Open Hole vs. Cased Hole and Production Strategies

Well completion is the critical phase that follows drilling, casing, and cementing. It determines how effectively hydrocarbons flow from the reservoir to the surface. Two fundamental completion types exist: open hole and cased hole. Further classification depends on production strategy, including single-zone, multi-zone, and dual-tubing completions. This article provides a technical overview of these methods, their applications, advantages, and limitations.

In open hole completion, production casing is set above the target zone, leaving the reservoir interval exposed to the wellbore. This method is primarily applied in tight carbonate or hard sandstone reservoirs characterized by fracture systems or thin permeable streaks that are difficult to identify on logs and are easily damaged during drilling and cementing. By avoiding damage, open hole completions maximize fracture intersection and inflow potential due to the large surface area.

Key Applications:

Low-cost/multi-well development

Deep wells with depletion drive

Naturally fractured reservoirs

Some horizontal and multilateral wells

Advantages:

Low-cost completion option

Avoids perforating costs and associated production restrictions

Allows use of non-damaging mud to drill the reservoir after setting casing above the zone

Log interpretation not critical since the entire zone produces

Maximum hole diameter across the reservoir

Easy to deepen the well

Simple conversion to other completion types (e.g., liner)

Limitations:

Little to no selectivity in reducing unwanted water or gas production

Cannot prevent cross-flow between zones or production impairment from back pressure

Not recommended for injection or production wells in areas with significant vertical permeability variations that affect sweep efficiency

If water or gas breakthrough occurs, isolation is difficult unless it happens at the bottom (where a cement plug might work)

Unstable formations may collapse or produce sand when swabbed

Important: Open hole completions should only be used in competent formations.

In cased hole completion, production casing is cemented across the reservoir interval, and then selective perforations are made. This is the most common and versatile completion method.

Advantages:

Better prevention and control of excessive gas or water production

Allows selective fracturing treatments

Still allows deepening

Casing prevents sand influx and accommodates specialized sand control techniques

Full diameter across the completion interval

Logging assists casing decisions

Suitable for multiple completion configurations

Minimum drilling time

Disadvantages:

Perforating costs can be high

Critical log interpretation required for selecting perforation intervals

Higher formation damage risk because the mud used to drill from the last casing shoe is typically the same mud used to drill the reservoir

Higher overall cost

Variation: Cased Hole with Liner

Production casing is set above the reservoir. The reservoir section is then drilled, a liner is run and cemented across the zone, and selective perforations are made.

Advantages: Minimizes formation damage by using a new, non-damaging mud to drill the reservoir; better control of water/gas production; allows selective stimulation; casing prevents sand influx.

Disadvantages: If production casing is 4-1/2", recompletion options for deeper zones are limited; critical log interpretation; additional costs for liner, cementing, perforating, and rig time.

In this method, casing is set above the completion interval, and an uncemented screen or pre-perforated liner assembly is installed across the reservoir. This is an alternative to open hole completions where formation sand production is a concern, provided the reservoir rock consists of relatively large, uniform sand grains. The screen or liner prevents produced sand from entering the production string.

Advantages:

No wireline perforating required (though pre-perforated liners can be costly)

Avoids cleanout problems if screen/perforation sizes are correctly selected

Log interpretation not critical

Accommodates unique sand control techniques

Gravel pack or pre-packed liner options available

Minimizes formation damage by using new, non-damaging mud to drill the reservoir

Disadvantages:

Same lack of zonal control as open hole completions (cannot control water/gas production because no cement behind pipe)

Only conditionally effective for sand control

Lower cost than full casing to surface with cementing and perforating (though high-quality sand control screens can reduce savings)

More rig time than open hole

Reduced diameter across the reservoir (e.g., 4-1/2" casing limits hole size to max 3-7/8" for deepening)

Used to increase effective sandface area and provide high-permeability flow paths to the wellbore, improving gas production from low-permeability rocks (<25 md). Risk: fractures may not be contained within the zone, requiring redesign of casing cementing and completion equipment to withstand stimulation pressures.

Used to control sand production in perforated completions. Gravel is placed between the cased wellbore and a sand screen, ideally squeezed into perforations to stabilize formation sand. Requires large-diameter perforating charges at maximum shot density to achieve sufficient flow area due to the limited permeability of gravel.

Based on production mode, completions are further classified.

Single-Zone Completion Options:

No casing, fluid flow

Casing and tubing flow

Tubing flow without annular isolation

Tubing flow with annular isolation (using a packer to isolate the annulus above the packer, which contains completion fluid with corrosion inhibitors)

Multi-Zone Completion:
Uses at least two packers to separate production zones. Fluids from all zones can be commingled, or upper zones can be shut off via sliding sleeves. Sliding sleeves are mechanically operated valves shifted by wireline or coiled tubing. Alternatively, operators may allow lower zones to deplete, set a plug, and produce only from upper zones.

Additional Considerations:

Dual-tubing completions allow two separate tubing strings to produce from two zones independently, avoiding commingling issues.

Intelligent completions (ICs) include permanent real-time remote pressure and temperature sensors and remotely operated flow control valves at each zone, used where intervention is costly or problematic.

Artificial lift (electric submersible pumps, rod pumps, gas lift) is required when reservoir pressure is insufficient to lift fluids to surface.

Injection wells may inject water or gas to push oil through the formation; producing wells may be equipped with inflow control devices (ICDs) to manage fluid influx.

Selecting the appropriate completion type depends on reservoir characteristics, production objectives, and economic considerations. Open hole completions offer cost savings and maximum exposure in stable formations but lack zonal selectivity. Cased hole completions provide flexibility for selective perforation, stimulation, and sand control. Screen liners and gravel packs address sand production. Multi-zone and intelligent completions enable efficient management of complex reservoirs. Understanding these fundamentals allows engineers to design completions that maximize recovery, ensure well integrity, and optimize long-term production.

For more detailed information, please don't hesitate to contact Vigor team for more detailed product information.