RIASSUNTO
Abstract
Extending motor, bit, MWD, and BHA component life is of primary importance to the drilling process. Frequently downhole vibrations can cause anywhere from minor to catastrophic failure in any of the components of a typical BHA.
Some types of vibrations are more detrimental to various components of the BHA. Most vibrations can be controlled with the alteration of surface parameters such as weight-on-bit and rotary speed in conjunction with downhole measurement tools such as downhole weight-on-bit and downhole torque as well as BHA alteration. The problem has always been knowing which parameters to alter without adversely affecting total drilling performance.
Recent advances in MWD technology have provided a means of measuring downhole vibration in multiple axes. This information, provided in real-time, allows the driller to control the proper parameters to minimize specific vibration effects thereby maximizing BHA life and total drilling performance.
Introduction
Attempts to drill vertical holes with 30/60 pendulum assemblies (MWD and straight-hole mud motors) or 60/90 pendulums (in rotary mode with only MWD) had met with catastrophic failures in the past year. Mud motors had been ""twisting off"" and several MWD failures resulting in junked electronics were common.
On the first case well, using a 30/60 pendulum assembly, the mud motor was twisted off at the AKO sub and the MWD failed. The MWD collar was even torn open. The MWD dump of the standard MWD shock counter revealed very high shocks. After this run, two rotary runs were made with only MWD and 60/90 pendulum assemblies. Both MWD tools failed. It was decided to employ the multi-axis vibration chassis to get a picture of what was happening downhole. The MVC data revealed violent episodes of mostly torsional and lateral vibrations characteristic of BHA whirl. These episodes occurred in hard, wet sands prevalent in this area. This MWD run failed as well.
On run five, the MWD was stabilized top and bottom. Using the MVC, the drillers were able to ""see"" the sands and adjust their weight-on-bit and rotary speeds to reduce vibrations to acceptable levels and prevent the BHA from going into a whirling state. Parameters were returned to normal in shales. All failures ceased. On run six a record BHA run for this area was recorded with the application of a straight-hole motor with a sleeve stabilizer and the MWD and LWD tools correctly stabilized. On two subsequent wells the assemblies were modified using straight-hole motors with sleeve stabilizers. The drillers duplicated their drilling practices of the previous well and both wells were drilled without failure.
Theory
Drilling requires energy. In drilling, this energy is obtained with three basic drilling parameters, weight on bit, RPM, and mud flow. Vibrations are always present during drilling in varying magnitudes and these detract from and re-direct some of the energy used for the drilling process. The usual primary goal in vibration detection and prevention is to minimize these vibrations in order to maximize ROP. In this field case, this statement has become secondary. The primary goal in this case is to prevent the destruction of critical components of the BHA in order to extend drilling life downhole.
To extend BHA life, it is critical to understand the mechanisms working against us and to be able to quantify their magnitude. With this knowledge, we can alter BHA construction as well as use surface drilling parameters to control these mechanisms. With a multi-axis vibration detection device we can identify the differing types of known mechanisms, measure their magnitude, and determine the effectiveness of our designs and efforts to control them. This case study will qualify these statements.