What is the Production Hole Diameter?

EMS - 22 October 2024 - 1:02 am

Definition of Production Hole Diameter

The production hole diameter refers to the final drilled diameter of a wellbore that will be used for oil and gas production. It is the diameter to which the well will be drilled to allow for the installation of production casing or liner and completion equipment. The production hole is drilled after the intermediate hole sections have been completed.

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Factors Influencing Production Hole Diameter

Several factors are considered when determining the optimal production hole diameter for a well:

Expected Production Rate

The anticipated production rate of oil and/or gas is a key factor. Higher flow rates generally require a larger diameter to minimize pressure losses and allow space for artificial lift equipment if needed.

Reservoir Characteristics

The properties of the hydrocarbon bearing formation impact the hole size. Thick, high-permeability zones can be produced through a smaller diameter compared to thin or tight formations. Horizontal and multilateral wells typically have smaller production holes than vertical wells.

Completion Design

The planned well completion type influences the production hole diameter. Key considerations include:

Casing vs liner
Cementing
Number of completion zones
Sand control requirements
Artificial lift strategy

Wellbore Stability

The mechanical properties of the formation being drilled impact the mAXImum hole diameter that can be achieved without excessive breakouts or hole collapse occurring. In weak formations, a smaller production hole may be required.

Drilling Capability

The technical limitations of the drilling rig and equipment constrain the maximum hole size that can be drilled. Very deep wells or HP/HT conditions require smaller hole diameters.

Cost

A larger production hole diameter is more expensive to drill and complete. The incremental cost must be justified by productivity gains.

Typical Production Hole Diameters

The table below shows typical production hole diameter ranges for various well types:

Well Type	Hole Diameter Range (inches)
Onshore vertical oil well	6 – 8-1/2
Onshore vertical gas well	4-1/2 – 7-7/8
Offshore vertical well	8-1/2 – 12-1/4
Horizontal well	4-1/2 – 6
HPHT well	4-1/2 – 6
Geothermal well	8-1/2 – 13-3/8

Production Casing and Liner

The production casing or liner is run into the production hole and cemented in place. It provides pressure integrity, wellbore stability, and an annular flow path for produced fluids.

Production Casing

Production casing extends back to surface and is hung from the wellhead. Typical casing sizes for the production hole diameters in the table above are:

Production Hole Diameter (in)	Production Casing Size (in)
6	4-1/2
6-1/8	5
7-7/8	5-1/2
8-1/2	7
9-5/8	7-5/8

The production casing is often the final casing string and provides the main pressure barrier for the life of the well. Premium threaded connections are used to provide sealing and resist high axial loads. Heavy wall casing is required across weak or fractured zones prone to buckling failure.

Production Liner

A production liner is hung off the previous casing string and extends only across the productive interval, not back to surface. Liners allow a larger production hole to be drilled as they can be deployed through the previous casing. After cementing, the top of the liner is sealed with a liner hanger packer.

A tie-back string is sometimes extended from the liner hanger back to surface. This allows control lines and monitoring instruments to be installed.

Typical liner sizes are 1/2″ or 3/4″ smaller than the production hole diameter. The liner diameter must allow sufficient clearance for running completion equipment inside it.

Open vs Cased Hole Completions

Cased Hole Completions

Most wells are completed with the production casing or liner fully cemented in place across the pay zone. The well is then perforated to establish communication with the reservoir. Cased hole advantages include:

Improved well control
Allows selective perforating of zones
Can perform stimulation treatments
Suitable for most artificial lift methods
Reliability and longevity

Open Hole Completions

In some cases, the production zone can be left uncased or uncemented (open hole). This is most applicable for competent rock like carbonates that can resist hole collapse. The open wellbore provides a large contact area with the formation. Slotted liners or screens can be used for sand control.

However, open hole completions provide limited options for well intervention and artificial lift. Acidizing or fracturing can be risky without casing to provide wellbore integrity. The lack of cement also increases the risk of environmental contamination from producing zones.

Effects of Production Hole Diameter on Productivity

Inflow Performance

According to Darcy’s law, the production rate is proportional to the contact area between the wellbore and reservoir. Contact area is maximized by increasing the production hole diameter.

However, friction pressure losses in the tubing and across downhole equipment also increase with higher flow rates. There is an economic balance between hole size, well productivity and associated drilling and completion costs.

Artificial Lift Feasibility

Most oil wells require artificial lift to maintain economic production rates as the reservoir pressure depletes. Common methods are:

Sucker rod pumps
Progressing cavity pumps
Gas lift
Electric submersible pumps (ESP)
Hydraulic pumps

All these methods require production tubing to be installed inside the production casing. The tubing dimensions are constrained by the casing inside diameter. For example, the table below shows tubing and rod sizes installable in common casing sizes:

Casing Size (in)	Max Tubing OD (in)	Max Rod OD (in)	Max Pump Size (in)
4-1/2	2-7/8	1	1.75
5-1/2	2-7/8	1-1/4	2.25
7	3-1/2	1-1/2	2.75

A small production hole may not allow a large enough pump to be installed to meet the well’s production potential. This is especially challenging in deep wells or those with viscous crude which require high pump power.

Increasing the production hole and casing diameter provides more options for artificial lift and allows higher production rates to be lifted.

Impact on Well Intervention

Production Logging

Production logs are run to evaluate well performance and diagnose production problEMS. Tools are run on wireline or coiled tubing. The production hole and completion dimensions limit the size of logging tools that can be used.

In smaller completions, miniature or memory tools may be required. These have reduced measurement capability compared to full size tools. Larger production holes simplify logging and improve data quality.

Coiled Tubing Interventions

Coiled tubing (CT) is used for various well interventions like clean-outs, stimulations and logging. The table below shows typical coil sizes compatible with different production tubing sizes:

Tubing Size (in)	Max CT OD (in)
2-3/8	1.25
2-7/8	1.5
3-1/2	1.75
4-1/2	2.0

In smaller production holes, CT operations are limited to small diameters. This restricts the downhole tools that can be deployed and the treatment rate for pumping. Heavier CT is also more fatigued when run in small completions with a high degree of curvature.

Larger diameter production casing allows larger CT to be run. This increases the available downhole pulling force and the pumping rate for chemical treatments or stimulation.

Multilateral and Extended Reach Drilling

Multilateral and extended reach wells are drilled to increase reservoir contact from a single wellbore. They employ smaller production hole diameters to allow a longer horizontal section to be drilled. The production hole must accommodate the completion while retaining pressure integrity.

Multilateral Well Completions

In multilateral wells, multiple drain holes are drilled from a common mother bore. Each lateral requires an individual completion string, typically 2-3/8″ to 4-1/2″ tubing. The mother bore production hole must be large enough to accommodate all the laterals. An 8-1/2″ to 9-1/2″ hole size is typical.

Larger hole sizes provide more options for multilateral junction construction. Both cased and open hole junctions can be employed. The junction must provide pressure isolation between laterals.

Extended Reach Drilling (ERD)

ERD wells employ a small diameter production hole to maximize reach from the drilling location. Modern ERD wells can extend beyond 35,000 ft MD. The main limitations are torque and drag, cuttings transport and equivalent circulating density (ECD) management.

To minimize these problems, ERD production holes are usually 6″ or smaller. Larger diameters risk getting the drillstring stuck. The completion design must be tailored to the hole size to reliably case and cement the production interval.

Specialized equipment like rotary steerable systems (RSS) and high-strength drill pipe enable long horizontal sections to be drilled in one run. This greatly improves drilling efficiency compared to tripping for bottom hole assembly (BHA) changes.

Environmental and Safety Considerations

Well Control

Production casing provides the primary well control barrier. It must withstand the maximum anticipated wellbore pressure (MAWP) during the life of the well. This requires high burst resistance.

The production hole diameter affects the casing collapse rating. Larger holes require thicker wall casing to resist buckling loads. Heavy wall casing increases the well cost.

In some cases, a production liner can be used to allow a larger hole size drilled below a restriction. This improves well control in deeper hole sections. A tie-back string provides a conduit for well kill operations if needed.

Wellbore Integrity

The production casing or liner is the main barrier preventing reservoir fluids from entering shallower formations or groundwater aquifers. Proper hole conditioning and cementing practices are critical to ensure zonal isolation.

Larger production holes are inherently more stable than slim holes. Key factors are:

Lower equivalent circulating density (ECD)
Reduced surge and swab pressures
Ability to reciprocate casing while cementing
Less sensitivity to centralization and hole cleaning

These factors make it easier to achieve a quality cement job in large hole sizes. This improves wellbore integrity and reduces environmental risk.

Production Safety Systems

The wellhead and production tree dimensions are based on the production casing size. Larger casing allows higher pressure-rated equipment to be used. This provides more overpressure protection.

Downhole safety valves (SCSSV) are installed in nearly all offshore wells and most onshore wells. They provide emergency shut-in at the production packer to stop uncontrolled flow. The SCSSV outside diameter must be compatible with the tubing drift and seal bore dimensions.

Larger production casing allows larger valves to be set. These can pass wireline or CT intervention tools to reduce production deferment during routine valve maintenance.

Specifying the Production Hole Diameter

The final production hole diameter is specified based on an integrated analysis of subsurface requirements and field development constraints. The main steps are:

Assess the expected production rates and artificial lift requirements
Select candidate hole sizes based on the reservoir properties and completion design
Perform drilling engineering analysis to verify hole stability and ECD limits
Determine the completion configuration and intervention needs
Estimate well costs for each hole size
Optimize the hole size based on productivity and economic metrics

Close collaboration between subsurface and drilling disciplines is key to achieving an optimal well design. The production hole diameter has far-reaching impact on the producibility and longevity of the well.

Frequently Asked Questions (FAQ)

What is the most common production hole diameter?

For onshore vertical wells, 6″ to 8-1/2″ production holes are most common. Offshore and deep wells typically use larger 8-1/2″ to 12-1/4″ hole sizes.

How does production hole diameter affect well productivity?

In general, larger production holes allow higher oil and gas production rates by providing a larger inflow area and reducing friction pressure losses. However, drilling and completion costs also increase with hole size.

Can the production hole size be changed after drilling?

Once drilled, the production hole diameter is fixed. The casing and completion designs must be compatible with the final hole size. In some cases, a well can be re-drilled to a larger diameter but this is very costly.

What is a monobore completion?

A monobore completion uses the same diameter production casing and tubing. This simplifies the well design and eliminates diameter restrictions. Monobores are most applicable for moderate flow rate wells.

What is the smallest possible production hole diameter?

Slim hole and coiled tubing drilling technology allows production holes as small as 3″ to 4″ in some cases. However, these require specialized equipment and provide limited options for completion and artificial lift. Most production wells use 4-1/2″ or larger holes.

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