INTRODUCTION
The resistance of steel to Sulfide Stress Cracking in Sour Service decreases when mechanical properties increase. The last decades have seen the successive development of high strength sour service grades with increasingly higher values of Specified Minimum Yield Strength (SMYS): 90 ksi, 95 ksi, 100 ksi and more recently 110 ksi. Today, a demand exists for Sour Service grades with 125 ksi SMYS for HPHT projects.
The paper first reviews the performances of a 110 ksi SMYS Sour Service grade developed in the 1990?s. In a second part the paper focuses on the SSC resistance of this same steel heat-treated to higher mechanical properties: 125 ksi SMYS. Various environments are explored according to the new edition of the NACE MR0175 / ISO15156 standard.
The challenge of obtaining High Strength Sour Service grades H2S is an acid gas that is quite commonly found in the fluids produced from Oil and Gas fields. It is responsible for a particularly dangerous form of corrosion known as Sulfide Stress Cracking (SSC), which can affect High Strength Low Alloy (HSLA) steels used for casing and tubing in the production wells. SSC resistance of such tubular goods is of particular importance for Oil and Gas companies since it directly involves the safety of their staff and assets.
The corrosive environments encountered in these H2S containing wells and susceptible to promote SSC on tubular goods, are the so called sour environments. The risk of SSC is induced by the presence of an acid water phase (produced or condensed water) containing dissolved H2S. HSLA steels that can resist SSC in such sour environments are called sour service grades.
The development of sour service grades has been challenged by the need to use these steels in increasingly deeper wells. Such deep, High Pressure High Temperature (HPHT) wells require tubes with high mechanical strength in order to withstand the reservoir pressures and heavy tubular string weights induced by well depths that are often in excess of 20,000 ft (6 000 m). An increase in mechanical properties of steel allows gaining strength while keeping wall thicknesses at a reasonable level.
At the same time, HPHT Oil & Gas fields often contain H2S which, even at relatively low levels (typically 10-100 ppm), can amount to quite high partial pressures, given reservoir pressures of the order of magnitude of 15,000 psi (1 000 bar). In these conditions, and since it is well known that the resistance of steel to SSC decreases when its mechanical properties increase, the challenge posed to the development of sour service grades is to combine high strength and SSC resistance.