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Openhole gravel packing

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Openhole completions provide another opportunity for sand control. Many engineers do not routinely think of performing an openhole completion when confronted with selecting a completion. This is true probably because cased-hole completions are so widely accepted and because they are not familiar with selection criteria and procedures. Openhole completions can provide excellent, high-productivity completions, but they must be applied under the right reservoir conditions. They avoid the difficulties and concerns of perforation packing and reduce the gravel-placement operations to the relatively simple task of packing the screen/openhole annulus. Because openhole gravel packs have no perforation tunnels, formation fluids can converge toward and through the gravel pack radically from 360°, eliminating the high pressure drop associated with linear flow through perforation tunnels. The reduced pressure drop through an openhole gravel pack virtually guarantees that it will be more productive than a cased-hole gravel pack in the same formation, provided they are executed properly.

Theoretical pressure drops

Fig. 1[1] illustrates the theoretical pressure drops experienced in openhole and cased-hole gravel packs. It reveals that openhole gravel packs result in virtually no additional pressure drop as the formation fluids converge at the wellbore.

Guidelines for selecting openhole gravel pack candidates

Despite their potential for creating high-productivity wells, openhole gravel packs are not suitable for all reservoirs and formations. One disadvantage of the openhole completion (including openhole gravel packs) is the inability to always isolate unwanted water and/or gas production. Unlike cased-hole completions that can be precisely and selectively perforated in the zones of interest, openhole completions sometimes offer less control over fluids (water, oil, and gas) exposed to the wellbore. In a cased-hole, remedial operations such as squeeze cementing, plugbacks, or straddle packoffs to isolate unwanted fluid production are can be carried out with a reasonably good chance of success with little to no planning. Such remedial operations in an openhole well (with the exception of a plugback) require additional planning to isolate undesirable fluids. With this in mind, openhole completions are best suited for thick reservoir sands rather than multiple sand reservoirs where there is water and/or gas to contend with.

Maintaining borehole stability during drilling and completion is an essential requirement for openhole gravel packs. Concern over the lack of borehole stability is a primary reason that openhole gravel packs are not used more often in unconsolidated, dilatant formations. Unstable boreholes make running of the gravel-pack assembly difficult and may prevent proper gravel placement if the formation flows in around the screen. Fortunately, state-of-the-art drill-in fluids are usually effective in maintaining borehole stability while performing a horizontal completion in dilatant-type formations.

Openhole gravel packs should be avoided in formations with several sand and shale laminations if the shales are prone to uncontrollable eroding and/or sloughing. During gravel placement, the shale can intermix with the gravel-pack sand, resulting in reduced gravel permeability and impaired well performance. Again, proper drill-in fluid selection can alleviate some of the problems associated with laminated sand and shale formations.

Openhole gravel-pack candidates include:

  • Formations where cased-hole gravel packing has unacceptable productivity.
  • Wells where increased productivity is required.
  • Reservoirs where long, sustained single-phase hydrocarbon flow is anticipated.
  • Situations where workovers for isolating gas or water cannot be accomplished.
  • Wells where high water/oil or gas/oil ratios can be tolerated.
  • Reservoirs with single uniform sands (avoid multiple sands interspersed with troublesome shale layers or water sands).
  • Formations that can be drilled and completed maintaining borehole stability in the completion interval.
  • Wells where cased-hole completions are significantly more expensive (i.e., long horizontal wells).

Top set openhole gravel pack

The most common type of openhole completion is referred to as “top set,” which is illustrated in Fig. 2. While this figure shows a vertical completion, this discussion is also pertinent to openhole horizontal wells. In this completion, the production casing is set at the top of the completion interval to isolate overlying strata. Once the casing is cemented, the following steps are undertaken:

  • Productive formation is drilled to total depth
  • Hole is cleaned and displaced
  • Gravel pack is installed

Critical issues

Critical issues in top-set openhole gravel packs include:

  • Selecting the casing seat
  • Drilling the open hole
  • Underreaming, if necessary
  • Cleaning the hole and gravel packing

Selecting the casing seat

Selecting the casing seat at the proper depth can have a significant impact on the success and cost of an openhole completion. Normally, the casing should be set at the top of the reservoir, just barely into the productive interval. If the overlying formation is an unstable or sloughing (heaving) shale, failure to isolate the shale behind casing may cause problems and delays throughout the remainder of the completion. Well logs should be run to ensure that all offending strata have been penetrated and will be cased before running the casing. In some instances, several logging runs may be required as the well is deepened to determine exactly when the casing should be run. In the case of logging while drilling, the casing point can be easily picked without multiple logging runs. Alternatively, the well can be drilled to total depth and logged to determine the appropriate casing depth. Then a sand plug can be placed across the productive interval before cementing the casing.

Drilling the open hole

Several options are available for drilling the openhole completion interval. How this is performed and the type of fluids used depend on the mineral and fluid content of the formation (i.e., whether it is sensitive to the drilling and/or completion fluid). Another factor is whether to enlarge the hole by underreaming. The fluid used for drilling the open hole is critical to the success of the completion. The general requirements of an ideal drill-in (or underreaming) fluid, which apply to any openhole completion and are not specific to gravel packs, are:

  • Compatibility with the reservoir rock and fluids (nondamaging)
  • Good suspension properties
  • Low friction loss
  • Low fluid loss
  • Easily controlled density
  • Ready availability
  • Low cost
  • Ease of mixing and handling
  • Nontoxicity
  • Thin friable filter cakes with low breakout pressure

While most fluids do not have all of these properties, some, such as calcium carbonate brine fluids, have performed well as drill-in and underreaming fluids. The critical issue is that the drill-in fluid should do minimal irreversible damage to the face of the formation. The solid-laden fluids should quickly form a filter cake to minimize filtrate losses. The filter cake should be easily removable before or after gravel packing. The ease with which it is removed is reflected in a low breakout pressure. Breakout pressure is reached when drawdown pressure, required to initiate production after the formation, has been mudded off with the drill-in fluid. In rare cases, clear brines have been acceptable as nondamaging drill-in fluids. If the open hole is to be underreamed, standard drilling mud may be used as a drill-in fluid, provided that the underreaming operation, using calcium carbonate brine-based systems, removes the mud-invaded, damaged portion of the formation.

Underreaming

Underreaming is the operation of enlarging the hole size below the casing shoe. One reason for underreaming an open hole is to remove damage present in the pilot hole. Underreaming may be unnecessary if the pilot hole is drilled with a nondamaging fluid. The larger-diameter hole also enhances the well productivity slightly, but in most cases, this is insignificant. Underreaming may be performed simply to provide greater clearance between the screen and the open hole. In any event, underreaming should be performed with a nondamaging fluid that keeps the hole stable. Traditional drilling muds should be used only as a last alternative, and damage-removal treatments should be planned before placing the well on production if these muds are used.

Underreaming is usually more of an annoyance than an incremental time, cost, or productivity issue because a cased-hole completion also requires changing over to a clean fluid before perforating. Perforating, of course, is unnecessary. Underreaming and perforating usually offset each other in incremental costs.

In the event that running a liner across the completion interval at a later date is an option to isolate unwanted fluids, underreaming probably should be avoided. The cement sheath in an underreamed hole will be much thicker than normal and will interfere with effective perforating or make perforating operations more difficult. The difficulties are that perforating, or ineffective perforations, will adversely affect gravel packing and, subsequently, will restrict well productivity.

Hole cleaning

The hole may contain solids that need to be removed. These solids can be any of the following:

  • Drill-in fluids
  • Drill solids
  • Gravel-pack sand

The importance of cleaning the hole and the filter cake is shown in Fig. 3. This bar graph is based on field data collected from 10 wells and shows the relationship between completion skin and hole cleaning. This relationship is not too surprising, but what is often overlooked is that once a well is damaged, subsequent acid treatments increase productivity but will not yield an undamaged well. Before running the screen in the hole and gravel packing, it is necessary to:

  • Remove the drill-in fluid
  • Drill solids from the hole
  • Clean the hole
  • Scour the filter cake to its dynamic thinness

Set through openhole gravel pack

When accurately setting the casing depth is difficult or secondary pay zones exist above the primary target, set-though openhole completions can be applied. In this type of completion, the casing is run through all formation pay zones and cemented in place. Cased- and openhole well logs are used to determine the exact location of the pay zones behind the casing, and windows are milled (with a nondamaging fluid) opposite the completion interval to create an “openhole” environment. The well can then be gravel packed.

Schematics of example set-through-type completions are shown in Fig. 4. Because of the amount of debris created by milling casing windows, it is recommended that all set-through openhole completions be underreamed to expose a clean, nondamaged formation face. A requirement in applying set-through-type completions is a good cement job. The casing must be securely cemented to facilitate milling operations and maintain alignment between the upper casing and the lower casing sections. Because a sump packer can be used, a set-through gravel pack assembly is basically the same as a cased-hole type. The only exception would be the use of bow-spring-type centralizers in long openhole sections. Set-through-type completions are especially well suited for recompletions in existing wells.

Gravel packing openhole completions

To gravel pack an openhole completion, follow the well preparation and gravel placement guidelines for cased-hole completions.

References

  1. 1.0 1.1 Penberthy, W.L. Jr. and Shaughnessy, C.M. 1992. Sand Control, 1, 11-17. Richardson, Texas: Monograph Series, SPE.

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