BHA stabilization

The main purpose of the BHA is to provide W.O.B. Another reason is to “control” drill, or in
other words, to try to keep the hole vertical (impossible) or to maintain a constant angle, to build or
finally to drop angle in a smooth, controlled manner.
It is of great importance to know if the formations to be drilled are dipping up or down relative
to surface location. It has been demonstrated that when drilling through up dipping formations with
dips up to 40°, the bit will try to climb uphill. Therefore, if a directional well is slated to be drilled, the
best possible location for W/C would be down dip to the target so as natural deviation could bring the
bit into the target or close. Sliding would be minimal or inexistent. R.O.P. would increase and hole
problems would be minimized and the client would benefit of performance drilling due to the use of
mud motors.

The first step would be to determine the W.O.B. needed. Assuming we need 18,000 daN, the
well bore has 30° Inc, and the mud weight is 1250 Kg/m3 we would use the following calculations:

Wt = Wt in air x BF x cos Inc, where:

 Wt in air is the weight in air of the total BHA
 BF is the buoyancy factor:
Mudweight
BF = 1- Steel density Roughly the steel density = 7850 Kg/ m3
 cos Inc is cosinus of the deviation of the well bore.

Also, we would use about 75% to 80% of the weight of the BHA for W.O.B.
This means we would need roughly 24,000 daN for the BHA buoyed.
The BF is 0.84.
This would result:

Buoyed Wt
Wt in air = = 32,961 daN
BF x cos Inc

Using the values for Wt/m in different API published tables, or the values given at the rig, one
can establish the number of drill collars and/or heavy weights or the combination of them to be used.

This is because if we want to stabilize the BHA, and in order to create side forces, you will
need to buckle the BHA, but not critical buckle. The extra amount of weight of the BHA will allow
you to do that. The stabilizers are tools that create side forces. The main type of stabilizers is:
 Straight blade stabilizers – they have “point contact” and are good for building
 Spiral blade stabilizers – they are “full contact” and will give less build and hence good
output for “hold” situations.
The spirals are at 45°, 90° and 270° designed to enhance the contact area. Some have left hand
blades and are used to create turbulence in the mud.
 Experience demonstrates that usually the PDC bits will have a left walk tendency and the roller
cone bits will have a tendency to go right. This has to be taken in account because if using stabilized
assemblies, we can control only inclination.
The critical parameters of using a stabilized BHA are:

 Gauge size of the stabilizers. The max undergauge is between ½” and ¾” under. and
minimum 1/16” under. This varies with the size of the bit and BHA. If the
stabilizers are too much undergauge, they are useless.
 The position in the BHA
 Drill collar OD (helps with momentum of inertia)
 Bit type: the smaller the gauge, the better the build. For locked assemblies use long
shanks.
 Controlling factors: W.O.B., R.O.P., r.p.m., and flow rates.

Stabilizer position and effects on BHA

Rotary BHA’s

The rule of thumb for a building BHA (Gilligan assembly) is showed in the schematics below.
 By applying more WOB, we will create a fulcrum effect between the lower 2 stabilizers,
resulting in pointing the bit upwards, and building angle. The resulting build rate is determined by the
controlling factors mentioned above. For best results, notes for WOB, rpm, pump rates should be made
for each single drilled, and tweaked as we drill ahead. The distance between the first two stabilizers
will dictate the amount of BUR, meaning that shortening the span will increase the build. When the
span gets too short, there will be no build effect.

A locked BHA is showed in the schematic below:

Drop assemblies (or pendulum) are shown in the schematics below:

 In highly deviated holes, (70° and above) the pendulum assembly listed above will not work.
Instead an undergauge near-bit stabilizer will do the trick because the bit will create undercut. Also,
the best results for a regular pendulum assembly will be achieved in well bores with a deviation up to
30°.
By using an Andergauge tool (variable Gauge Stabilizer), the driller can control build, hold and
drop of the well using:

Full Gauge – 9.5m to 19m – Variable Gauge Stab – 9.5m to 19m – Full Gauge.

The drilling parameters used to build or to drop when utilizing rotary stabilized BHA’s are:
 Low WOB, high rpm. Result: drop
 High WOB, low rpm. Result: build.

Stabilized Motor assemblies

 With all the stabilizers 1/8” under gauge, the stabilized motor assembly behaves exactly the
same as the rotary stabilized BHA’s.

Bit Size Sleeve Dimension

17 ½” 17 3/8”
12 ¼” 12 1/8”
8 ½” 8 3/8
6 ½” 6 3/8”
6 1/8” 6”
6” 5 7/8”
5 7/8” 5 ¾”

Some successful examples of using stabilized BHA’s are:

  Horizontal drilling in the Middle East. After running surface casing, the well is
deflected up to 25° - 30° Inclination with a motor assembly. When POOH for bit, the
motor is laid down and a stabilized Gilligan assembly is run and the well is landed at
90°. This is suitable when the formations are flat.
 Shallow horizontal wells using pull-downs to reach desired reach. After 700m it is quite
hard to steer the bit even with pull-downs. What I used to do in the Hay River area was
to use WOB / rpm combination to climb/hold/drop. Having a PDC bit with left turn
walk, I usually set me up around 650m about 3.5° to the right of the planned azimuth,
after which I rotated to TD. The WOB could not be read accurately, so I mostly used
ROP & differential as a measure. With a lower differential and ROP, spinning the string
15-20 rpm’s, I usually dropped. To hold I used a mid-ranged differential and ROP, the
best string rpm being around the 25 value. At 30-35 rpm and higher ROP, I was
building. Before attempting this, it is recommended to do some trials before so as to
have a feeling of what kind of build rates/drop rates you can expect. Also, it is very
useful to write down all those parameters every single, so when you will have to do
some tweaking data is readily available.
 Some open hole sidetracks are very easy to achieve using a Gilligan assembly, with the
following adjustment: Big bit OD, near-bit stabilizer, small Drill Collars. This type of
BHA will allow for more WOB and will be more aggressive in the first stage of the
sidetrack.

Examples of stabilized BHA’s

Rotary stabilized BHA’s

Pendulum Assemblies

1. Case 1: Should be used in soft formations, 17 ½” holes or larger. No directional control.

2. Case 2: Should be used in soft formations, 12 ¼” or larger hole sizes. Good application
for dropping back to vertical in low angle well bores. Low directional control
3. Case 3: Low directional control. Fit for 8 ½” or larger hole sizes. Good application for
dropping back to vertical in low angle well bores.
4. Case 4: Can be used in any hole size. Low directional control. Good application to drop
rapidly from higher inclinations – drop rates decreasing as the well bore approaches
vertical.
5. Case 5: Slight directional control. Good application in achieving a gradual drop rate from
high angle. At lower angle, drop rate will decrease and/or even stall.
 Packed Assemblies

1. Case 1: Very good directional control. Tendency to build angle.

2. Case 2: Good directional control. Same as previous, only slower build rate
3. Case 3: Good application in slim hole sizes (6 ½” to 8 ½”). Good directional control.
Uses near-bit stabilizer (Dog sub)
4. Case 4: Good application in 12 ¼” or larger hole size. Good directional control. Uses
near-bit stabilizer (Dog sub)
5. Case 5: The output of the performance can be fine tuned by changing the span between
the Dog sub and the first string stabilizer. Used in highly deviated well bores using high
WOB.
6. Case 6: Same as Case 5, good application at high angles and high loads (WOB).

For all the packed assemblies, if there is a drop tendency, then a slight adjustment at the first
string stabilizer level can be made (under gauge) and build can be achieved through the fulcrum effect.
If the near bit stabilizer is under gauge, a drop assembly will result, therefore it is necessary to gauge the
dog sub every time a bit trip is called.
Build (Gilligan) Assemblies

1. Case 1: Fair directional control. Depending on under gauge stabilizer placement it can be
used in drop, hold or build situations. Drilling parameters have great impact.
2. Case 2: Fair directional control. Build rates vary pending on gauge of first string
stabilizer. Can be used to build inclination in any hole size.
3. Case 3: Fair directional control. Build rates vary pending on gauge of first string
stabilizer. Can be used to build angle in 8 ½” or larger hole sizes.
4. Case 4: little directional control. Should be used immediately following a motor run on a
sidetrack off a cement plug around a fish. No or very little directional control.
5. Case 5: Fair directional control. Accelerated build rate due to increased distance between
bit and near bit stabilizer (extension sub). Very good in jetting applications.

Again, never forget the rule:

 Low WOB, high rpm = drop
 High WOB, low rpm = build.
 Hybrid BHA’s – using Adjustable Gauge Stabilizer (Andergauge)

This can be a very effective build-hold-drop assembly, and in conjunction with any kind of
MWD tool (even DMT or AMS) good results can be achieved. The drawback is that no azimuth control
can be obtained. The Adjustable Stabilizer can be positioned near bit or in the 1st stabilizer position.
Close to the bit a greater build / drop rate will be achieved but with less directional control. When using
the walk tendency of the bit or formation (in under gauge position), 1st string stabilizer position is
desired. The span of the Pony DC between the near-bit Adjustable Stabilizer and the 1st stabilizer is
critical for acquiring drop. If the span is too long, in full gauge the assembly will build and not drop.
Again never forget the controlling factors: WOB, rpm, and pump rate.
 Performance Drilling BHA (straight motor)

These BHA’s are designated to build or drop at rates between 0.5°/30m to 1.5°/30m. When the
Adjustable Gauge stabilizer is near-bit positioned, high rpm will reduce its life, but will give the expected
1.5°/30m drop or build rate.
 Stabilized Turbine

Excellent directional control. Sliding is minimal in hold situations. Good build and turn response.
The assembly to work best needs a diamond impreg bit (long shank). The distance between the near-bit
stabilizer and the 1st stabilizer varies with the turbine size and is preset by the vendor. The 1st stabilizer
can be under gauge or in gauge. Keep in mind that the build ability of the turbine increases
exponentially with the inclination of the well bore. The position of the under gauge stabilizer is field
interchangeable.