Abstract

Kuqa foreland basin, which is located in the western of China, has the characteristics of HPHT with high CO2 partial pressure, and covered Yaha, Kela, Dina, Dabei, and Keshen gas fields. This story dated back to 2000 year that Yaha gas field was put into production, and the reservoir temperature 140°C, pressure 56 MPa, depth 4900-5300 m, and CO2 concentration 0.7-1.3 %. Carbon steel was selected for tubing material in the early stage of field development. After about 3 years, tubing perforation was caused by serious corrosion. Then carbon steel wasupgrade to 13Cr. However, it was found that there was serious corrosion in the connection part of tubing, and the corrosion was caused by the CO2 and condensate water.

Based on the experience in Yaha gas field, the modified 13Cr tubing was used in the Dina2 gas fields, and the reservoir temperature 140C, pressure 110 MPa, depth 5200 m, and CO2 concentration 0.26-1.02 %. Although the well condition is less harsh, serious corrosion still occurred concentratedly on tubing pin end. Considering premium tubing leak in Dina field happened during acidification operations, and the results of series simulation tests conducted, the understandings were achieved that acid will cause serious corrosion to the inner wall of tubing, aslocal corrosion is dominant factor of stainless steel.

The super 13Cr material was used in Keshen gas field which has reservoir temperature 150-188C, pressure 105-136 MPa, depth 6000-8038 m, and CO2 concentration 0.1-1.1 %. However, tubing fracture happened one by one, which originate from stress corrosion cracking caused by mixture of phosphate packer fluid and killing mud. Therefore, material selection needs to considerthe compatibility of different fluids, and formate was chosen as packer fluid. By December 2020,it has been used in 103 wells of Kuqa foreland basin, abnormal annular pressure is presented in 6 Wells, and the longest service time is six years.

As the rapid exploration and development of Kuqa foreland basin, the proper material selection become more difficult for gas reservoir temperature more than 190C and its pressure greater than 140 MPa, the past practices about material selection may provide the reference, and the story about material selection will be continued.

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