The use of CO2 rich gas mixtures as injection fluid is nowadays considered in order to enhance the oil recovery process. Flexible pipes, which have been used to transport oil and gas in offshore applications for more than 35 years, are attractive products for the transportations of such fluids. The CO2 content encountered for these applications can be higher than 90%. This value being superior to what is found in standard applications (<15%), and CO2 being known to behave as a supercritical fluid as soon as the temperature and the pressure are higher than 31.1°C, 72.8 atm, the compatibility of the pipe materials with the CO2 rich gas mixture had to be verified.
This paper will present the qualification work carried out so far on several thermoplastic materials used for flexible pipe pressure sheathes and stainless steel carcasses. This qualification work covers application as high as 90°C/600bar/100%CO2.
The formation of blisters (blistering) is known as a possible degradation mode of thermoplastic materials when subjected to rapid gas decompression. This phenomenon is directly linked to the fluid solubility in the polymer. Different materials have been tested for their ability to withstand rapid gas decompression in CO2 rich gas mixtures.
Long time exposure tests have been carried out. The objectives of these tests were to check the impact of CO2 on the physical, mechanical and chemical properties of the polymer grades. A special attention has been brought to the plasticizer effect of CO2. The impact of supercritical CO2 on the hydrolysis degradation mode has also been assessed.
Finally, CO2 being a small molecule, it will permeate through the pressure sheath to reach the annulus. The metallic layers which are constitutive of this annulus are potentially sensitive to corrosion phenomenon in acidic conditions. A good knowledge of the permeation properties of the polymer materials when subjected to high pressure CO2 is necessary in order to assess the severity of this annulus and to select the appropriate steel grade. The results of such tests will also be presented.