Location :The first thing to do is to define a local coordinate system originating at the structure reference point. In many land wells, this will be the surface location. The target location is then converted to this local coordinate system, if necessary.
Target SizeCost versus Accuracy
In many cases, operator companies adopt an arbitrary in-house target size
As the target size get smaller due to certain limitation ( geological or political) you need to be more accurate and cost will increase
Good communicationThe first step of any plan to correct the azimuth of a well should always be consultation with the Geology Department
Wellbore ProfileA- Knowing the position of the surface location and given the location of the Target, its TVD and rectangular coordinates
B- wells can be either:
- The build and hold profile is the most common deviated well trajectory and is the most simple trajectory to achieve when drilling. technologists in exploration and appraisal wells since it is easier to assess the potential productivity of exploration wells, or the efficiency of stimulation treatments when the productive interval is entered vertically, at right angles to the bedding planes of the formation.
- The deep kick-off profile may be required when drilling horizontal wells or if it is necessary to drill beneath an obstacle such as the flank of a Salt Diapir. This well profile is the most difficult trajectory to drill since it is necessary to initiate the deviated trajectory in deeper, well compacted formations.
- The S-shaped well is more complex but is often required to ensure that the well penetrates the target formation vertically. This type of trajectory is often required by reservoir engineers and production
C- The type of profile selected will depend upon the Geological objective and production mechanism of the well
The Parameters Defining the Wellpath
(a) The Kickoff Point (KOP)
- The kick off point is the along hole measured depth at which a change in inclination of the well is initiated and the well is orientation in a particular direction (in terms of North, South , East and West).
- In general the most distant targets have the shallowest KOPs in order to reduce the inclination of the tangent section of the well (see below).
- It is generally easier to kick off a well the shallow formations than in deep formations. The kick-off should also be initiated in formations which are stable and not likely to cause drilling problems, such as unconsolidated clays.
Buildup Rate (BUR) and Drop Off Rate (DOR)The build up rate and drop off rate (in degrees of inclination) are the rates at which the well deviates from the vertical (usually measured in degrees per 100 ft drilled).
The build-up rate is chosen on the basis of drilling experience in the location and the tools available, but rates between 1 degree and 3 degree per 100ft of hole drilled are most common in conventional wells.
Since the build up and drop off rates are constant, these sections of the well, by definition, form the arc of a circle. Build up rates in excess of 3 degrees per 100 ft are termed doglegs when drilling conventional deviated wells with conventional drilling equipment. The build up rate is often termed the dogleg severity.
It is determined by one or more of the following:
- The total depth of the well.
- Maximum Torque and Drag limitations.
- High dogleg severity in the build section of the well results in high torque and drag while drilling the remainder of the well. This can be a severe limiting factor in deeper wells.
- The formations through which the build section must pass. Higher build rates are often not possible to achieve in soft formations.
- Mechanical limitations of the drill string or casing.
- Mechanical limitations of logging tools and production strings.
- Formation of “Keyseats" in the Kick-off arc.
The tangent angle (or drift angle)The tangent angle (or drift angle) is the inclination (in degrees from the vertical) of the long straight section of the well after the build up section of the well. This section of the well is termed the tangent section because it forms a tangent to the arc formed by the build up section of the well.
The tangent angle will generally be between 10 and 60 degrees since it is difficult to control the trajectory of the well at angles below 10 degrees and it is difficult to run wireline tools into wells at angles of greater than 60 degrees.
Defining the Points on the WellpathHaving fixed the target and the rig position, the next stage is to plan the geometrical profile of the well to reach the target.
The most common well trajectory is the build and hold profile, which consists of 3 sections - vertical, build-up and tangent.
The trajectory of the wellbore can be plotted when the following points have been defined :
- KOP (selected by designer)
- TVD and horizontal displacement of the end of the build up section.
- TVD and horizontal displacement of the target (defined by position of rig and target)
Since the driller will only be able to determine the along hole depth of the well the following information will also be required:
- AHD of the KOP (same as TVD of KOP)
- Build up rate for the build up section (selected by designer)
- Direction in which the well is to be drilled after the KOP in degrees from North (defined by position of rig and target)
- AHD at which the build up stops and the tangent section commences and
- AHD of the target
These depths and distances can be defined by a simple geometrical analysis of the