Prepacking perforations with gravel

Prepacking can be defined as any method that intentionally places gravel into the perforation tunnels. Filling of perforation tunnels can be accomplished either with a dedicated operation before performing the gravel pack or simultaneously with it. The technique used is normally dictated by well parameters.

Cased-hole gravel packs

Gravel packing cased-hole completions in vertical and deviated wells are more common than openhole completions, particularly in shaley reservoirs. Reasons for this are several-fold:

• Cased-hole completions are the norm in almost any development because the reservoir is usually easier to manage, so remedial operations are simplified
• Wellbore stability issues are minimal
• If multiple intervals are involved, openhole completions will not provide the necessary isolation

However, cased-hole gravel packs have an important requirement that is easily overlooked. The perforations must be prepacked with gravel if productivity and completion longevity is desired. Not until the late 1980s was the importance of prepacking fully appreciated. The illustration shown in Fig. 1 is an example of prepacked perforations. Note that the gravel is packed through each perforation and into the perforation tunnel beyond the cement sheath. Fig. 2 shows the benefit of prepacking. This information was taken from large-scale laboratory testing studies that illustrated the pressure drop across perforations filled with 1-darcy formation sand, 20:40 gravel and 20:40 gravel that was prepacked in the perforations. Tables 1 and 2 provide additional information. The lowest pressure drop through the perforations occurs when they are prepacked. Lower pressure losses across the perforation not only affect flow from the reservoir, but the larger wellbore pressure provides additional inflow pressure to lift fluids to the surface. Cased-hole gravel packs that have not been prepacked are usually damaged. There is no remedial treatment that can remove the damage (a frac pack can bypass the damage), leading to a well that will be permanently restricted unless a workover is performed to prepack the completions and complete the well properly. Table 3 verifies this scenario with field data and shows the superiority of wells that were prepacked.

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  Fig. 1—Prepacked perforations.

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  Fig. 2—Effect of perforation packing (0.5-in. perforation on pressure loss).

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  Table 1

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  Table 2

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  Table 3

Well considerations

The prepacking technique used is normally dictated by well parameters, such as:

• Excessive fluid loss
• Extended rathole area
• Reservoir acid sensitivity
• Zone length

An additional concern that must be addressed is the question of what transport fluid to use for the prepacking operation. Regardless of the technique selected, to effectively pack the perforations, one critical condition must be met: there must be fluid loss through the perforation. Fig. 3 shows the effects of the leakoff rate on the amount of gravel prepacked. Data also show that the well deviation is not a factor on the amount of gravel placed.

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  Fig. 3—Effect of leakoff rate to perforation filling efficiency (courtesy of Baker Oil Tools).

Choice of fluids

Provided that there is leakoff, any fluid can be used. The packing sequences, 1 to 7, when brine and viscous fluids are used, are shown in Figs. 4 and 5. The two are slightly different because of the viscosity of the fluid. Viscous fluids suspend and transport the gravel completely to the end of the perforation tunnel and then pack back toward the entrance of the perforation. Note the node at the entrance of the perforation caused by viscous forces in Fig. 4. With brine, the gravel is initially deposited at the entrance of the perforation, and subsequent packing takes place over the top of the dune until it reaches the end of the perforation. The last volume to be prepacked is that over the dune. The obvious question at this point is which fluid should be used, or which is the best? The question has many operating implications. However, field data from prepacking operations, conducted at matrix rates, show that brines are superior because they pack more gravel.

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  Fig. 4—Perforation with viscous transport fluid.

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  Fig. 5—Perforation filling with brine transport fluid.

Prepacking below fracture pressure

To prepack below fracture pressure, the perforations must be clean and contain no debris. There must be leakoff into the formation. A void outside the perforation is desirable.

These completions consist of gravel packing with viscous gels—slurry packs in which there is no dedicated procedure to prepack the perforations. Any prepacking that occurs is simultaneous with the gravel pack. Example field results using this approach (Fig. 6a) for a project in southeast Asia reflect the performance in terms of the skin factors measured after completion. Some wells performed exceptionally well (i.e., the skin factor was small), while others were disappointing. Completion success was inconsistent. The average of the data indicated a skin factor of about 24 or a flow efficiency of 25%, which is common for gel packs. Whether the problem with well performance was a lack of prepacking or damage caused by other factors is not known. Acidizing is probably the only alternative for restoring production for this example, but it will never restore reservoir capacity if the perforations are not prepacked.

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  Fig. 6a—Distribution of gel-pack skins.

Acid prepacking

Acid prepacking has been used to improve productivity. A critical aspect of a successful damage removal procedure is that the acid must come into contact with the entire interval. In addition, it has been commonly thought that contact time must be sufficient to allow all of the damage to be dissolved. With these assumptions, during the mid-1980s, acid prepacking quickly evolved into a process in which a diverted acid treatment was pumped at a low rate. Several studies indicated that one of the most effective diverters for acid prepacking is to carry relatively small quantities of sand in an HEC gel. While this combination did provide good diversion, the well test results, shown in Fig. 6b, tended to be inconsistent. Poor perforation filling from injecting a sand/gel slurry into the perforations at a low rate, coupled with formation damage, resulting from the use of HEC, are the most likely causes for the elevated skins. The detrimental effects of questionable perforation filling can easily overpower any benefit obtained from using the acid.

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  Fig. 6b—Distribution of conventional acid prepack skins.

Dedicated prepack operations

High matrix injection rates and the use of nonviscous transport fluids are two techniques that have been demonstrated to improve perforation filling. The traditional acid prepacking techniques violate both of these conditions. If the perforation filling is indeed critical for cased-hole gravel packs, completion methods that focus on filling perforations should prove superior to those that sacrifice perforation filling for damage removal. Fig. 7 illustrates this point. Here the skin factors from 55 Gulf of Mexico wells are shown, 42 of which were prepacked at matrix rates with a 20 lbm/1,000 gal HEC (hydroxyethylcellulose or slickwater) fluid. Typical prepack volumes were about 40 lbm/ft. An annular brine gravel pack followed prepacking. The remaining wells were completed with a water prepack and an annular brine pack. The wells completed with the gel prepack required post-gravel-pack acid to achieve the performance reported in Fig. 7. However, the transport fluid was able to easily leak off to the formation, and high injection rates were used to enhance placement of gravel in the perforation tunnels. The data presented indicate that not only are the average skin factors reduced compared to slurry packing and acid prepacking (Figs. 6b and 7), but the overall consistency was also improved (especially for high-permeability thick formations). These data demonstrate that when prepacking below fracture pressure, it is more important to ensure that as many perforations as possible are completely filled with gravel-pack sand than for the damage to be removed. However, it must be remembered that improved well performance will result if damage can be effectively removed without jeopardizing the filling of the perforations.

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  Fig. 7—Distribution of dedicated brine-pack skins.

Prepacking above fracture pressure

One of the main detriments to prepacking below fracture pressure is that gravel can only be placed into voids created during underbalanced perforating or perforation cleanup. If the amount of penetration into the formation does not extend completely through the near-wellbore damaged zone, restricted well productivity results. To overcome this difficulty, it becomes necessary to remove the damage with acid. This is not always easily accomplished if sufficient gravel has not been prepacked. Another technique to eliminate the effects of the damaged zone is to bypass it rather than to attempt to remove it. This is accomplished by hydraulically inducing a fracture in which the orientation is normal to the least principal stress in the formation.

Techniques available to create these fractures include brine fracturing or a frac pack. To allow frac packing and water fracs to be distinguished, a description of these techniques is discussed next.

Frac pack

A fracture with a length of about 100 ft can be created with a viscous transport fluid, but typical lengths are usually shorter. High pump rates are typically used (15 to 20 bbl/min), with proppant concentration increasing from 12 to 15 lbm/gal. The total amount of gravel pumped is typically in excess of 1,000 lbm/ft. Horsepower requirements may exceed 5,000 hydraulic horsepower (hhp) but are commonly lower.

Water frac

A fracture with a length between 5 and 15 ft can be created with a low-viscosity (brine) transport fluid. Pump rates are higher than for conventional gravel packing but usually lower than a frac pack. Typical pump rates are in the range of 8 to 12 bbl/min. Proppant loading is held constant between 1 and 2 lbm/gal, and total job size is typically from 100 to 150 lbm/ft. These treatments can be multistaged to further enhance the ability to effectively treat several sand subintervals with a single treatment. Horsepower requirements are typically about 1,000 hhp.

Treatment comparison

From the description of these prepacking treatments, frac packs are significantly larger than water fracs. The frac packs appear to reach much farther out in the reservoir as a consequence of the longer fracture lengths, while the water fracs focus is near the wellbore. The amount of fracture length required is a question that arises. Many propose that bigger is better.

When water is used as the fracturing fluid, short, narrow fractures are created because of the fluid’s low viscosity that results in a hydraulic fluid efficiency less than 5%. With frac packs, the fluid efficiency is in the range of about 25% because viscosified fluids reduce leakoff. Also, frac packs are designed for a tip screenout that ceases fracture length extension before the end of the treatment. Continued pumping with high gravel concentrations is intended to increase the width of the fracture to increase fracture conductivity.

The gravel placement geometry in a water-frac treatment forms an equilibrium gravel bank similar to that shown in Fig. 8. Frac packs pumped in viscous fluids at high gravel concentrations also probably have a small equilibrium gravel bank, but substantially more of the gravel tends to be suspended in the fracture at higher concentrations, which provides for the wide fractures after closure.

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  Fig. 8—Equilibrium bank formation within a fracture (courtesy of Baker Oil Tools).

Both treatments can be pumped in either a single step or two steps. In the single-step approach, the formation is fractured and subsequently gravel packed in one pumping sequence. In the two-step method, the fracturing and the annular gravel are performed separately. Of the two alternatives, the single-step method is preferred because it is less expensive and time consuming.

There are proponents of both fracture prepacking methods. Some prefer the frac packs because they believe that the longer, wider fractures provide less risk of a low-productivity well. Proponents of water fracs cite lower costs and operations conducted with platform-based equipment as advantages. From the standpoint of productivity improvement (stimulation) in the high-permeability wells, long fractures are not required, and fracture conductivity is more significant than length, provided the fracture extends past the damage.

Probably the best way to compare the benefits of the various prepack treatments is to compare their relative performance based on experience in the field. Figs. 9 and 10 compare frac packs and water fracs. Because there is a wide discrepancy in their designs and fracture geometry, one might think that the frac packs with long, wide fractures would provide a superior result. While there are similarities between the techniques, comparing the results of the frac packs to the water fracs reveals that the skin distributions are almost identical. These data strongly suggest that the main benefit of either treatment is perforation prepacking and damage bypass, regardless of which prepack technique is implemented. Credence to this viewpoint is that neither of the fracture prepack methods produces completions with large negative skin factors that have been achieved with conventional fracturing in consolidated formations. Skin factors below –1 are rare for any cased-hole gravel pack.

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  Fig. 9—Distribution of water-frac skins.

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  Fig. 10—Distribution of frac-pack skins.

Noteworthy papers in OnePetro

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External links

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See also

Gravel pack design

Gravel placement techniques

Gravel pack equipment and tools

Openhole gravel packing

History of gravel packs

Sand control

PEH:Sand_Control