Investigation of the Serviceability of Tubing Strings Made of Steel of Strength Group K
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INVESTIGATION OF THE SERVICEABILITY OF TUBING STRINGS MADE OF STEEL OF STRENGTH GROUP K O. I. Radkevych, H. V. Chumalo, R. M. Yurkevych, A. V. Vasylyk, and I. M. Antoshchak
UDC 620.194
The resistance of steel of strength group K to corrosion, hydrogen-corrosion, and stress-corrosion fracture is analyzed for hydrogen-sulfide-containing media of the Lokachi gas field and in the NACE solution. It is shown that the indicated type of steel is not susceptible to hydrogen-induced cracking, its resistance to sulfide fracture σ ssc ≈ 0.8 σ0.2 min , and the rate of corrosion is much lower than the dangerous threshold value. This enables us to conclude that the tubing and casing strings made of this type of steel can be safely exploited at the Lokachi gas field.
In the oil-and-gas production, it is customary to use three main types of oil pipes, namely, drill pipes, casing pipes, and tubing strings. They operate under severe and variable conditions in broad ranges of operating temperatures (from – 60 to 150–200°C) and under the action of alternating (applied to drill pipes and tubing strings) or impulsive (applied in the case of cumulative perforation of casing pipes) loads and suffer stress corrosion in aggressive media [1]. To evaluate the corrosion and corrosion-mechanical resistance of the materials of casing pipes and tubing strings, we studied steel of strength group K (in what follows, referred to as K-steel). This steel belongs to the class of structural materials with elevated strength and, hence, the tubing and casing strings made of this metal can operate in wells down to a depth of 2727 m. The system used in the wells of the Lokachi gas field is based on the application of hanging tubing strings (without packers). Therefore, the pipes are mainly subjected to the action of tensile loads caused by the weight of the string. The depth of the wells in the gas field does not exceed 1000 m. The natural gas in the field contains hydrogen sulfide (up to 0.18%) accelerating the general corrosion process and the hydrogen-corrosion and stress-corrosion fracture of steels [2]. The seam pressure is equal to 8.7 MPa. The stratal water belongs to the calcium-chloride class and contains the following ions (mg-eq / liter): 890.7 Na+ + K+, 142 Ca2+, 72 Mg2+, 1078 Cl–, 4.2 SO24 − , and 22.5 HCO3− . The total mineralization is equal to 2209.4; pH = 8.71. The equipment of all wells is protected with corrosion inhibitors. Tubing and casing strings of K-steel are mounted in some wells. We studied the corrosion and stress-corrosion properties of K-steel in sections of the pipes supplied by the “L’vivgazvydobuvannya” GPU. K-steel was produced according to GOST 633-80, the pipes had the following dimensions: ∅ 73 × 5.5 mm; the material was subjected to thermal treatment by normalization; the pipes were produced by rolling. According to the indicated GOST, the content of detrimental impurities in K-steel is restricted to ≤ 0.045% of sulfur and ≤ 0.045% of phosphorous. The content of the other alloying elements (C, Mn, Si, Cr, Ni, etc.) is
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