Environmental Cracking Of High Strength Octg In Mud And Its Mechanism

Environmental cracking of high strength casing in mud has been experienced but its mechanism has not been clarified. Degradation of lignate mud with time and its effects on steel in the mud were experimentally investigated in a laboratory. Sulfide concentration of mud increases and pH value of mud decreases with aging time. In an aged mud, hydrogen absorption and cracking of steel were observed when pH of the mud was lowered. Sulfide which was formed from mud change into H₂S by a decrease in pH which was caused by dissolution of C0₂. C0₂ was observed to form from mud. As the result, sulfide stress cracking occurs, which is a proposed mechanism of cracking of casing in mud.

Environmental cracking of high strength OCTG exposed to drilling mud have been experienced. The cracking tends to occur for very high strength steels such as V150 grade. One of the cases is a 7“OD casing of V150 grade which cracked after the outer surface was exposed to lignosulfonate mud left on the top of the cement^l). The cracking was related to sulfide stress cracking (SSC) because sulfide was detected. If the mechanism of the cracking in mud was SSC, it could be easily understood that the cracking occurs more frequently for higher strength steels. However, no direct evidence for the mechanism was reported.

In the present paper, the mechanism of the cracking is examined and the cracking of steel in mud is reproduced in a laboratory.

EXPERIMENTAL

Materials

Mud. Lignate mud which was used conventionally for drilling was used. The main ingredients are listed in Table 1.

OCTG material. As representatives of high strength OCTG, V150 grade steel (V) and C11O SSC resistant grade (S) were used. V150 was the primary test material. The chemical compositions and the tensile properties are shown in Table 2.

Experimental Procedure

Measurement of Sulfide Concentration in Mud. By decreasing pH of the mud with an addition of concentrated nitric acid (HNOS), all sulfides were converted into hydrogen sulfide (H₂S). Argon (Ar) gas was bubbled into the mud to expel H₂S from the mud and carry H₂S gas into a fixing solution ( (CH₃COO)₂Zn + CH₉COOH solution ) as shown in Fig. 1. Then the H₂S in a fixing solution was titrated with iodine ( I₂ ) and sodium thiosulfate ( Na₂S₂O₃ ). A concentration (ppm) is indicated as a weight ratio of H₂S to mud (the specific gravity of the mud was 1.06).

Measurement of Hydrogen Absorption in Steel. Hydrogen absorption (content) in steel was measured using am electrochemical permeation method with nickel (Ni)- plated specimens on the anode-side surface²⁾. Test apparatus is illustrated in Fig.2. The cathode-side chamber was filled with a test solution (mud) or an electrolyte when a diffusion constant was being measured. The solution in the anode side contained 1 N NaOH solution. The specimen was set at 0.2 V vs SCE. Diameter of the specimen was 20 mm, and the thickness was 1.2 mm.