Planning Directional Wells Through a High-Density Cluster of Existing Wells
- Zhihua Wang (Heriot-Watt U.) | Tom A. Inglis (Heriot-Watt U.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling Engineering
- Publication Date
- December 1990
- Document Type
- Journal Paper
- 291 - 293
- 1990. Society of Petroleum Engineers
- 1.1 Well Planning, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.6 Drilling Operations
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Summary. This paper describes a method for determining the optimum well path through a high-density cluster of existing wells by considering the close proximity of the other wells. A simple 2D or 3D path is proposed, checked for proximity to the other wells, and then modified if necessary according to dogleg severity and the proximity of neighboring wells. An example using North Sea data is presented.
Planning directional wells from large, multiwell platforms in the North Sea is critically important to offshore oilfield developments. Up to 60 wells may he drilled from one central location to drain an entire reservoir covering a large area. Each well may have a horizontal displacement of 10,000 ft and may reach a depth of 13,500 ft below the seabed. The most critical planning and drilling stages focus on the kickoff point (KOP), which usually occurs 1,500 to 2,700 ft below the platform, where all the wells are very close to each other. As more wells are drilled from the platform, planning the next well to prevent interference or collision with other wells becomes increasingly difficult. This paper describes a procedure (Fig. 1) to plan new well paths by considering the proximity of the other wells. The procedure consists of four major steps: studying the space below the platform with multiwell plots, planning the new well path, checking the clearance between the new well path and adjacent well paths, and optimizing the well path. The multiwell plots include horizontal-projection, vertical-section, traveling-cylinder, and single-well plots. These plots can be used to draw the trajectories of planned and existing wells. They can also be combined to study the relationship of existing and planned wells and to determine the possible parameters for the new well. Planning a new path is the most important and the most difficult step. A 3D well-planning method is used. This method can also be used to plan a 2D path. Once the new well path is planned, it should be compared with those of adjacent wells to ensure that clearance is sufficient to prevent collision. If clearance is insufficient, the new path should be modified or replanned. Finally, after all possible new well paths are investigated, an optimized one can be chosen.
Planning a New Well Path
Once the initial parameters (KOP, buildup rate, and azimuth) are chosen by study of the available information about formation conditions, deflecting tools, and the space below the platform, initial planning can begin. Because of the proximity of drilled and planned wells to the platform, it is very difficult to plan a 2D well path to reach the target without any risk of interference with other wells; however, 2D well paths are always tried first. Any well path is generally a combination of various sections. These sections may be marked by one of the following basic trajectories: simultaneous changes in inclination and direction, angle building or dropping in a vertical plane, helical turns with constant inclination, or straight lines in space. Ref. 1 presents the mathematical formulas for each basic trajectory. The parameters needed for the whole well planning are as follows. For the first point (KOP), the measured depth (MD), true vertical depth (TVD), north and east coordinates, and inclination and azimuth are needed. For the last point (target), TVD and the north and east coordinates must be known. For each section, TVD at the end of the section and buildup rate and/or turn rate for the first three types of basic trajectory are necessary. During planning, the well path is broken down into many sections. The designer plans the well path to be as simple as possible. The number of sections and the type of each section are determined by the particular situation. The parameters for each section are determined according to the formation to be drilled and practical engineering considerations.
To plan a practical, cost-effective well path through the high-density well cluster, it is important to define a reasonable "critical distance" (i.e., a minimum distance between the new well path and adjacent well paths that will prevent interference). Many factors should be considered when the critical distance is defined-e.g., the types of adjacent wells, the accuracy of the directional-survey instruments, and the reliability of the drilling techniques. The critical distance used in this paper accounts only for the borehole-position uncertainty caused by the inaccuracy of survey instruments. The uncertainty model described in Ref. 2 is used to calculate borehole-position uncertainty. The critical distance at the vertical depth of interest is the sum of the uncertainty values of the new well and the existing well at that vertical depth.
Modifying the Proposed Well Path
When the proposed well path is planned, the corresponding data file is set up, and the borehole-position uncertainty of the proposed well path can be calculated with the surveying instruments. Then, the clearance between the proposed well and the existing wells is checked. If the clearance is not sufficient to prevent collision, the well path must be replanned or modified. While the clearance at the vertical depth of interest is checked, the distance between the proposed well and the existing wells is calculated and compared with the critical distance. If that distance is less than the critical distance, the proposed well path will be modified, depending on the proximity of adjacent wells, the capability of deflection tools to be used, the inclination and azimuth at that depth, and allowable dogleg severity. At the depth of interest Oust above the trouble spot), the inclination, azimuth, and allowable dogleg severity determine the maximum possible area for the next point. By selecting one particular combination of inclination and azimuth from within this area, one can calculate the new position of the wellbore. The proximity of this modified position with respect to all other wells can be checked. If the clearance is greater than the critical distance, the new position is accepted and the same procedure is repeated for the next depth. If an acceptable point cannot be determined because of the proximity of other wells, something must be altered (e.g., the slot or type of sections). Suppose that the well is to be drilled from Depth D1, with Inclination and Azimuth , to Depth D2, with Inclination and Azimuth . The maximum allowable dogleg severity over the interval is (in degrees/100 ft MD). The acceptable area at D2 is determined by the maximum changes in inclination and azimuth as follows.
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