The application of calcium polysulfide to increase corrosion resistance of the timbering of wells

The cement slurries used in well casing should ensure the tightness of the lining and protect the casing from aggressive formation fluids. At the same time, cement and water based solutions are used for cementing, with a water-cement ratio of 0.45–0.9, which predetermines their high porosity and low corrosion resistance. The technologies for increasing the durability of concrete used in con-struction practice when fixing wells are not acceptable. One of the most aggressive components of reservoir fluids is hydrogen sulfide, depending on thermodynamic conditions, can be both dissolved and gaseous. The analysis of various types of corrosion of cement stone shows the ki-netics is determined by the diffusion of aggressive ions into the stone. To reduce the porosity of the cement stone, calcium polysulfide has been proposed, which can precipitate in the pores of the stone during hardening of the cement slurries, clogging the pores, reducing their size, slowing down the diffusion rate of aggressive ions into the cement stone. In addition, calcium polysulfide adsorbing in the pores on cement hardening products will ensure the inhibition of pore space. Three methods of calcium polysulfide injection into the cement slurries were tested: into the tempering fluid (water), directly into the prepared cement slurry and dry method, through cement powder, which was subsequently subjected to disintegrator treatment. The results of the experiments showed calcium polysulfide make an impact on the rheological properties and pumpability of cement slurries, increasing the strength of the obtained stone and reducing its permeability. The stability of a stone in an acidic environment was estimated by its corrosion by one molar hydrochloric acid. The depth of stone corrosion with increasing concentration of PSS decreased from 30 to 6 mm in 21 days. When testing the cement stone under the action of dissolved hydrogen sulfide for 45 days, it was shown that the increased concentration of calcium polysulfide from 0 to 5% reduced the corrosion depth from 5.0 to 0.3 mm, depending on the technology of adding the reagent. Micrographs of cement stone samples proved that calcium polysulfide, being distributed in the pore space, blocks the surface of the cement stone, limiting the contact of aggressive ions with hardening products.