The drilling industry has never been more challenged than it is currently; consequently, we need to go back to bases and have a closer look at the root cause of many of the drillstring and borehole related problems we are currently confronted with on a daily basis.
The majority of drillstring and borehole related problems can be traced back to drillstring vibration and borehole instability and spiralling. As we push the envelope of possibilities ever further in extended reach drilling applications and complex borehole trajectories, it is understandable that the driveshaft of the entire system i.e. the drillstring requires adequate stabilisation.
A poorly designed stabilizer with no specific design criteria in combination with insufficient stabilisation points is the root cause of vibration in its many forms.
Bit and BHA whirl are a result of inadequate stabilisation; BHA whirl is a very disruptive sequence identified by and contributing to eccentric wear on BHA components. BHA whirl is the main contributor to lateral shock loads and the onset of stick-and-slip cycles.
Many of the vibration related problems confronting our industry are a combination of interrelated events which can be avoided or minimised if identified and managed.
Problems, when POOH for instance, in many incidences are a result of borehole patterns/spirals which create pinch points on near gauge stabilizers as the BHA inflexible by tension is pulling through the spiral borehole profile. Practical consideration should be given to the maximum outer diameter of string stabilizers to avoid pinching. Food for thought: the average ignition key on a motor car is 3mm = 1/8” of an inch, if the string stabilizers in the BHA are ¼” under gauge, the total clearance when pulling through a spiral profile wellbore is twice the thickness of your car key.
Boreholes are rarely or never cylindrical and boreholes drilled with PDC bits all have whirl pattern profiling to some degree. The reactive forces created by the rotation of PDC bits generates a tortuous reactive motion between the bit and the first stabilization point; this tortuous displacement is one of the root causes of bit whirl and borehole patterns. The greater the distance between the bit and the first stabilization point, the greater the amplitude of the whorl patterns.
“Avoid running pendulum assemblies in vertical hole sections”– A quote taken from a publication by Fred Dupriest on the same subject: “Packed good, Pendulum bad”.
Practical analysis of BHA modelling software, combined with experience and local knowledge are key components in improving BHA design. By optimising the placement stabilizer in the drillstring, identifying critical rotary speeds windows, are all vital when striving to improve BHA and drilling performance, minimising vibration and BHA component failures.
However, when modelling to find the optimum outer diameter gauge of Stabilizers to minimise the hazards of pinching when tripping out of the hole….. A word of caution here: there are too many variables in the complex phenomena of borehole spirals for the results modelling alone to be taken as a definitive solution.
The oil and gas industry has evolved considerably over the past four decades; we are drilling holes thirty percent faster today than we were in the mid-eighties. Yet there is one important element in the drill string where the design and profile have not changed in over 40 years.
Our goal is to design a range of stabilizers flexible enough to meet the challenges of today’s market.
This is what we are striving to change.