The objective of all sand exclusion techniques is to arrest the movement of formation sand into the well. Sand production can cause erosion of well equipment, well restrictions, and sand deposits in flowlines and other surface installations.

In the case of gravel packing, the gravel should be placed directly against the formation so that formation grains loosened from the rock can be arrested immediately. As the production exerts forces in the direction of the well bore, the gravel should be packed against the formation face and  the pack itself should be without voids to prevent the gravel (and formation sand) to move.

A third requirement for gravel is that the pack should be tight. The grains of the gravel should rest on each other in such a way that the grains cannot move or re-arrange themselves during production.

1 Internal Gravel Pack (IGP)

1.1 Successful Internal Gravel pack

• to tightly pack the perforations with gravel. This is essential to prevent formation sand movement and create a highly permeable flow path for the produced fluids. A perforation tunnel that is not packed may collapse when the well is on production. This perforation will then be impaired due to the high pressure drop caused by the presence of formation sand;
• to maintain a clean gravel/sand interface to avoid permeability impairment associated with mixing of gravel and sand;
• to place a dense homogeneous gravel sheath around the screen in order to prevent backflow of the gravel placed in the perforations and control formation sand opposite poorly packed perforations;
• to ensure that the near wellbore formation remains unimpaired by drilling and gravel packing operations.

Good gravel placement can be ensured by proper control of all of the following variables:

• mechanical variables, i.e. equipment selection: screen size, washpipe dimensions;
• hydraulic variables, i.e. the slurry design: rheology, gravel concentration, viscosity breakback;
• operational variables, i.e. circulating or squeezing, leak-off rate, pump rates and pressures.

1.2 Gravel placement methods

1.2.1 Squeeze pack

In a squeeze pack, the slurry is forced to dehydrate in the perforation tunnels thereby ensuring gravel placement in cavities behind the casing. This can be achieved either by placing the gravel pack tool in the squeeze position or by closing-off the returns.

A squeeze pack promotes a radial build-up of the gravel pack inside the screen/casing annulus as gravel nodes build-up on the packed perforation entrances, with a consequent high risk of bridging. Non uniform injectivity profiles increase the risk of bridging as the slurry preferentially dehydrates opposite high permeability zones. Bridges may not be so problematic in vertical wells as they may collapse as soon as pumping stops. However, the perforations below the bridge run a high risk of being poorly packed. In deviated wells and/or long intervals, however, neither the perforations nor the annulus below the bridge will be adequately packed.

Also in a squeeze pack, it is not possible to dehydrate slurry below the lowest perforation. This will create a void which may be subsequently filled when the pack settles with the risk of leaving voids elsewhere in the pack. Squeeze packs are generally not recommended and should in any case be restricted to gravel packing very short intervals, i.e. less than 5 meters.

1.2.2 Circulation-squeeze pack

Lower tell-tales are frequently used to force a self-induced squeeze pack. The crossover tool is left in the lower circulating position for the entire gravel placement operation.

The pressure drop for returns to flow through the telltale increases dramatically as the annulus packs-off and the circulating pack effectively becomes a squeeze pack, which is undesirable in most cases.

1.2.3 Circulation pack

The objective of a circulation pack is to fill the annulus and the perforations from the bottom up. The wash pipe must be spaced out so that return circulation occurs as close as possible to the bottom of the screen. As the gravel pack begins, slurry fills the annular space outside the screen and the slurry starts to dehydrate at the bottom of the screen opposite the end of the washpipe. As packed gravel accumulates around the bottom of the screen, a higher pressure drop is required for the fluid to enter the screen and to flow in the screen/washpipe annulus to the end of the washpipe, thereby inducing fluid leak-off through the perforations.

Fluid leak-off can also be controlled by action on the fluid returns. However, when high gravel concentrations are used, the pack can be completed in a few minutes and little time is available to effectively control the operations.

1.2.4 Perforation prepacking

Conceptually, the ideal cased hole gravel packing method is to separate the perforation and annular packing process, i.e. to prepack the perforations. Prepacking methods are claimed by other operators to be very effective at packing perforations, controlling fluid loss and to result in high well productivity. Gravel can be squeezed into the perforations prior to running the gravel pack liner assembly. This method is to some extent a revival of the old "washdown" gravel packing technique. If it is necessary to control fluid losses after squeezing gravel behind the casing the perforations can be sealed with fluid loss control pills.

Prepacking - the concept of splitting the process of perforation packing from gravel packing the annulus is not new, and has been carried out routinely by a number of operators, although it was not until recently (1990) that a number of Opcos decided to follow up the "success" of other operators with prepacking.

Filling the perforation tunnels immediately after perforating is thought to reduce the likelihood of tunnel damage by:

• minimising the possibility of tunnel/cavity collapse;
• invasion of incompatible (possibly unclean) fluids.

Furthermore the chance of filling the perforations is increased as the annular packing stage often results in additional packing of perforations. Prepacking followed by circulation packing is the currently recommended technique for difficult wells: long highly deviated zones, reservoirs which exhibit high permeability contrasts, and zones where excessive losses are anticipated.

The following is a summary of the basic prepacking techniques:

• Prepacking without acidising - commonly referred to as the BP technique. This involves perforating the zone underbalance, pulling guns above the completion interval and packing the perforations with low concentration slurry. Perforation tunnels are then sealed using a LCM (Lost Circulation Material) system. Thereafter the annulus is conventionally packed. If required an additional LCM pill is spotted inside the wire wrapped screen prior to pulling the work string.
• Prepacking while acidising - as above this operation can be carried out before or after running the gravel pack liner. However, in order to minimise losses and the risk of tunnel collapse it is preferred to pack perforations prior to pulling the perforating guns. Again LCM can be spotted where required.

Uniform treatment is difficult as acid will preferentially flow into the first set of open perforations or into high permeability zones. Possible diversion techniques include gelled or foamed acids. A relatively new but promising method is to use viscous pills loaded with gravel to divert acid stages and simultaneously prepack perforations.

A typical gravel diverted acidisation procedure is as follows:

• The treatment should be carried out in stages with a gelled prepack diverter slurry placed between stages. The diverter pill containing the gravel is required to pack the perforations opened by the preceding stage. Each acid stage should be over flushed far enough into the formation to reduce the potential for damage due acidisation side effects.
• The final acid stage should be followed by the gravel pack slurry required to complete the pack.
• A typical design practice is to target between 10-20 ft of perforations, with gravel concentrations in the range of 1-2 lb/gal.
• Auger system - this system involves spotting gravel across the completion interval, and "screwing" a specially designed screen into the gravel bed. It is claimed that this technique promotes tighter more effective packing of perforations.

1.3 Operational considerations

1.3.1 Pumping the slurry

As the progress of a gravel pack job is monitored by the volumes of fluids pumped, it is essential that the slurry arrives at the crossover tool in one homogeneous slug. Velocities of 500 ft/min have been recommended to prevent premature sand outs or roping of the slurry during the pumping operations, but the evidence supporting this criterion is weak. According to this criterion the minimum pump rate required in a 3 1/2inch work string is some 4.5 bbl/min. Further experimental and theoretical work is being progressed.

In many cases the slurry will free-fall as soon as it enters the work string. This is due to the difference in density between the slurry and the completion brine which is generally high especially with high gravel loadings. As the fluids in the wellbore are unbalanced the system "U-tubes", in other words voids are generated in the work string. If "U-tubing" occurs then the slurry may reach the cross-over port long before expected and volumetric control of the placement operations is lost. This will prevent proper gravel placement in the case of an IGP. One practical method to ensure that U-tubing does not occur is to keep the pump rate high enough to maintain positive pressure at the wellhead (the pump pressure may only indicate the friction pressure loss in the lines from the pump to the wellhead).

An alternative is to spot the slurry with the crossover tool in the reverse circulating position. If a low bottom hole pressure ball is used in the crossover tool then the formation is completely isolated from circulating pressures while spotting the slurry. U-tubing of the wellbore fluids can be prevented by choking the fluid returns. This method has one drawback as great care has to be taken to shift the crossover tool in the circulating position before slurry reaches the crossover port (use a healthy safety factor e.g. 5 barrels). Failure to do so may result in a stuck gravel pack string because of gravel in the annulus above the packer.

When the pad is within 5 barrels of the GP port, the pump rate can be slowed and the crossover tool lowered into the circulating position. The pump operator should react quickly to any unexpected sharp pressure increase. This may be caused by bridging of the gravel in the casing/screen annulus. The blank liner can easily be collapsed under excessive pressures and an expensive fishing job will result.

1.3.2 Placing the gravel (circulation pack)

The pump rate affects the annular and perforation gravel packing efficiency. In highly deviated wells, circulation rates should be kept high (2-3 bbl/min) to prevent premature bridging. In less deviated wells, circulating rates of 1/2to 2 bbl/min are adequate and allow more time to for the perforations to fill.

Circulation packs are in a sense a gradual squeeze. As the gravel pack begins, slurry fills the annular space outside the screen. As gravel accumulates at the bottom of the screen opposite the end of the washpipe, the pressure drop required for the fluid to enter the washpipe increases. Hence leak-off increases while fewer perforations remain unpacked.

Returns must be monitored during placement of the pack to ensure that adequate leak-off occurs. As gravel placement may be over very quickly, a real time measurement of the fluid returns (positive displacement flowmeter) is required to be able to adjust the balance between fluid returns and leak-off. There is no fixed guideline for the ratio of fluid leak-off to fluid returns. An acceptable range is 50 to 70%.

1.3.3 Screen out

When screen-out occurs, pump pressures will increase sharply. The screen out pressure is supported by the gravel fill in the casing/screen annulus. This pressure is transmitted only a short distance within the pack and the formation is in principle not exposed to screen-out pressures.

Screen-out pressures should not exceed 1000 to 1500 psi over the initial circulating pressure for the following reasons:

·it is unlikely that more gravel can be placed as the pack cannot be compacted in this fashion;

·excessive pressure can damage downhole equipment. Incremental pressures must stay well within the blank liner collapse pressure.

Once screen-out occurs, pumping is stopped and the bleed-off rate should be observed. A slow bleed-off is a sign of a good pack as it indicates that carrier fluid leak-off is impeded by the presence of gravel in the annulus and the perforation tunnels.

A screen-out should be reconfirmed immediately, e.g. a few minutes after the initial screen-out. The reason being that if a premature bridge has formed, it often collapses as soon as pumping stops and packing can then be resumed without unnecessary time loss.

1.3.4 Reversing excess slurry

If the screen-out is confirmed, the annulus should be pressured-up to 500-1000 psi and the crossover tool picked-up to the reverse circulation position. Pressurising the annulus avoids having slurry entering the annulus as this may result in a stuck crossover tool. Connections should be spaced out to avoid stripping a connection through the Hydrill. To avoid bridging in the tubing do not wait too long before reversing the excess slurry. Circulate 1 1/2to 2 tubing volumes at high rates. It is best to have a check valve (low bottom hole pressure tool) in the crossover tool to isolate the pack from circulation pressures.

The volume of gravel returns should be measured in order to estimate the volume of gravel placed outside the casing. It is, however, difficult to accurately measure return volumes. The following procedure, developed by Baker, is recommended.

Returns are diverted to the blenders when gravel is spotted. With the blenders thoroughly mixing the return slurry, several samples of the fluid are taken. Using a centrifuge or a graduated cylinder, an average volume percentage of gravel is determined. The blenders can be shut down momentarily to determine the total volume of return slurry. Multiplying the total volume by the volume fraction of gravel reversed out gives a reasonably accurate value of the total volumes of gravel reversed out of the well. The amount of gravel placed in the perforations can then be estimated.

1.4 Repacking

After the excess slurry has been circulated out, the pack should be left to settle. If a viscosified carrier fluid was used, then the pack should be left undisturbed for the time corresponding to the design viscosity break back time, ususally an hour.

Screen-out should then be reconfirmed by an injectivity test. The "confirmed screen-out" criterion is then based on the circulation rate and/or pump rate achieved during the injectivity test. There are no firm guidelines to give here. Each operator should rely on field experience to establish threshold values.

If an additional batch of slurry is required, then a batch of no more than half of the original slurry volume should be mixed. Consideration should be given to reduce gravel concentration and viscosity to ease secondary placement operations. Slurry for repacking can best be placed with the gravel pack tool in the reverse position.

1.5 Reporting

Concise and accurate job reporting is essential to allow for subsequent analysis and review of gravel packing operations.

The Perforation Pack Factor (PPF) or amount of gravel placed behind casing is a qualitative indicator of the success of an IGP. This can be estimated if the volume of gravel reversed out of the well is known.

It should be noted that it is notoriously difficult to accurately measure the volume of the gravel returns, nevertheless it is recommended that as accurate measurements as possible are made to enable the quality of the technique and operation to be evaluated.

2 External gravel packs (EGP)

2.1 Factors affecting gravel placement

EGP completions only require that the annular space between the screen and the underreamed hole be packed with gravel. Hence fluid leak-off into the formation is not essential to place the pack but contributes to compacting the pack by slurry dehydration.

Basically, the factors affecting gravel placement in the casing/screen annulus for an IGP also apply here. The main difference is that the annular clearance is much larger in the case of an EGP.

Especially with thin fluids, there is an inherent risk of mixing gravel with sand as the slurry is being circulated downhole. Hence circulation rates are generally limited to about 4 barrels per minute in normal applications, where fluid velocity in the annulus is not a critical factor for gravel placement. In highly deviated intervals fluid velocity in the annulus should be designed with a view to optimise gravel transport in the annulus.

As with an IGP, gravel placement is increasingly difficult with increasing deviation, interval length and fluid leak-off variations along the interval.

2.2 Gravel placement methods

The one trip crossover circulation method is normally used to place an EGP. The cheaper option is to use the "over the top" system together with low viscosity fluids (conventional packing).