Numerous challenges exist in Total's Central Graben Area (CGA). The main reason being that they present extremely "hard" conditions: high temperature (with 195°C to 225°C due to reverse Joule Thomson effect), high pressure (with 1,100 bar), high salinity (with more than 250g/L in TDS, more than 20g/L in calcium and circa 450mg/L in iron) along with several scale types (with sodium chloride, calcium carbonate, zinc sulphide and lead sulphide).

In the wells of CGA fields, the highest scaling risk is the formation of sulphide scale in the subsurface safety valve region. Commercially available scale inhibitors have proven incapable to perform under such "hard conditions" (see SPE 173761).

This paper describes the design process of suitable new chemicals (scale inhibitors and nanoparticles scale inhibitors) for both downhole continuous injection and squeeze treatment applications.

Such compounds are based on cationic polymers and sulphonated anionic polymers. A chemical approach has been used for the synthesis of silica nanoparticles. The sol-gel method, in addition to its low cost, allows controlling both the size and morphology of the particles by varying certain parameters of the reaction. Extensive laboratory tests have been performed for the validation of these products; these tests include scale inhibitor/brine compatibility, static and dynamic tests, thermal ageing, post ageing analysis and performance tests as well as coreflood tests using real core from the CGA formations. These laboratory testing have allowed the tuning of the chemical design of these novel products in order to improve the performance and the thermal-stability.

This paper describes the considerable advancement in chemical performance under these extreme conditions, including specific test development for lead sulphide which has to date proven more difficult than other sulphide scales to assess under field representative conditions in the laboratory. The newly developed chemicals are now ready for trial on the field.

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