RIASSUNTO
Abstract
Drill string dynamics has become an important topic in recent times. This has proven to be particularly critical in slimhole drilling where the understanding and control of vibration has become essential. Conventional rigs have the strength and brawn to over-come most vibration problems and to get to TD even if not all that efficiently. In slimhole drilling, vibration can destroy the string. This paper is a case study into the experience of a major drilling contractor with severe vibration problems on their newly built slimhole rig, at the start of a multi-well contract with a major operating company to drill exploration wells in Venezuela.
The bottom hole assembly (BHA) was catastrophically damaged as a result of vibration, culminating in a number of fishing jobs and extensive repair work to the drill string and BHA. The postmortem on the damaged components and other wear indications on the string confirmed that the string had been rotating, at high speed, in a buckled mode.
This paper describes the various solutions considered and the actions taken. Particular focus is placed on the analysis of the BHA employed and the development of a new BHA design using the experience of a slim hole drilling consultant with the help of available modeling technology and how the limits to the operating parameters were set. With only theory as a guide, we needed reassurance that what had been done was the answer. What was actually going on down hole and what were the implications of our actions at the surface?
Technology currently available to measure vibration offered us the opportunity to see what was going on down hole in real time, giving us immediate feedback on the effects of our actions on surface. The standard vibration sensing equipment available consists of an instrumented tool installed in the string at surface. Unfortunately this was too big to fit under the top drive of the little slimhole rig and another approach was needed.
The paper describes how the team from the vibration monitoring service company came up with the innovative idea of instrumenting the top drive itself to act as the tool and how they managed to achieve their objectives of providing a reduced level of their standard service to focus on the specific problem.
The paper concludes with some examples of how the monitoring system was able to tell us what the acceptable operating limits for weight-on-bit (WOB) were, by observing the growth of axial strain energy at specified frequencies. Some other valuable spin-offs have been seen and are highlighted in the paper.
Introduction
The application of the slimhole drilling technique in the oil and gas environment has drawn on the developments achieved in the mining industry. This has been particularly the case in the adoption of wireline coring, a feature that has excited oil and gas explorers, which is fortunately highly developed in the mining industry. Unfortunately, with slimhole drilling, there are a number of compromises that have to made in order to take advantage of the cost saving benefits. Mining has been able to drill to extremely deep depths with thin wail pipe and low torque connections. Normally flush or almost flush pipe Is common in mining, and to perform correctly, it requIres the support of the well bore to provide lateral stability with weight applied to the bit, particularly at high rotation speeds. The mining industry does not generally use bottom hole assemblies (BHA's) which are a combination of drill collars to provide weight for the bit and stabilizers to control direction. A major oilfield drilling contractor was the first to develop a hybrid system for slimhole drilling which offers the slimness in size, together with wireline continuous coring capabilities, but with the use of specially developed drill string and BHA, and oil field techniques for the application of weight and directional control.
The main advantage of the hybrid over the mining system is that the stiffer BHA will take compressive loads, to a degree, without lateral deflection of the pIpe which is essential in drilling washed out hole that is typical in sedimentary rock. The lateral deflection, if it exceeds a relatively low value, will very quickly subject the string to cyclic stress that will result in fatigue failure. In the case discussed here, the first indication of this phenomena manifested itself in the form of vibration of the drill string clearly discernible at the surface.
Recently, a number of suppliers have been developing and manufacturing high tech drill rods for use in oil and gas environments.
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