Design method and fracturing method for lacustrine shale gas wellbore

Abstract

The present application belongs to the field of oil and gas field development, aiming at the technical problem of difficult longitudinal layer penetration of hydraulic fractures in lacustrine shale, providing a design method and fracturing method for lacustrine shale gas wellbore, drilling horizontal wellbore according to the designed W-shaped wellbore track, so that the horizontal wellbore forms a W-shaped wellbore track; after the completion of the W-shaped horizontal wellbore, hydraulic fracturing operation is carried out in the W-shaped horizontal wellbore to form a fracture. By using W-shaped well in lacustrine shale gas well, the longitudinal layer penetration of hydraulic fractures is solved by longitudinal layer penetration of wellbore, and the hydraulic fracturing operation is carried out by using the fixed face continuous jet perforation process, which maximizes the longitudinal layer penetration effect of hydraulic fractures, thereby improving the volume fracturing reconstruction effect.

Description

Technical Field

[0001] This invention belongs to the field of oil and gas field development and relates to wellbore design methods and wellbore fracturing methods for lacustrine shale gas. Background Technology

[0002] Currently, with the successful exploration and development of marine shale gas, lacustrine shale gas has also gained significant attention from the industry. Early approaches generally adopted those used for marine shale gas, but these proved unsuitable. The main reason is that lacustrine shale gas typically features multi-lithological interlayers and high clay content, making fracture initiation and propagation difficult, especially in the fracture height direction (perpendicular to the horizontal bedding plane). As the number of perforation clusters within a section increases, the extension of the fracture height becomes even more limited due to flow diversion. Therefore, under conditions of limited fracture height, it is difficult to increase the volume of fracture modification. In the past, methods to increase the vertical cross-layer effect (i.e., fracture height) of lacustrine shale mainly included acid pretreatment, pre-placement of high-viscosity adhesive, and mid-placement of adhesive. However, these methods also have many limitations. For example, acid pretreatment makes it difficult to ensure uniform acid distribution at the fracture openings of each cluster of fractures, so uneven fracture initiation and extension of fracture heights in each cluster are still difficult to completely avoid. When pre-placed with high-viscosity adhesive, it is impossible to guarantee true pre-fracture, because a volume of low-viscosity completion and washing fluid in the wellbore will preferentially create fractures before the high-viscosity adhesive. Moreover, the viscosity of the high-viscosity adhesive will decrease accordingly due to the increase in temperature when it migrates to the wellbore in the early stage of fracture creation, thus significantly reducing its promoting effect on fracture height extension. Although the temperature inside the fracture has dropped significantly and the viscosity retention rate is relatively high when the adhesive is placed mid-placed, the impact on fracture height extension is relatively small because the geometric size of the fracture is relatively large.

[0003] In addition, the perforation method used in the fracturing process also plays an important role in the longitudinal penetration effect of hydraulic fractures.

[0004] Chinese patent (CN106285606A) provides a rotatable nozzle type dual-cluster hydraulic jet fracturing string and fracturing method. It designs an Archimedes double helix structure on the internal structure of a conventional hydraulic jetter, with four spherical nozzles distributed on the double helix groove of the jetter body. This changes the flow state and direction of the sand-carrying fluid to facilitate uniform entry into each nozzle, effectively solving the problem that the nozzles of a horizontal well dual-cluster hydraulic jetter cannot be adjusted in position. However, it cannot solve the problem of how to improve the longitudinal penetration effect of hydraulic fractures.

[0005] In addition, the project "Performance Improvement of Hydraulic Jet Fracturing Process String (Oil & Gas Well Testing, April 2019)" addressed the issues of low performance indicators of fracturing tools and sand jamming during the application of hydraulic jetting technology in Daqing Oilfield, which could lead to the inability to retrieve the tubing. Solidworks software was used to optimize the rubber sleeve structure and improve the rubber compound formulation, increasing the tensile strength of the rubber sleeve by 23% and reducing the residual deformation rate to below 3%. Ansys software was used to simulate and optimize the stress state of the slips, increasing their ability to withstand alternating loads by 50% and improving the reliability of the fracturing tools. A packer sand removal structure was designed, and the tubing was integrated into the design, reducing the tubing outer diameter by 16% and the overall length by 25%, reducing the risk of sand jamming to zero. Fluent simulation was used to optimize the nozzle orifice diameter to 4.5mm, the number of orifices to 4, and the phase angle to 180°, improving perforation capability. A "simply supported beam" type coupling locator was designed, achieving 100% positioning accuracy and improving the safety of the process tubing. This article studies improvements to conventional jetting tools, but it also fails to address how to improve the longitudinal penetration effect of hydraulic fractures.

[0006] In summary, the existing spiral perforation technology is no longer sufficient to meet the requirements for promoting fracture height extension. At the same time, the existing processes for promoting fracture height extension are also difficult to achieve the goal of significantly increasing the fracture height of multiple clusters of fractures in lacustrine shale. New alternative solutions must be developed to overcome the limitations of the above-mentioned problems. Summary of the Invention

[0007] The purpose of this invention is to solve the problems existing in the prior art and provide a design method for lacustrine shale gas wellbore, which designs a wellbore track that can improve the longitudinal penetration effect of hydraulic fractures.

[0008] The present invention is achieved through the following technical solution: a design method for lacustrine shale gas wellbore, comprising designing the wellbore trajectory of the horizontal wellbore as W-shaped: four consecutive build-up sections traverse the top and bottom of the target layer in a downward, upward, downward, and finally upward trajectory.

[0009] Furthermore, the W-shaped wellbore trajectory is as follows: after passing through the top of the target layer at the build-up point, it is driven downward once, reaching 1 / 4 of the horizontal projection of the horizontal wellbore and then passing to the bottom of the target layer. Then it is driven upward once, reaching 2 / 4 of the horizontal projection of the horizontal wellbore and then passing to the top of the target layer. Then it is driven downward a second time, reaching 3 / 4 of the horizontal projection of the horizontal wellbore and then passing to the bottom of the target layer again. Finally, it is driven upward a second time, reaching 4 / 4 of the horizontal projection of the horizontal wellbore and then passing to the top of the target layer again.

[0010] Furthermore, the design principles for the W-type wellbore track are as follows:

[0011] Target point design: The starting point is taken as the first target point, and the points at 1 / 4, 2 / 4, 3 / 4 and 4 / 4 of the horizontal projection of the horizontal wellbore are the second to fifth target points, respectively.

[0012] Draft rate design: With the goal of minimizing the unused volume of the reservoir, the maximum probability of crossing the vertical multi-lithological layers is considered, and the draft rate of each draft section in the W-shaped wellbore trajectory is considered to ensure the smooth entry and exit of subsequent hydraulic fracturing tools.

[0013] Furthermore, by simulating the process of hydraulic fracturing tools being lowered and raised under different build-up rates in each build-up section of the W-shaped wellbore track on the ground, the build-up rate of each build-up section in the W-shaped wellbore track that meets the design principles of the W-shaped wellbore track can be determined.

[0014] This invention also provides a fracturing method for lacustrine shale gas wells, which can effectively improve the longitudinal penetration effect of hydraulic fractures and increase the extension of fracture height.

[0015] A fracturing method for lacustrine shale gas wellbore, wherein a W-shaped wellbore trajectory is designed according to the design method of lacustrine shale gas wellbore proposed in this invention, so that the horizontal wellbore forms a W-shaped wellbore trajectory: four consecutive build-up sections pass through the top and bottom of the target layer in a downward, upward, downward again, and finally upward trajectory; after the W-shaped horizontal wellbore is completed, hydraulic fracturing operation is carried out in the W-shaped horizontal wellbore to create fractures.

[0016] Furthermore, a fixed-surface continuous jet perforation process is adopted to carry out hydraulic fracturing operations.

[0017] Furthermore, a combination of directional perforation and fixed-surface continuous jet perforation is used to carry out hydraulic fracturing operations: when the wellbore is at the top of the target layer, downward directional perforation is used to create fractures; when the wellbore is at the bottom of the target layer, upward directional perforation is used to create fractures; and when the wellbore is at the middle of the target layer, fixed-surface continuous jet perforation is used to create fractures.

[0018] Furthermore, a fixed-surface continuous jetting perforation process is implemented using an injector: the injector includes a tube with a disc body, the tube body being used to connect to a continuous oil pipe; a plurality of nozzles are distributed on the outer circumference of the disc body, and a flow channel is provided inside the disc body corresponding to each nozzle, through which the continuous oil pipe and the nozzle are connected.

[0019] Furthermore, the size and shape of the ejector channel were optimized: with the optimization objectives of minimizing pressure consumption and reducing the corrosive effect of sand-containing liquid on the channel, the size and shape of the channel were determined by a comprehensive trade-off based on ground simulation experiments and fluid-structure interaction simulation software.

[0020] Furthermore, the rear end of the injector tube is connected to the continuous tubing via an internally hollow universal joint, allowing the angle between the injector and the continuous tubing to be adjusted.

[0021] An eccentric counterweight is threaded to the front end of the injector tube. Adjusting the eccentric counterweight to align with the nozzle axis ensures that the injector's spray direction remains perpendicular to the horizontal stratification direction under gravity.

[0022] Furthermore, two injectors are connected in series via a conduit. The injector directly connected to the continuous tubing is the rear injector, and the flow channel of the rear injector is smaller in size than that of the front injector.

[0023] Furthermore, the injection flow rates of the two injectors are measured. If the difference in injection flow rates between the two injectors is greater than 10%, the continuous injection perforation process parameters are re-optimized, including the flow channel and / or the angle between the injector and the axis of the continuous tubing.

[0024] Furthermore, numerical simulations are conducted before the actual hydraulic fracturing operation. If the simulated fracturing effect is less than 30% better than the effect of using conventional completion tubing in the adjacent well, the W-shaped wellbore trajectory and / or fixed-face continuous jet perforation process parameters are redesigned until the simulated fracturing effect is not less than 30% better than the effect of using conventional completion tubing in the adjacent well.

[0025] Furthermore, let N be the number of perforation clusters in the inclined section. When N is even, the number of injections is N / 2. When N is odd, a ball seat is installed between the two injectors. When the number of injections reaches N-1 / 2, the ball seat is sealed with a ball to block the liquid inlet channel of the front injector, and then one last injection is performed.

[0026] Compared with the prior art, the beneficial effects of the present invention include:

[0027] 1. Due to the limited vertical penetration effect of hydraulic fractures, in existing horizontal wells with a horizontal, straight wellbore trajectory, the fracture height is difficult to penetrate the entire target layer. However, this invention overcomes the limitations of vertical penetration of hydraulic fractures by using a W-shaped horizontal wellbore in lacustrine shale gas wells, leveraging the vertical penetration effect of the W-shaped horizontal wellbore itself.

[0028] 2. Based on the W-type horizontal wellbore, this invention adopts a fixed-face continuous jet perforation process for fracturing. Compared with the existing spiral perforation process, the energy of the fixed-face continuous jet perforation process is concentrated on a circumferential surface. Therefore, the fixed-face continuous jet perforation process can maximize the penetration effect of hydraulic fractures in the vertical direction, further solve the problem of difficult fracture penetration in lacustrine shale gas reservoirs, and thus improve the volumetric fracturing effect.

[0029] 3. Hydraulic fracturing operations are carried out by combining directional perforation technology with fixed-surface continuous circumferential jet perforation technology, which avoids injecting into non-target layers when the wellbore passes through the top and bottom of the target layer, thus saving energy.

[0030] 4. The ejector creates gaps by injecting high-speed pumped abrasive material. The size and shape of the ejector are optimized with dual objectives to ensure minimal pressure consumption and the lowest abrasive effect of the sand-containing liquid on the flow channel.

[0031] 5. By designing the angle between the injector and the coiled tubing axis, the direction of fracture extension is ensured to be perpendicular to the horizontal bedding plane, which can minimize the frictional effect of near-wellbore bending fractures.

[0032] 6. The extension of the toe crack is generally insufficient. The front injector is close to the toe crack. By increasing the flow channel size of the front injector, the spray area is larger, which helps to improve the extension of the toe crack.

[0033] 7. Control the difference in spray flow between the two injectors to within 10% to improve the operational coordination between the two injectors.

[0034] 8. Conduct numerical simulations before actual hydraulic fracturing operations to guide practice with theory, predict post-fracturing effects, and ensure actual construction results. Attached Figure Description

[0035] Figure 1 This is a structural schematic diagram of a W-shaped wellbore trajectory;

[0036] Figure 2 This is a schematic diagram of the injector's structure;

[0037] Figure 3 for Figure 2 AA cross-section view;

[0038] Figure 4 This is a schematic diagram of a structure with two injectors connected in series.

[0039] Figure 5 This is a schematic diagram of the adaptive slope generation for coiled tubing. Detailed Implementation

[0040] The present invention will now be described in further detail with reference to the accompanying drawings:

[0041] The overall concept of this invention is:

[0042] 1) In the well completion string design, the traditional straight horizontal well (whether horizontal or highly deviated) is changed to a W-shaped wellbore trajectory: four consecutive build-up sections traverse the top and bottom of the target formation in a downward, upward, downward, and finally upward trajectory. For example... Figure 1As shown, within the entire length of the horizontal wellbore projected from top to bottom, after passing through the top of the target layer at the start-up point, the wellbore is driven downwards once, reaching 1 / 4 of the horizontal projection length of the wellbore and then passing through the bottom of the target layer. Then, the wellbore is driven upwards once, reaching 2 / 4 of the horizontal projection length of the wellbore and then passing through the top of the target layer. Then, the wellbore is driven downwards a second time, reaching 3 / 4 of the horizontal projection length of the wellbore and then passing through the bottom of the target layer again. Finally, the wellbore is driven upwards a second time, reaching 4 / 4 of the horizontal projection length of the wellbore and then passing through the top of the target layer again, thus completing the W-shaped horizontal wellbore completion.

[0043] The aforementioned W-shaped well design spans the entire projected length of the wellbore, which is typically 1500m or even longer. This ensures a relatively low build-up rate in the wellbore and does not affect subsequent operations such as installing bridge plugs.

[0044] The build-up rate, also known as the build-up strength, is the total bend angle of the borehole formed by the drilling footage during the build-up drilling. Commonly used units include degrees / meter, degrees / 10 meters, degrees / 100 meters, etc.

[0045] The reason for adopting the above-mentioned W-shaped well structure is mainly because the effect of hydraulic fractures penetrating longitudinally is not obvious. The limitation of hydraulic fractures penetrating longitudinally is overcome by penetrating the well shaft longitudinally.

[0046] 2) In order to further enhance the longitudinal fracturing effect, a continuous tubing with an ejector is used to change the previous spiral perforation to a fixed-surface continuous jet perforation. The streamline direction of the ejected liquid is perpendicular to the horizontal bedding plane, which changes the conventional practice of always being perpendicular to the wellbore axis, so as to maximize the longitudinal fracturing effect of hydraulic fractures.

[0047] Example 1

[0048] A fracturing method for lacustrine shale gas wellbore involves drilling a horizontal wellbore along a designed W-shaped wellbore trajectory. The wellbore is first drilled downwards at the start-up point, reaching the bottom of the target layer at 1 / 4 of the horizontal projection of the wellbore. Then, it is drilled upwards once, reaching the top of the target layer at 2 / 4 of the horizontal projection. Next, it is drilled downwards a second time, reaching the bottom of the target layer again at 3 / 4 of the horizontal projection. Finally, it is drilled upwards a second time, reaching the top of the target layer again at 4 / 4 of the horizontal projection. After completion of the W-shaped horizontal wellbore, hydraulic fracturing operations are performed within the wellbore to create fractures.

[0049] 1) W-type wellbore trajectory optimization design

[0050] The design principles for W-type wellbore tracks are as follows:

[0051] Target design: based on the creation of angled points (reference) Figure 1Point A is taken as the first target point. The points at 1 / 4, 2 / 4, 3 / 4, and 4 / 4 of the horizontal projection of the wellbore are the second to fifth target points, respectively. Figure 1 Points B, C, D, and E in the middle.

[0052] Build-up rate design: With the goal of minimizing the unused volume of the reservoir (requiring a larger build-up rate), the maximum probability of crossing vertical multi-lithological layers is considered, and the build-up rate of each build-up section in the W-shaped wellbore trajectory is considered to ensure the smooth entry and exit of subsequent hydraulic fracturing tools (requiring that the build-up rate of each build-up section does not change abruptly).

[0053] By simulating the process of lowering and lowering hydraulic fracturing tools under different build-up rates in each section of a W-shaped wellbore track on the ground, the build-up rate of each section in the W-shaped wellbore track that meets the design principles of the W-shaped wellbore track can be determined.

[0054] 2) Based on the optimization results of step 1), implement directional drilling and run casing for cementing and completion.

[0055] 3) Hydraulic fracturing operations are carried out in a W-shaped horizontal well to create fractures.

[0056] Example 2

[0057] This embodiment is a further optimization of embodiment 1. Based on the W-shaped wellbore trajectory, a fixed-plane continuous jet perforation process is used to carry out hydraulic fracturing operations.

[0058] A fracturing method for lacustrine shale gas wells involves drilling a horizontal wellbore along a designed W-shaped wellbore trajectory. The wellbore is first drilled downwards at the start-up point, reaching the bottom of the target layer at 1 / 4 of the horizontal projection of the wellbore. Then, it is drilled upwards once, reaching the top of the target layer at 2 / 4 of the horizontal projection. Next, it is drilled downwards a second time, reaching the bottom of the target layer again at 3 / 4 of the horizontal projection. Finally, it is drilled upwards a second time, reaching the top of the target layer again at 4 / 4 of the horizontal projection. After completion of the W-shaped horizontal wellbore, hydraulic fracturing operations are performed within the wellbore using a continuous jet perforation technique.

[0059] 1) W-type wellbore trajectory optimization design

[0060] The design principles for W-type wellbore tracks are as follows:

[0061] Target design: based on the creation of angled points (reference) Figure 1 Point A in the middle is taken as the first target point. The points at 1 / 4, 2 / 4, 3 / 4, and 4 / 4 of the horizontal projection of the horizontal wellbore are the second to fifth target points, respectively. (Reference) Figure 1Points B, C, D, and E in the middle;

[0062] Build-up rate design: With the goal of minimizing the unused volume of the reservoir (requiring a larger build-up rate), the maximum probability of crossing vertical multi-lithological layers is considered, and the build-up rate of each build-up section in the W-shaped wellbore trajectory is considered to ensure the smooth entry and exit of subsequent hydraulic fracturing tools (requiring that the build-up rate of each build-up section does not change abruptly).

[0063] Considering that the injection direction is perpendicular to the bedding fracture direction of different lithologies, the build-up rate between any two of the five target points in the W-type wellbore trajectory is designed to be different.

[0064] By simulating the process of lowering and lowering hydraulic fracturing tools under different build-up rates in each section of a W-shaped wellbore track on the ground, the build-up rate of each section in the W-shaped wellbore track that meets the design principles of the W-shaped wellbore track can be determined.

[0065] 2) Based on the optimization results of step 1), implement directional drilling and run casing for cementing and completion.

[0066] 3) Optimization of jet flow path parameters

[0067] The injector creates cracks by spraying high-speed pumped abrasive material. (Reference) Figure 2 and Figure 3 As shown, the injector 1 includes a tube with a disc 101 for connecting to a continuous oil pipe; a plurality of nozzles 102 are distributed on a portion of the outer circumferential surface of the disc 101, and a flow channel 103 is provided inside the disc 101 corresponding to each nozzle, through which the continuous oil pipe and the nozzle are connected.

[0068] The optimization objectives are to minimize pressure consumption and the corrosive effect of sand-containing liquid on the flow channel. The size and shape of the flow channel (such as an arc flow channel) are determined by a comprehensive trade-off based on ground simulation experiments and fluid-structure interaction simulation software (such as FLUENT).

[0069] 4) Design injectors for different clustering orifices within the section.

[0070] Considering the uniform extension of different burst hole cracks within the segment and cracks near the toe (reference) Figure 1 The insufficient extension of the crack at point E can be addressed by designing flow channels with varying diameters, with the flow channel diameter being larger near the toe. For details, refer to... Figure 4 As shown, two injectors are connected in series via ball seat 2 (the ball seat and the injector are connected by a thread). The injector that is directly connected to the continuous tubing is the rear injector. The flow channel of the rear injector 2 is smaller in size than that of the front injector.

[0071] 5) Design and installation of the angle between the ejector and the coiled tubing axis at different build-up sections of a W-type wellbore trajectory: (Refer to...) Figure 5 As shown, the attitude of the coiled tubing relative to the horizontal bedding fracture in each build-up section of the W-shaped wellbore trajectory is determined based on the build-up rate of each build-up section. A plane is cut at any position along the coiled tubing in the corresponding attitude, perpendicular to the horizontal bedding fracture. The angle between this plane and the coiled tubing axis is the angle between the ejector and the coiled tubing axis. This plane is a slanted section for coiled tubing with a certain slope, but for vertical fractures, it represents the direction of fracture extension, minimizing the frictional effect of near-wellbore bending fractures.

[0072] Regarding the installation of the injector, refer to... Figure 5 As shown, the rear end of the injector tube is connected to the continuous tubing via an internally hollow universal joint 4, allowing the angle between the injector and the continuous tubing to be adjusted.

[0073] refer to Figure 4 As shown, the front end of the injector tube is threadedly connected to an eccentric counterweight 3. Adjusting the eccentric counterweight to be aligned with the nozzle axis ensures that the injector's spray direction remains perpendicular to the horizontal stratification direction under the action of gravity.

[0074] The injector is installed on the coiled tubing. The coiled tubing adapts to the inclination rate of each inclination section. Since the angle between the injector and the coiled tubing can be adjusted, the injector will not hinder the angle change of the coiled tubing. At the same time, due to the effect of the eccentric counterweight, the injection direction of the injector remains perpendicular to the direction of the horizontal bedding joint.

[0075] By rotating the injector to reverse the nozzle, the direction of spraying towards the top and bottom layers of the target layer can be switched.

[0076] 6) Assemble and test the above-mentioned pipe string on the ground, and install flow meters at the corresponding injection outlet positions. If the flow difference between two adjacent injector outlets is greater than 10%, return to steps 3)-5) to adjust the relevant parameters until the above requirements are met.

[0077] 7) Conduct on-site testing. Construction parameters are executed according to the design plan. Let N be the number of perforation clusters within the inclined section. When N is even, the number of injections is N / 2. When N is odd, a ball seat is installed between the two injectors. When the number of injections reaches (N-1) / 2, the ball seat is sealed to block the fluid inlet channel of the front injector, and then one final injection is performed. If the improvement in fracturing effect compared to the adjacent well using conventional completion tubing is less than 30%, then return to steps 1)-6) for further optimization and adjustment until the simulated improvement in fracturing effect compared to the adjacent well using conventional completion tubing is not less than 30%.

[0078] The following example, Well A, is used to illustrate the well fracturing method of the present invention in more detail:

[0079] 1) Optimization design of W-type wellbore trajectory for Well A

[0080] The design of five target points for the W-shaped wellbore trajectory takes into account the design of the build-up point and the maximum build-up rate (to minimize the unused volume of the reservoir) to reach the top of the target layer, the maximum probability of crossing the vertical multi-lithological sub-layers, and the smooth insertion and retrieval of the subsequent bridge plug and perforating gun due to the wellbore inclination.

[0081] If necessary, simulations of the insertion and removal of bridge plugs and perforating guns under different inclinations of the W-shaped wellbore trajectory can be conducted on the ground to determine the optimal inclination of the W-shaped wellbore trajectory.

[0082] 2) Based on the optimization results of step 1), implement directional drilling and run casing for cementing and completion.

[0083] 3) Injector design

[0084] At the end of the coiled tubing, an ejector is installed. The size and shape of the flow channel are selected based on ground simulation experiments and commonly used fluid-structure interaction simulation software such as FLUENT, and the objective function of optimization is to minimize pressure consumption and the abrasive effect of sand-containing fluid on the flow channel.

[0085] 4) Design and manufacture of the flow channel morphology and dimensions within the above-mentioned injectors at different cluster injection holes within the section.

[0086] Considering the balanced extension of different burst hole cracks within the segment and the generally insufficient extension of cracks near the toe, the above-mentioned flow channels can be designed with different diameters, with the flow channel diameter near the toe being larger.

[0087] In manufacturing, two injectors can be connected in series, with the inlet pipe closer to the heel injector having a smaller diameter to match the smaller diameter flow channel size inside the injector. The other inlet pipe is connected to the first inlet pipe in a two-way configuration and directly to the end of the coiled tubing.

[0088] 5) Design and installation of the angle between the ejector and the coiled tubing axis at different build-up sections of the W-type wellbore trajectory: Cut a plane along a certain position of the coiled tubing in a direction perpendicular to the horizontal bedding fracture. This plane is a slanted slab for the coiled tubing with a certain slope, but for the vertical fracture, it is the extension direction of the vertical fracture, which can minimize the friction effect of the near-wellbore bending fracture.

[0089] 6) Assemble and test the above-mentioned pipe string on the ground, and install flow meters at the corresponding injection outlet positions. The flow error between two adjacent injector outlets should be within 6%.

[0090] 7) Numerical simulation tests were conducted indoors. Construction parameters were executed according to the design plan. The post-fracturing effect was 50% better than that of adjacent wells using conventional completion tubing.

[0091] Example 3

[0092] This embodiment is a further optimization of embodiment 2. The only difference from embodiment 2 is the injector: the injector includes a tube with a disc body, which is used to connect to a continuous oil pipe; a plurality of nozzles are evenly distributed on the outer circumference of the disc body, and a flow channel is provided in the disc body corresponding to each nozzle, through which the continuous oil pipe and the nozzle are connected.

[0093] Hydraulic fracturing operations are carried out by combining directional perforation technology with fixed-surface continuous circumferential jet perforation technology: when the wellbore is at the top of the target layer, downward directional perforation technology is used to create fractures; when the wellbore is at the bottom of the target layer, upward directional perforation technology is used to create fractures; and when the wellbore is at the middle of the target layer, fixed-surface continuous circumferential jet perforation technology is used to create fractures.

[0094] The fixed-surface continuous jet perforation process can simultaneously jet perforations at the top and bottom of the target layer in the wellbore without repeatedly reversing the nozzle direction, thus achieving higher efficiency.

[0095] In the description of this invention, unless otherwise stated, the terms "upper," "lower," "left," "right," "inner," "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0096] The above technical solution is only one embodiment of the present invention. For those skilled in the art, based on the principles disclosed in the present invention, it is easy to make various types of improvements or modifications, and not limited to the technical solutions described in the specific embodiments of the present invention. Therefore, the foregoing description is only a preferred option and is not restrictive.

Claims

1. A fracturing method for lacustrine shale gas wellbores, characterized in that, The wellbore trajectory of the horizontal wellbore is designed as a W-shaped wellbore trajectory: four consecutive build-up sections travel through the top and bottom of the target layer in a downward, upward, downward, and finally upward trajectory. After the W-type horizontal well is completed, hydraulic fracturing operations are carried out inside the W-type horizontal well to create fractures. Hydraulic fracturing operations are carried out by combining directional perforation technology with fixed-surface continuous jet perforation technology: when the wellbore is at the top of the target layer, downward directional perforation technology is used to create fractures; when the wellbore is at the bottom of the target layer, upward directional perforation technology is used to create fractures; and when the wellbore is at the middle of the target layer, fixed-surface continuous jet perforation technology is used to create fractures.

2. The fracturing method for lacustrine shale gas wellbores according to claim 1, characterized in that: The W-shaped wellbore trajectory is as follows: after passing through the top of the target layer at the build-up point, it is driven downward once, reaching 1 / 4 of the horizontal projection of the horizontal wellbore and then passing to the bottom of the target layer. Then it is driven upward once, reaching 2 / 4 of the horizontal projection of the horizontal wellbore and then passing to the top of the target layer. Then it is driven downward a second time, reaching 3 / 4 of the horizontal projection of the horizontal wellbore and then passing to the bottom of the target layer again. Finally, it is driven upward a second time, reaching 4 / 4 of the horizontal projection of the horizontal wellbore and then passing to the top of the target layer again.

3. The fracturing method for lacustrine shale gas wellbores according to claim 2, characterized in that: The design principles for W-type wellbore tracks are as follows: Target point design: The starting point is taken as the first target point, and the points at 1 / 4, 2 / 4, 3 / 4 and 4 / 4 of the horizontal projection of the horizontal wellbore are the second to fifth target points, respectively. Draft rate design: With the goal of minimizing the unused volume of the reservoir, the maximum probability of crossing the vertical multi-lithological layers is considered, and the draft rate of each draft section in the W-shaped wellbore trajectory is considered to ensure the smooth entry and exit of subsequent hydraulic fracturing tools.

4. The fracturing method for lacustrine shale gas wellbores according to claim 3, characterized in that: By simulating the process of lowering and lowering hydraulic fracturing tools under different build-up rates in each section of a W-shaped wellbore track on the ground, the build-up rate of each section in the W-shaped wellbore track that meets the design principles of the W-shaped wellbore track can be determined.

5. The fracturing method for lacustrine shale gas wellbores according to claim 1, characterized in that, A fixed-surface continuous jetting perforation process is implemented using an injector: the injector includes a tube with a disc body, which is used to connect to a continuous oil pipe; a number of nozzles are distributed on the outer circumference of the disc body, and a flow channel is provided in the disc body corresponding to each nozzle, through which the continuous oil pipe and the nozzle are connected.

6. The fracturing method for lacustrine shale gas wellbores according to claim 5, characterized in that, The size and shape of the jet's flow channel were optimized: with the optimization objectives of minimizing pressure consumption and the corrosive effect of sand-containing liquid on the flow channel, the size and shape of the flow channel were determined by a comprehensive trade-off based on ground simulation experiments and fluid-structure interaction simulation software.

7. The fracturing method for lacustrine shale gas wellbores according to claim 6, characterized in that, The rear end of the injector tube is connected to the continuous tubing via an internally hollow universal joint, allowing the angle between the injector and the continuous tubing to be adjusted. An eccentric counterweight is threaded to the front end of the injector tube. Adjusting the eccentric counterweight to align with the nozzle axis ensures that the injector's spray direction remains perpendicular to the horizontal stratification direction under gravity.

8. The fracturing method for lacustrine shale gas wellbores according to claim 7, characterized in that, Two injectors are connected in series by a conduit. The injector that is directly connected to the continuous tubing is the rear injector. The flow channel of the rear injector is smaller in size than that of the front injector.

9. The fracturing method for lacustrine shale gas wellbores according to claim 8, characterized in that, Measure the injection flow rate of the two injectors. If the difference in injection flow rate between the two injectors is greater than 10%, then re-optimize the continuous injection perforation process parameters, including the flow channel and / or the angle between the injector and the axis of the continuous tubing.

10. The fracturing method for lacustrine shale gas wellbores according to claim 9, characterized in that, Numerical simulations are conducted before the actual hydraulic fracturing operation. If the simulated fracturing effect is less than 30% better than that of the adjacent well using conventional completion tubing, the W-shaped wellbore trajectory and / or fixed-face continuous jet perforation process parameters are redesigned until the simulated fracturing effect is no less than 30% better than that of the adjacent well using conventional completion tubing.

11. The fracturing method for lacustrine shale gas wellbores according to claim 8, characterized in that, Let N be the number of perforation clusters in the inclined section. When N is even, the number of injections is N / 2. When N is odd, a ball seat is installed between the two injectors. When the number of injections reaches (N-1) / 2, a ball is thrown to seal the ball seat to block the liquid inlet channel of the front injector. Finally, one more injection is performed.

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