Research of cement slurries for sealing casing strings in underground hydrogen storage facilities in depleted hydrocarbon reservoirs

The article presents issues related to the technology of cement slurries for sealing casing pipes in underground hydrogen storage facilities in depleted hydrocarbon reservoirs. Ten recipes of slurries containing various ingredients (including nanomaterials, i.e. nano-SiO2, nano-Al2O3, latexes, high-molecular polymers) were selected for laboratory tests. The tests were carried out at a temperature of 60 degrees C and a pressure of 25 MPa, using defoaming, fluidizing, antifiltration admixtures and setting time retardant in the slurry compositions. The tests were carried out on two types of cement: Portland CEM I 42.5 and class G drilling cement. Technological parameters of fresh and hardened cement slurries were determined by examining the following: density, water retention (free water), rheology, thickening times as well as compressive strength, porosity and hydrogen tightness of cement cores. The liquid cement slurries had the correct technological parameters (they were well pumpable under HPHT conditions and their densities ranged from 1.80-1.91 g/cm(3)). The compressive strength of cement stones in the period from 2 days to 28 days of hydration, especially for samples with the addition of nanocomponents, was very high (after 28 days exceeding 40 MPa). The samples of cement stones had a very low content of capillary pores, which limits the possibility of forming channels in the cement sheath of the borehole. For most samples, the smallest pores (below 100 nm) accounted for the vast majority (over 95-97%) of the total number of pores in the cement matrix. The most favorable technological parameters were obtained for samples containing nano-SiO2 (nanosilica) and the optimal water-cement ratio for such slurries was around 0.46-0.48, depending on the type of cement used. The lowest hydrogen permeability values were obtained for formulations containing nanosilica (nano-SiO2). Recipes offering the best technological parameters, containing nanocomponents (after detailed tests), may be used when sealing casing pipes in holes drilled for hydrogen storage.