Wellhead Isolation Tool Nozzle Design Reduces Erosion Of Well Tubing
Authors: Surjaatmadja Jim B. , Ripley Harry E. 
Published: March 2010

Improperly designed wellhead isolation tool (WIT) nozzles can cause substantial wear on well tubing. This is especially true when stimulation treatments contain abrash1e particulates such as proppants and are conducted at high flow rates. Multiple treatments can create cumulative damage.

This paper presents the results of an extensive research effort to evaluate different nozzle designs. The data show that many nozzle designs may cause severe weakening of the tubing. One design which minimizes wear is also presented. Laboratory tests and field data indicate that the Use of this design minimizes concerns for tubing failures during or after the job. The paper also includes results and discussions on the effects of treatments on various grades of tubing using different fluids and proppants.

Introduction

Wellhead equipment often is the weak point during high-pressure stimulation treatments. It can be designed to take relatively low pressures and is generally quite expensive. When fracturing of the formation is desired, wellhead equipment may become a constraint because it will not allow high-pressure treatments through the equipment Even if the wellhead tree can withstand the treatment pressures, there is the problem of wellhead tree erosion caused by the proppant used in propped fracture treatments. Fracturing operations therefore may require either installation of different wellhead equipment or protection for the wellhead.

Wellhead isolation tools (WIT) protect high-cost wellhead equipment from damage by erosive fluids and from high pressures used during fracturing. This type of equipment have been widely used in the past few decades and, in most cases, the jobs were completed satisfactorily.

As fracturing treatments continue to demand increased pumping rates with higher proppant concentrations, erosion damage becomes more relevant in areas where moving fluids experience dimensional changes. The most critical area is in the tubing directly below the exit of the WIT. Erosion damage can range from insignificant to extremely serious; in fact, total failure of the well tubing has been experienced. Thus, the WIT, which is designed to protect wellhead equipment, may itself become a threat to the well tubing integrity.

This paper will discuss WIT nozzle designs and their effectiveness in preventing tubing damage. Extensive laboratory data and some field data are also presented. The authors will show that some WIT nozzle designs have tubing wear levels equivalent to instances where no WIT was used. The conclusion will discuss how to apply the data for selecting the proper WIT nozzle design. Other means for reducing tubing wear are also discussed.

WIT Basics

All currently existing wellhead isolation tools share similar basic design features. Essentially, the tool uses a mandrel, which is insened through the vertical passage of the wellhead equipment (Fig. 1). The mandrel must pass all wellhead equipment and seal in the well tubing; therefore, all parts of the mandrel, including sealing elements, need to be small enough to pass all wellhead equipment yet large enough to seal in the tubing. From experience, the authors have found that many tubing hangers are smaller than the tubing ID, so the mandrels used were designed smaller than the tubing hangers.