We designed and developed a constant angular acceleration flexible drill pipe made of 40CrNiMoA alloy structural steel. However, tests have revealed a lot of problems when the horizontal section exceeded a certain length, such as frequent occurrence of string break, and failure of WOB and torque transmission to the bit. These problems limit the length of horizontal section. Therefore, considering the requirement of flexible sidetracking horizontal well stimulation technology, the structure and material of key parts of the flexible drill pipe were optimized.

The subs of the flexible drill pipe are connected by threads (Fig. 1). Since the threads have lower strength than the pipe body, the drill pipe is frequently subjected to thread off and sliding at normal working loads, due to the bucket effect. First, hardness tests were conducted on the materials of three key parts of the flexible drill pipe: universal joint housing, ball head and end shield. Three specimens were randomly selected for each part, and full wall thickness strip samples were taken along the axial direction from the middle part of each specimen and then tested for Rockwell hardness (HRC) according to the standard GB/T 230.1-2018 ^[11]. Then the HRC was converted to Brinell hardness (HB). The test results are given in Table 1. It is indicated that the hardness of the end shield material is low, and the nitriding process needs to be further optimized.

Second, mechanical performance tests were conducted on the material of flexible drill pipe. For this purpose, the indentation test was used instead of traditional tensile test, because the flexible drill pipe is too small to yield qualified samples. During the indentation test, an indenter was used to continuously load and unload the sample to obtain the load-indentation depth curve. Then, the elastic modulus of the material was determined based on the unloading segment data of the curve. Finally, the stress-strain curve, tensile strength and yield strength of the material were obtained depending on the relationship between the indentation load and depth. In this study, column plane indentation tests were conducted on three parts: ball head, end shield, and universal joint housing, and each part was tested at 3 points. The test results are listed in Table 2.

Cracks were observed macroscopically on the surface of the end shield, so samples were taken from the crack location shown in Fig. 2a for metallographic examination. The root crack is shown in Fig. 2b. As shown in Fig.2a, there are several cracks, starting from the internal surface, on the internal thread section of the end shield, and no anomaly is observed in the texture near the cracks.

Based on the analysis results of material properties and cracks, stress simulation analysis was conducted on the broken parts. Axial compressive and torsional loads were applied on the broken part model. The torque was 6, 8, 10 and 12 kN·m, respectively, and the axial pressure was 40 kN. Simulation was conducted on the stress status of the model under four working conditions using the explicit dynamic finite element method. The equivalent stress distribution of the flexible drill pipe was obtained by elasto-plastic analysis. Fig. 3 shows the equivalent stress distribution of the end shield when the torque is 12 kN·m. Obviously, the maximum equivalent stress of the flexible drill pipe is located at the escape root of the end shield thread, which is consistent with the root crack position of the thread. When the torque is 12 kN·m, the maximum equivalent stress is 909 MPa, exceeding the yield strength of 40CrNiMoA alloy structural steel (835 MPa). When the torque is 10 kN·m, the maximum equivalent stress is 757 MPa, which is less than the yield strength of 40CrNiMoA alloy structural steel. It is inferred that the thread structure of the end shield is relatively safe when the working torque of the flexible drill pipe is controlled below 10 kN·m. However, the downhole torque cannot be accurately controlled during field operations. Therefore, the thread structure needs to be optimized depending on the crack analysis and stress simulation analysis results.

The thread pitch was increased from 6 mm to 10 mm, and the thread angle was increased to 30°, effectively increasing the root width. Furthermore, a circular arc transition was added at the root to improve the root strength and alleviate stress concentration while ensuring sealing ability. The inspection shows that the optimized thread does not have apparent damage under a torque of 20 kN·m, and its torsional capacity is significantly improved compared to the existing structure ^[10].

The original material 40CrNiMoA of the flexible drill pipe body is a kind of medium-carbon low-alloy high-strength steel. This material after oil quenching at 850 °C and tempering at 600 °C has the yield strength of 840-850 MPa, tensile strength of 983-997 MPa and elongation of 11%-13%. Despite of good mechanical properties, it may be susceptible to fatigue failure and severe wear when it is used as the material of flexible drill pipe body. The current material needs to be improved. By means of quenching + high temperature tempering tests, a low-carbon low-alloy high-strength malleable steel was selected. After oil quenching at 850 °C and tempering at 540 °C, this steel material exhibits the yield strength of 930-945 MPa, tensile strength of 1 090-1 170 MPa and elongation not less than 10.5%. In addition, considering the relatively low carbon content in the material, addition of Ni and Mo alloying elements allows for refinement of the martensitic structure and the sorbitic structure subsequently formed by high-temperature tempering, thus improving the impact toughness after tempering (Fig. 4). According to laboratory test results, the impact energy reaches 71 J. Therefore, the improved material has better mechanical properties, and can effectively enhance the strength of the drill pipe.

To solve the problems such as high transmission drag and severe wear at the hinge joint of flexible drill pipe, according to the concept of bionics, by imitating the structure of skeletal joints, the outer surface of the ball socket in direct contact with the drill pipe ball head was replaced with a softer copper-based composite material, achieving the goal of reducing friction and drag. Friction/wear tests were conducted, showing that during the friction process, the wear scar depth, debris quantity and oxide quantity of the copper-based composite material are small, and the wear rate, friction coefficient and fluctuation are apparently lower than those of the original material. These features help effectively reduce the torque transmission loss between drill pipes, and allow the bit to obtain sufficient rock breaking energy even if the horizontal section is overlong. Finite element analysis was conducted to obtain the stress distribution at hinge assembly of the copper-based composite material (Fig. 5), showing that the maximum equivalent stress of the ball head on the assembly is 386.32 MPa, located at the seal groove of the ball head. The equivalent stress of the contact surface between the copper-based alloy ball socket and ball head is 150-160 MPa (the yield strength of the copper-based alloy is 350 MPa). After interference assembling, neither the ball head nor the ball socket undergoes plastic deformation, indicating that the hinge joint can still maintain the required flexibility under normal load.

The effectiveness of the flexible sidetracking horizontal well technology depends on equipment and tools, and also on the optimization of drilling process. The flexible drill pipe adopts a hinged structure, which enables it to have excellent bend throughput capacity. However, the lack of rigidity makes the flexible drill pipe far inferior to conventional drill pipes in terms of force transmission efficiency, strength and structural stability, resulting in a series of problems such as low transmission efficiency, high drilling risk and poor directional control accuracy. To solve these problems, the drilling process was improved from various aspects such as dynamic analysis and laboratory experiment optimization.

First, a multi-body dynamic analysis was conducted to explore the downhole dynamic properties and load transfer patterns of flexible drill pipe, and clarify the relationships between drilling process parameters (e.g. WOB, rotation speed) or wellbore geometry parameters (e.g. wellbore curvature radius, horizontal section length, and wellbore diameter) and the motion and stress states of flexible drill pipe, laying a foundation for the design of drilling process. Second, a test bench for sidetracking scaled model of flexible drill pipe was built (Fig. 6) according to the similarity theory. Combined with a high-speed camera, the eddy current displacement sensor can collect the displacement at any section of the flexible drill pipe. The dynamic torque sensor, tension sensor and compression/torsion sensor can synchronously collect WOB, torque and friction. The real downhole motion and stress states of drill pipes were restored through scaling transformation, thereby assisting in improving the setting of multi-body dynamics model parameters and boundary conditions.

The above research results were applied to guide the field operations of drilling process. With the help of the improved flexible drill pipe, the WOB and rotation speed were properly controlled through easing the bit in light pressure, so that the problems of low transmission efficiency and high drilling risk of flexible drill pipe were effectively solved. At present, the maximum length of horizontal section reaches 80 m, and the risk of drill pipe breakage and falling into the well is significantly reduced.

Considering the problem of "inaccurate drilling" caused by poor directional control accuracy, a causal chain analysis was conducted. It is found that the primary cause is the overlong rigid length of the downhole motor, which is not suitable for directional build-up during flexible sidetracking. Under current technical conditions, the sidetracking azimuth is orientated only relying on whipstock, and the tool face is uncontrollable during kick off and build-up processes ^[10]. A flexible turbodrill directional drilling technology was proposed. The conventional turbodrill is transformed for flexibility to allow it to penetrate deep into the curved portion during build-up process. With the help of gyroscopic orientation measurement, the face of the drilling tool is controlled to make the azimuth error below ±10°. Meanwhile, the application of flexible turbodrill realizes downhole drive during the process of build-up and horizontal drilling. The upper flexible drill pipe only bears the reactive torque of turbine. Since this torque is smaller than the torque of rotary table, the drill pipe breakage risk is reduced.

To accurately understand the distribution of remaining oil and further evaluate the oil reservoirs encountered by sidetracking horizontal well, the flexible sidetracking and sealed coring technology was developed. In contrast to early coring tools which take cores with small diameter and susceptibility to downhole fluid contamination, an ultrashort-radius large-diameter flexible sealed coring tool (Fig. 7) was innovated by integrating the articulated flexible drill pipe and sealed coring barrel. The articulated structure was optimized to improve the integrity of the core, and the core barrel was prefilled with sealing fluid to isolate drilling fluid contamination. According to design, the coring diameter is 42 mm, and the coring length in a single trip reaches 1.5 m.

The coring tool contains a sealing mechanism, which releases sealing fluid to enwrap the core when it enters the core barrel, so as to preserve the information contained in the core as much as possible. The core barrel is made of TC4 pipe with an outer diameter of 55 mm and an inner diameter of 42 mm. Sealing fluid is prefilled, and a core barrel sealing mechanism with check valve as its main structure is designed near the bit to ensure the sealing fluid not to flow out. During the coring process, the sealing mechanism is opened by the compression of the core, so that the sealing fluid flows out to wrap around the core. The mechanism gradually enters the core barrel along with the core. After core drilling is completed, the ball-dropping pressurizing method is used to push the core catcher to shrink radially and cut the core. To verify the coring effect, laboratory coring experiment was conducted using the flexible sidetracking and sealed coring tool, with results shown in Fig. 8. In the experiment, the rotation speed was 40 r/min, the WOB was 1 t, the torque was 300 N·m, the coring footage was 1 500 mm, and the drill time was 48 min. The experimental results show that the sealing fluid can uniformly enwrap the core. After removing the sealing fluid on the surface of the core, the core diameter and length were measured to be 41.8 mm and 1 450 mm, respectively, and the core recovery reached 97%.

The horizontal section is sidetracked to the remaining oil enriched area depending on the remaining oil distribution in low-permeability reservoirs. Due to small curvature radius and borehole size, open hole completion is adopted for the sidetracked horizontal section. It has become the key to effective production enhancement by optimizing the fracturing scheme to efficiently stimulate the open hole section and thus significantly improve the well production ^[16-17]. The optimization of fracturing scheme mainly includes two parts: optimization of fracture parameters, and optimization of fracturing parameters. The main purpose is to clarify the reservoir fracture parameters (number, size and conductivity of fractures, etc.) required to achieve the maximum post-fracturing production under the current remaining oil distribution and stress field conditions, as well as the fracturing parameters (displacement, fluid volume, proppant volume, etc.) required to achieve the optimal fractures mentioned above.

For flexible sidetracking horizontal well with small wellbore curvature radius (2-4 m), conventional jetting tool with large size cannot pass through the build section when open hole completion mode is adopted. Therefore, a new type of hydraulic ejector was designed (Fig. 12), with a length of 150 mm, 6 holes (5 mm in size), and a phase angle of 60°. One end of the ejector is designed with a universal joint structure, in which a dynamic sealing structure is adopted at the fitting surface. This sealing structure has a sealing pressure exceeding 50 MPa, and can smoothly pass through the build section to the open hole section of the sidetracking horizontal well, while meeting the seal requirements under fracturing conditions. A circular arc transition is used inside the joining part between the ejector body and the universal joint to reduce the frictional resistance caused by the sudden change in structural size when liquid flows through. Meanwhile, antifriction materials are sprayed on the liquid passage, extending the service life of the ejector by more than 30%. In addition, a centralizing structure is equipped to effectively reduce the energy loss caused by vibration during jetting, thus improving the effectiveness of jetting.

Since the curvature radius of ultrashort-radius sidetracked hole is less than 4 m, a conventional packer with large rigidity cannot smoothly pass through the build section to the designed setting position. To ensure the smooth throughput in sidetracked hole ^[19], the conventional expanding open hole packer was optimized structurally. Similar to the flexible drill pipe transmission structure, the upper and lower connection joints of the packer all adopt ball head design (Fig. 13). The mandrel of the packer is made of titanium alloy. When passing through the build section, the packer body can be elastically bent within a range of 7°, forming a flexible expanding open hole packer. This packer has a length of 620 mm, the outer diameter of 95 mm, the pressure resistance of 50 MPa, and the minimum allowable throughput curvature radius of 1.8 m. During hydraulic fracturing, the flexible expanding open hole packer is inserted deep into the horizontal section, and the designed setting point is more than 10 m away from the original wellbore to avoid the initiation of old fractures. The distance between the first jet perforation position and the original wellbore is increased from 10 m to 15 m, reducing the risk of connection of propagated fractures to old fractures.

By means of laboratory experiments and numerical simulation, combined with physical property characteristics of target reservoirs, the hydraulic jet pumping rate is optimized as 1.0-1.5 m³/min, the nozzle jet velocity as 142-198 m/s, and the sand concentration of jetting fluid as 6%-10% ^[20⇓-22]. During field fracturing operations, the hydraulic jet string is run in hole to conduct hydraulic jetting at the optimized perforation position, forming a mechanical weak surface at the jetting position to induce hydraulic fracture initiation and propagation. The hydraulic jetting string can be lifted in turn to achieve hydraulic jetting at multiple perforation positions. Then, after the jetting is completed, the hydraulic jetting string is tripped out, and the fracturing string is run in hole for fracturing process. The fracturing string is composed of new hydraulic ejector, flexible expanding open hole packer, flexible drill/composite pipe, and hydraulic safety joint, etc., from bottom to top. During the fracturing process, the intra-fracture temporary plugging agent is added to increase the net pressure in the fracture, facilitating the uniform propagation of multi-cluster fractures and the opening of natural fractures ^[23⇓-25].

The Chang 6 oil reservoir in the Ansai Oilfield of Changqing is a typical ultra-low permeability reservoir, with an average buried depth of 1 365 m, a formation temperature of 44.5 °C, an initial formation pressure of 9.13 MPa, a saturation pressure of 6.23 MPa, a formation-saturation pressure difference of 2.90 MPa, and a pressure coefficient of 0.7-0.8. The reservoir thickness, porosity and permeability are 2-10 m, 8%-14% and (0.2-1.0)×10⁻³ μm², respectively, and the oil saturation is 40%-60%. The oil reservoir has been developed by water injection for more than 30 years. Currently, the oil reservoir is being developed by a row well pattern, with a well array spacing of 160 m×200 m, the reserves recovery percent of 16.86%, and the composite water cut of 72.5%. It is in the middle-late development stage, facing the challenges such as low well productivity and difficult establishment of an effective pressure system. In this context, the flexible sidetracking horizontal well stimulation technology is designed to reduce the oil/water well array spacing, establish a pressure displacement system, realize pressure conduction, and ultimately enhance the production of low-yield and low-efficiency wells ^[26-27].

The observation of cores taken from inspection wells such as An 201 and Bai 209 shows that the cores are slightly washed at 25 m from the water line, and not washed beyond 40 m. The vertical washing thickness is less than 1 m. The injected water rushes along natural fractures, and remaining oil is enriched in a large area both horizontally and vertically. Based on remaining oil distribution law and the characteristics of flexible sidetracking horizontal well stimulation technology, ultra-short radius horizontal wells are deployed: (1) in the blocks with lateral local massive remaining oil distributed on the sides of fractures, to effectively improve the control degree of reserves and the well productivity; and (2) in the ultra-low permeability blocks characterized by large well array spacing, ineffective long-term waterflooding, and poor adaptability of basic well pattern, where the reserves recovery percent is less than 6%, the recovery rate is less than 4%, and the composite water cut is less than 50%, to reduce well array spacing, and quickly establish an effective pressure displacement system, thereby improving the stimulation effect and the well productivity.

According to the characteristic of remaining oil enrichment on the sides of fractures in low-permeability reservoirs, flexible sidetracking horizontal wells can directly communicate with the remaining oil enriched area. Therefore, accurately evaluating the distribution of remaining oil to guide the determination of sidetracking horizontal well azimuth and length is an important prerequisite for improving post-fracturing production and achieving efficient recovery of remaining oil ^[28⇓-30]. Numerical simulation was conducted on the remaining oil distribution in the demonstration area. First, based on a great deal of actual data, the reservoir numerical simulation software was used to build a geological model for the injection-production well group determined according to the location of the target well. Then, based on well logging and primary fracturing data, the model was refined by setting the parameters such as permeability, porosity and initial fracture. Finally, a history matching was conducted on the production performance of the well group to determine the remaining oil distribution of the well group, and the sidetracking horizontal well azimuth was optimized based on the remaining oil distribution results. In this case, the remaining oil is mainly enriched within 110 m range of the minimum horizontal principal stress of the target well, the oil saturation is 42%-57%, and the remaining oil saturation is higher in the upper layer. Therefore, the optimized sidetracking horizontal well azimuth is the minimum horizontal principal stress azimuth, that is, the azimuth is 337° or 157°. Based on the distribution of remaining oil saturation, the length of the sidetracking horizontal well is further optimized: if the sidetracking horizontal well is drilled along the 337° azimuth, the length of the horizontal section should be limited to 50 m to prevent from communicating the waterflooding front; if the sidetracking horizontal well is drilled along the 157° azimuth, the length of the horizontal section can be increased to 100 m.

From 2020 to 2021, the flexible sidetracking stimulation technology was successfully applied in 10 wells of the Ansai Oilfield of Changqing. The length of sidetracked horizontal sections is 24.0-43.3 m, the azimuth of sidetracking horizontal wells is controlled at 328°-24° or 155°-192°, the fracture spacing is 9-12 m, the number of clusters is 2-4, the pumping rate is 3-4 m³/min, the pumped-in fluid volume is 300-400 m³, and the sand addition volume of single fracture is 15-25 m³ (Table 4). By the end of December 2024, the incremental oil of 10 wells after fracturing was 1 172.0 t individually, and 11 719.6 t in total, indicating good results (Table 5). Due to the low cost of the technology, the break-even point of cumulative incremental oil per well is 400 t ($45/bbl), proving that the technology is economically beneficial.

Based on field operation data, the hydraulic fracturing software was used to perform net pressure fitting for six wells (PJ49-16, PJ48-17, PJ39-41, PJ21-07, PJ13-12 and PJ23-11). The fitting results show that after temporary plugging, the net pressure in the fracture is increased by 4-5 MPa, the fracture height is 15-20 m, and the fracture half-length is 70-80 m.

Post-fracturing comparative analysis indicates two key factors deciding the post-fracturing production of sidetracking horizontal wells. One factor is the azimuth of sidetracked hole. For Well W399-101, the length of sidetracked horizontal section is 30.9 m, and the angle between the sidetracking azimuth and the minimum principal stress direction is 65°-77°. For Well WJ41-0261, the length of sidetracked horizontal section is 26.5 m, and the angle between the sidetracking azimuth and the minimum principal stress direction is 0°. For the two wells, the proppant consumption is 50.4 m³ and 54.2 m³ respectively, and the proppant volume of single cluster is 16.8 m³ and 18.1 m³ respectively, with a difference of about 7.7%. Similar results are found for other reservoirs and operation parameters such as horizon, cluster number, pumping rate, fracturing process, fracturing fluid type and proppant grain size. By the end of 2024, the incremental oil of the two wells was 967.6 t and 2 245.7 t respectively, with a difference of 132.3%. It is believed that a smaller angle between the sidetracking horizontal well azimuth and the minimum principal stress direction is more conducive to communicating the remaining oil and improving the stimulation effect. The other factor is the length of sidetracked horizontal section and the scale of effective stimulation. For Well PJ48-17, the length of sidetracked horizontal section is 42 m, the number of clusters is 4, the proppant volume is 70.0 m³, and the proppant volume of single cluster is 17.5 m³. For Well PJ49-16, these parameters are 24 m, 2, 50.4 m³ and 25.2 m³, respectively. Similar results are obtained for other reservoirs and operation parameters such as horizon, pumping rate, fracturing process, fracturing fluid type and proppant grain size. By the end of 2024, the incremental oil of the two wells was 2 021.2 t and 773.8 t, respectively. Obviously, the longer the sidetracked horizontal section and the more the effective fractures, the higher the post-fracturing initial production and the cumulative production are.