Simulation and Application of Windowing for High-Grade Steel Thick-Wall Casing in Ultradeep Wells: A Case Study of ST102 Well

Windowing processes in ultradeep wells (> 7000 m) with high steel grade (TP155V) and thick-wall (15.83 mm) casing not only face complex underground environments with high temperature and pressure but also have the difficulty of long operation time and high cost. To improve the success rate and operational efficiency, CAE software was used to establish the casing windowing side-drilling simulation model and optimize the windowing milling cone. Field applications were used to further demonstrate the simulation results. The main conclusions are as follows. The simulation shows that the milling shoulder and the outer circle of the cone top are the main working faces in window opening and window repair. The conical milling cone has the largest windowing stress, highest windowing efficiency, and regular shape of the window. A casing with a depth of 7473 m, wall thickness of 15.83 mm, and steel grade TP155V was successfully windowing using six conical milling cone in the ST102 well, taking 51.76 h, and generating 94 kg iron chips in total; the average milling speed was 0.06 m/h; The casing windowing progress can be determined by observing the return state of the cuttings. ST102 well set three records in China for the deepest drilling depth, highest steel grade, and thickest wall thickness.


Introduction
Ultradeep oil and gas resources, such as those in the Penglai gas area in the Sichuan Basin, Shunbei oil field in the Tarim Basin, Kuqa piedmont structure, and southern margin block of the Junggar Basin, constitute an important direction for oil and gas exploration and development in China [1,2].
Window-sidetracking technology can not only be used to tap into the potential of old wells [3][4][5], reduce drilling costs [6,7], and enhance oil recovery [8][9][10][11][12][13], but also to handle sticking and other downhole complex accidents [14][15][16].The common casing windowing methods include section milling and cone milling [17,18].Section milling has a large window area and is convenient for later sidetracking operations; however, section milling is opened by hydraulic pressure, and the application of the well depth is limited [19,20], (only suitable for well depths less than 5700 m).Cone milling has the advantages of no electronic and transmission components, strong temperature and pressure resistance, simple operation, and a high success rate; therefore, it is suitable for deep wells [21,22].
Theoretical research on window sidetracking has mainly focused on section mills.Fluid mechanics simulation and dynamic simulation are used to optimize the flow channel, blade arrangement, and cutting teeth of the section mill to improve the cutting and forging milling efficiency of the section mill and avoid the formation of iron wire and winding milling tools during section milling, which leads to downhole complex accidents such as sticking [23][24][25].The structure of the cone-milling tool is simple, and the operation process is mature.However, most reported studies in this regard are field application reports, and there are few theoretical studies.The 193.7 mm casing of Yuanba 12-1H was milled and sidetracked at a depth of 6330 m.The casing was steel grade TP110TSS with a wall thickness of 12.7 mm [26].Milling window and sidetracking were conducted at a depth and casing of 6465.82 mm and 177.8 mm, respectively, in Well TP17CX of the Tahe Oilfield.The steel grade of the casing was TP110TS, and the wall thickness was 12.65 mm [27].
There are few reports on the field application of milling and windowing at a depth of more than 7000 m, casing steel grade greater than the P110 steel grade, and wall thickness greater than 13.00 mm.The windowing and sidetracking of the ST102 well are faced with the problems of ultra-depth, high temperature and pressure, high steel grade, and large wall thickness.To ensure the success of windowing, this study adopts simulation to optimize the milling tools and carry out field applications.

Simulation
The specific windowing process is as follows: (1) Down into the whipstock using the anchoring mechanism.(2) Determine the orientation of the whipstock according to the windowing orientation, and set the whipstock.(3) Down into the milling cone, and start the windowing operation.Figure 1 shows the common types of milling cones suitable for sidetracking in the ST102 well, including the conical milling cone, spiral milling cone, and bit-type milling cone.

Geometric and Mesh Models
The main parameters of the geometric model are as follows.The milling cone had an outer diameter of Ф150 mm, the whipstock had an outer diameter of 146 mm, the whipstock concave had an angle of 4°, and the casing had an outer diameter and wall thickness dimensions of Ф184.15 mm × 15.83 mm.The milling cone and whipstock adopted tetrahedral meshes, and the casing adopted hexahedral meshes, as shown in figure 2.

Figure 2.
Three-dimensional mesh models of different tools.

Dynamic Simulation Results
According to the contact relationship between the milling cone and the casing, the windowing process can be divided into three stages: (1) the downward stage of the milling cone; (2) milling casing stage; (3) out of the window stage.

Equivalent Plastic Strain Analysis.
Figure 4 shows the equivalent plastic strain results for the conical, spiral, and bit-type milling cones during the windowing.It can be observed from the figure that the maximum stress of the three milling cones occurs outside the window stage, and the maximum plastic strains of the conical, spiral, and bit-type milling cones are 1.24, 0.83, and 0.88 mm, respectively.The equivalent plastic strain of conical milling was the largest.In summary, the use of a conical milling cone windowing operation is recommended.As milling progressed, the casing-damage strain gradually increased and reached a maximum at the out-of-window stage.In the downward stage, the milling shoulder is the main working face.In the milling stage, the outer circle of the milling cone head and end face of the milling cone side are the main working faces.In the out-of-window stage, the milling cone side end face is the main working face.

Corn Point Stress Analysis.
In the milling stage, the milling cone passes through the corn point above the whipstock concave.At this time, a large stress is generated at the 'corn point'.Figure 8 shows the change in casing stress with time when different types of milling cones pass the "corn point."It can be seen that the maximum stresses of the casing at the "corn point" of the conical, spiral, and bit-type milling cones are 965.2,799.5, and 677.3 MPa, respectively.And the conical milling cone has the shortest time to pass through the "corn point" and the maximum stress on the casing.Therefore, it is demonstrated that the conical milling cone has the highest milling casing efficiency, which is recommended.In conclusion, the simulation dynamics results of the three types of milling cones showed that the stress and strain generated by the conical milling cone were the largest, the casing damage was the largest, and the milling efficiency was the highest.Therefore, a conical milling cone is recommended for casing windowing.

Field Application and Analysis
The ST102 well is a directional evaluation well in the buried structure of Yandianchang in the Shuangyushi-Hewanchang structural belt of the northwestern Sichuan Basin.The purpose of drilling is mainly to explore the Qixia and Maokou Formations and the oil and gas of the Devonian.The design and actual drilling of the ST102 well are all six-opening and six-completion casing structures.During the oil test, the drill encountered complex stuck faults and failed many times.The fish top position was 7497.84 m, and windowing and sidetracking were conducted at 7419.62 m.The parameters of the windowing casing were as follows: the outer diameter was Ф184.15 mm, TP155V steel grade was used, and the wall thickness was 15.83 mm.
Referring to the results of the dynamic simulation, a conical milling cone was used to open the window in the ST102 well, and a composite milling cone was used to repair the window.Six conical milling cones and two composite milling cones were used to complete the windowing operation.The milling cones are shown in figure 9.
milling cone spiral milling cone bit-type milling cone Table 1 presents the summary of each milling trip of the ST102 well, with 1-6 trips for casing milling and 7-8 trips for window repairing.It can be found that during the casing milling stage, the footage is 3.21 m, taking 51.76 h, and generating 94 kg of iron chips.During the window repairing stage, footage is 3.93 meters, taking 22.49 h, and generating 48.41 kg of iron filings.In total, most of the iron chips come from casing, and a small part comes from milling cone, whipstock and drill pipe.The wear of the drill pipe is due to friction between the drill pipe and the upper part of the window.The strength of the whipstock is highest, so it is least worn.In conclusion, facing the high steel grade thick-wall casing of ST102 well, the conical milling cone recommended by efficiently completes the windowing operation, and the average milling speed is 0.06 m/h.

Conclusions
In this study, three different types of milling cone windowing of ultradeep wells were simulated using CAE simulation software.Based on field application, the following conclusions were drawn: (1) CAE software can simulate the stages in the windowing process (cone milling down stage, casingmilling stage, and out of the window stage), indicating that CAE software is suitable for simulation analysis of windowing and sidetracking.In the windowing and window repair processes, the shoulders of the milling cone and the outer circle of the cone top were the main milling faces.
(2) Through a comprehensive analysis of the stress, equivalent plastic strain, casing-damage displacement, and corn point stress analysis of different milling cones during windowing, the conical milling cone was found to be more suitable for windowing operations.The conical milling cone exhibited a uniform stress distribution during the windowing process.The conical milling cone had the maximum stress at each windowing stage, the smoothest window, and the highest stress through the "corn point.";therefore, it is recommended that a conical milling cone be used for windowing.
(3) The window was successfully opened using a conical milling cone in the ST102 well.The casing windowing progress can be determined by observing the return state of the cuttings.This study shows that the simulation analysis of windowing is reliable and can guide field operation.

2. 2 . 1 .
Stress Analysis.Figure3shows the stresses of the conical, spiral, and bit-type milling cones.It can be seen from the figures that the maximum stresses of the conical, spiral, and bit-type milling cones are 1565.3,799.5, and 632.5 MPa, respectively.Only the milling stress of the conical milling cone exceeded the yield stress of the TP155 V casing (1069 MPa).This shows that the conical milling cone has the largest milling stress and highest milling efficiency.In summary, it is recommended to use a conical milling cone in the whipstock windowing operation.

Figure 8 .
Figure 8. Variation of casing maximum stress with time when different milling cones pass the "corn point".

Table 1 .
of each milling trip.