Surface Casing

What is Surface Casing?

Surface Casing is a type of OCTG casing. It is the second layer of casing lowered into the wellbore (the first is the Conductor Casing) and the primary casing forming the wellbore framework. It is lowered from the wellhead, penetrating soft surface layers and any shallow freshwater formations, and resting on a sufficiently hard and stable formation (usually bedrock) beneath.

Depth of lowering: The specific depth varies from tens to hundreds of meters, depending on the depth of the freshwater formation and geological conditions. Its primary purpose is to isolate deeper freshwater formations and shallow gas formations, which is crucial for well control safety. The surface casing is designed to penetrate shallow water-producing formations (such as confined aquifers in Tertiary clastic rocks) to a depth sufficient to completely isolate it from the formation through cementing.

Structure: The surface casing consists of a string of high-strength steel pipes connected by special threads. After lowering, cement is injected into the annular space between the string and the wellbore wall to permanently secure it.

As the first barrier in a multi-layer casing system, the surface casing must meet technical specifications for resistance to external extrusion forces and wellhead suspension. In engineering practice, cement slurry must be returned to the wellhead to ensure annular sealing and reduce the risk of oil-water crosstalk. Inspection technology requires periodic monitoring for failure modes such as corrosion perforation and mechanical deformation.

Functions of Surface Casing:

Surface casing plays a crucial role in the drilling process. Its specific functions are as follows:

1. Isolating shallow confined aquifers and shallow complex formations. 

This blocks the path between oil and gas and groundwater, preventing chemicals in the drilling fluid or formation fluids from contaminating precious freshwater resources. This is a legal requirement.

2. Stabilizing the Wellbore. 

This strengthens the wellbore wall in loose surface rock formations, preventing wellhead collapse and drilling fluid loss, and ensuring stability for subsequent drilling.

3. Providing a supporting foundation for the installation of well control equipment such as blowout preventers and casing heads. 

The heavy, high-pressure blowout preventer assembly is directly mounted on the flange at the top of the surface casing. It serves as the "gate" for well control, and the surface casing is the solid foundation of this gate.

4. Isolating shallow geological hazards.
Shallow layers of gas or high-pressure water may exist. Surface casing provides the first reliable barrier, preventing these high-pressure fluids from escaping uncontrollably to the surface and causing catastrophic events such as blowouts, fires, or explosions.

Distinguishing Features of Surface Casing:

1. Large Diameter: Its outer diameter is typically second only to the casing, with common sizes being 13 3/8 inches, 16 inches, and 9 5/8 inches (for shallower wells).
2. Shallow Depth but Significant Impact: Although its depth is not deep, it serves as the first substantial barrier for well control safety and environmental protection.
3. Connecting the Upper and Lower Layers: It serves as the foundation for installing the heavy-duty blowout preventer stack and subsequent wellhead equipment, and all subsequent drilling operations depend on its stability.
4. Strict Cement Return Height Requirements: During cementing, cement must be returned from the bottom of the casing all the way to the surface to ensure complete isolation of the shallow layer. This requires rigorous verification through well logging.

Surface Casing vs. Intermediate Casing vs. Production Casing:

1. Surface casing: Emphasis on "protection"

Its core objectives are to address issues near the wellhead: protecting the environment (freshwater formations), establishing a safety foundation (installing blowout preventers), and preventing shallow-level disasters. Its function is outward-oriented, oriented toward the surface and shallow-level environments.

2. Intermediate casing: Emphasis on "security"

Its core objective is to ensure the safety and smoothness of the drilling process. When complex underground conditions arise that cannot be balanced by drilling fluid pressure (such as a blowout upon encountering a high-pressure zone or a loss of return upon encountering a lost-flow zone), intermediate casing must be lowered to isolate it; otherwise, drilling cannot continue. It is a process-oriented approach, acting as a "safety valve" to address unknown underground risks.

3. Production casing: Emphasis on "durability"

Its core objective is to serve the entire production lifecycle of the oil and gas well (which can be decades). It must ensure channel integrity, corrosion resistance, and effectively isolate the producing strata from other layers. It is a result-oriented approach, serving as the vehicle for ultimately achieving the extraction goal. 

On-Demand or Off-Demand: The Difference in Running Logic

Surface casing and production casing are mandatory for almost every well:

Without surface casing, operations are illegal and extremely dangerous.
Without production casing, effective production is impossible.

Intermediate casing (also called technical casing) is optional:

If the geology is simple and there are no complications from the surface casing shoe all the way to the target pay zone, then intermediate casing is not necessary and production casing can be run directly to the planned wellbore depth.
The more complex the geology, the more intermediate casing layers may be required, and the higher the cost of the well.

What are the typical sizes of surface casing used in drilling?

Surface casing sizes are typically expressed by their outside diameter. The most common size ranges are as follows:

Most Common Sizes:
13-3/8 inches – This is the most widely used size in onshore and offshore drilling worldwide.
16 inches – Also very common, it is often used for wells requiring larger boreholes or more complex geology.

Other possible sizes:
9-5/8-inch—used in shallower wells with simple geology.
20-inch—often used in very deep or offshore wells as surface casing, with larger conductor pipe sizes (such as 36 inches) below.

These sizes adequately protect freshwater aquifers and provide adequate structural integrity for the wellbore during the initial drilling phase. Furthermore, the surface casing diameter is selected to allow for the installation of subsequent casing strings, which will be extended deeper into the wellbore during drilling.

Note: There must be enough annular space (usually 1-2 inches) between each layer of casing and the wellbore, and between the casings, so that the oil casing can be smoothly lowered and the cement sheath outside can be guaranteed to have sufficient thickness and strength.

Surface casing structure and design requirements:
Wellbore positioning: This serves as the first layer of the casing system, with intermediate casing and production casing connected below.

Casing parameters:
Typical running depth is 15-20% of the wellbore depth (300-400 meters of casing for a 2,000-meter well).
Anti-collapse design requires calculation of the equilibrium point between formation pore pressure and fracture pressure.

Cementing technology:
Use G-grade oil well cement to inject the casing annulus. The cement slurry must be returned to the surface to form a continuous seal.
Casing shoe depth is determined by optimizing mathematical models of pore gradient and fracture gradient.

Construction specifications:
Clean mud must be used during the running process to reduce the risk of cuttings blockage.
Casing head installation includes steps such as setting the anti-wear sleeve and pressure testing. The slip hanger must withstand the weight of the subsequent casing string.
The wellhead assembly uses injection-molded sealing grease to achieve a BT seal. Under extreme operating conditions, kill fluid can be injected through the side holes of the casing head.

Environmental Impact:
Defects in cementing quality can cause oil-water crossover, leading to the infiltration of benzene and heavy metal pollutants into the aquifer.
A case study in a 3,000-meter-deep well in Xinjiang demonstrated that 600 meters of surface casing combined with cement isolation can eliminate 90% of the contamination risk.

Inspection and Maintenance:
The 2025 inspection standard requires the use of acoustic imaging and electromagnetic flaw detection technologies to assess pipe corrosion and deformation.
Key monitoring areas include stress concentration areas such as the casing shoe transition and threaded connections.
Regular inspection intervals are dynamically adjusted based on the casing corrosion rate.

In short, although the surface casing is short, it is the cornerstone of the safety and environmental protection of the entire well. Its design and construction quality are prerequisites for the success of the drilling operation. 13-3/8-inch or 16-inch diameters are commonly used, but the specific size depends on the engineering design of each well.

Read more: Types of Drilling Casing Pipe or API 5CT Steel Casing and Tubing Chart