Directional Drilling

How do they drill those crooked holes?……………………Directional Drilling for the pre-spud meeting

 

No doubt that when drilling first started, the deeper they went they made deviated holes without realizing it. This was due to a variety of reasons such as the bending of the drill pipe, formation dips, bedding planes, etc. These crooked holes led to accusations of drillers stealing each other’s oil from their wells. In fact there were several lawsuits in the 1920’s about ‘getting their straw into their reservoirs to suck on it’.

Interesting fact: In 1902, the first rotary drilling in California did not get off to a great start. The hole was so crooked that they had to use a cable tool rig to redrill the well.

 

What happens when we drill a deviated/horizontal hole?

There are many reasons why we would want to drill a horizontal well.

Surface Reasons

There may be obstructions in the way such as a town or unsuitable terrain. Even offshore where a permanent platform can drill multi-wells from a single location.

Economic Reasons

It can hit multiple targets just using a single well bore. It can also increase the surface area that the wellbore is exposed to in the producing zone. This is particularly useful for zones that are narrow and/or run the risk of water conning if the drawdown is too high.

Sub-Surface Reasons

Sidetracking a fish. Sometimes if we drop or leave something that we did not plan to, it may be cheaper to bypass the ‘fish’ than to retrieve it.

Formation dip effects. If the formation dips more than 45° it can cause the drill bit to drill upwards. In one case in South America where the formation dips below 20° the Directional Driller is used to drill a vertical hole.

Sometimes it is easier to drill a deviated well to avoid a fault.

Multiple targets from a single well.

Avoid colliding with other wells. Indeed, reports of wells colliding, some with surface locations several thousand feet apart, begin as early as the 1920’s

Special Needs

Drill relief well for a blow-out. The Macondo well in the Gulf of Mexico was finally brought under control by using a relief well.

Interesting Fact: The first patent for directional drilling was for dentistry. John Smalley Campbell in 1891 patented a tool that had a flexible shaft that could rotate a drilling bit. This technology could also be applied to the oil industry.

 

Well Profile Terminology

RKB – This is the rotary Kelly bushing. This is on the rig floor and is where we start counting the dept. So the rotary Kelly bushing is 0ft.

KOP – Kick Off Point – this is where we start to build angle.

EOB is the end of build up rate. This is where we either hold the angle (tangent) or drop.

TVD - True Vertical Depth. If we drill at an angle then we are going to have two depths TVD and TMD.

TMD - Total Measured Depth

DLS - Dog-Leg Severity – is a term that describes the build rate. How much change in inclination and direction. The bigger the casing you use, the stiffer the casing. If the DLS is severe, then it will be hard for the casing to get down to bottom.

 

BUR - Build-Up Rate. It is the rate of change (degrees/100 feet or degrees/30 metre) of the increasing angle in the hole.

 

Inclination - The Angle from Vertical. This is the slope of the deviated hole.

 

Azimuth - The Direction of the Well. In other words is it heading north, south, east or west?

Interesting Fact: The first deliberate recorded deviated well was in 1928 by John Eastman. He would become famous in 1934 for drilling the first directional well to put out a blowout in Texas. He worked with George Failing along with his patented mobile rig. John Eastman is recognized as the father of directional drilling. His company, Eastman Whipstock merged with Norton Christensen in 1986 to form Eastman-Christensen. This was then taking over by Baker-Hughes in 1992.

 

Basic Well Profiles

There are 4 basic well directional profiles.

Build and Hold

This is the basic directional profile. It drills vertical and then reaches the desire KOP and builds angle. Once the desired angle is achieved it then holds that angle. This is common for shallow wells with a single producing zone.

 

Build Hold and Drop (S profile).

This starts to drill vertically then the KOP is reached and it build angle. The it holds (tangent) and then it drops angle. This is difficult to do as it generates more torque and drag. It is also difficult to control.

It is commonly used for wells that have multiple producing zones, avoid faults and/or to minimize the inclination in the formation where it will be fractured for completion.

 

Horizontal Continous build (J profile)

These can also be referred to as deep kick off wells. They start drilling vertically and then either continuous build up leading to a horizontal section or build and hold and then build to go horizontal.

This type of well is generally used for multiple sand zones, fault drilling, salt dome drilling, and stratigraphic tests.

 

 

Undercut Profile ( the inverted question mark)

This is where you drill vertically and then build angle in the opposite direction that you plan to go. Then you revert back where you can drill horizontal directly below the rig.

It can be used for wells where the producing zones have a high dip.

 

Interesting Fact: The first deviated holes were drilled simply by dropping a wedge (whipstock) in the hole. Commonly it was used when someone dropped something they should not have. The cure then was to drop wedge and force the bit to sidetrack.

 

How in the heck do we drill those crooked holes?

When we are directional drilling we need to use some directional drilling assembly. What that amounts to is that we use a downhole motor to turn the bit. So, we have a bit and a motor. On the motor part we have a ‘bend’. This allows us to point the bit in the direction we want to drill. A typical bend on the motor would be between 1-2°.

How the drill bit turns is by pumping drilling mud. This goes through the motor that contains a rotor and stator. There are several combinations of motor/stator where you have as little as 1 rotor to 2 stator to as many as 9 rotors to 10 stators. The higher the combination the higher the power(torque) but the speed of the bit is less.

We have these different motor/stator configurations so different bits that perform best at different speeds and power, we can choose the best rotor/stator for that bit that will give those criteria.

Interesting Fact: Downhole motors where first developed by the Soviets. Modern motors use the progressive cavity or the Moineau pump, named after its inventor René Moineau, 1930.

Typically roller (tricone bits) bits tend to operate with heavier bit weight but lower RPM or bit speeds. So when we are designing our BHA, we use a performance chart to pick the best motor/stator for our bit. For PDC bits, they require more horsepower however when the bit is faster they will drill faster.

So how do we drill?

As we mentioned previously, we have a bit and a motor. We also have stabilizers. These have the same OD has the bit.

When we want to drill in a certain direction, we turn the pipe on surface until we are in the direction we want to drill. We use MWD tools to let us know that we are in the desired direction.

Once we are in the right direction, then we start the mud pumps which in turn rotates the bit. Note that drill string itself is not rotating only the bit itself. This is called sliding. Now we start to build angle.

The directional driller uses the pump pressure on how well he is drilling. He will have a specific pressure which gives the maximum drilling rate. If this pressure is too low, the bit is not at bottom so he puts more WOB and pressure should increase. If it is too high the bit has stalled.

So we want to build angle……..

Then we use the fulcrum principle. This has a stabilizer above the drill bit which acts as a fulcrum and the drill collars above act as the lever. As we build up angle, there is enough WOB to cause the drill collars to bow and therefore force the drill bit upwards, thus increasing the fulcrum effect.

When we build angle we slide. We do not rotate the drill string.

 

Now we want to hold……

Then we use the stabilization principle. Here we have a series of stabilizers that stiffens the BHA and stops it from bending or bowing and make the bit drill straight ahead.

Here we can rotate the drill string and this helps us drill ahead.

 

Now we want to drop……….

Then we use the pendulum principle. Here we have the stabilizers high above the bit where it forces the bit to drop.

 

Next generation of Directional Drilling – The Rotary Steerable System

We talked about using a motor for directional drilling but there is another choice. This is where we have a Rotary Steerable system. This does not use a motor and does not need to slide to make hole. It uses cams that makes the bit change direction.

These systems are able to carry out the entire directional path (including any course corrections on the horizontal plane) without having to trip-out to change the geometry and the rigidity of the drill string.

There are 2 concepts in the RSS – 1 Point the bit and 2 Push the bit. The former is similar to the bent housing. This is contained in the collar above the bit. The latter uses force either hydraulically or mechanically to force the bit against the borehole.

Interesting Fact: The first horizontal well was drilled in 1929 by Robert E Lee in Texas.

Recap

For directional drilling, we have 4 basic profiles; Build and Hold, Build Hold and Drop (S profile), Continuous Horizontal (J profile) and Undercut profile.

Directional Drillers use standpipe pressure gauge for directional drilling and use the following three principle for directional drilling – fulcrum, stabilizing and pendulum.

The new generation of directional drilling tools uses robotic technology to help them drill those crooked holes.

 This is part of the Petroleum Technology Workshop course run by Fogerty Training Solutions.