CONSIDERATIONS WHEN PLANNING THE DIRECTIONAL WELL PATH
When planning a directional well a number of technical constraints and issues will have to be considered. These will include the:
• Target location
• Target size and shape
• Surface location (rig location)
• Subsurface obstacles (adjacent wells, faults etc.)
In conjunction with the above constraints the following factors must be considered in the geometrical design of the well:
• Casing and mud programmes
• Geological section
(a) Target Location
The location of the target is chosen by the geologists and/or the reservoir engineers. The target location will be specified in terms of a geographical co-ordinate system such as longitude and latitude or a grid co-ordinate system such as the UTM system. The grid reference system, in which the co-ordinates are expressed in terms of feet (or meters) north and east of a local or national reference point, is particularly useful when planning the directional well path, since the displacement of all points on the wellpath can be easily calculated.
The depth of the target is generally expressed by the geologist in terms of true vertical depth, TVD below a national reference datum such as Mean Sea Level. The difference between this national reference point and the drilling reference datum (such as the Rotary table) must be computed so that the driller can translate the computed TVD of the borehole below the rotary table elevation, into depth below mean sea level, and therefore proximity to the target.
(b) Specification of Target, Size and Shape
The size and shape of the target is also chosen by geologists and/or reservoir engineers. The target area will be dictated by the shape of the geological structure and the presence of geological features, such as faults. In general the smaller the target area, the more directional control that is required, and so the more expensive the well will be.
(c) Rig Location
The position of the rig must be considered in relation to the target and the geological formations to be drilled (e. g. salt domes, faults etc.). If possible the rig will be placed directly above the target location. When developing a field from a fixed platform the location of the platform will be optimised so that the directionally drilled wells can reach the full extent of the reservoir.
(d) Location of Adjacent Wells
Drilling close to an existing well can be very dangerous, particularly if the existing well is on production. This is especially true just below the seabed on offshore platforms, where the wells are very closely spaced. The proposed wellpath must be designed so that it avoids all other wells in the vicinity. It is essential that the possible errors in determination of the existing and proposed wells are considered when the trajectory of the new well is designed.
(e) Geological Section
The equipment and techniques involved in controlling the deviated wellpath are not suited to certain types of formation. It is for example difficult to initiate the deviated portion of the well (kickoff the well) in unconsolidated mudstone. The engineer may therefore decide to drill vertically through the problematic formation and commence the deviated part of the well once the well has entered the next most suitable formation type. The vertical depth of the formation tops will be provided by the geologists.
(f) Casing and Mud Programmes
The trajectory of the well will be designed so that the most difficult parts of the well are drilled through competent formations, minimising problems whilst drilling the well. It is very common to initiate the kick-off just below the surface casing and possibly to change out to oil-based mud when drilling the build-up section. In highly deviated wells the build-up section of the well may also be cased off before drilling the long, tangent section of the well. Oil-based mud may also be used in the long tangent sections of the well. The trajectory of the well will therefore be designed so that these operations correspond to the casing setting depths which have been selected for many other reasons. This is an iterative process taking into account all of the considerations when designing the well.