HORIZONTAL COMPLETIONS-1

Kevin D. Smejkal,W.L. Penberthy Jr.
Baker Oil Tools Houston

Proper drilling and displacing of an open hole completion may make the difference between a well that will produce at the reservoir capacity or one that is completion limited as a consequence of a dirty, unstable hole.

Horizontal open hole completions require drilling the open section with fluids that deposit thin filter cakes that exhibit excellent return permeability after the well starts producing. Displacement of the drill-in fluid with brine by reverse circulating at high annular velocities prior to running the completion equipment provides the best chance for an undamaged well.

These procedures have been successful in many wells, particularly horizontal gravel packs.

This first in a series of three articles on the design and procedures for horizontal open hole completions covers drilling and displacing the open hole.

The next two will discuss horizontal well stand-alone screens and gravel packs.

Horizontal open holes

Horizontal wells are becoming common in unconsolidated formations either as an initial completion or re-entry. Stand-alone screens such as slotted liners, prepacked screens, or special screens can be run to prevent formation sand production. More recently, long horizontal open holes have been completed with gravel packs.

Surprisingly, the trend has been towards completing horizontal wells open hole instead of with casing as in vertical wells drilled in the same reservoir.

In horizontal wells, stand-alone screens for excluding formation sand contrasts with the usual gravel pack used for controlling sand in vertical wells. As a matter of fact, stand-alone screens have been disappointing in conventional vertical wells.

In horizontal wells, the initial productivity of stand-alone screens has usually been good but experience shows that the screens may plug with time whereas gravel packs maintain productivity and completion longevity.

Drilling, displacing

Drilling and displacing the well bore is critical for a clean, stable well bore. To accomplish this, drill-in fluids should exhibit good drilling properties and excellent formation return permeability upon flow back.

Displacing the drill-in fluid with brine by applying reverse circulation techniques is the most efficient way for cleaning the hole. Once the open hole is effectively displaced, the well can be completed by either running a stand-alone screen or with a gravel pack.

Open hole completions

Whether to complete a well open or cased hole depends primarily on the need to isolate unstable intervals that contain undesirable fluids such as water or gas from the well bore. Provided that the completion results in a clean, stable borehole and that there are no undesirable formation fluids such as water or gas (or if they are present, they do not create a problem), an open hole completion is the most productive option.

The productivity indices (PI) in Table 1 document the superiority of open hole productivity in two Miocene reservoirs in Venezuela. While each reservoir has a different PI, the trend is the same.

On the other hand, if borehole stability is poor and excessive gas or water production cannot be tolerated, a cased hole completion isolates offending intervals behind cemented casing and allows production through perforations from only the oil or gas intervals.

Cased hole gravel-pack productivity can be enhanced by prepacking gravel outside of the perforations, but the PI is not as great as in an open hole completion (Table 1 [7,330 bytes]).

While both completions have advantages and disadvantages, the primary reasons for selecting either deal with the need for isolation, costs, and productivity. However, from a productivity stand point, an open hole gravel pack excels. This is the main reason for selecting it over a cased hole gravel pack.

Horizontal wells

Horizontal wells can improve productivity, enhance reservoir maintenance, or produce reservoirs which would be uneconomical with vertical wells. Provided that a horizontal lateral is in the same strata, intervals in the borehole may not require isolation.

Because lateral lengths often are more than 2,000-3,000 ft, costs for casing, cementing, and perforating can be significant in cased holes. Open holes simplify the completion and are economically attractive because of cost and productivity.

From one perspective, the horizontal lateral can be viewed as a gigantic perforation with the exception that tools can be run in it. This is appealing from a productivity perspective because greater perforation penetration is usually related to greater flow capacity in spite of having only a single perforation.

Guidelines

Open hole completions until recently were perceived to be passé however, their greater productivity and lower cost have renewed operator interest.

Open hole completions are reasonably simple. However, conducting each completion step correctly and in the proper sequence significantly affects the completion and subsequent well performance. By contrast, a cased hole can be completed with sloppy procedures. While the well may be completion limited (damaged) when it is placed on production, at least, the completion equipment can usually be run and operated without complications. Such is not the case in many open hole completions.

One main requirement for sand-control is well bore and fluid cleanliness. Hence, drilling must deliver a clean, stable, and undamaged well bore.

The first step in an open hole completion is to set production casing so that shales and other strata contributing to borehole instability are behind pipe. The next step is to drill into the reservoir with a drill-in fluid. The well must then be displaced with a brine in preparation for running completion equipment. Generic steps are as follows:

• Set production casing to case off shales
• Drill the productive formation with a drill-in fluid
• Displace the casing with brine
• Displace the open hole interval with brine
• Run screens
• Gravel pack the open hole, if gravel packing is the choice.

Drill-in fluids

Drill-in fluids enable open holes to be completed without formation damage caused by conventional drilling fluids. Drill-in fluids are relatively new and have been specifically developed for drilling open hole intervals.

The definition of a drill-in fluid is vague, but these fluids should possess the desirable physical properties of a drilling mud as well as having attributes of a completion fluid such as excellent return permeability after the well starts producing. This normally is not possible with conventional water or oil-based drilling fluid unless the formation damage is subsequently treated.

After the production casing is set, the drilling mud is displaced with drill-in fluids, which are used to drill the open hole interval.

One must remember that the first opportunity to damage the formation is when the drill bit enters the pay section. Unfortunately, this will not be the last. Selection of a drill-in fluid that will not cause formation damage is important for obtaining high productivity wells.

Currently, there are two generic water-based drill-in fluids available: calcium carbonate and saturated-salt systems.1 2 Water and oil-based drilling fluids have been used to drill the open hole interval but are not usually viewed favorably when compared to the generic drill-in fluids.

On occasion, water has served as a drill-in fluid but normally water results in excessive fluid loss that precludes its use.

Both the calcium carbonate and salt systems are water-based fluids containing polymers to increase viscosity and suspend particulates. Calcium carbonate or salt particles also achieve fluid-loss control. In concert with polymers, these systems should exhibit an API fluid loss of less than 5 cc/30 min. These low fluid-loss values are routine with some calcium carbonate-polymer filter cakes that are only about 1-mm thick.

Regardless of which drill-in fluid is used, it should meet the following requirements:

• Wide density range
• Compatible with the formation
• Low fluid loss
• Thin, friable filter cake
• High return permeability.

One main role of a drill-in fluid is to develop a thin, nondamaging filter cake that can be easily removed after placing a well on production. The breakout pressure, defined as the pressure differential required to initiate backflow, should be less than 20 psi and reflect a return permeability greater than 80%.

While the two generic drill-in fluids, mentioned previously, have been used widely, they differ in that the salt systems require a saturated aqueous phase to prevent dissolving the suspended salt particulates. Because the salt system is water soluble, it will dissolve in produced water or can be dissolved with a water overflush or acid treatments.

Several calcium carbonate drill-in fluids are available. From a drilling performance perspective, they are similar, but the return permeabilities vary considerably. Some require acidizing to restore permeability while others develop a thin, friable filter cake that usually differentially lifts off the sand face and does not require acid clean up.

Hence, the friable nature of some calcium carbonate filter cakes allows unrestricted fluid flow through a screen or gravel pack after placing a well on production without acidizing.

Other desirable drill-in fluid properties include a wide density and temperature range, both of which present problems with currently available drill-in fluids when fluid densities exceed 13 ppg and temperatures surpass 275° F. But research and development are extending the operating limits.

Displacing

Once the open hole reaches total depth, the drill-in fluid should be displaced by brine in preparation for running a screen into the well. At this point, the well bore should be clean and stable.

The drill-in fluid may contain as much as 10%, or more, solids which is unacceptable for the completion because these solids will likely plug the screen and/or contaminate the gravel pack. Hence, the reason for the displacement is to completely remove all solids from the open hole that potentially could plug a screen or gravel pack.

One approach for some stand-alone screen completions, which currently enjoys some use, has been to condition the drilling fluid so that it will backflow through the screen.^3-6 While some particulate backflow through the screen probably does occur, complete flow through the screen is unlikely with the consequences of flow impairment due to plugging.

Another approach is to run the screens into the drill-in fluid and to remove damage by circulating, washing, and acidization. However, there are concerns such as:

• Washing does not remove all debris.
• Acid often disturbs the formation, tends to mobilize plugging materials, and may adversely affect the screen.

These methods may be moderately acceptable for some stand-alone screen completions, but they are unacceptable and incompatible with gravel packing because particulates will plug and contaminate the completion.

Brine is the displacement fluid of choice. It should have a density of at least the same as the drill-in fluid and be sufficient to achieve an overbalance of 300-500 psi.

Exceeding the original formation breakdown pressure (fracturing) is almost never recommended. Also, under no circumstances should the system be underbalanced because this will either cause hole collapse or well-control problems.

With brine in the open hole, the filter cake acts as an impermeable seal with the overbalance pressure keeping the borehole from collapsing (Fig. 1 [16,290 bytes]). Furthermore, the filter cake facilitates a sharp pressure interface at the sand face and low fluid loss because the lack of a sharp pressure discontinuity at the sand face promotes borehole instability and enlargement (Fig. 2 [15,269 bytes]).

After drilling the open hole to total depth, the next step is to displace the casing to the shoe with brine. The most effective way is to reverse circulate with a large diameter, open-ended (mule shoe) workstring which is equipped with casing scrapers. Because there is no drill bit, the pressure on the open hole interval is minimal when reverse circulating-provided that the workstring diameter is not too small. Also, this is the most effective way of lifting debris to the surface because it is a high-velocity conduit.

To assist the initial displacement, a push pill can be pumped ahead of a casing sweep followed by filtered brine of 5 NTUs (nephelometric turbidity units), as illustrated in (Fig. 3 [20,932 bytes]). The push pill consists of altered drill-in fluid that is viscosified so that its yield point is 1.5-2.0 times more than the drill-in fluid. The pill volume should be designed to contact 300 annular ft.

The casing sweep is typically calcium hypochlorite at a concentration of 1.5 lb/bbl. The solution is a strong oxidizer which is designed to break residual polymer in the casing. The sweep volume should be designed to provide a contact time of at least 5 min or 300 annular ft (Fig. 4 [14,218 bytes]).

Once the casing is displaced, the next step is to displace the open hole. The displacement operations are similar, but no casing sweep should be used because the casing has already been cleaned and the oxidizer has adverse effects on the polymer in the filter cake. (Fig. 5 [18,159 bytes]) shows the displacement sequence that consists of a push pill followed by filtered brine pumped at 300-400 fpm; however, the workstring is now positioned at the toe of the horizontal section (Fig. 6 [15,240 bytes]).

The displacement is complete when brine return flow has a turbidity reading of 20 NTUs or less or has stabilized at an acceptable level. At this point, the casing and open hole interval are ready for completion operations to begin.

Contrary to some beliefs, water pumped at high annular velocities is considerably more effective than viscous fluids for open hole displacement, particularly in scouring "fluff" at the filter cake surface.

Effective displacement requires annular velocities in the 300-400 fpm range. These velocities accelerate the cleaning-displacement process as illustrated in (Fig. 7 [16,295 bytes]). Reverse circulation should be continued until returns to surface have about 20 NTUs.

Some operators are reluctant to displace the hole with brine for fear of excessive fluid loss. But provided that proper drill-in fluids and procedures are followed and a 300-500 psi overbalance is maintained, field experience supports the laboratory studies that indicate that the filter cake remains intact and is not eroded from the sand face.⁷ However, downhole assemblies should be used that will not mechanically disturb the sand face or contribute to hole enlargement.

Experience with open hole horizontal gravel packs, where a calcium carbonate-polymer drill-in fluid was used for drilling the open hole interval, has consistently demonstrated that when pumping at 5 bbl/min the returns to surface were about 4 bbl/min. Situations where problems have occurred are usually associated with active shales that are exposed to the open hole. Active shales should be cased off, if possible.

In situations where brine displacement disturbed the shales, some success with preventing shale movement has been achieved by displacing the open hole with drill-in fluid without the fluid-loss package.

If a well cannot be reverse circulated, the entire well should be displaced in a single step by pumping down the workstring with returns up the annulus (circulation mode).

Cleaning the casing first, such as with the reverse method, has no benefit because drill solids and debris must be pumped through the casing annulus twice. Hence, the workstring should be positioned at the toe of the well on the initial displacement. Pump rates must generate an annular velocity of 300-400 fpm for an efficient displacement and a reasonable clean out time.

Because of its adverse affects on the filter cake, the casing sweep should be eliminated when the well is cleaned by circulating down the workstring and up the annulus.

Isolating shales

As previously mentioned, in most open hole completions where problems have occurred, unstable shales invariably have been the main source of dirty fluids, borehole instability, and low productivity. The cause seems to be that some shales react adversely with the drilling fluid and promote swelling and entry in to the circulation path.

Also, a filter cake does not form because there is no fluid loss across the shales. The consequence is that the exposed shale continues to erode and creates plugging and fluid-cleanliness problems. Casing-off troublesome shales seems to be the best solution (Fig. 8 [17,746bytes]) .

Fortunately, not all shales are sensitive and present problems, and when these situations exist, intervals have been successfully completed even with exposed shale. However, should shale problems occur after setting casing, an uncemented liner can be run across the shale section to take it out of the circulation path.

References

1. Halliday, W.S., "Drill-in Fluids Control Formation Damage," World Oil, December 1994.
2. Donovan, J.P., and Jones, T.A., "Specific Selection Criteria and Testing Protocol Optimize Reservoir Drill-in Fluid Design," SPE Paper No. 30104.
3. Brown, S.V., et al., "Simple Approach to the Cleanup of Horizontal Wells with Prepacked Screen Completions," JPT, September 1995, pp. 794-98.
4. McLarty, J.M., Dobson, J.W., and Dick, M.A., "Overview of Offshore Horizontal Drilling/Completion Projects in the Gulf of Mexico," SPE Paper No. 24842, 1992.
5. Pardo, C.W., and Patricks, A. N., "Completion Techniques Used in Horizontal Wells Drilled in Shallow Sands in the Gulf of Mexico," SPE Paper No. 27350, 1992.
6. Brown, S.V., and Smith, P.S., "Mudcake Cleanup To Enhance Productivity of High-Angle Wells," SPE Paper No. 27350, 1994.
7. Johnson, M.H., Ashton, J.P., and Nguyen, H., "The Effects of Erosion Velocity on Filter Cake Stability During Gravel Placement of Open hole Horizontal Gravel Packed Completions," SPE Paper No. 23773, 1992.

The Authors