Sand control completions in cased hole require careful design and installation to ensure optimal well economy and performance. In general, a cased and perforated completion will not perform as well as an open-hole completion, and so there are normally specific reasons which drive the operator to install such a cased sand control screen completion. These reasons are usually connected with zonal isolation and flow control and/or maybe imposed by local legislation or other considerations. The CHESS (Cased Hole Expandable Sand Screen) installation offers certain distinct. Advantages over internal gravel packs, especially multiple packs with zonal isolation, but (as with all such cased hole completions) there are concerns which need to be addressed well in advance of installation to make sure the completion performs as required. Cased hole issues include:
- Well productivity & economics
- Perforation strategy
- Casing and cementing
- Other common cased hole sand control factors
Well Productivity & Economics
In general terms, a cased and perforated well cannot outperform an openhole well. This is because of the additional pressure drop associated with flow to and through the perforations. An operator would normally select a cased and perforated completion because of zonal isolation requirements or other such concerns. A CHESS installation provides a large ID completion, which in turn allows excellent well operability (the ability to run tools/equipment through and control the fluid flow) and reduced frictional fluid pressure drop. Perhaps more importantly with CHESS, multiple zones can be completed in one trip with a minimum of rig time. In general, a CHESS completion will be better than a gravel pack completion.
With any cased hole sand control solution, erosion of the screens can become a major issue. Evaluation of the erosional risks associated with a screen completion is based upon the potential downhole velocities and whether the perforation tunnel is open or packed. In more competent formations, the tunnel may stay open and also in certain situations, the possibility of cavities behind the casing may exist, especially in old wells. In such cases, the produced sand/fines will be largely retained at the ESS surface assuming the mesh is sized correctly. The formation of a pack of sand particles at the screen surface back to the casing OD will effectively protect the screen from further erosion by the larger particles. Erosion from fines is a much slower process as the erosion rate is also dependent on particle mass. As the ESS is pressed directly against the casing, the sand pack builds in the tunnel. If an annulus exists between the ESS OD and the casing ID, the sand pack can fall off into this annulus and the ESS should then be treated as a stand-alone screen. For this reason, it is essential to ensure that the perforation internal burrs and debris are cleaned and removed. Note also, that modern guns develop very considerable pressures and in some circumstances liner ovality might occur, the use of compliant expansion systems is therefore recommended for CHESS installations.
The ideal perforation for a CHESS completion is one that provides a stable tunnel deep into the undamaged zone with a maximum diameter entry hole and no burrs. Unfortunately, most of these ideals conflict with one another. A large diameter hole is more likely to collapse with applied stress. Long, deep holes although more stable allow the sand particles to accelerate and the entry hole diameter is reduced, thus concentrating the erosive energy of the fluid stream. Most perforations fill with formation sand and other particles and also produce burrs, even specifically designed burrfree charges and therefore, thorough casing cleaning, scraping and if necessary, milling is recommended for CHESS installations.
Wax, scale, asphaltenes and other chemical deposits can have a seriously adverse effect on well productivity by damaging the tunnel permeability. In such cases, the operator is forced to remove this damage using an acidic formulation for example, which would dissolve or break up the deposit and allow it to flow freely through the screen. A CHESS installation has advantage in such cases because the stimulation fluids can be delivered directly to the damaged area. Stand-alone screens and gravel packs introduce an annulus between the screen OD and the casing ID, this allows the stimulation chemicals to find their path of least resistance and thus some badly damaged perforations would be missed.
It is worthwhile noting that corrosion in sand screens normally requires more careful attention than non-sand control completion. This is because the woven metal mesh or wire-wrap sand screen has a relatively high surface area and a small cross-sectional area. Corrosion (and alsoerosion) may have an effect on sand screens with respect to their sand retention effectiveness. For this reason, the filtration material (meshes, weaves and wires) of most sand screens used in the oilfield is made of 316L stainless steel. Conventional sand screens are however built on standard oilfield base-pipes (N80, etc). For ESS screens, the base metallurgy for the pipe, weave and shroud is 316L stainless steel (roughly 18% Cr), except the connections which are Super Duplex 25%Cr. This broadly eliminates galvanic corrosion concerns over dissimilar materials.
Casing and Cementing
As mentioned in the above sections on erosion and perforation, it is advisable to have a clear understanding of the state and condition of the casing and the cement behind it. To achieve the maximum benefit from a CHESS completion, the objective must be to allow full casing contact and to prevent damage to the screen and formation. It is recommended to perform casing cleaning and scraping operations prior to installing the CHESS screen. This will remove burrs from the perforations and also the cement and perforation debris from the well. Cement evaluation logs are also recommended to ensure a good bond between the casing and the formation. A void or channel occurring behind the casing will not only provide an avenue for fluid migration, but might also allow sand to pass and thus increase erosional risks.