Operators face multiple screen choices

SAND CONTROL-Conclusion

M. Babs Oyeneyin
Robert Gordon University
Aberdeen

Operations engineers face the problem of selecting the right screen for a specific job. A variety of screens are available to choose from.

This concluding part of a series of three articles (started in OGJ, Jan. 26, 1998) covers some of the screen options available for sand control without installing a gravel pack.

Screen completions

Barefoot screen control, without a gravel pack, involves placing the selected screen in the well bore within the pay zone region specifically designed to allow reservoir fluids to be produced through the screen slots while the formation sand is restrained by the screen.

Barefoot screen completions generally are used in open hole horizontal wells. No industry standard criteria exist for the design of screens.

Experiences to date have shown that many of the early screen systems were susceptible to plugging by fines and cake debris. There have also been reported cases of mechanical failure and installation difficulties especially in high angle and horizontal wells.

These failures have contributed to the proliferation of different screens. However, in-depth performance evaluation of screens has been rather limited.

But now, these problems are being addressed by a number of joint industry projects.^{1 2}

Screens currently available, with their trade names, include:

Conventional wire-wrap
Conslot
Low-profile prepack
Slimpak
Dual prepack
Excluder
Stratapac
Poroplate
Sinterpak.

Wire-wrapped screens

Wire-wrapped screens are manufactured by wrapping wires around specially constructed tubulars (pipe-based wire wraps) or special stainless-steel rods (rod-based wire wraps).

Wedge-shaped screen slots provide less susceptibility to clogging and also provide much higher flow capacity.

These screens are useful for applications in clean, medium-to-coarse clean sands with very low pore fluid velocity. They may, however, allow the production of fines and some load-bearing sand grains.

Application in gas wells is not recommended due to ease of erosion. These screens are now being widely used for sand control in horizontal oil wells.³

Conslot screen

Conslot screens are mainly modified wire-wrapped screens with a slot configuration that has improved flow capacity and self-cleaning ability.

For the wire-wrapped and Conslot screens, the key design criteria are:

A screen OD that provides a maximum 0.5-in. annular clearance between borehole and screen.
A screen slot gauge of about 2-3 times the 10 percentile size of the sand. For Conslot screens, a gauge size equal to the d₅₀ of sand has been recommended by the manufacturers.
A base-pipe perforation density and size.

Prepacked screens

Prepacked screens are manufactured by filling the annulus between two concentric wire-wrapped screens or the annulus between a perforated pipe and wire-wrapped screen with properly sized resin-coated gravel. Different types available include:

Low-profile (LP) pre packed screen (Fig. 1a [54,804 bytes]), which is basically all welded screen with a thin prepack placed between the screen and base pipe.
Slimpak, which is similar to the LP prepack in design but also has an extra layer of wire mesh on the base pipe for secondary control.
Dual-screen prepack (Fig. 1b ), which is a special system in which the resin-coated gravel or even loose proppant is placed between two screens. This screen is popular for open hole, long horizontal sections.
Other screens no longer popular are the perforated pipe-base prepack and the dual-screen pipeless prepack. These were run in slim horizontal wells or zones with tight restrictions.

Design features

Key design features of prepacked screens are:

Screen OD
Screen gauge
Prepack thickness
Prepack gravel size and type
Base pipe bore and perforation density.

Recommended screen ODs were discussed in the second part of this series (Table 2, OGJ, Feb. 2, 1998, p. 60).

The screen gauge should be 2 times the 10 percentile (d₁₀) percentile size of the formation sand to minimize flow restriction and plugging by fines.

With the adoption of shallow internal-bridging criteria for gravel selection, there will be a tendency for an "inside out" secondary filter or sandpack around the prepacked screen by the formation sand grains that in the longer term should minimize screen erosion, especially in gas wells.

The prepack thickness should have an average size of about 0.4-0.5 in. However for poorly sorted sands, it is recommended that this should be increased to about 0.75 in., if possible, especially where low-profile prepacks are used. This will allow increased effective flow capacity.

A prepack gravel size and type needs to be determined as in gravel-pack design. The same criteria apply to the selection of gravel size and type.

For open hole completions and to promote good control, all sand intervals should be sampled but gravel size selection should be based on the finest formation sand size distribution in the interval. This should promote an "inside out" secondary sand pack around the screen.

The base-pipe bore is dictated by the pack thickness, and many of these screens are designated by their nominal base-pipe diameter. Dual screens are, however, designated by their pipe OD and outer screen OD.

Conventional perforation densities do vary but for better inflow control and to minimize friction loss in especially long horizontal wells, a minimum of 500 perforations/m of 10-mm diameter may be adequate.

Choice should be based on effective friction loss analysis.

Recent studies⁴ have demonstrated the impact of shot density and perforation size on the relative roughness of perforated pipes. The correlations developed can be adapted for screens because no current industry standards exist.

A joint-industry venture¹ is expected to provide some appropriate friction-loss correlations for different screens.

Experiences with many prepacked screens have shown that they are more susceptible to damage by fines and cake debris. The more relatively recent screen developments include:

Sinterpak screen that has a powdered metal outer layer on a perforated inner base pipe. The powdered metal outer layer acts as the filtering medium for the formation sand.
Stratapac (Fig. 2a [48,206 bytes]) that has a membrane screen and works on the principle of total sand exclusion. This tended to restrict its application because there was no variety of membrane selection to cater for different sand sizes. But now, newer designs for use with different sand sizes have been developed.⁵
Excluder screen (Fig. 2b) is claimed to have better sand control efficiency, and the more rugged and less susceptible to plugging than other screens. It is, however, too early to judge its long-term performance. There currently appears to be no limitations on the use of Excluder screens. Applications are known to include oil and gas wells and all sand types.
Poroplate (Fig. 2c) is another new screen in the market. It is claimed to be less susceptible to plugging by fines and its pleated-wire configuration allows for self cleaning. The screen can theoretically be used for both oil and gas wells to control all types of sands, both well sorted and poorly sorted, as well as coarse or fine sands. Manufacturer recommendations on slot sizes are equivalent to the mean size of sand.

Performance evaluation

Screens can be evaluated in terms of the inflow pressure drop as a function of time. For prepacked screens, this will be similar to the evaluation requirement for gravel packs. However, the effective permeability will be that of the entire prepack including the wire mesh.

Additional evaluation criteria is the total friction loss along the screen bore.

Success of any screen completion depends largely on the completion strategy in terms of well bore cleanup, drilling/completion fluid displacement, and handling of fines/filter cake debris.

Screen Installation

Screen installations should:

Ensure good well preparation, including adequate cleanup of filter cake and other debris.
Avoid displacement of filter cake debris and other junk through the screens as these contribute largely to screen plugging even before the well is put on production.
Avoid displacing the screens to heavy solids-laden drilling fluids. Screens should be displaced to a good completion brine only with relatively low solids content.
Optimize the size distribution of completion fluid solids to avoid any screen plugging. This calls for optimum choice of LCM and other additives.

References

Horizontal Well Technology Unit, Evaluation of the performance of screen options for the completion of horizontal wells, joint venture industry project, initiated in 1995.
Rogaland Research Institute, Screens Evaluation Project.
Los, A., Schiehallion Development, SPE monthly meeting, Aberdeen, October 1996.
Ze, S., and Gudmundsson, J.S., "Friction Factor of Perforation Roughness in Pipes," SPE Paper No. 26521, 1993.
Butler, D., personal communications.