Abstract

Downhole plugs are now commonly used for isolating multistage stimulation jobs. These plugs allow for more timely completion, yet they must also be removed in a timely fashion. This paper discusses how coiled tubing (CT) can efficiently mill out these plugs.

The three case histories in this paper demonstrate the successful use of coiled tubing to mill out bridge plugs set in multistage stimulation jobs. Specifically, each case history provides a brief history of the well, a description of the equipment used to meet downhole and surface needs, and the operating procedure used for the job. The paper concludes with procedures to improve CT millout efficiency.

Introduction

As oil companies search for ways to lower costs, the completions area has become a prime target for change. CT services have helped reduce costs by eliminating the need for workover rigs in certain applications. When coiled tubing units (CTU's), are used, operators can arrive on location, rig up the CTU, perform the service, and be off location more quickly and economically than they can when they use a workover rig. Bridge-plug milling is definitely an application where a CTU can save revenue.

Milling plugs can be a cost-effective operation if the proper planning occurs before the job. When planning a milling operation, operators must consider equipment and treatment options (type of CTU, workstring, bottomhole tool assembly, and milling fluid) on the basis of plug information, bottomhole pressures, bottomhole static temperatures (BHST's), hydraulic fracturing information, and other data. When an operator reviews and adheres to the best practices for all parameters, a successful milling operation is possible.

Well History

Before performing a CT milling operation, operators must understand the well's history. In addition to knowing information regarding well tubulars, depth, location, and other data, operators should consider other key factors. For example, if the well has been stimulated, it may be energized as a result. If the well has been produced for a long period, it may be depleted. If fluid has been injected into the well for a prolonged period, deposits may have built up in the well.

Another key factor is the well's bottomhole pressure (BHP). By knowing the BHP, operators can determine the proper equipment and the proper fluid for transporting cuttings to the surface. If the BHP is low enough that the formation cannot support a column of fluid, nitrogen must be added to the milling fluid to help increase annular velocity and transport cuttings to the surface.

Another important factor is "fish," or debris left in the well from previous operations. For example, ball sealers used on an acid ball-out or tools lost from wireline operations can greatly increase the milling time and often increase the chance of the assembly becoming stuck in the hole.

Equipment Considerations
CT/Pumping Equipment.

When performing a millout, operators must review equipment selection. Selecting the appropriate CTU, CT string size and length, as well as the needed pumping equipment will aid in milling efficiency (Fig. 1, Page 5). If a high-pressure well environment is present, special equipment such as additional flow crosses and external blowout preventers (BOP's) may be needed.

CT/Pumping Equipment.

When performing a millout, operators must review equipment selection. Selecting the appropriate CTU, CT string size and length, as well as the needed pumping equipment will aid in milling efficiency (Fig. 1, Page 5). If a high-pressure well environment is present, special equipment such as additional flow crosses and external blowout preventers (BOP's) may be needed.

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