Reservoirs are naturally losing pressure as oil and natural gas is extracted, causing a slowdown in the flow of fluids out of the well. To counteract this problem, operators commonly enlist a practice called seawater injection. They will drill wells to inject seawater into the reservoirs to help increase the pressure and keep wells producing at a higher rate for longer.
Corrosion is a concern in the presence of water, especially in an offshore seawater environment where there is a higher potential for oxygen contamination. Offshore seawater injection projects require careful planning and selection of materials to ensure the integrity of the well.
PREN stands for Pitting Resistance Equivalent Number. It provides a relative comparison of the resistance of alloys to localized corrosion based on their chemical composition. Various empirical formulas have been developed for the calculation of the PREN. NACE MR0175 requires the use of the following equation:
PREN = CR% + 3.3 x (Mo% + 0.5 x W%) + 16 x N%
It is recommended that materials have a PREN > 40 for seawater injection applications. Such alloys include 25CR super duplex and CRA 2550E (see below).
To calculate a PREN number using the formula mentioned above, check out our PREN calculator here.
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