This article describe the completion packer selection, specifications, classification, setting mechanism, etc.
Packer selection/specification
packer selection must take into account:
- type of hole: open, cased, liner completed;
- type of well: producing, appraisal, injection;
- well content: oil, gas (sweet, sour), water, steam, abrasive material;
- natural well pressure: high, low, flowing, shut-in - in the tubing;
- imposed well pressure: high, low - in the annulus - especially during completion pressure testing;
- well temperature: flowing, shut-in - range of temperature changes;
- vertical: straight, deviated - small angle, large angle; ·production method: natural flow, gas lift, pump;
- drawdown rate: high, low;
- completion method: tubing latched in tension, setdown in compression, multiple straddle pack, tailpipe, extension required below packer;
- tubing hanger design: suitable for packer setting/releasing method;
- minimum bore: ability to pass tools and equipment required further downhole;
- packer function: annulus/tubing isolation, zone isolation, damage straddling, cement squeezing;
- also to be taken into consideration are the pressure and temperature changes, especially during stimulation operations;
- casing damage caused by the slips;
- hang-off requirements (tailpipe assembly).
Packer classification
Retrievable:
The packer is run as an integral part of the tubing. Except for the retrievable bridge plug, the tubing cannot be pulled without pulling the packer. The packer is set mechanically, hydraulically or a combination of both. It is released by manipulation of the tubing, either rotating or pulling (shearing lock pins). Generally used where the well may have to be worked over regularly (i.e. electrical submersible pump applications), temporary completions (i.e. production testing) or well intervention activities (i.e. stimulation or casing leak detection).
·The following aspects need to be considered when running retrievable packers:
-pulling the packer out of the well may swab the well in;
-equalisation of pressure across the packer before pulling may be difficult (care should be exercised on shallow set during unseating operations);
-straight pull release packers may prematurely shear and release due to tubing contraction;
-deposits above the packer may render it non-retrievable.
Permanent:
The packer is set within the casing and the setting mechanism (tubing/wireline) can be released from the packer. Except for the case of a permanent bridge plug the tubing can be run and resealed in the packer. The packer may be set mechanically (by tubing), hydraulically or electrically (by wireline). As the name implies it cannot be retrieved, but can be destructively removed (i.e. milling). generally used in high pressure differential applications.
·Permanent/Retrievable: This class of packer combines the advantages of the permanent packer (i.e. large bore, withstands higher pressure differentials etc.) but when required can be released and recovered, entire, from the well.
In general, a permanent packer will be selected if:
·the predicted maximum differential pressure across the packer exceeds 5000 psi;
·the temperature at setting depth exceeds 225°F;
·H2S is present and the temperature at the packer is less than 160°F;
·infrequent workovers are envisaged.
Otherwise a retrievable packer may be recommended.
Packer setting
Mechanically
The packer may be set by one or combination of: ·rotation (standard 'J' slot latch arrangement); ·compression (slacking of the tubing weight into the packer; ·tension (pulling, overweight, up on the packer).
Hydraulically
The packer is set by applying pressure to the tubing so as to cause a pressure differential between the tubing and annulus. Commonly used in deep or highly deviated wells, or offshore environments when the platform motion plays a significant role. It is also a consideration if control lines are used with the subsurface safety valve or permanent downhole monitoring applications.
Electrically
The packer is set by a setting tool on electric wireline (wireline set). The wireline setting tool is released and recovered with the wireline. This method is more commonly employed for setting bridge plugs or when the exact location of the packer is critical.
Packer bore
·No bore - bridge plugs. To isolate the casing or tubing. Sometimes referred as cement retainers.
·Single bore - for use with a single conduit.
·Dual bore - for use with two conduits in dual completions.
Packer forces
There are two prime forces acting on a packer:
·hydraulic pressure forces (differential pressure acting )
·tubing-to-packer forces.
The tubing-to-packer forces need to be calculated at the design stage.
Force needed to prevent unseating. Permanent packers will withstand pressure differentials from above and below. Retrievable packers may be either compression set, tension set or both.
The assumptions and actual tubing set-down force need to be documented to prevent mishaps during subsequent well intervention activities
Operating envelopes (safe performance window)
The permanent packer rating envelope is a means of describing the functional limits of a packer under combined pressure and applied axial (tensile/compressive) loading. When requesting an envelope for a permanent packer, specify the packer model, size, material and casing size. The ratings derived from envelope graphs are for unplugged packers. Plugged packer ratings can be significantly lower.
Recommendations
- ·Select a packer with element metal shoe and shoe support systems (metallic back-up rings) in high-pressure applications to provide anti-extrusion back-up for the elements.
- ·Typical packer element combination is a 90-70-90 Schure hardness combination.
Schure hardness is a rating system to determine the suitability of rubber to a pressure environment. The higher the number, the greater the hardness and the more suitable for use with higher pressures. The hardness rating system is used for 'O'rings, stripper rubbers and packer elements.
- ·No 'O'-rings.