There are four basic well profiles considered while planning a directional well. Here we are only going to have basic preview of these profiles and the design considerations will be covered in the coming posts.
TYPE I WELLS
Type I wells are made up of a kick off point, one buildup section and a tangent section up to the target. They are also called Build and Hold Trajectory or L Profile Wells (as it is L - shaped). These wells are drilled vertically from the surface to kick-off point at a relatively shallow depth. From the kick off point, the well is steadily and smoothly deflected until a maximum angle and the desired direction are achieved (BUILD). Then, if desired, casing is run and cemented. Further, the established angle and direction are maintained (HOLD) while drilling upto the target depth.
Usually this method is employed when drilling shallow wells with single producing zones.
TYPE II WELLS
Type II wells are made up of a vertical section, a kick- off point, a build-up section, a tangent section, a drop-off section and a hold section upto target. They are also called S Profile Wells (as they are S - shaped). Like Type I Wells, the Type II wells are drilled vertically from the surface to the kick-off point at a relatively shallow depth. From the kick off point, the well is steadily and smoothly deflected until a maximum angle and the desired direction are achieved (BUILD). The angle and direction are maintained until a specified depth and horizontal departure has been reached (HOLD). Then, the angle is steadily and smoothly dropped (DROP) until the well is near vertical. Finally the angle and direction is maintained till we reach the target depth.
A disadvantage of the Type II is that it will generate more torque and drag for the same horizontal departure.
Usually this method is employed to hit multiple targets or to avoid faulted region or to minimize the inclination in the zone which will be fractured during completion or for sidetracking.
TYPE III WELLS
Type III wells are made up of a vertical section, a deep kick off and a build up to target. They are also called Deep Kick off wells or J Profile wells (as they are J - shaped). They are similar to the Type I well except the kickoff point is at a deeper depth. The well is deflected at the kickoff point, and inclination is continually built through the target interval (BUILD). The inclinations are usually high and the horizontal departure low.
This type of well is generally used for multiple sand zones, fault drilling, salt dome drilling, and stratigraphic tests. It is not used very often.
TYPE IV WELLS
Type IV wells are made up of anyone of the above profiles plus a horizontal section within the reservoir. They are also called Horizontal wells or Horizontal Directional Wells. A horizontal well is a well which can have any one of the above profiles plus a horizontal section within the reservoir.
The horizontal section is usually drilled at 90 degrees and therefore the extra maths involved is quite simple as we only need the measured length of the horizontal section to calculate the total well departure and total measured depth.
The hole total TVD usually remains the same as the TVD of the well at the start of the horizontal section. However, if the horizontal section is not drilled at 90 degrees or there are dip variations within the reservoir, then the total hole TVD will be the sum of the TVD of the horizontal section and the TVD of the rest of the well.
Horizontal drilling is used to produce thin oil zones with water or gas coning problems, used to increase productivity from low permeability reservoirs by increasing the amount of formation exposed to the wellbore, used to maximize production from reservoirs which are not being efficiently drained by vertical wells and to connect the portions of the reservoir that are productive.