Deviation is essentially departing from the intended drill path and ending up somewhere entirely different. Any kind of deviation is time-consuming and costly, and more so when you can’t see where you are deviating, as is the case with directional drilling. Directional wells are subject to problems along the borehole such as bore profile and a reduced axial component of gravity.

As the angle of inclination of the borehole increases, the problems faced with drilling also increase. The benchmark to determine the difficulty in drilling a bore is the total time taken in completing the borehole and the cost incurred. Some deviations are planned as the borehole progresses, but some are caused by problems while drilling.

Monitoring Technical Hole Deviations

Calculated deviation during drilling is a costly affair but unplanned deviation can be disastrous for the project. Survey instruments that can measure the borehole inclination and its direction as the bore path progresses are used for the purpose of preventing deviation from the planned route.

These instruments make it possible to monitor a real-time position of the borehole as it progresses, guide deflection tools in the correct direction if a deviation is observed or while corrections are applied, prevent cross-boring of other utility lines in the area and help in determining dog-leg severity along the bore path.

A directional bore is designed to meet or exit at a certain point and must be guided along a preplanned trajectory. Minimal turning in either direction is permissible as long as the borehole will exit at the designated point. Periodic readings taken during drilling from probes inserted in the drill collar close to the drill bit determine the inclination and azimuth of the leading edge. A wire running inside the drill string transmits readings to the surface and is read along with readings from the survey measurements. These readings are used to determine vertical and horizontal coordinates along the pilot bore relative to the point of entry.

Since azimuthal readings are taken relative to the earth’s magnetic field, magnetic interference from drill pipe, down-hole tools and magnetic fields created by surrounding structures should be eliminated by inserting the probe in a non-magnetic collar to isolate it from their influence. Incorrect choice of the bottom-hole assembly (BHA) and weight on bit (WOB) can cause deflection of the bit from the planned path.

Types of Deviation Encountered During Directional Drilling

Collectively, the different types of deviations are called technical hole deviations (THD). THD consists of eight components namely:

  • Vertical deviation (msVD)
  • Relative change in vertical deviation (rcVD)
  • Inclinational deviation (msID)
  • Relative change in inclinational deviation (rcID)
  • Horizontal deviation (msHD)
  • Relative change in horizontal deviation (rcHD)
  • Azimuthal deviation (msAD)
  • Relative change in azimuthal deviation (rcAD)

VD, HD, ID and AD Deviations

VD and HD are lineal deviations while ID and AD are angular deviations.

Vertical deviation (msVD) – It is the difference between the planned bore path and the actual bore path in the up and down direction at the time of drilling.

Horizontal deviation (msHD) – it is the difference between the planned bore path and the actual bore path in the left and right direction at the time of drilling.

Azimuthal deviation (msAD) – it is the direction of the bore on a horizontal plane measured from the North reference in the clockwise direction. It is measured from 0 North to 360° and can also be expressed as a quadrant system from 0-90°.

Inclinational deviation (msID) – it is the angle of a bore defined by a tangent line at any point on the bore path and a vertical line parallel to the earth’s gravity, i.e. 0° inclination is vertical and points downward and the 90° inclination is in the horizontal direction.

Deviation Categories

VD and HD are known as lineal deviations and ID and AD are known as angular deviations. They are categorized as,

1st order lineal deviation – VD and HD

2nd order lineal deviation – rcVD and rcHD

1st order angular deviation – ID and AD

2nd order angular deviation – rcID and rcAD

These components are defined by the properties of the three-dimensional nearest point between current true depth (TD) and planned path. This is known as measured depth (MD) and associates with it the planned values of North, East, inclination, azimuth and true vertical depth (TVD). Below is a brief look at the components of THD.

1st Order Lineal Deviation – VD and HD

VD and HD give an idea of the location of the wellbore in space relative to the planned location and can be easily visualized because it defines the position in terms of left, right, up and down. Assuming that we walk along the planned path at MD and point to the borehole we would get the components of the pointing vector as VD and HD with respect to the designed path at MD.

Minimizing deviation to the left and right becomes important when drilling is carried out near lease lines or in tight spaces.

2nd Order Lineal Deviation – rcVD and rcHD

Visualizing rcVD and rcHD is not as simple as VD and HD but it is capable of providing important details, predictive information and can be used for projecting VD and HD. It helps to understand how VD and HD vary and help to guide directional tool setting adjustments. For example, positive rcVD means that VD is increasing.

1st Order Angular Deviation – ID and AD

ID and AD present the difference between actual and planned bore path in the tangential direction. It provides important directional control information along with other components of THD. Minimizing deviation vertically is very important in directional drilling. When VD and rcVD are approximately equal to zero, it is possible for ID to be much greater than or less than zero. Even if the depth has been maintained, it is not necessary for VD to remain zero, unless ID is also zero.

2nd Order Angular Deviation – rcID and rcAD

rcID and rcAD are similar in design to rcVD and rcHD and quantify the change in ID and AD as the hole is drilled. It is possible to directly control rcID and rcAD via steering than is possible to control any other components of THD.

Directional drill operators determine their tool setting control actions by using forecasting methods that are quantitative such as linear and path projection, and qualitative such as numerical or graphic information. However; all methods are limited by unknown soil conditions ahead of the drill. Combined with THD, better information on directional control can be obtained to perform the next action. Though it is difficult to predict the drilling direction accurately, it can be controlled by making real-time decisions while steering by using available information.