Packers

The packer (more accurately described as the 'upper completion production packer') is a key piece of downhole equipment in many completions - a sealing device that isolates and contains produced fluids and pressures within the tubing string; it is a well barrier element, usually part of the well's primary well barrier, protecting the casing and creating an A-annulus. The packer is essential to the basic functioning of most wells, injectors or producers. Alternatives to using a production packer include a dynamic seal assembly, a cemented completion and a packerless completion.  

Uses of packers

In addition to providing a seal between the tubing and casing, other aspects of a packer are as follows:

·         Prevent downhole movement of the tubing string, generating considerable axial tension or compression loads on the tubing string.

·         Support some of the weight of the tubing where there is significant compressive load on the tubing string 

·         Allows the optimum size of well flow conduit (the tubing string) to meet the designed production or injection flowrates

·         Protect the production casing (inner casing string) from corrosion from produced fluids and high pressures

·         Can provide a means of separating multiple producing zones

·         Provided the tubing string and packer maintain integrity, well control is focussed on the tubing flow, allowing the downhole safety valve to shut-off flow fron the reservoir. 

·         Hold well-servicing fluid (kill fluids, packer fluids) in the casing annulus.

·         Facilitate artificial lift, such as continuous gas lifting through the A-annulus.  

Packer components

Packers have four key features:

·         Slip

·         Cone

·         Packing-element system

·         Body or mandrel.

The slip is a wedge-shaped device with wickers (or teeth) on its face, which penetrate and grip the casing wall when the packer is set. The cone is beveled to match the back of the slip and forms a ramp that drives the slip outward and into the casing wall when setting force is applied to the packer. Once the slips have anchored into the casing wall, additional applied setting force energizes the packing-element system and creates a seal between the packer body and the inside diameter of the casing.

Packer classification

Production packers can be classified into two groups:

·         Retrievable

·         Permanent.

Permanent packers

Permanent packers can be removed from the wellbore only by milling. The retrievable packer may or may not be resettable, but removal from the wellbore normally does not require milling. Retrieval is usually accomplished by some form of tubing manipulation. This may necessitate rotation or require pulling tension on the tubing string.

The permanent packer is fairly simple and generally offers higher performance in both temperature and pressure ratings than does the retrievable packer. In most instances, it has a smaller outside diameter (OD), offering greater running clearance inside the casing string than do retrievable packers. The smaller OD and the compact design of the permanent packer help the tool negotiate through tight spots and deviations in the wellbore. The permanent packer also offers the largest inside diameter (ID) to make it compatible with larger-diameter tubing strings and monobore completions.

Retrievable packers

The retrievable packer can be very basic for low pressure/low temperature (LP/LT) applications or very complex in high pressure/high temperature (HP/HT) applications. Because of this design complexity in high-end tools, a retrievable packer offering performance levels similar to those of a permanent packer will invariably cost more. However, the ease of removing the packer from the wellbore as well as features, such as resettability and being able to reuse the packer often, may outweigh the added cost.

Packer selection

Before selecting either tool, it is important to consider the performance and features of each design, as well as the application in which it will be used. Perhaps in some instances, the permanent packer is the only option, as may be the case in some HP/HT applications. However, in those instances in which either will suffice, the operator must decide which features offer the best return over the life of the well.

When selecting a packer for a cased-hole completion, the differential pressure and temperature requirements of the application must be considered. The well depth, deployment and setting method desired, and final tubing landing conditions are also factors that come into play. The various operational modes (flowing, shut-in, injection, and stimulation) that are anticipated over the life of the well are critical and must be considered carefully in the design phase. The changes in the operational modes that influence changes in temperature, differential pressure, and axial loads all have a direct impact on the packer. Understanding the uses and constraints of the different types of packers will help clarify the factors to consider when making a selection.