Abstract

Tubular restrictions from scale buildup can significantly reduce hydrocarbon production and the overall production capacity of a well. Before production can be restored, this buildup must be removed. Although scale buildup is a global problem, the material composition and characteristics of scales can vary drastically, even within the same field. Therefore, most tools traditionally used for scale removal cannot efficiently remove scale in all well conditions. This paper discusses a recently developed, customized jet cleaning tool that resolves some of the problems inherent with scale-cleaning operations.

Traditional jet cleaning tools have not adequately controlled jet dwell time, which has severely impacted their operational performance. Depending on the type and hardness of the scale, many removal methods cannot control the jet dwell time required to accomplish scale removal. Because the required jet dwell time increases with the hardness of the buildup material, cleaning efficiency can be measured by the capability of the scale-removal method to control dwell time. A new, slow-rotating jet cleaning tool maximizes jet dwell time without introducing repetitive cyclic effects on a coiled tubing (CT) workstring.

The slow-rotating jet cleaning tool can also be used for cutting tubulars. Previously, tubulars could only be cut by mechanical, explosive, or chemical methods. CT was used either for deploying positive-displacement motors (PDM's) for rotating mechanical cutters or to deploy pressure-activated firing devices that initiated the firing sequence in explosive or chemical cutters. The slow-rotating jet cleaning tool can be used to cut tubulars when limitations exist for other cutters.

This paper discusses the evolution of CT cleaning procedures, alternatives to cleaning with CT, and the design and application of the new slow-rotating jet cleaning tool. Case histories discuss the use of the jet cleaning tool for removing scale and production buildup on tubing walls. The new tool can be used for performing through-tubing cuts and can help reduce service costs for difficult applications, such as those involving long bottomhole assemblies (BHA's). The capacity of the new jet cleaning tool to control jet dwell time and pump abrasive fluids helps reduce the risks, obstacles, and costs associated with traditional cleaning and cutting methods.

Introduction

Scale deposits on tubing walls and perforations can stop well production, inhibit the injection of well fluids, and cause downhole production equipment to become lodged. If left untreated, these problems will compound, and hydrocarbon production will continue to decrease.

Early Use of CT for Cleaning.

The choice of a scale-removal treatment is often affected by cost. Treatment methods such as acidizing or milling with small-diameter tubing can be expensive. Because CT operations could be performed under live-well conditions, CT was introduced in the 1970's to provide a lower-cost option for scale removal.

CT provided a washing action for removing scale materials. When acids were run through the tubing, they reacted chemically with the scale, extracted waste materials from the tubing, and restored the fluid flow path to the base pipe. However, using CT alone to remove scale was insufficient. Several factors affected the cleaning efficiency of CT:

  • The fluid-pumping equipment delivered more hydraulic horsepower (hhp) to the CT than the CT could withstand.

  • Because of the friction pressure of the small string diameters and pressure ratings of the tubing, appoximately 80% of the available hhp was lost.

  • The tools that were placed on the end of the CT to assist with cleaning caused an additional 15% loss of available power. Consequently, only a small percentage of the hhp from surface pumping equipment was available for removing buildup.

Early Use of CT for Cleaning.

The choice of a scale-removal treatment is often affected by cost. Treatment methods such as acidizing or milling with small-diameter tubing can be expensive. Because CT operations could be performed under live-well conditions, CT was introduced in the 1970's to provide a lower-cost option for scale removal.

CT provided a washing action for removing scale materials. When acids were run through the tubing, they reacted chemically with the scale, extracted waste materials from the tubing, and restored the fluid flow path to the base pipe. However, using CT alone to remove scale was insufficient. Several factors affected the cleaning efficiency of CT:

  • The fluid-pumping equipment delivered more hydraulic horsepower (hhp) to the CT than the CT could withstand.

  • Because of the friction pressure of the small string diameters and pressure ratings of the tubing, appoximately 80% of the available hhp was lost.

  • The tools that were placed on the end of the CT to assist with cleaning caused an additional 15% loss of available power. Consequently, only a small percentage of the hhp from surface pumping equipment was available for removing buildup.

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