open hole, Oil & Gas Glossary

What Is Open Hole?

Open hole refers to the portion of a wellbore that has been drilled through a formation but has not been lined with cemented steel casing, leaving the borehole wall in direct contact with drilling fluid and subsequently with wellbore fluids during production, used as the context for open-hole logging (wireline and LWD formation evaluation before casing is run), open-hole completions (producing directly from the exposed formation face), and open-hole testing (formation pressure and fluid sampling from uncased formations).

Key Takeaways

Open hole is contrasted with cased hole: open hole has no casing across the producing or evaluated formation; cased hole has cemented casing requiring perforations for communication.
Open-hole logging is performed before casing is run — it provides the highest-quality formation evaluation data because the tool reads through the formation directly rather than through steel and cement.
Open-hole completions (barefoot completions) expose the formation face directly to the wellbore without perforations, typically used in competent carbonates and horizontal wells in hard rock.
Open-hole interval stability depends on formation strength, wellbore pressure balance, and time — shales and weak sandstones may collapse into the wellbore if open hole is maintained too long without casing.
Open-hole packer completions in horizontal wells provide zone isolation between multiple open-hole segments without perforating, using mechanical packers to separate producing intervals.

Open-Hole Logging: Formation Evaluation Before Casing

The most critical use of open hole in oil and gas operations is the logging window between completing drilling of a reservoir section and running and cementing casing. In this open-hole interval, wireline logging tools can be lowered to the formation on a cable and pressed directly against the borehole wall without the signal attenuation and geometric complexity introduced by steel casing and cement. Open-hole formation evaluation provides the primary data for characterising reservoir porosity, fluid saturations, lithology, permeability, and pressure that guide the completion and production strategy for the well. The standard open-hole logging suite includes gamma ray, resistivity (laterolog or induction), neutron porosity, bulk density, sonic transit time, and caliper, with additional tools added as required: formation tester for pressure and samples, nuclear magnetic resonance for producibility, borehole imaging for fracture and sedimentary structure characterisation.

The time window for open-hole logging is constrained by borehole stability: formations left open for too long without hydrostatic overbalance from the drilling fluid develop wall failure, washouts, and in extreme cases wellbore collapse, that degrade log quality and may prevent tools from reaching target depth. Shales with reactive clay minerals begin swelling and sloughing within hours to days of open-hole exposure; salt formations creep and close the borehole continuously; soft sandstones erode under fluid circulation. Operators therefore aim to complete all required open-hole logging within hours to a few days of reaching total depth in each section, before running casing to stabilise the borehole. This time pressure on open-hole logging operations is a primary driver of well cost and operational efficiency on exploration wells where multiple logging runs may be required.

Open-Hole Applications Across International Jurisdictions

In Canada, open-hole wireline logging is standard practice in WCSB exploration and appraisal wells before setting production casing. AER well licence conditions require formation evaluation in exploration wells; the open-hole log data submitted to the AER is archived in the Alberta Energy Regulator's well data repository and released publicly after a confidentiality period. Montney horizontal wells use open-hole LWD logging during drilling for real-time geosteering followed by a wireline open-hole run for final formation evaluation before casing is set and the lateral is cemented and perforated. Devonian carbonate reservoirs in the Foothills use open-hole completions in competent limestone and dolostone because the hard formation provides adequate wellbore stability without casing, and the matrix porosity and natural fracture permeability allow direct production without perforations.

In the United States, open-hole conditions in Gulf of Mexico deepwater exploration wells require careful management of pore pressure and fracture gradient windows — the narrow drilling margin in deepwater Miocene and Pliocene sections often requires multiple casing strings to isolate shallow hazard zones and maintain open-hole stability through the primary reservoir target. BSEE well operations regulations require documentation of the open-hole logging programme as part of the well operations plan. Eagle Ford and Barnett shale horizontal wells use open-hole packer completion systems in some formations to stage hydraulic fracturing without perforating. In Norway, Sodir log submission requirements mandate that open-hole logs from all exploration wells on the Norwegian Continental Shelf be submitted to the Diskos national data repository within a defined timeframe after well completion. In the Middle East, the Khuff Formation carbonate reservoirs in Saudi Arabia and Qatar are completed open hole in long horizontal sections that maximise contact with the natural fracture network, with formation stability provided by the competent dolomitic carbonates.

Fast Facts

An open-hole wireline logging run in a deep exploration well (5,000-7,000 metres total depth) can take 12-48 hours to complete from rigging up the logging unit to retrieving the tool. During this time, the well is held in a static, overbalanced condition with the drilling fluid providing hydrostatic pressure to prevent formation fluid influx and maintain borehole stability. The cost of the logging rig time during open-hole logging (typically USD 50,000-500,000 per day depending on rig type and location) motivates optimising the logging programme to include all required measurements in a single run where possible, using combinable tool strings that run multiple sensors simultaneously. Missing a required open-hole measurement after casing is set requires either a through-casing tool run (lower quality data) or a casing perforated logging interval (destructive and expensive).

Open-Hole Completions: Advantages and Limitations

An open-hole completion allows the producing formation to communicate directly with the wellbore without perforations, benefiting from the full contact area of the open borehole wall rather than the limited perforation tunnel area of a cased and perforated completion. This is advantageous in high-permeability carbonate reservoirs where any restriction to flow limits production — the limestone or dolostone formation face provides structural integrity without casing, and the natural fracture network that provides the primary flow paths is undamaged by the mechanical impacts and explosive debris from perforation. In horizontal wells drilled into hard carbonate or tight sandstone, open-hole packers allow zonal isolation between segments of the open lateral, enabling selective stimulation or production testing of individual intervals without the cost of full cemented and perforated casing installation.

Tip: When deciding between an open-hole and a cased-hole completion for a new horizontal well, evaluate the formation's mechanical strength and clay content before committing to open hole. A formation with unconfined compressive strength (UCS) below approximately 20-30 MPa (weak to moderate sandstone), or with smectite clay content above 5-10% (swelling potential), is unlikely to maintain long-term borehole stability as an open-hole completion — the initial production may be good, but progressive sand production, wall collapse, or fines migration will degrade the completion within months to years. Request a geomechanical assessment and formation stability analysis using the caliper log, sonic log, and any available core mechanical data before finalising completion design. In general, carbonates with UCS above 50 MPa are excellent open-hole completion candidates; weak or clay-rich sands are not.

Open hole is also referenced as:

Uncased hole — the explicit descriptive term used when contrasting with cased hole in technical documents; "uncased interval" specifies a section of wellbore without casing in a well that has casing above and/or below the specified interval
Barefoot completion — used for open-hole completions where the wellbore terminates in open hole without any completion hardware (screens, packers) and the formation produces directly through the open borehole face; most common in hard carbonates
Open-hole section — the specific interval of borehole currently exposed to open-hole conditions; "the open-hole section below the 9⅝-inch casing shoe" refers to the freshly-drilled portion of the wellbore below the last cemented casing string

Frequently Asked Questions

Why is open-hole logging preferred over cased-hole logging for formation evaluation?

Open-hole logging provides significantly better formation evaluation data quality than through-casing logging for most measurements. Resistivity tools in open hole read formation resistivity directly with depth-of-investigation controlled by the tool design; through casing, the highly conductive steel casing shorts out the formation resistivity signal and requires specialised pulsed induction tools that achieve much lower accuracy. Neutron and density tools in open hole press against the formation wall for direct contact measurement; through casing, the steel and cement between the tool and the formation attenuate the neutron flux and gamma ray count rates, degrading porosity accuracy. Borehole imaging is only possible in open hole (the tool must contact the formation wall directly). The exception is that through-casing perforation and production logging can supplement open-hole data for specific objectives (production profiling, cement evaluation), and in some old wells where no open-hole logging was run at the time of drilling, through-casing evaluation may be the only option for characterising the formation.

What is the risk of leaving a formation open hole for an extended period?

The stability of an open-hole interval decreases over time due to several mechanisms. Shale hydration and swelling: water-based mud filtrate invades reactive shale sections, causing clay minerals to absorb water, swell, and slough into the borehole, reducing hole diameter and potentially causing differential sticking of any tool in the hole. Stress relaxation: time-dependent creep of the rock around the borehole concentrates stresses that may exceed the formation's compressive strength, causing spalling and wellbore ovaling. Cyclic loading: repeated piston effects from tripping tools in and out of the hole cause pressure pulses that can destabilise already-weakened borehole walls. In practice, operators aim to case off and cement reactive shale or salt sections within 24-72 hours of drilling them, and the decision to run wireline logs before or after setting casing in a given section depends on whether the expected open-hole stability time is adequate to complete the logging programme safely. Wellbore stability modelling, caliper data from LWD, and offset well experience are used to predict and manage this stability risk.

Why Open Hole Matters in Oil and Gas

Virtually every development decision in the life cycle of an oil or gas field — whether to perforate or not, which zone to produce first, where to set the production packer, how many fracture stages to pump, what reserves to book — is based on data acquired in the open-hole logging window before casing is set. The quality and completeness of the open-hole formation evaluation programme determines how accurately the operator understands the reservoir, how effectively the well can be completed, and ultimately how much oil and gas can be produced from the investment. Gaps in open-hole data that cannot be recovered by through-casing logging or production testing translate directly into higher development risk, less optimal completion designs, and potentially uneconomic wells in marginal reservoir quality intervals where the difference between wet and productive zones may be 10-20 ohm-metre in resistivity — a distinction that requires a properly run open-hole resistivity log to make reliably.

Open Hole: Definition, Open-Hole Completions, and Formation Evaluation