Barefoot: Open-Hole Wellbore, Completion Architecture, and Formation Stability
In drilling and well completions, the term barefoot describes a borehole or a section of a wellbore that has been drilled but not cased, not lined with a perforated or slotted liner, and not screened: it is exposed open hole from the drilled face of the bit to the bottom of the last casing string above. The formation rock is in direct contact with the wellbore fluid across the barefoot interval, with no tubular steel between the reservoir and the flowing stream. The word "barefoot" is North American field vernacular for what more formal engineering documentation describes as an open-hole completion or an unlined open-hole section. It appears in completion and drilling reports in the context of individual wellbore sections (for example, "the Montney B was drilled barefoot to 5,230 m MD and left open for the horizontal completion") and also as a descriptor of the overall completion architecture when no casing or liner is run across the producing interval at all.
The barefoot condition can arise at three distinct points in the life of a wellbore. First, it is the temporary state of every new wellbore section after the bit has advanced below the casing shoe and before the next casing string or liner is run: the borehole is barefoot in the interval between the shoe and the bit until the next string is cemented. Second, it is an intentional completion design choice for wellbores targeting mechanically competent reservoirs where borehole stability is expected throughout the production life, and where the absence of casing or perforations across the pay zone is judged to improve productivity and reduce completion cost. Third, it can be an unintended consequence of a casing-running failure or an equipment loss that prevents the planned liner from being landed, leaving the open-hole section without its intended liner until a remedial operation can be performed. In the context of completion design, the barefoot wellbore refers almost exclusively to the second case: an intentional engineering decision to produce from open hole rather than from perforated casing or screened liner.
Key Takeaways
- Formation requirements for barefoot stability: A barefoot wellbore is only stable in formations that can sustain the induced mechanical stresses of the wellbore without spalling, sloughing, or plastic deformation. The key parameters governing barefoot stability are unconfined compressive strength (UCS) of the rock (minimum approximately 20 MPa for a stable wellbore at 2,000 m depth in a normal stress regime), the ratio of maximum to minimum horizontal stress (lower ratios mean less anisotropic stress around the wellbore and less tendency to fail on a preferred plane), the rock's tensile strength (governs hydraulic fracturing risk during production drawdown), and the presence and orientation of natural fractures and bedding planes relative to the wellbore trajectory. Dense carbonates such as the Devonian Leduc and Nisku reefs in central Alberta, some tight Cardium sandstones, and chalk reservoirs in the North Sea (Viking Graben producers) routinely sustain barefoot completions because their high UCS (40 to 200 MPa) prevents spalling even at elevated production drawdowns. Weak sandstones and clay-rich shales are incompatible with barefoot completions because they slough into the wellbore under production-induced stress changes, plugging the flow path and requiring sand control measures.
- Flow area advantage of barefoot over perforated casing: A barefoot wellbore presents the entire drilled surface area to the formation, providing a flow area equal to the circumference of the wellbore bore times the net pay length in the barefoot interval. For a 216 mm (8.5 inch) wellbore in 50 m of Leduc carbonate, the effective flow area is approximately 0.034 square metres (pi x 0.216 m x 50 m = 33.9 square metres of cylindrical surface, minus the fraction occupied by impermeable tight zones). This compares with approximately 0.004 to 0.008 square metres of open hole created by a standard perforation pattern in the same interval (24 perforations at 4 shots per 0.3 m across 50 m, each perforation with 12 mm entry diameter). The barefoot flow area advantage of 5 to 10 times over perforated casing translates directly to lower near-wellbore pressure drop (lower skin factor) and higher productivity index, particularly in moderate-permeability carbonate formations where skin from perforation friction is a meaningful fraction of total drawdown.
- Wellbore integrity and production life challenges: While a barefoot completion avoids the skin damage associated with perforating, it introduces different integrity risks over the production life of the well. The primary long-term risk is mechanical weakening of the borehole wall as production drawdown progressively decreases pore pressure in the reservoir, reducing the effective confining stress on the formation rock and potentially causing shear failure in the near-wellbore zone (spalling or breakout). A borehole that was stable at initial reservoir pressure of 25 MPa and 200 MPa UCS may experience wall failure at economic production drawdown of 15 MPa if the rock's intact UCS has been reduced by diagenetic alteration or if the stress state in the reservoir changes as adjacent compartments are depleted by offset wells. Wellbore stability monitoring using downhole caliper surveys run on wireline during workover operations detects enlargement of the barefoot section, with enlargement rates of more than 5 mm per year typically triggering consideration of running a sand exclusion screen or a perforated liner across the affected interval even in an originally casing-free design.
- Zonal isolation limitations of barefoot completions: A barefoot wellbore offers no mechanical means of isolating individual zones within the open-hole section. All formations exposed in the barefoot interval communicate directly with the wellbore, and fluid from any zone can commingle with fluid from any other zone without restriction. In a multi-zone barefoot carbonate well, this means that gas from an upper zone may re-enter and interfere with oil production from a lower zone, or that water from a basal aquifer may breakthrough to the wellbore and immediately mix with hydrocarbons from the pay zone above, since there is no packer, liner, or cement sheath to create a barrier. Selective zone treatment, stimulation, or shut-in is not possible in a barefoot completion without running a packer or liner retroactively. This limitation makes barefoot completions unsuitable for formations where production from specific zones must be controlled individually, where gas-oil or water-oil contact management is important, or where regulatory requirements mandate zone-by-zone production allocation for royalty or environmental purposes.
- Cost economics and application decision: The capital cost advantage of a barefoot completion over a perforated or screened liner is the elimination of the liner string itself (casing material, running costs, cementing, and potentially perforation costs), which can total CAD 200,000 to CAD 500,000 per well on a deep Devonian carbonate or Nisku reef well in Alberta where liner equipment and cementing operations are major cost components of the completion programme. Against this saving, the operator must weigh the risk of borehole instability requiring a remedial liner run (which would incur the full liner cost plus a workover rig mobilisation premium), the risk of reduced selectivity if zonal control is ever required, and the regulatory requirements for zonal isolation and casing barrier records under AER Directive 020 for well abandonment. In competent reef carbonates with a 30-year production history of barefoot operation in offset wells, the risk-adjusted economics strongly favour the barefoot approach; in formations with less proven stability or where regulatory compliance requirements mandate casing, the economics shift toward perforated liner or perforated casing design.
Barefoot Well Sections During Drilling Operations
Every wellbore is temporarily barefoot in the section between the last casing shoe and the current bit position while drilling is in progress. The management of wellbore stability, fluid loss, and well control in this temporary barefoot section is a primary concern of the drilling programme. In the Montney formation of northeast BC, the barefoot Montney section from the 7-inch liner shoe to total depth is drilled with oil-based mud (OBM) to prevent clay swelling in the Buckinghorse shale overlying the Montney, to provide borehole stability in the Montney siltstone itself, and to reduce the torque and drag penalty of a long horizontal barefoot section that would be significantly higher with water-based mud due to differential sticking tendency. The barefoot Montney section during drilling is 2,000 to 3,000 m long and exposed to OBM filtrate invasion for 3 to 10 days from first penetration of the Montney top to completion of the lateral. Managing the barefoot section during this period requires monitoring of drill cuttings returns for signs of instability (spalling cavings, blocky cuttings, or persistent bridging) and maintaining mud weight above a calculated minimum of 1.25 to 1.35 SG equivalent mud weight to prevent shear failure in the tight siltstone.
In conventionally cased vertical wells targeting Devonian carbonates, the barefoot section during drilling is the reef interval below the Devonian shale casing shoe, which may be only 50 to 150 m long and drilled with minimal mud weight (near formation pressure balance) to preserve porosity in the reef without inducing lost circulation into the vuggy carbonate system. The temporary barefoot drilling condition transitions to a permanent barefoot completion condition if the operator elects not to run a liner across the pay zone, or to a perforated casing condition if the operator runs and perforates a production liner after cementing. The drilling team's observations of borehole condition during the temporary barefoot state (caliper readings, LWD density image data, drilling mud returns) inform the completion engineer's decision on whether the permanent barefoot state is likely to be stable throughout the production life.
Barefoot Horizontal Sections in WCSB Unconventional Completions
In the Montney and Duvernay formations, horizontal wells are drilled through the target formation with the lateral section in a barefoot condition during the drilling and logging phase before the production liner is run and cemented. In ball-operated sleeve completions, the production liner replaces the barefoot state with a cemented and perforated liner that allows multi-stage hydraulic fracturing; the barefoot lateral section during drilling is a temporary engineering necessity, not an intended production configuration. In some operators' Montney and Duvernay completions, however, an open-hole liner system (pre-perforated or slotted liner without cement) is run to control borehole stability and prevent late-stage collapse while still preserving a large flow area, representing a hybrid between barefoot and fully cemented liner designs.
The Bakken formation in southeastern Saskatchewan and southwestern Manitoba is one of the few truly barefoot horizontal completions in the WCSB context, where some operators run horizontal laterals in the tight Bakken silicified siltstone without a production liner, relying on the formation's high UCS (greater than 100 MPa in the silicified facies) and its self-sustaining natural fracture network to maintain wellbore integrity over a 20 to 30-year production life. Bakken barefoot wells in the Weyburn-Estevan area have production histories spanning 20 years without observed borehole collapse events, validating the barefoot design for this specific formation and region. The same approach applied in the softer, clay-rich Queen Elizabeth facies of the Bakken in other areas would not be viable, illustrating that barefoot completion stability is a formation-specific assessment rather than a universal Bakken characteristic.
Regulatory and Abandonment Considerations
AER Directive 020 in Alberta and BC OGC Drilling and Production Regulation Section 44 in British Columbia impose specific requirements for wellbore abandonment that affect barefoot completions. A wellbore that was produced as a barefoot open-hole completion must be abandoned with cement plugs placed across the open-hole production interval to achieve the same zonal isolation that casing and cement would have provided had the well been completed conventionally. The abandonment cement volume required to fill a barefoot reef wellbore of 216 mm diameter and 100 m length is approximately 3.7 cubic metres of neat Class G cement, compared to the volume of annular space that would have been behind a production liner in the same interval (much less, since the liner would reduce the annular cross-section). The higher abandonment cement cost for a barefoot well (typically CAD 40,000 to CAD 80,000 more per open-hole interval than an equivalent cased interval abandonment) is a lifecycle cost that must be included in the capital budgeting for any barefoot completion design, offsetting a portion of the upfront completion cost saving.