Stinger or Tailpipe Placement of Cement Plugs
Authors: Carpenter Chris 
Published: February 2015

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 168005, ’Don't Get Stung Setting Balanced Cement Plugs: A Look at Current Industry Practices for Placing Cement Plugs in a Wellbore Using a Stinger or Tailpipe,’ by Justin Roye, Schlumberger, and Sam Pickett, Chesapeake, prepared for the 2014 IADC/SPE Drilling Conference and Exhibition, Fort Worth, Texas, USA, 4-6 March. The paper has not been peer reviewed.

The most common method used for the placement of cement plugs is the balanced-plug method using drillpipe, tubing, or a combination of both. A stinger with a smaller diameter than the drillpipe is commonly run on the bottom of the drillpipe for setting a balanced plug. However, a mathematical analysis of what occurs once dynamic conditions are initiated by pulling out of hole (POOH) with a small-diameter stinger shows that the initially balanced system quickly becomes unbalanced.

Introduction

The balanced-plug method of calculating plug-placement volumes has long been a standard industry practice for setting plugs in the wellbore. Volumes are calculated in such a way that all fluids both inside and outside of the pipe are at the same height, thus resulting in a hydrostatically balanced system.

The basic assumption behind the balanced-plug calculation method is that the fluid is going to remain in place while the drillpipe is simply pulled through the fluids with minimal falling of the fluid level to fill the void left behind by the metal displacement of the drillpipe. This assumption is correct, neglecting the frictional drag forces on the fluid, when the drillpipe outer diameter (OD) and inner diameter (ID) are the same from top to bottom. However, when the drillpipe includes a stinger at the bottom, there is a disruption in the hydrostatic equilibrium between the column of fluid inside and outside the drillpipe, resulting in flow at the bottom of the stinger between the two regions.

As soon as this dynamic situation is imposed on the fluids in the wellbore, the fluids downhole begin to experience a continual shifting in hydrostatic conditions. When a stinger is used at the bottom of a larger-diameter drillpipe, the difference in volumetric capacities per annular length can be substantial. The difference in the volumetric capacities inside the pipe per length can also be substantial. As a given length of larger-diameter drillpipe is pulled out at surface, all the fluid that was contained within that drillpipe, and the fluid in the annulus that was surrounding it, will attempt to stay in place. As noted earlier, there will be some drop in the fluid level caused by the removal of the volume of steel in the drillpipe. However, a much longer length or column height of fluid per unit volume is displaced within the smaller-diameter stinger because of its smaller capacity. After pulling out several hundred lineal feet of drillpipe at the surface, a substantial difference in lineal footage of fluid has passed through the stinger. If these fluid-column-height changes are not taken into account in the placement calculations, as they are not in the balanced-plug method, the result will be a plug that has had other displacement fluids dumped into the middle of the cement as the drillpipe is pulled out of hole.