This is one of excellent VDO demonstrating shock and vibration in the drillstring by Schlumberger. In this VDO, you will learn about mechanisms of shock and vibration, types of whirls and how these will damage your drillstring and BHA. Moreover, we provide the VDO transcript to help you get more understand about the content of this VDO as well.
Full VDO Transcript
As you know both shock and vibration are prime culprits in many drilling problems, today we are going to discuss how to minimize their effects and thereby improve drilling performance. Shock in a drilling environment is the sudden input of energy from impacts of the bit, BHA or drill pipe with the wellbore. Vibrations can result from the shocks. Rapid and continuous vibrations result in fatigue of the drill string connections to the point of twisting off. This is why many drill string failures are the result of accumulative fatigue due to vibration and have regular inspection periods. Shocks are measured in G’s by an accelerometer. 1G equals the force of gravity. Shock magnitudes registered via down hole tools can exceed 200G’s.
The severity of a shock depends on three parameters;
1- the magnitude of the shock,
2- the duration or length of time of the shock and
3- the frequency or number of shocks.
Magnitude is that a force the tool sees when impacts the borehole well. Duration measure how long the shock last in seconds. Frequency is the number of times the tool seize a shock greater than the accelerometer’s threshold. Shock and vibration and poor drilling mechanics can adversely affect ROP, slowing the drilling process.
So how do we detect potentially harmful shock and vibration? Surface and down hole data in both depth and time formats are used to diagnose down hole conditions. Looking at the drill string and bit after the run will also help you determine the severity and type of shock and vibration. Schlumberger measures lateral shocks and all of their tools; these tools have shocks sensors that measure lateral vibration shocks with magnitudes greater than 50G’s. Our discussion will look into six BHA dynamic motions; axial, lateral, torsional, BHA Whirl, Stick-slip and eccentric. These can exist separately or can be present together. Proper identification of the vibration mode is essential in order to recommend the correct cure.
Axial shocks arise from movement of the drill string along the axis of the drill string. in the most severe form, it is sometimes referred to as a bit hopping or bit bounce. However in most cases, there is not enough force to allow the drill string to come up the bottom and bounce. Instead the shocks are transmitted up the drill string which harmonically increases and decreases the weight on bit.The consequences of axial shocks could be broken bit teeth, damaged down hole tools and slowed ROP.
Lateral shocks result from lateral motion of the BHA in one side of the well bore to the other, causing it to bounce randomly against the sides of the bore hole walls. Torsional shocks result from the momentarily slowing down and stopping of the drill string which occurs when the bit digs into the formation deeply enough to slow it down relative to the drill string or when destabilized into the formation. This causes a whirling effect on the drill string which can fatigue the drill string and BHA.
BHA Whirl is very complex or exists as a result of under-stabilized drill string that forces acting on the BHA such as rotating, resonance rpm on large bore hole. Whirl occurs most frequently but is not limited to near vertical walls. It occurs when there is an side wave or lateral movement in the BHA contacting the walls. In the video, we see a rolling shaft test with BHA component mounted horizontally or anchored at each end of rotating.
There are three main types of BHA whirls; Forward, Backward and Chaotic Whirl. Forward whirl is when the BHA robs the formation along the same part of the collar as the drill string rotates. If the formation is abrasive, excessive wear will occur along the part of drill collar that rubs the formation this wear is seen as plat box one side of drill collar or as a single worn blade or stabilizer.
In Forward Whirl, the BHA still rotates in the same direction as the drill string. Forward Whirl can destroy bits and BHAs. Backward Whirl is very similar to Forward Whirl except friction between the formation and BHA is greater. This increased friction results in increased torque on the BHA, which causes the BHA to rotate in the opposite direction of the rotation of the drill string. If whirling is backwards, then the collar connections can flex and fatigue at a very fast rate, resulting in accelerated fatigue cracking, washouts and possible twist offs.
In Chaotic Whirl, there is no preferential side of the collars of BHA contacting the formation. The torque will be above average along with the lateral vibration and shocks. Chaotic whirl can occur when changing rotary rpm to try and address forward or backward whirl. Bit whirl is associated with PDC bits because of their aggressive side cutting action on harder rocks and near vertical holes. It is caused by non-symmetric cutting action of a real formation that displaces the bit from its center of rotation, and then allows the bit to move. Verification of shock and vibration can often be obtained by examining the bit after the trip. When dealing with BHA whirl, it is sometimes necessary to stop drilling completely to cure it- depending on the severity of the shocks and difficulties with drilling.
Stick-slip is a non-uniform rotation of the drill string. This is the rotational slowing down and acceleration of the BHA. In extreme cases, the BHA can stop, even reverses its direction. During drilling, friction builds up causing the BHA to momentarily turn at a slower rate than the surface pipe or even stop. As this happens, the string stores the energy imparted by the rotary of top drive in the drill string, excess energy builds up, the friction slowing BHA rotation will be overcome. When the stored rotation since it is now going to beyond the number of rotation the rotary has made, it will now spin backword for short period.
When this happens, an inadvertent unscrewing of the connector can occur. Our review of shocks and vibration effects on drilling is a good starting point for your understanding of the issues raised this phenomenon in the drilling environment. Schlumberger wants you to develop your knowledge of these issues, how to detect them and how to mitigate them. These different types of shocks occur simultaneously, creating serious drilling problems which often requires drilling to stop and then restart. Careful planning, execution and post-drive analysis will often reduce the hazards of these shocks and vibrations. We hope this presentation will assist you in this endeavor. Thanks for your attention
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