: ； In the above formula, α is the fracture length, R is the wellbore radius, L is the distance from the center of the wellbore to the end of the fracture, L is equal to the sum of the wellbore radius R and the fracture length α, and D is the distance from the center of the wellbore to the sealing layer, D is equal to the sum of the wellbore radius R and the position S of the sealing layer inside the fracture.

5. The evaluation method for the wellbore sealing and strengthening ability of shale water-based drilling fluid according to claim 1, characterized in that: The formula for calculating the weighting coefficient is as follows: ； In the above formula, This is the weighting coefficient for any crack direction among all crack directions with a probability greater than a specified value, where n represents the number of crack directions with a probability greater than the specified value. This represents the probability distribution of any crack direction among all crack directions with a probability distribution greater than the specified value.

6. The evaluation method for the wellbore sealing and strengthening ability of shale water-based drilling fluid according to claim 5, characterized in that: The specified value is 15%.

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