How to determine casing pipe size based on well depth and formation pressure?

How to determine casing steel grade and wall thickness based on well depth and formation pressure?

As an oil well casing manufacturer, we typically guide our clients through a systematic engineering process. Simply put, this isn't a simple lookup table, but a rigorous design process based on data calculations and standard specifications. Determining the casing steel grade and wall thickness is essentially a process of "load analysis → strength matching → economic optimization."

Common selection of casing steel grades for different oil well conditions:

Shallow casing (conduit/surface): Typically controlled by resistance to compression and tension, often using lower steel grades (such as J55/K55), which have sufficient strength, the lowest cost, and good weldability.
Deep technical casing and production casing: Typically controlled by resistance to internal pressure (especially in gas wells) and tensile strength, tending to use higher steel grades (such as N80/L80/P110).

Sulfur-containing environment: Please directly select from sulfur-resistant steel grades such as L80, C90, and T95, and then calculate the wall thickness.

Below, we will explain in detail how to determine the casing steel grade and wall thickness:

1) Collect basic data

1. Well depth structure: The depth of each casing layer and the corresponding wellbore dimensions.
2. Formation pressure profile: Pore pressure and fracture pressure throughout the entire well section (from the surface to the target layer). This determines the external and internal pressure limits that the casing needs to withstand.
3. Drilling fluid density: The planned drilling fluid density for different well sections.
4. Geological and fluid information: Presence of salt deposits (generating abnormal external extrusion pressure), hydrogen sulfide/carbon dioxide (determining steel grade type), temperature (affecting steel strength).

2) Calculate the load on the casing

Based on the data, calculate the worst-case conditions the casing may encounter downhole.

1. External extrusion load
Source: External formation pressure and drilling fluid column pressure.
Most dangerous operating condition: Casing internal cavitation (e.g., during production or after cementing, when the annular fluid level drops). At this time, the external pressure is at its maximum, and the internal pressure is at its minimum.
Calculation: Take the maximum external pressure that the bottom of this section of casing can withstand as the design external extrusion pressure.

2. Internal pressure load resistance
Source: Internal fluid pressure during well kick, shut-in testing, and fracturing operations.
Most dangerous operating condition: Wellhead shut-in, with formation fluid (gas) filling the casing.
Calculation: Take the maximum internal pressure that the top of this section of casing can withstand as the design internal pressure.

3. Tensile load resistance
Source: The suspended weight of the casing string itself.
Calculation: Consider the total weight of the casing string in air under drilling fluid buoyancy.

3) Applying safety factors to determine the required minimum strength

According to industry standards and specifications, set safety factors for each of the three major loads, amplifying the design load to ensure absolute safety.

Extrusion resistance safety factor: typically 1.0 - 1.125
Internal pressure resistance safety factor: typically 1.0 - 1.1
Tensile resistance safety factor: typically 1.6 - 2.0

Strength calculation formula: Design strength requirement = Calculated load × Safety factor

4) Casing Standards and Selection

Engineers will use formulas and data from the API TR 5C3 standard, or specialized casing design software.

For a given casing outer diameter, in the "Casing Performance Table," find all combinations of steel grades and wall thicknesses whose extrusion resistance, internal pressure resistance, and tensile strength are greater than or equal to the "Design Strength Requirement" calculated in step three.

At this point, we will have several feasible combinations. For example, for 9-5/8” oil casing, the following combinations are possible:
Combination A: N80, wall thickness 47.0 lb/ft
Combination B: P110, wall thickness 40.0 lb/ft
Combination C: P110, wall thickness 47.0 lb/ft (greater strength margin)

5) Economic Optimization and Final Decision

Compare the costs of multiple combinations that meet the strength requirements and select the optimal solution.
Principle: Compare the "low steel grade + thick wall" solution with the "high steel grade + thin wall" solution. Total Cost of the Solution.

Considerations:
Unit Price: Higher grade steel casing is more expensive, but thinner-walled casing is lighter.

Total Weight: Wall thickness directly affects steel consumption and total price.
Inner Diameter: Thinner-walled casing has a larger inner diameter, which is beneficial for subsequent operations.

Availability: Commonly used specifications are available faster.

Example: A 3000-meter deep well requires production casing designed to withstand an internal pressure of 70MPa.

Option 1: N80 steel grade, requiring a thicker wall thickness (e.g., 11.99mm).
Option 2: P110 steel grade, requiring only a thinner wall thickness (e.g., 10.03mm).
Although P110 has a higher unit price, the reduced wall thickness and weight may result in a lower total procurement cost, and a larger inner diameter. Option 2 is generally the better choice.

Steel Grade and Wall Thickness Selection for Different Types of Casing Pipe:

1. Steel Grade Selection for Conductor Casing
Conductor casing is used to establish a wellhead and isolate soft surface layers.
Common outer diameters: 30 inches, 36 inches (commonly used offshore), 20 inches (for harder onshore formations).
Characteristics: Largest diameter, thickest wall, lower steel grade requirements (commonly J/K-55 or higher strength), often installed using the "driving method".

2. Steel Grade Selection for Surface Casing 

The core function of surface casing is to isolate shallow, loose layers and freshwater layers, and to provide a stable foundation for installing blowout preventers (BOPs). It lies relatively shallow, and the external pressure from the formation and its own tensile force are relatively small. Surface casing has the largest diameter (e.g., 20 inches, 13-3/8 inches). Using economical steel grades can significantly reduce the material cost per well, conforming to the principle of engineering economy.

Common outer diameters: 20 inches, 13-3/8 inches (most commonly used globally), 16 inches.
Common steel grade: J-55. K-55 (mainstay, best cost). N-80 is used when encountering shallow, high-pressure formations.
Common weights: Taking 13-3/8″ as an example, common weights are 61.00 lb/ft, 68.00 lb/ft, 72.00 lb/ft, etc.

3. Steel Grade Selection for Technical Casing 

Technical casing is used to isolate complex formations encountered during drilling (such as high-pressure zones, lost circulation zones, and salt deposits), creating conditions for drilling into more important producing formations below. It is a temporary but crucial safety barrier. Its steel grade is mainly determined by the pressure and fluid properties of the isolated formation, emphasizing pressure resistance and safety during drilling.

A wide range of outer diameters is available, depending on the size of the upper casing and the designed well depth:
First layer technical casing: Commonly 9-5/8 inches, 10-3/4 inches.
Second layer technical casing: Commonly 7 inches, 7-5/8 inches.
Common steel grades: N-80 (most common), P-110 (for deep, high-pressure sections), L-80 (for sulfur-bearing formations).

Flexible wall thickness selection: Requires rigorous calculation based on specific well depth and pressure; multiple weights are available for the same outer diameter.

4. Steel Grade Selection for Production Casing 

As the "final liner" of an oil and gas well, the production casing provides access throughout its entire production lifecycle (potentially decades) and withstands various pressures during production, enhancement, and shut-in periods. It is a permanent and demanding barrier. Its steel grade is primarily determined by formation pressure, fluid properties, and enhancement operation pressure, emphasizing integrity, sealing, and corrosion resistance during long-term production.

Common outer diameters: 7 inches, 5-1/2 inches (as a tailpipe or for shallower wells), 4-1/2 inches.
Common steel grades: Conventional/High-Pressure Wells: P-110 (strong resistance to internal pressure, the preferred choice for deep well production casing).
Medium-Deep Wells: N-80.
Sulfur-containing oil and gas wells: L-80 (mandatory requirement).
Wall thickness characteristics: To withstand high-pressure operations such as fracturing, a combination of higher steel grade and thicker wall thickness is typically selected.