Thru-tubing sand control remediation expands

Lance Hebert
Chevron U.S.A. Inc.
New Orleans

Chris Malbrel
Pall Well Technology
Saint-Germain-en-Lay, France

Over the past few years, industry trends indicate that thru-tubing methods are being used more frequently for well completions requiring sand control remediation.

This increase correlates well with the development of novel multilayer screens, tailored to the specific needs of thru-tubing sand control applications, such as screens that are rugged, have improved inflow performance, and resist plugging.

These advances in screen design use filtration technology techniques that have been extensively proven in other industries.

A survey of 250 thru-tubing sand control remediations performed worldwide with coiled tubing and wire line shows that for this type of application, coiled tubing takes the greatest share of sand control remediation, but wire line intervention still remains significant (Fig. 1 [65,849 bytes]).

The survey is based on a review of Pall Corp.'s installation data base. Survey results are deduced from information gathered on actual sand control screens used for each documented installation, including length, diameter, deployment method, and basic completion information such as gravel pack or no gravel pack.

Filtration fundamentals

Sand retention, flow performance, and rugged design are three fundamental criteria for designing sand control systems.

Sand retention

Whether thru-tubing screens are the sole sand control device or are in conjunction with a gravel pack, they must be sized to retain formation sand. This requirement is obvious in the case of non-gravel pack applications.

In the case of gravel pack applications, this rule still remains valid because thru-tubing gravel pack integrity is always problematic. The gravel placement is uncertain because the annulus around the screen is often very narrow (such as for a repair inside a failed gravel pack) and/or because some very large voids may appear from sand production around the well bore.

Although wire-wrapped screens have been used in thru-tubing applications, porous metal filter media with much higher pore volume provides several performance benefits, as follows (Fig. 2 [43,081 bytes]):¹

• Decreased fluid velocity through the screen and improved inflow because of reduced screen pressure drop
• Extended service life because of more pore space for particles to bridge across
• Minimized erosion risk because of delayed plugging, which might increase fluid velocity through screens and lead to screen abrasion.

Flow performance

Flow through the screen and flow through the screen inner tubing can characterize inflow performance. High media porosity reduces fluid velocity through the screen, and contributes in reducing the resulting pressure drop.

Selection of media constructed of sintered fine powders or fine fibers can result in media with a porosity of 50-70%. By comparison, media constructed of nearly spherical particles, such as sand used in prepacked screens, results in filtration media with an average porosity of 30-35% (Fig. 3 [48,772 bytes]).

The importance of screen ID to flow restriction can be illustrated with a simple calculation. Assuming single-phase flow, the pressure drop induced by liquid flowing in a pipe is given by:

DP = 0.0000115 (fL g_LQ _L²)/d ⁵

where:

DP = Pressure drop, psi

f = Friction factor

L = Pipe length, ft

g_L = Liquid specific gravity

d = Pipe diameter, in.

Q_L = Liquid flowrate, b/d

The increased production associated with an increased tubing ID can be calculated using the following equation:

Q ₂ = Q ₁ (d ₂/d ₁) ^2.5

Thus, improving tubing ID by 20% increases liquid flowrate by almost 60%. In thru-tubing applications, the thru-tubing screen tends to have a very small ID, which can significantly restrict flow. Therefore, the screen must be designed to minimize this restriction.

Porous metal filter media selected in the design of thru-tubing screens are very thin. As a result, screens can be made with very low sidewall profiles, maximizing the screen ID for a given OD. Further improvement in wall thickness is obtained by using low profile, non-API base pipe.

Rugged design

Thin filter materials allow a good compromise between mechanical strength and sand control performance. The saving in wall thickness, achieved with thin filter media, is partially spent in the selection of a rugged cage that protects the screen during deployment.

In addition, filter materials fabricated prior to screen assembly are more reliable because their quality can be controlled in advance. They can also be designed to withstand deformation and mechanical stresses without losing sand retention characteristics.

The sintering process bonds the powder or fibers to the mesh substrate, resulting in a medium which has the tensile strength of the mesh, and fixed pore structure that does not deform and unload sand under stress.

Weight considerations also contribute to screen design. For example, if a long screen is run inside a horizontal liner, torque and drag limitations are affected by screen weight. The lighter the string, the longer it can be pushed before reaching its buckling limit.

It is, therefore, important to design a screen that is constructed as light-weight as possible without compromising the screen's stiffness to allow easy deployment in horizontal wells.

Thru-tubing sand control

Examples of successful thru-tubing installations of wells requiring remedial sand control include:

• Screen deployment in an initial completion
• Screen deployment inside a failed horizontal prepacked screen
• Coiled tubing gravel pack
• Squeeze gravel pack
• Coiled tubing and wire line deployment of stand-alone screens.

Initial completion

In California, a horizontal well was drilled with a coiled tubing unit using a 33/4-in. bit in a highly unconsolidated turbidite formation. The well was drilled to 3,480 ft measured depth with a 500-ft horizontal open hole section at about 2,350-ft true vertical depth. Build-up rate to the open hole section was about 15°/100 ft (Fig. 4 [66,998 bytes]).

The well was completed using coiled tubing by running 500 ft of alternating 23/8-in. screen and blank pipe into the hole filled with mud. A circulating shoe with a check valve was included at the bottom of the assembly to allow circulation around the screen.

The mud (xanthan gum with calcium carbonate and starch) was displaced with a potassium chloride solution, followed by breaker soaks (bleach and acid) to remove the filter cake.

This well has been producing for 2 years at about 740 b/d of fluid.

Failed prepacked screen

In the Gulf of Mexico, a well was originally drilled in a "fish-hook" fashion to intersect three clinoforms within 3,000 ft departure of the platform. ² Hole trajectory presented two 10°/100 ft doglegs, combined with a 2,500-ft inverted horizontal section at an average incline of 98°.

After a few months of production, the well started making sand. This led to the setting of an inflatable bridge plug inside the failed prepacked screen.

After a few more months, the well started making sand again and was shut in.

Coiled tubing assisted in the hole clean out. A polymer pill sweep was performed to achieve an overbalance (230 psi) to minimize sand flow in the well. Then 11/2-in. coiled tubing was used to run 1,250 ft of a multilayer screen (1.94 in. OD) and blank pipe inside the 51/2-in. screen (Fig. 5 [93,399 bytes]).

The packer was set on the XN nipple inside the 27/8-in. tubing, and the well was nitrogen lifted with coiled tubing to restore production at its pre shut-in rate. Sustained production was 400 b/d of fluid.

Rig-assisted workovers performed on the same platform to repair similar well failures, provided a basis for a fair cost comparison. The operator estimates that it saved $1.3 million by using coiled tubing for the remediation work.

Gravel pack

In Trinidad, a well was initially completed as a 185 ft open hole gravel pack using 40/60-mesh gravel placed around a low profile 31/2-in. prepacked screen inside an 11-in. under-reamed well bore. ³ Pump problems during the water-pack operation caused a 10-ft void in the gravel, about 120 ft from bottom.

A bridge plug was set in an unsuccessful attempt to stop sand production that was suspected to be coming from the washdown shoe. Sand problems persisted, and an inflatable isolation assembly was set to correct mechanical screen damage that was identified by electric line logging at the same depth as the void, noted during the completion.

Original production rates were restored for 12 months, until production was restricted again by sand production.

After washing out sand down to the isolation assembly, the operator preformed a circulating gravel pack around 230 ft of thru-tubing multilayer screen and blank pipe assembly inside the failed gravel pack (Fig. 6 [48,409 bytes]). Screenout was achieved after pumping 128 cu ft of gravel, well in excess of the theoretical 3 cu ft annular volume available.

A small thru-tubing gravel pack packer was set inside the 31/2 in. tubing. Xanthan gum was selected to reduce friction and provide good carrying capacity because small tubular and circulating ports are more susceptible to bridging and require gel strength to minimize risk of sticking the coil.

Initial production after the workover was 475 bo/d, which was subsequently restricted to 250 bo/d to protect the longevity of the workover.

It should be noted that the washdown technique has also been used successfully; however, it tends to be limited to shorter intervals (less than 50 ft or on average 20-30 ft).

Squeeze gravel pack

In the Gulf of Mexico, a well started making sand from a perforated/non-gravel pack interval that had been producing for more than 10 years. Reserves were not enough to justify pulling the tubing and recompleting the interval. But there was enough oil remaining to justify some form of coiled tubing intervention.

The location of the tubing (2,300 ft above the perforations) precluded the use of a standard circulating thru-tubing gravel pack because of excess pressure drop through 1.66 in. tubing. This tubing size was needed because of a nipple restriction in the 31/2-in. tubing.

After setting a plug beneath the perforations, a combination of bull plugs, screens, and blank pipe was set across and above the perforations (Fig. 7 [49,901 bytes]). The assembly was centralized in the 5-in. liner with eight bow-spring centralizers.

Gravel was bullheaded through the tubing after acidizing the perforations. Enough gravel was pumped to cover the screen assembly, about 70 ft above the vent screen.

The gravel was restressed and coiled tubing was used to remove gravel below the vent screen. The length of blank pipe between the downhole screen and the vent screen was designed so that the pressure drop across 40/60-mesh gravel in the annular space would force produced fluid to preferentially flow towards the screen instead of up the annulus.

The choke size on the well was increase slowly (2 weeks). The well produced up to 575 bo/d and 2.1 MMscfd of gas through a 24/64-in. choke before it started producing traces of sand.

It is believed that voids may have been formed during gravel placement. These provided flow channels for the fluids. Future intervention is planned to cap the gravel with a cement plug.

One lesson learned is that the bow-spring centralizers should be aligned to ease gravel placement in the annulus.

Finally, painted centralizers should be avoided as the squeeze method forces all debris into the perforations and could potentially damage the near well bore formation.

Stand-alone screens

In some circumstances, wells are completed without any sand-control device because the formation is relatively consolidated. Later in the well's life, however, sand production can occur from circumstances such as reservoir depletion or water production.

In these instances, stand-alone thru-tubing screens have been used for remedial sand control.

As an example of a coiled tubing intervention, an onshore Louisiana gas well was originally completed in two zones. The lower zone was completed with open perforations and open-ended tubing. The upper zone used open perforations, with blast joints across them and a sliding sleeve above.

Over time, significant sand production forced the well to be shut-in and written off.

But 2 years later, coiled tubing intervention was selected.

It was decided to install a bridge plug above the sliding sleeve, reperforate the 23/8-in. production tubing, and install two joints of multilayer screen above the new perforations (Fig. 8 [75,821 bytes]).

Production was slowly increased, to allow the formation of a permeable filter cake on the screen. Typical production was around 50-70 bo/d, 170-200 bw/d, and 3-3.5 MMscfd of gas.

Original production, before the well was shut in, was 4.5 MMscfd gas.

As an example of wire line-deployed intervention, a Gulf of Mexico well, originally completed with perforated casing and open-ended tubing, started making sand within months. To remediate this problem, the well was previously washed out with coiled tubing nine times over a 10-year period.

But the last intervention consisted of simply using a wire line to set a few joints of a multilayer screen at the end of 27/8-in. tubing (Fig. 9 [55,459 bytes]).

The screen had to be set about 130 ft above the perforations because of a restriction in the casing that was suspected to be a liner collapse. Production was slowly increased to allow the development of a permeable filter cake.

Because reservoir sand has a relatively high permeability, and reservoir pressure is maintained by a water drive, there was no concern about the well being choked-up by sand fill. Initial production was 195 bo/d, 155 bw/d, and 155 Mscfd of gas.

Design recommendations

Depending on the deployment method (wire line or coiled tubing), the complexity of associated services (sand clean-up, gravel pack, etc.), and the location, thru-tubing sand control interventions costs may vary between $50,000 and $500,000. Pay back will obviously vary depending on the amount of incremental production.

One of the most critical keys to success is the selection of the right well candidates. Considerations such as reserves and production potential, assessment of the risk involved in the intervention, and the ability to maintain access to the well for future intervention should be weighed carefully.

Experience shows that in the case of stand-alone screen deployment, the presence of a rathole is very important to allow sand to fall out so that it does not accumulate around the screen and restrict production. If a rathole is not present, pumping gravel is preferred unless the economics do not allow it.

In addition, it is better not to place the screen across perforations to prevent cutting the screen, especially in gas wells.

In the case of circulating through tubing gravel pack, success will depend on circulating rate. All parameters that favor high circulating rates to pack perforation will increase the chance of success. These include:

• Large tubing size
• Shallow depth (to reduce friction in coil tubing)
• Reduced intervals (up to 40-50 ft) in the case of thru-tubing gravel pack inside casing.

In the case of gravel pack inside tubing or inside a failed gravel pack, the recompleted length can be much longer because the tubing or the initial gravel pack screen ID is already small.

Screen media selection

From the screen selection standpoint, media selection is the key to optimum performance. While screens must resist plugging, they also need to retain sand.

The sizing rules developed over the years to size gravel pack and wire-wrap screen slots are the basis of this selection. However, other factors need to be taken into consideration.

In addition to the average formation sand size, sand sorting plays a critical role in media selection as both sand retention performance and filter cake permeability will be affected.

Another factor is the type of produced fluid and its production rate. If the fluid is gas, or oil with a high GOR, the impact of sand production will be more critical. There is a higher risk of erosion with gas production.

Gas well completions

The risk of screen erosion in gas wells is very important to consider, especially if the screen is installed without a surrounding gravel pack. In this case, whether the completion is cased hole or open hole, lack of gravel packing will affect gas velocity and erosion risk.

In a cased hole completion, installing screens directly across perforations will potentially lead to screen failure and sand production, especially when the gas velocity is very high. Screens should preferably be installed above the perforations, and the screen diameter should be selected to provide enough annular space around the screen to allow flow distribution over the entire screen section.

Screen length must also be sized to reduce gas velocity to a safe level throughout the life of the well. In these calculations, the terminal reservoir pressure should be used because gas velocity will increase at lower pressure.

In open hole completions in a horizontal well, for example, sizing must take into account some screen plugging associated with screen installation (drill-in fluid residue), and again the low-pressure situation is a factor. This will primarily affect screen length.

Drill-in fluids

Drill-in fluids are known to damage sand control screens by causing partial plugging. While the extent of screen impairment is unknown, drill-in fluids are suspected to be the main cause for the high drawdowns experienced worldwide in horizontal completions.

Effective well bore clean-up is critical, and a number of options are available, such as:

• Reverse circulation of the drill-in fluid around the screen
• Production of the fluid through the screen (not recommended in the case of fine screen media), which requires proper mud conditioning (circulation through fine shaker screen)
• Deployment in clear brine, using an overbalance pressure of several hundred psi to maintain the filter cake on the walls during deployment⁴
• Use of breaker soaks to dissolve polymers and sized solids.

Although each case must be studied individually, it is critical to account for the screen media when designing the completion procedure.

Acknowledgments

The authors thank Pall Corp. and Chevron USA Inc. for permission to publish this article. In addition, the authors wish to express their appreciation to Alex Procyk for his efforts in collecting the data for the case histories.

References

1. Pall Process Filtration Co., Principles of Filtration, 1993.
2. Crow, W.P., Hill, P.H., and Johnston, R., "Coiled tubing cuts horizontal screen repair cost," World Oil, January 1996.
3. Holder, G.M., Hadeed, J.E., and Crow, W.P., "New Thru-Tubing Sand Control Technology Returns Oil Wells to Productivity," Paper No. SPE 36091, 1996.
4. Smejkal, K.D., and Penberthy, W.L., "Proper drilling, displacing critical for open hole completions," OGJ, July 27, 1997.

The Authors