Method of determining drill string stiffness

Details Method of determining drill string stiffness Method of determining drill string stiffness

1999-08-25

2001-12-04

Assouad, Patrick (Department: 2857)

Data processing: measuring, calibrating, or testing

Measurement system in a specific environment

Mechanical measurement system

C702S009000, C702S145000, C073S152430, C073S152470, C175S040000

Reexamination Certificate

active

06327539

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a method and system for determining the rotational stiffness of a drill string for drilling a borehole into an earth formation.
BACKGROUND OF THE INVENTION
During rotary drilling the drill string, and in particular the lower part thereof which is termed the bottom hole assembly (BHA), can be subjected to undesired rotational vibrations also referred to as oscillations. The magnitude and frequency of such rotational vibrations depend on parameters such as the length and stiffness of the drill string, the number and positions of the drill string stabilisers, the shape of the borehole, and the weight of the BHA. Stick-slip is a mode of rotational vibration which is particularly undesirable as it leads to a reduced penetration rate of the drill bit and to enhanced wear and damage to the drill string. During stick-slip the movement of the drill string is characterised by repeated cycles of deceleration and acceleration whereby in each cycle the drill bit comes to a halt and subsequently accelerates to a speed significantly higher than the nominal speed of the rotary table.
EP-A-0443689 discloses a system for controlling drill string vibrations, which varies the rotary speed gradually in response to rotational vibrations of the string so as to damp the vibrations. The drill string is driven by a drive system which in most cases includes a rotary table driven by an electric motor, or by a top drive driven by an electric motor. The control system operates on the principle of controlling the energy flow through the drive system and can be represented by a combination of a rotational spring and a rotational damper associated with the drive system. To obtain optimal damping, the spring constant of the spring and the damping constant of damper are to be tuned to optimal values. It will be understood that the rotational stiffness of the drill string plays an important role in tuning to such optimal values. However, the actual rotational stiffness of the drill string is generally unknown as it changes during the drilling process due to, for example, the drill string being extended as the borehole becomes deeper.
It is therefore an object of the invention to provide a method and a system for determining the rotational stiffness of a drill string for drilling of a borehole in an earth formation.
SUMMARY OF THE INVENTION
In accordance with the invention there is provided a method of determining the rotational stiffness of a drill string for drilling of a borehole in an earth formation, the drill string having a bottom hole assembly (BHA) and an upper end driven by a rotational drive system, the method comprising the steps of:
determining the time derivative of the drill string torque during drilling of the borehole at a selected time when stick-slip of the BHA occurs;
determining the nominal rotational speed of the drill string at an upper part thereof at said selected time; and
determining the rotational stiffness of the drill string from a selected relationship between said time derivative of the drill string torque and said nominal rotational speed at the upper part of the drill string.


REFERENCES:
patent: 5864058 (1999-01-01), Chen
patent: 6205851 (2001-03-01), Jogi
patent: 443689 (1991-08-01), None
patent: 816629 (1998-01-01), None
patent: 0443689 A2 (1991-08-01), None
patent: 2705801 (1994-12-01), None
patent: 2311140 (1997-09-01), None

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