Carbon Steel OCTG Casing Pipe for Oil and Gas

OCTG Steel Casing Overview

OCTG (Oil Country Tubular Goods) refers to oilfield-specific tubing, a general term for key tubing used in drilling, well completion, and oil production during oil and gas exploration and development. Among them, carbon steel OCTG casing, as the most widely used and largest-volume basic tubing, plays a crucial role in supporting the wellbore, isolating the formation, controlling oil and gas flow, and protecting the wellbore, earning it the reputation of being the "steel backbone" of oil and gas wells.

From a compositional perspective, carbon steel OCTG casing is based on low-carbon steel or low-alloy steel. Through composition optimization and heat treatment process control, it achieves mechanical properties and corrosion resistance suitable for different service conditions. According to API (American Petroleum Institute) standards, carbon steel casing is mainly divided into steel grades such as J55, K55, N80, L80, C90, T95, and P110, with different steel grades corresponding to different yield strengths, tensile strengths, and applicable operating conditions.
 

Special Requirements for Steel Casing in Natural Gas Development

As a clean energy source, natural gas has a significantly different exploration and development environment compared to oil, placing more specific demands on the performance of OCTG casing. Natural gas wells typically feature high pressure, high corrosive media content, and long service periods. The application of carbon steel casing in these environments requires careful consideration of the following factors:
High Pressure Characteristics:Natural gas reservoirs have high pressure coefficients, with some wellhead pressures exceeding 70 MPa, requiring casing with sufficient internal pressure resistance and sealing performance.
Complex Corrosive Environment:Natural gas often contains corrosive media such as H₂S, CO₂, formation water, and Cl⁻, and gas well production cycles are long (up to 20-30 years), resulting in a significant risk of corrosion failure.
Temperature Fluctuations:Temperature changes during gas well production can lead to thermal stress, affecting the mechanical integrity of the casing string.
Gas Sealing Requirements:Natural gas is prone to leakage, placing significantly higher demands on the gas sealing performance of casing threaded connections compared to oil wells.

Typical Application Scenarios of Carbon Steel Casing in Natural Gas Wells

Surface Casing and Guide Pipes
In the shallow section of a natural gas well, low-grade carbon steel casing such as J55 and K55 is typically used as the surface casing. Its main function is to isolate loose formations and install wellhead equipment. Due to the relatively low corrosion risk and short service life in shallow sections, economical carbon steel casing is a reasonable choice. However, it should be noted that some shallow gas reservoirs may also contain H₂S, requiring an assessment of corrosion risk based on the actual gas composition.

Technical Casing
Technical casing is used to isolate complex formations, often needing to traverse high-pressure gas layers, water layers, or lost circulation zones in natural gas wells. N80 and P110 grade carbon steel casing are commonly used, with increased wall thickness and optimized thread structure (such as trapezoidal threads) to improve resistance to crushing and internal pressure. For H₂S-containing gas wells, technical casing must be selected with SSC-resistant steel grades (such as L80-1 type) to ensure safety during drilling and completion.

Production casing is a critical tubing string that directly contacts the natural gas production reservoir fluids. Its material selection requires comprehensive consideration of multiple factors, including strength, corrosion resistance, and economic efficiency. The application strategy for carbon steel casing in natural gas production wells is as follows:
Conventional natural gas wells: For dry gas wells with no H₂S or extremely low H₂S content (partial pressure <0.3 kPa), conventional carbon steel casing such as N80 and P110 can be used. Corrosion risk can be reduced by controlling production parameters (such as dehydration and adding corrosion inhibitors).

H₂S-containing acidic gas fields: When the H₂S partial pressure ≥0.3 kPa, SSC-resistant grade carbon steel casing (L80, C90, T95, etc.) must be selected, and the material hardness (HRC≤22) and sulfur and phosphorus content (S≤0.010%, P≤0.020%) must be strictly controlled. For high-sulfur gas fields (H₂S partial pressure > 1 MPa), higher-grade corrosion-resistant alloys or composite pipes are required, limiting the applicability of carbon steel casing.

For high-CO₂ gas fields: when CO₂ partial pressure > 0.2 MPa, carbon steel casing may experience severe uniform corrosion and pitting. Lifespan needs to be assessed through corrosion rate calculations, and if necessary, internal coatings, corrosion inhibitor injection, or the use of corrosion-resistant alloy-lined composite pipes should be employed.