Barefoot Completion: Definition, Open Hole, and Wellbore Stability

A barefoot completion is a well completion method in which the producing interval is left as open hole, with no casing, liner, screen, or perforations across the reservoir section. Reservoir fluids flow directly from the formation face into the wellbore without any tubular or cemented annulus in between. The term "barefoot" is widely used in North American drilling parlance and is synonymous with open-hole completion in this specific context. The technique is employed in competent, mechanically stable formations such as dense carbonates, some tight sandstones, and chalk reservoirs, where the borehole walls can sustain the stresses induced by drilling, completion, and long-term production without support from a cemented casing string. In the right geological conditions, barefoot completions offer meaningful cost savings, eliminate near-wellbore drilling damage associated with perforating, and maximize the flow area open to the reservoir. However, they require careful pre-drill wellbore stability analysis and carry limitations in zonal control and remediation flexibility that make them inappropriate for complex or heterogeneous reservoirs.

Key Takeaways

• A barefoot completion leaves the production interval as open hole with no liner, casing, screen, or cement; reservoir fluids flow directly through the exposed formation face into the wellbore.
• The approach eliminates perforation skin, reduces completion costs, and avoids cement filtrate invasion of the near-wellbore zone, but sacrifices the ability to selectively control, isolate, or re-stimulate individual zones.
• Formation mechanical competence is the primary qualification criterion: unconfined compressive strength (UCS) greater than approximately 20 to 40 MPa (2,900 to 5,800 psi) is a common minimum threshold, though this varies with reservoir pressure, fluid type, and drawdown.
• Barefoot completions are widely used in horizontal wells drilled through competent carbonate reservoirs, including Khuff gas in the UAE, Ekofisk chalk in Norway, and various carbonate plays in the Middle East and North Africa.
• Regulatory frameworks in Canada (AER Directive 051), the United States, Australia, and Norway each require demonstration of mechanical integrity and wellbore stability before barefoot completions are approved.

How It Works: The Barefoot Completion Mechanics

In a conventional cased-hole completion, the operator runs a production casing or liner to total depth across the reservoir, pumps cement into the annulus between the casing and the borehole wall, and then perforates the casing and cement to establish flow communication with the reservoir. The perforating process creates small tunnels through the cement and casing into the formation, and each of those tunnels introduces perforation skin, a near-wellbore flow restriction that partially offsets the benefit of having a stable, controlled completion. The cement job protects the casing from reservoir fluids and allows the operator to isolate individual zones for selective stimulation or water shutoff. This system provides maximum flexibility but adds significant cost and introduces multiple potential failure modes including poor cement bonding (channeling), incomplete perforation coverage, and cement filtrate invasion that can impair near-wellbore permeability.

In a barefoot completion, all of these intermediate elements are eliminated. The drill bit reaches the base of the production casing shoe, and the open hole section is drilled through the reservoir with a suitable completion fluid designed to minimize formation damage. When the target depth is reached, the drillstring is pulled, and the well is completed simply by allowing formation fluids to flow into the open hole and up through the production tubing string set above the casing shoe. There is no liner run, no cement job, no perforating gun run, and no perforation skin. The entire cross-sectional area of the borehole is open to flow, giving a theoretical skin factor of zero (or even slightly negative if the drilled hole is larger than the production casing inner diameter). In practice, some formation damage from the drilling fluid filtrate invasion exists, but in tight, low-porosity carbonates this invasion is typically shallow and partially self-remediated by the initial gas or oil influx.

The open hole interval in a barefoot completion typically ranges from a few tens of meters in a short vertical producer to hundreds or even over a thousand meters in a long horizontal well. For horizontal wells in thin but laterally extensive carbonate reservoirs, the barefoot approach is particularly attractive because it allows the entire horizontal section to contribute to production without the cost of running and cementing a liner across a potentially kilometers-long horizontal wellbore. The flow regime shifts from the Darcy linear flow through perforation tunnels typical of cased completions to a nearly radial or elliptical flow from the entire open-hole face, which is theoretically more efficient in a homogeneous reservoir.

How It Works: Wellbore Stability Requirements

The critical engineering prerequisite for a barefoot completion is confidence that the open hole will remain mechanically stable throughout the well life, from the moment the drill bit exits the formation to final abandonment. Wellbore stability analysis combines rock mechanical properties with in-situ stress characterization and mud weight selection. The primary failure mode in barefoot wells is compressive wellbore breakout or tensile spalling, where the concentrations of hoop stress around the borehole exceed the rock strength, causing small slabs or fragments to fall into the wellbore. In extreme cases, progressive breakout collapses the hole entirely, trapping the completion tubulars or blocking production.

The unconfined compressive strength (UCS) of the formation is the most widely used first-pass screening criterion for barefoot completion suitability. Rock mechanics practitioners generally require UCS above approximately 20 MPa (2,900 psi) as a minimum; many carbonate reservoirs targeted for barefoot completion have UCS values of 50 to 200 MPa (7,250 to 29,000 psi), well above this threshold. However, UCS alone is insufficient: the stress anisotropy of the in-situ stress field, the pore pressure, the drawdown magnitude, the well trajectory relative to the principal stress axes, and the presence of pre-existing natural fractures all modulate whether a given formation will remain stable in open hole. A full Mohr-Coulomb or Drucker-Prager failure envelope analysis, calibrated with wireline sonic log data, multi-arm caliper data, image logs, and where available core plug test results, is the industry standard approach. In many jurisdictions, regulatory approval of a barefoot completion requires submission of this stability analysis to the relevant authority.

Paradoxically, one of the most famous barefoot completion success stories involves a formation that is mechanically very weak: the Ekofisk chalk of the Norwegian North Sea. Ekofisk chalk has UCS values as low as 2 to 10 MPa (290 to 1,450 psi), far below typical barefoot thresholds. Yet Ekofisk wells have been successfully completed as barefoot (open hole) horizontal wells because chalk behaves plastically rather than brittly under confinement. Instead of spalling off and blocking the hole, chalk tends to deform by grain rearrangement and compaction, creating a zone of plastically deformed material around the wellbore that actually acts as a filter cake supporting the borehole wall. This ductile behavior is exceptional and should not be generalized: most weak sandstones or chalks in other basins would collapse catastrophically in barefoot completion. The Ekofisk analog requires careful qualification before application elsewhere.

How It Works: Comparison to Other Open-Hole and Cased-Hole Completions

Barefoot completions occupy one end of a spectrum of open-hole completion options. Moving from barefoot toward increasing wellbore support, the next option is a slotted liner, where a liner with pre-cut slots is run into the open hole without cement; the slots are sized to let reservoir fluids in while preventing large formation fragments from entering. A slotted liner provides some structural support to the wellbore walls and allows zone-by-zone production measurement if enough isolation is achieved. Beyond slotted liners come wire-wrapped screens, which offer finer solids exclusion, and open-hole gravel packs, which pack gravel between the screen and the formation to provide comprehensive solids control in unconsolidated sands. All of these options maintain the open-hole philosophy of direct fluid contact with the formation without cement, but add progressively more mechanical support and completions complexity.

At the opposite extreme is the fully cased and cemented completion with perforations and selective hydraulic fracturing or acid stimulation. Cased completions offer the highest degree of zonal isolation and flexibility for later intervention, including re-perforating, squeeze cementing, selective hydraulic fracturing, and mechanical water shutoff. The trade-off is higher upfront cost, perforation skin, and the irreversibility of the cement job. In formations where water breakthrough risk is high, where multiple separate pay intervals exist at different pressures, or where the reservoir is unconsolidated, cased and cemented completions with selective perforations are generally preferred over any open-hole approach. The barefoot completion is specifically suited to the narrow window where the formation is both mechanically competent and relatively simple in its reservoir architecture.