A GPO guideline to create a wider understanding of the many variables involved in the selection and optimisation of stabilisers in the BHA.
G.P.O. Guideline
The job description of a drill string stabiliser is multi-functional, its main function is to stabilise the bottom hole assembly (BHA) in the wellbore, while enabling the drill string weight and rotary torque to be transferred past the stabilisation point with the minimum amount of weight and energy loss. The blade design should enable drilling fluid and the transportation of drilled cuttings past the stabiliser as efficiently as possible. In addition, the profile of the blades should allow interaction with the borehole wall, minimising the amount of shock and vibration generated when stabilisers with sharp edges collide with the formation, this scraping action can severely damage the filter-cake.
Although widely acknowledged as an essential component of any BHA, the stabiliser is still often overlooked. If properly designed the stabiliser will also contribute to improvements in performance. This is achieved by enhancing BHA stability in the borehole, minimising rotary torque and vibration and maximising energy transfer to the bit.
When optimising the sometimes-complex process of BHA stability, bearing in mind the many variables such as wellbore trajectory and profile, formation type, possible borehole instability, bit selection, drive type, and maximum drill-string RPM. It is obvious there are too many compromises in a one-fits-all scenario.
With the GPO guideline, to help simple BHA stabilisation and stabiliser selection, we highlight 3 areas that require additional attention.
- Geometry, i.e. the taper angle, blade profile, and flow-by area
- Position of the stabiliser in the BHA, to optimise BHA stability and minimise whirl and vibration
- Optimal outer diameter of the stabiliser blades, to avoid too close to gauge stabilisers from pinching on the borehole spiral patterns when POOH. Make allow for the difference between actual borehole diameter and effective borehole diameter
G – Geometry
Orientation, position, and profile of the blades (dome profile, no sharp edges) are all part of the design features of this different type of stabiliser. The 17-1/2” and 16” Switchblade and Fixedblade stabiliser have 8 blades, the 12-1/4” size and under all have 6 blades, this fore and aft blade configuration ensures all-round contact with the wellbore, improving BHA stability while providing a low pad pressure on the formation. The dome profile of the blades creates a plastering effect on the filter cake when rotating which compresses and limits the further growth of the wall cake.
The area profile between the two sets of blades creates a jetting effect accelerating the fluid and drilled cuttings around the blades, reducing the tendency of cuttings to accumulate build-up. The open profile, low pad-pressure, and spacing of the blades enable this design stabiliser to displace and bypass cuttings accumulated in cutting beds, with ease compared to the conventional spiral blade design.
P – Position
The optimal position of stabilisers in the BHA will change depending on the application i.e. hole size, well profile and trajectory, drive type, and maximum drill string rotational speed.
Contrary to common belief, vertical wells require equal attention to stabilization and stabilization points, if not more, than deviated wells, if downhole drilling dysfunctions are to be maintained at a manageable level. Inadequate BHA stabilization is also the main cause of Bit, and BHA whirl, resulting in shorter life cycles of downhole tools and compromised quality of the wellbore. Borehole spiralling, in turn, often leads to problems pulling out of the hole. This is particularly valid when the initial period and following spacing is neglected and close to gauge stabilisers end up pinching on the borehole spiral patterns.
Since the average diameter of a spirals hole is under gauge, spirals borehole patterns can create problems when running casing.
Never use a pendulum assembly to drill a vertical wellbore, it generates excessive lateral shocks and vibration. This theory is backed up by a quote from Fred Dupriest: “packed good, pendulum bad”.
The position and outer diameter of stabiliser in deviated hole-sections will differ depending on drive type i.e. motor or RSS.
For motor applications the outer diameter of the stabiliser positioned above the motor suitably under gauge to maintain a build or hold trajectory when drilling in the rotary mode.
For this application our Switchblade stabiliser is an ideal choice, as the OD of the blades can be varied by one inch in increments of 1/8” of an inch, enabling the operator to fine-tune the assembly to suit the required build rate when drilling in the rotary mode.
To improve BHA stability and minimise whirl when rotating, we recommend having at least two stabilisation points above the motor (observing critical rotary speed windows will improve performance and minimise tool failures).
Likewise, when drilling with an RSS we always recommend at least two stabilisation points above the RSS to minimise BHA whirl, which is the main cause of borehole spiral patterns, and the main contributor to time-consuming back reaming issues, when close to gauge stabilisers pinch and hang-up on the spiral profiles when pulling out of the hole.
Again, observing critical rotary speed windows are important. There is a tendency when drilling with RSS systems, to compensate for the combined bit speed of the motor- drill string combination, by rotating the drill string at speeds above the critical rotary speed threshold overlooking the centrifugal forces and buffeting created as a result of the excessive rotary speeds.
O – Outer diameter
The outer diameter and the position of stabilisers for deviated hole-sections will differ depending on the drive type, i.e. RSS or Motor.
For motor applications, the outer diameter of the stabiliser positioned above the motor will depend on the well-plan for the section i.e. build only or build and hold.
For a build only section, the outer diameter of the stabiliser above the motor should be gauged so the assembly has a build tendency when drilling in rotary mode, reducing the number of slides required to reach the planed hole angle, this practice will deliver a smoother build-up curve.
Motor assemblies for build and hold sections requires more finesse and a degree of local knowledge to fine-tune the assembly to where it has a slight build or neutral tendency in rotary mode. If the assembly has a dropping trend when rotating it will require additional upward slides to maintain the angle and produce a more tortuous build section.
For RSS applications, some RSS systems are more inclined to create borehole spiralling than others, yet they still continue to run stabilisers 1/8” of an inch = 3mm under gauge.
Summary
If you can minimise borehole spiral patterns when drilling, you will improve performance by minimising friction and consequently improving weight and energy transfer to the bit.
Spiral patterns in the borehole are also the main cause of tight hole problems when POOH, when too close to gauge stabilizers pinch on the spiral profiles.
All boreholes have spiral patterns to some degree, this fact should be considered when selecting the optimum outer diameter of stabilisers for your particular application.
Sufficient stabilisation points strategically placed in the BHA will minimize BHA whirl and spiral hole patterns.
The geometry of the stabilisers should be an important part of stabiliser selection. The profile of the blades (no sharp edges), the wrap angle (should have a clear line of sight and preferably an open profile) and an optimum taper angle of 30 degrees or below should be taken into account (Most is well documented by Paul Pastusek in ExxonMobil’s stabiliser guidelines).
Critical rotary speeds windows and optimum flow rates for hole cleaning should always be taken into consideration. Moreover, there is a balance that must be reached between the optimum rotary speed to clean the hole and damaging centrifugal forces created by detrimental rotary speeds.
It’s all about balance.
Hope this helps.
Tom
