Selection of material for downhole completion is a challenging process for appraisal wells, where the uncertainties associated with the reservoir parameters need to be weighted and economically reasoned. This is particularly true for high sour, high temperature, high-pressure (HS-HPHT) gas appraisal wells, where the probability associated with potential release of gases needs to be as low as possible, as the consequences of a release can be very high and potentially fatal.
A corrosivity assessment was carried out for the material selection of tubing and liner of a HS-HTHP gas well with H2S and CO2 in excess of 40% and 10%, respectively. Uncertainties in the simulations were related to the amount of water, and condensate. Previous reservoir data suggested that elemental sulphur would not be present. The well was completed with a 25Cr35Ni alloy for a flow test period of 1 year.
Test separator data revealed a minor amount of water. After the test was completed, the tubing string was pulled out of hole. Visual inspection showed scattered shallow pitting, with some connections revealing crevice corrosion.
This paper describes post-exposure analysis of the completion tubing, relating pitting and crevice corrosion with the fluid and reservoir conditions.
Results of this real-time study allowed setting the requirements and standards for selecting downhole materials, considering the presence of water and elemental Sulphur in the HPHT sour atmosphere downhole.
Furthermore, recommendations were incorporated in the test of FFP materials in such harsh conditions to test against crevice corrosion in challenging reservoir conditions, as a fundamental parameter in the material selection process for FFD. This study represents a valuable document in the current ADNOC strategy of attraction of business partners for development of new concession blocks, especially for HS-HPHT gas and condensate reservoirs.