A fitness for purpose stress corrosion cracking (SCC) assessment was performed to evaluate the suitability of a 13Cr-5Ni-2Mo 110 ksi (758 MPa) grade martensitic stainless steel as a potential well tubing material for oil and gas production from deep water reservoirs. Conditions were chosen to reflect those expected in producer wells, including the possible presence of H2S as a result of reservoir souring due to seawater flooding. A detailed analysis of existing martensitic stainless steel test data for sulphide stress corrosion (SSC) and SCC revealed the need for a qualification test program to fully cover the conditions identified.
Both static Creviced C-ring and dynamic Cyclic Slow Strain Rate (CSSR) tests for assessing SCC resistance were undertaken in concentrated brine at 200°F (93°C). The test program and the results obtained are detailed and compared against existing data, thereby further defining the safe operating envelop for SCC and SSC of 13Cr-5Ni-2Mo 110 ksi grade martensitic stainless steels in environments that are outside of current experience.
For standardization of production tubing materials used for two reservoirs in a deep water field: Reservoir A and Reservoir B, two materials were identified as candidates for use: 13Cr-5Ni-2Mo 110 ksi (Super 13Cr-110) martensitic stainless steel and 22Cr duplex stainless steel (UNS(1) S32205). Assessment with existing laboratory test data and field experience suggests the 22Cr duplex stainless steel is qualified for use for the given well conditions. However, it is twice as expensive and requires much longer lead time to order than Super 13Cr-110. Therefore, if safety and/or integrity cannot be compromised, Super 13Cr-110 is the preferred choice.
In evaluating Super 13Cr-110, two environments must be considered to properly address environmentally assisted cracking (EAC). A condensed water environment at wellhead during well shut-in is used to evaluate SSC at low temperatures. A production environment at the bottom-hole for a particular reservoir was used to evaluate SCC at elevated temperatures. Design conditions for each of the two reservoirs are presented in Table 1, and the main parameters of consideration are temperature, pressure, H2S, and CO2 contents in gas phase and chloride content and pH in the water. Localized pitting is also evaluated as this is associated with the onset of cracking.