Which of the following is NOT a main cause of swabbing?

The main cause of swabbing during operations is typically related to the interaction between the drilling fluid, the wellbore, and the pipe being tripped in or out. When considering the options, increasing annular clearance does not inherently contribute to swabbing. In fact, increasing the distance between the drilling assembly and the wellbore can provide more space for the drilling fluid to circulate and may help reduce the effects of swabbing.

Swabbing primarily occurs when the pipe is pulled out of the well too quickly or when there's an issue related to the hydraulic balance within the wellbore. High viscosity and gel strength of mud can create resistance when trying to pull the pipe out, while tripping out too fast can create a vacuum that leads to formation fluids being sucked into the wellbore, causing swabbing. Similarly, pulling through tight spots with the pump off can also cause a drop in pressure that may lead to the influx of fluids.

Thus, increasing annular clearance does not contribute to these dynamics and is recognized as not being a main cause of swabbing.