Directional Drilling Tools and Techniques

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Azimuth Change

When there is a need to change the azimuth of the well course – for instance, as a result of an over optimistic trajectory setting, unforeseen events, or errors in estimating natural deviation tendencies—the first steps are to determine the required change based on an initial feasibility study, and to determine the orientation of the tool face.

To reset the toolface properly, the directional driller must have certain information:

The present inclination and azimuth of the hole.
The required change in inclination or azimuth to correct the trajectory or kick off the well.
The expected rate of change (i.e., the dog-leg) that the deflecting tool can provide.

A large change in azimuth is easier when the inclination is small. If the inclination is greater than 30 degrees, and the planned azimuth correction is greater than 15 degrees, it is best to change azimuth as soon as possible (for instance on the first bit replacement), especially if the inclination has not reached its maximum value. A rule of thumb gives

$dA = \frac{180}{L}$

Where L = present inclination and dA = possible change in azimuth per 100 ft drilled with constant inclination.

Azimuth changes can be determined using any of three methods (Byrum, 1982; Short, 1993).

Ragland diagram.
“Ouija Board” nomograph.
Direct calculations.

The main consideration for the directional manager is to avoid being faced with the impossibility of reaching the target. Graphic construction helps to solve the problems associated with a hurried, last-minute decision.

Ragland Diagram

The Ragland Diagram is comprised of four elements (Figure 1):

A line OA indicating the initial azimuth of the wellbore, and marked off in increments of inclination from vertical.
A dogleg circle with its center at the present angle of inclination (α1) and its radius equal to the dog-leg angle (DL).
A line AB indicating the orientation of the deflection tool with respect to the current azimuth OA.
A line OB indicating the direction obtained with respect to the original direction.

Where L = present inclination and dA = possible change in azimuth per 100 ft drilled with constant inclination.

Azimuth changes can be determined using any of three methods (Byrum, 1982; Short, 1993).

Ragland diagram.
“Ouija Board” nomograph.
Direct calculations.

Ragland Diagram

The Ragland Diagram is comprised of four elements (Figure 1):

A line OA indicating the initial azimuth of the wellbore, and marked off in increments of inclination from vertical.
A dogleg circle with its center at the present angle of inclination (α₁) and its radius equal to the dog-leg angle (DL).
A line AB indicating the orientation of the deflection tool with respect to the current azimuth OA.
A line OB indicating the direction obtained with respect to the original direction.

Ragland Diagram, Directional Drilling Tools and Techniques, Directional Drilling Tools, Directional Drilling Techniques — **FIGURE 1**

Suppose that we have drilled to point O, with an inclination α₁ of 4 degrees at an azimuth of N 40°E. We want to know what toolface orientation is needed to direct the well course to a new azimuth (α₂) of N 55 ^°E, within the constraint of DL = 2 degrees.

We would begin by drawing a line from O at an angle of (40°+15°), or N 55° E.
Next, we would draw a circle, with its center at α1, having a radius corresponding to DL.
We would then draw a line from A to B and measure its angle to determine the required toolface setting (TF), which in this instance is 45^°.

Thus, for these parameters – TF, DL and α₂ – if any two of them are known, we can determine the one that is unknown.

An established method that is used as a standard on every drilling site is to use a Ragland diagram right from the start of the build up phase, and to continuously analyze the Ragland diagram and the result of successive plots. This keeps the driller aware of the efficiency of the bent sub/downhole motor combination (i.e., the angle change gradient and the roll-off of the downhole motor) for predetermined weight-on-bit and flow rate conditions. The driller can then orient the tool face without hesitation, avoid errors, and by increasing inclination, reach the desired azimuth with minimal difficulty.

Using a Ragland diagram helps to determine:

The azimuth and inclination resulting form a deflected hole, in accordance with the initial orientation of the deflection tool.
The roll-off of a deflection tool.
The orientation to be given to a downhole motor, knowing its roll-off and the real dog-leg created by the downhole motor / bent sub combination with NDMC (non-magnetic drill collar) of a certain size.
The total dog-leg between two surveys in order to determine the API dog-leg calculated over a distance of 100 ft.

Ouija Board

The Ouija board involves a similar technique to that described for the Ragland diagram, but it is much quicker since no diagram needs to be drawn. In Figure 2, β corresponds to the overall angle change over the drilled interval (i.e., DL), while γ represents the tool face orientation (TF). The current and new inclination angles (α and α_N, respectively), as well as the change in azimuth (Δε) are known.

Direct Calculation

As an alternative to the graphical methods describe above, a series of equations can be derived to calculate the necessary angles directly. To calculate the change in azimuth in a given toolface hading , dog-leg angle and inclination

$\Delta \beta = \tan ^{-1}\left ( \dfrac{\tan DL\ \cdot \ \sin TF}{\sin \alpha _{1}+\tan DL\ \cdot \ \cos \alpha _{1}\ \cdot \ \cos TF} \right)$

where

Δβ = change in azimuth

DL = dog-leg angle

TF = toolface setting

α₁ = present inclination.

The new inclination angle can be given by

$\alpha_{2}=\cos^{-1}\left ( \cos \alpha _{1}\cdot \cos DL -\tan DL \cdot \sin \alpha _{1} \cdot \sin DL \cdot \cos TF \right )$

o calculate the toolface heading from the dog-leg angle and the expected change in inclination, the following equation can be used:

$TF = \cos^{-1}\left ( \dfrac{\cos \alpha _{1}+\cos DL-\cos\alpha _{2}}{\sin \alpha _{1}\ \cdot \ \sin DL} \right )$

Once the required toolface setting has been determined, the effect of reactive torque must be considered in re-orienting the toolface. Reactive torque is the twisting effect caused by the stator of the downhole motor turning anticlockwise in response to the rotor turning clockwise. Most manufacturers provide tables with estimates of how much left-hand turn can be expected under certain conditions. From these tables or from experience, the directional driller must compensate for reactive torque be pointing the toolface to the right of the calculated heading. As soon as the bit begins to drill, the scribe line will turn to the left to bring it back to the calculated heading.