Directional drilling techniques have been used for many years to reach subsurface objectives that have had inaccessible surface locations. Economic considerations and increased environmental concerns have increased the number of directional wells drilled in recent years. Directional drilling techniques have also been applied to horizontal drilling with interest in this area increasing greatly over the last several years.
In the past, there have been two major methods utilized in planning the directional well. These methods are the use of planning the directional well. These methods are the use of buildup or composite buildup charts and the use of several directional well planning equations, each method depending on the particular wellbore geometry desired. The use of buildup charts is particular wellbore geometry desired. The use of buildup charts is a tedious process that often yields inaccurate results since it requires using preplotted graphs that require interpretation and interpolation. The equation approach is often confusing to use due to the similarity of the various equations with the selection of the proper equation dependent on the desired wellbore geometry.
This paper presents the derivation of a single equation for planning the trajectory of any directional well and a similar planning the trajectory of any directional well and a similar equation for any horizontal well. Several examples are used to demonstrate the application of the equations to various wellbore geometries.
Directional drilling is the purposeful deflection of a wellbore from the vertical along some preplanned trajectory to a predetermined target. The necessity of directional drilling is usually dictated by economic and environmental concerns. Applications for directional drilling are varied with the most common application of directional drilling techniques probably being in offshore waters. Here, several wells are drilled from a single platform to different bottomhole locations, allowing the optimization of development costs. Similarly, directional wells have been drilled in remote areas from artificial islands and drilling pads. This type of operation is common in places like Alaska, the Canadian Arctic and the jungle areas of South America. Drilling multiple wells from a single location can greatly simplify the installation of gathering and production facilities.
Inaccessible surface locations also require the use of directional drilling techniques. The inaccessibility may be the result of natural barriers, such as rivers, lakes and mountainous terrain, or man-made barriers, like highways and populated areas. In these situations, single wells may be directionally drilled to reach the objective target. Directional drilling techniques are also used in drilling relief wells, drilling in saltdome areas and in some fishing operations.
Another and rapidly growing area requiring directional drilling techniques is horizontal and extended reach drilling. Generally, horizontal wells are drilled for economic reasons. Applications include consolidated, naturally fractured reservoirs where the wellbore may intersect multiple fracture systems. They have also been drilled to reduce coning problems in reservoirs that have large gas caps or strong water drives. In some reservoirs the horizontal well may improve drainage by increasing the area of the wellbore in contact with the reservoir.
Deflecting a wellbore involves many factors which must be considered individually. As a result, careful planning is the key to successful directional drilling. One of the first steps in planning the directional well should be the design of the wellbore planning the directional well should be the design of the wellbore trajectory. Wellbore trajectories can be categorized into two classes, the directional well and the horizontal well.
In the directional well class, there are three basic types of wellbore geometries: Type I, Type II and Type III. Fig. 1 shows these major types of wellbore configurations. A Type I well is basically a "build and hold" trajectory where the wellbore is deflected from the vertical at some kickoff point and the angle built until a maximum angle is reached and then held until the target is intercepted by the wellbore. A Type II well is a "build, hold and drop" or "S" trajectory where the wellbore is deflected to some angle, the angle is held and then dropped in a manner such that the target is penetrated vertically. A modified Type II well differs in that the wellbore is not returned to the vertical in the drop portion.