The existence of substantial vibrations in the drillstring have caused much trouble time during drilling in slim (and conventional) holes. Minimising drillstring vibrations during the drilling of slim and conventional holes means that the dynamic components of the loads in the drillstring are reduced, with the following consequences:

• The peak loads can be reduced reducing twist-offs, etc.
• The cumulative fatigue loads are reduced, giving longer component life.
• The dynamic loads on the formation are reduced. These loads can contribute to borehole instability and washouts.
• The predictability of directional drilling devices and assemblies is improved.
• Smooth running bits drill faster and last longer.
• More mechanical and hydraulic power can be transmitted to the bit, with a consequent improvement in penetration rates.
• Open hole times are reduced, due to less trouble time and faster penetration rates.
• The level of mechanically induced hydraulic vibrations are reduced, thus facilitating improved mud pulse transmission.
• The dynamic loads on surface equipment are reduced.
• Damage to the casing, cement sheath and wellhead are reduced.
• The risk of losing downhole radioactive sources from MWD tools is reduced. The less rugose hole and shorter open hole time reduce the risk of losing wireline run sources (as well as facilitating good interpretation).

2 Types of drillstring vibrations

"A drillstring is an extremely slender structure with a ratio between length and diameter larger than in a human hair. Because the string has a smaller diameter than the borehole it is free to vibrate laterally. These lateral or bending vibrations are especially important in the lower part of the drillstring. Higher up, the high tension in the drillstring causes the string to be in continuous contact with the borehole wall in most places, because boreholes are always slightly curved. Furthermore, the damping caused by borehole wall contact, and the diminishing amplitude of upward travelling bending waves caused by the increasing tension in the drillstring, render a detection of downhole lateral vibrations at surface very difficult. As a result, the importance of this type of motion has been overlooked for a long time. An important cause of lateral vibrations are out/of balance forces in the drill collars, resulting in a whirling motion, just as in an unbalanced centrifuge. Another cause of lateral vibration is the friction between the rotating drillstring and the borehole wall, which can produce a backward rolling motion of the drillstring along the wall. Typical frequencies of lateral vibration are from 0.5 to tens of Hz. In case of lateral shock loads much higher frequency components may be present."

"The second type of drillstring vibration is torsional or rotational vibration. In its most drastic form, the bit comes to a standstill while the top of the drillstring rotates with a constant rotary speed, thus increasing the torque in the drillstring until the bit suddenly comes loose again. This "stick-slip" type of torsional vibration has a typical frequency of 0.05 to 0.5 Hz. The vibrations are caused by a non-linear relationship between torque and rotary speed at the bit (and stabilisers): the torque required to rotate the bit (and stabilisers) is less than the torque required to break it loose from standstill. This causes self-excited vibrations where a constant rotational motion of the drillstring produces oscillations, very much like a constant translational motion of a piece of chalk on a blackboard sometimes produces a squeaking sound."

"The third type of drillstring vibration is axial or longitudinal vibration. In the extreme case the bit periodically comes loose of the bottom of the hole, a mode of motion called "bit-bounce". When drilling with a three-cone bit, the initial cause may be some hard spot in the formation, causing a periodic displacement at the bit with a frequency of three times the downhole rotary speed. If the resulting periodic force at the bit is almost in phase with the imposed displacement, the unevenness of the hole bottom will be amplified, leading to a "three-lobed" bottom-hole pattern and sustained axial vibration (at three times the rotary speed) with a typical frequency between 1 and 10 Hz. Other types of axial vibration occur, caused by e.g. pressure fluctuations in the drillpipe or the cutting process of PDC bits.

Vibration can also be observed when the drillstring is not rotating. For example, when using a steerable mud motor in oriented mode longitudinal stick-slip friction can induce irregular vibrations. In some circumstances this can become so severe that it is impossible to hold a constant tool face

3 Minimising the effects of drillstring vibrations

There are two complementary ways to counter drill string vibrations and both methods have a place in an integrated slim hole drilling system. They may be summarised as:

• make it stronger
• minimise and control the vibrations.

4 Make it stronger

This is the approach which has been historically followed in the drilling industry; make things stronger, more fatigue resistant and develop operating procedures to safely and effectively live with the consequences of random and unpredictable failures.

5 Minimise and control the vibrations

There are four requisites to drilling performance improvement by vibration minimisation:

• Recognition that vibrations exist.
• Recognition that overall drilling performance will be improved if they can be minimised.
• Recognition that time and money are required to investigate the nature of the vibrations and to develop and apply methods to minimise them.
• Evaluation of the costs savings of improved overall drilling performance and being able to drill a smaller well against the costs of vibration minimisation.

There are two basic complementary principles of vibration minimisation:

• Minimise the generation of vibrations.
• Minimise the life of the vibrations by damping and avoiding reflections, coupling and resonance.