THROUGH-TUBING SAND CONTROL IS NOW IMPORTANT OPTION

Sept. 9, 1991
Michael H. Mayer Dowell Schlumberger Inc. Montrouge, France J. David McLaurin Dowell Schlumberger Inc. Tulsa Darrel G. Gurley Dowell Schlumberger Inc. New Orleans Completion with through-tubing sand control is an idea that is becoming more attractive with the advances in tools, carrying fluids, resins, and the understanding of Nodal analysis.
Michael H. Mayer
Dowell Schlumberger Inc.
Montrouge, France
J. David McLaurin
Dowell Schlumberger Inc.
Tulsa
Darrel G. Gurley
Dowell Schlumberger Inc.
New Orleans

Completion with through-tubing sand control is an idea that is becoming more attractive with the advances in tools, carrying fluids, resins, and the understanding of Nodal analysis.

No longer is through-tubing sand control considered as an alternative only when the expense of a major workover cannot be justified. Typical applications are wells that have produced large amounts of formation sand, due to bottom hole pressure depletion, and the onset of water production.

One of the most economically attractive areas is offshore, where a workover rig can be avoided by using a coiled-tubing unit or an hydraulic-snubbing unit with concentric tubing.

Another attractive area is in locations that previously have not required sand control until the zones start to produce formation water. Frequently, the production of water is the harbinger of formation sand production.

Many of these wells are older and were shot through-tubing with wire line guns that had diameters of 0.25-0.31 in. The procedures that are followed are simple and most often do not require the disturbance of production tubing or packers.

If properly engineered, the production realized from a through-tubing sand control completion will be very near (80-91%) that expected from a major workover or a new well. This assumes that all variables remain constant.

SAND-CONTROL TECHNIQUES

There are four basic techniques in sand control, and all four can be considered in through-tubing completions.

  1. Gravel pack is a mechanical means of filtering formation sand with a graded-resieved gravel/screen combination.

  2. Resin-coated gravel is another mechanical means of filtering formation sand from the well bore by means of an epoxy-coated gravel that forms a consolidated barrier. The gravel eliminates the need for a screen.

  3. Sand consolidation is a means of chemically forming with resin an artificial sandstone of the formation sand. This renders the formation sand immobile.

  4. Prepacked screen is a mechanical filter formed by the addition of graded-resieved gravel placed between two wire-wrapped screens.

All of the precautions that are consistent with proper sand-control techniques should be followed when using through-tubing methods.

THROUGH-TUBING PROCEDURES

A coiled-tubing unit (CTU), or an hydraulic-snubbing unit with concentric tubing, is moved to location. After control procedures are followed, the tubing is run to bottom and the fluid filtered to the acceptable quality of that normally used in the area to perform conventional sand-control techniques.

The through-tubing sand control procedures are the same as those in a conventional sand-control treatment, All the fluids and the casing/tubing treating equipment must be contaminant free. Any contaminant in the well bore, tubing, or pumping equipment will be carried into the perforations with the ensuing treatment. Contaminants will impede production once the treatment has been completed. These contaminants are virtually locked in place by the treatment.

Once the completion fluid is satisfactorily filtered, any formation sand that is left in the well bore can be removed by washing. The hole is stabilized with either fluid-loss additives alone or with a prepack followed by fluid-loss additives.

This operation is performed with high-pressure pumping equipment. Normally the equipment consists of:

  • A triplex pump

  • A stainless-steel blender

  • An hydraulic power pack (optional)

  • Tanks of presand control acid, that is pumped to help clean up the tubulars, casing, or perform a matrix treatment to improve leakoff into the perforation tunnels prior to the sand-control method.

PRODUCTION COMPARISON

A Nodal analysis can be used to compare the same zone completed in different configurations. In these examples, the wellhead pressure (Pwh) is held constant at 1,000 psi. The other parameters are shown in Table 1.

Because a successful acid treatment is assumed prior to the sand-control technique, these examples use a zero skin factor.

Most wells that are candidates for through-tubing sand control do not allow the operator to shoot underbalanced or flow test to clean up the perforation tunnels effectively.

Each example (Figs. 1-2) is a typical completion method with varied inflow area. The inflow area is defined as:

IA (sq in./ft) = area of one perforation x shot density

Nodal analysis is run to determine the optimum inflow area. The graph is then used to determine the shots per foot and the entrance hole diameter for that inflow area.

The analysis, Table 2, indicates that with the proper design a through-tubing gravel pack can be placed in the well bore and in the perforation tunnels (the largest source of pressure drop) without significant reduction in productivity.

As previously discussed, through-tubing completions generally have limited inflow area due to the perforating gun limitations. Table 2 illustrates productivity for a through-tubing completion having an inflow area of 0.2 sq in./ft (4 shots/ft, 0.25 sq in. diameter) with no gravel pack. This is compared to the same completion with a through-tubing gravel pack installed using U.S. 40/60 mesh gravel.

The through-tubing gravel packed completion has only 28% (2,498 Mcfd) of the 9,014 Mcfd production of a completion without a gravel pack. Table 2 also indicates that by increasing the inflow area to 0.6 sq in./ft a production rate of 6,256 Mcfd is possible. Further, increasing the IA to 1.15 sq in./ft will yield 8,971 Mcfd.

The same production rates can be achieved if the IA is increased (allowing for the pressure drop of the gravel) prior to the through-tubing gravel pack. In this case, the zone was reperforated with an additional 12 shots/ft of 0.31 in. diameter. This was accomplished with two wire line runs of 6 shots/ft each.

The lower pressure drop due to the additional inflow area would normally mean greater productivity; however, in the case of the through-tubing gravel pack, the productivity is lower due to the smaller ID of the gravel-pack assembly.

As the Nodal analysis demonstrates, the inflow area is the major influence on the production rate. Fig. 2a shows that the through-tubing gravel pack can produce 28% of the completion without a gravel pack. But with added inflow area (Fig. 2b) the through-tubing gravel pack production increases to 98% of the completion without a gravel pack.

This demonstrates that the major influence on the production rate is the inflow area and not the small-diameter hardware necessary to gravel pack without removing the previously installed hardware. A Nodal analysis run on an oil well will have similar results.

Understanding the effect of inflow area on production is necessary to determine the inflow area needed to cause the gravel in the perforation tunnels to be "transparent" to the production. This is true, not only for the gravel-packed completions, but for all sand-control completions, even the conventional completions if gravel is left in the perforation tunnels.

It also shows that through-tubing gravel packed completions offer the operator a well that can produce at very close to the potential of a conventional completion.

DESIGN

The steps that are followed in a successful through-tubing sand control completion are essentially the same as those followed for a normal full-scale control treatment. They are:

  • Obtain a representative formation sand sample. If possible, go to the original cores if available. If unavailable, a produced or bailed sample can be used.

  • Run a sieve analysis on the formation sand.

  • Determine the necessary size of commercial gravel-pack sand that will completely stop the formation with the lowest pressure drop across the completion.

  • Determine the necessary screen that will completely stop the gravel-pack sand.

  • Determine the inflow area (IA) needed to allow the zone to be produced with a minimum pressure drop across the completion.

  • Order the necessary downhole equipment to perform the through-tubing gravel pack, with backup.

  • rder the surface mixing and pumping equipment to perform a matrix acid pretreatment prior to the sand-control work. This should be dedicated equipment for acid and sand-control services.

  • Clean perforation tunnels by reversing, unbalanced flow, or matrix acidizing.

  • The slurry concentration and gravel or gravel substitute are other factors that must be designed to achieve the maximum productivity from a through-tubing gravel pack.

SLURRY CONCENTRATION

The gravel and gel concentration of through-tubing slurries is usually lower than in a conventional gravel pack. The lower viscosity and lower gravel loading have been used in through-tubing gravel packing for several years, primarily because of the friction encountered inside the coiled or concentric tubing.

The typical gel loading is 40-60 lb of HEC (hydroxyethylcellulose)/1,000 gal of fluid. This gel exhibits viscosities between 50 and 150 cp at 170 sec-1.

Viscoelastic surfactants such as Permac and Xanvis polymers have also been used as carrier fluids. The gravel concentrations have been from 2 to 5 ppg of carrier fluid.

GRAVEL OR SUBSTITUTES

Although gravel is still the particle of choice, bauxite and intermediate-strength particles (ISP) have been used recently for the washdown method. The Krumbein roundness makes this material easier to wash through.

However, because the bauxite particles are heavier than gravel particles (specific gravity 3.1 vs. 2.65) and the ISP is equal in density, these proppants are harder to place in the perforation tunnels. The lower pump rates and heavier particles make controlling the slurry difficult during the placement process.

Without gravel-pack particles in the perforation tunnels, the formation sand will fill the tunnel and increase the pressure drop across the completion. For this reason, an LDP (low-density particle, specific gravity 1.55-1.65) has been introduced.

The high friction pressures and lower pump rates encountered in the through-tubing work make the LDPs a natural choice.

The LDPs can be placed with lower viscosity carrier fluids. Because the LDP is lighter, viscosities can be lowered with little effect on the sand falling rates. The low-viscosity slurry provides greater leakoff into the perforations. The resultant particles left in the perforation tunnels allow the pressure drop to be lowered, and higher production is achievable.

OTHER METHODS

Several other through-tubing methods are also possible. These include resin-coated gravel, sand consolidation, and prepacked screens.

RESIN-COATED GRAVEL

The use of resin-coated gravel in through-tubing sand control does not require a screen and liner. The resin coatings (either placed on the surface by agitation or applied to the gravel at manufacturing) form a permeable filter that allows production to move freely into the well bore yet restrains the formation sand in place.

This permeable filter is placed in the perforation tunnels and outside of the casing. Most often these systems are run in water-based carrying fluids. The slurry is squeezed through the perforations until a screenout occurs. After curing, the permeable network of bonded gravel remaining in the casing is drilled out and the well is returned to production.

Another way in which resin-coated gravel has been used in the through-tubing market is to drill only to the top of the perforations and accept the pressure drop created by the resin-coated gravel left in the casing. This has been successful with gas wells.

With oil wells, the practice has been to drill out the casing ID down to the perforations and then drill a pilot hole through the perforations or drill out the entire casing ID.

Fig. 3 shows an analysis of the production from the same example well using a resin-coated gravel for sand control.

In Fig. 3a, the pressure drop across the resin-coated casing plug is large, and production is only 16% (1,428 Mcfd) of the well without a gravel pack.

But this method allows producing with the confidence that the formation sand will not be a problem.

Notice that in Fig. 3b the productivity available through resin-coated gravel once it has been drilled through with a 1.75-in. ID is comparable to that of the through-tubing gravel pack.

This indicates the available production with 4 shots/ft or IA of 0.2 sq in./ft.

With an IA of 1.15 sq in./ft of perforations, production capacity increases to 8,489 Mcfd.

A resin-coated gravel is available that can be used in wells with temperatures ranging from 80 to 225 F. Its use is usually limited to intervals less than 20 ft thick. Compressive strengths for both 20/40 and 40/60 gravel typically range between 2,500 and 3,500 psi. The resin-coated gravel is resistant to most produced fluids and retains high compressive strengths.

SAND CONSOLIDATION

Sand consolidation resins control formation sand in situ. The permeable synthetic sandstone that is formed from the resins bonding with the unconsolidated formation retains about 60-80% of the native permeability (Fig. 4).

The native permeability has been damaged by 40%. However, the production is only reduced by 22%.

If the zone were perforated with 4 shots/ft more (0.25-in. diameter), the production would be 99% of completion without a gravel pack.

Sand consolidation is favored in completions where using downhole tools may not be desirable. Placement has always been a problem in the past, but with today's selective treatment tools many of the placement fears are being overcome.

These tools (today capable of expansion ratios of 2:1) can be run through-tubing and expanded below the production tubing, isolating the intended perforations. Once the treatment has been completed, the tools can be pulled out of the hole or moved to a different location in the well bore.

Dowell Schlumberger's K200C sand consolidation system is externally catalyzed, requiring an overflush of catalyst and carrier to activate the resin. The high resin content of this system makes it suitable for zones that contain higher than normal amounts of clay. This system is also highly resistant to produced fluids.

PREPACKED SCREEN

Prepacked screen is another option that has been used to control the production of formation sand. This option is generally the least expensive of the four options. However, the savings are not always justified.

By looking at Fig. 5a, it is apparent that the prepacked screen option will reduce the productivity to 1,056 Mcfd if the interval is not reperforated. Reperforating the interval to the same inflow area (Fig. 5b) used in the through-tubing gravel pack (1.15 sq in.) will allow production of 5,300 Mcfd or 60% of the through-tubing gravel pack.

Copyright 1991 Oil & Gas Journal. All Rights Reserved.