Stick-slip and whirl are the devastating vibrations that significantly limit drilling performance. They not only cause equipment failures, but also increase non-productive time driving up field development costs. Although the industry has achieved tremendous improvements in fighting against these dysfunctions, the root causes of stick-slip and whirl are still not fully understood.
Stick-slip is characterized by the fluctuation of rotary speed of the bottom hole assembly (BHA) while whirl is known as the severe lateral vibration that may occur at bit and/or BHA. A driller's dilemma emerges when increasing weight-on-bit (WOB) induces stick-slip whereas increasing revolutions /minute (RPM) induces whirl. Keeping both WOB and RPM low reduces vibration levels but it negatively affects ROP. As a result, the drilling operation either suffers low ROP or experiences higher ROP but with severe vibrations. The latter can cause damage to the bit and rotary steerable system (RSS), motor (PDM), measurement while drilling/logging while drilling (MWD/LWD) leading to a lack of steerability and poor borehole quality.
New research strongly suggests the above dilemma is related to the drillstring's tendency to couple stick-slip and whirl. This coupling is induced by improper bit selection and unfavorable bit-BHA interactions. For a given bit and rock, the critical values of WOB and RPM triggering stick-slip and whirl can be predetermined assuming other drilling conditions are known and fixed. Plotted in a rectangular coordinate with abscissa of RPM and ordinate of WOB, these critical values represent the boundaries of stable drilling parameters. The other important boundaries include:
Maximum torque limited by rig capability
Minimum ROP specified by operators
Maximum WOB limited by drillstring buckling and directional control
These boundaries together define a closed domain in the space of WOB and RPM. This domain is called the "optimum zone" in this paper. The drilling parameters in the optimum zone theoretically guarantee BHA/bit stability. The scope of the optimum zone depends on the bit and the mechanical properties of the rock to be drilled. The drillstring dynamics, however, reduce the optimum zone significantly by creating bit-BHA interactions. In an extreme case, these interactions can push the boundaries of stick-slip and whirl to cross, completely eliminating the optimum zone. In this situation, any attempt to mitigate vibrations by varying drilling parameters will most likely fail. To solve this challenge, an advanced drillstring model was developed and applied to quantify the following effects on the optimum zone:
Mechanical rock properties
Bit design including cutter, body and gauge profile
BHA design and its interaction with bit
The ability to obtain superior stability and ROP simultaneously depends on two key factors: (1) maximizing the optimum zone through bit optimization, and (2) minimizing bit-BHA interactions by BHA optimization. The objective of both is to decouple potential stick-slip and whirl. The authors will present several case studies to illustrate the concept of optimum zone and its effectiveness on decoupling stick-slip and whirl to increase overall drilling performance.