Avaliação de pastas de cimento reforçadas com fibras de basalto moídas para aplicação em poços de petróleo

Abstract

The dispersion of short fibers to oil well Portland slurries may improve the compressive strength and fracture energy of the hardened cementing material. This study aimed to investigate the effect of the addition of milled basalt fibers to Portland slurries. The fibers were added at varied average sizes and obtained by ball mill. Initially, samples were prepared with milled basalt fibers 5% (BWOC) and silica flour 40% (BWOC) in the composition, cured for 7 days under different Bottom Hole Static Temperatures (BHST): 80 °C (176 °F) and 300 °C (572 °F). Subsequently, samples were prepared with milled basalt fibers 5% and 10% (BWOC), and different types of silicas, silica flour and/or metacaulim, in contents of 30%, 35%, 40% and 45% (BWOC) in the composition, cured for 7 days under BHST 80 °C. The characterization of the additives was carried out by X-ray diffraction, scanning electron microscopy, X-ray fluorescence and thermogravimetry. The mechanical properties and the microstructure of the hardened pastes were evaluated by compressive strength tests, X-ray diffraction and scanning electron microscopy. The results showed that milling basalt fibers was efficient method to adjust the length of the basalt wool fibers, assuring slurry mixing and, therefore, adequate pumpability. In addition, the surface roughness of the fiber obtained after milling may be responsible for better fiber-matrix adherence. The combined additions of silicas and basalt fibers improved the fracture energy of samples cured at 80 °C, therefore below the strength retrogression temperature. The samples with milled basalt fibers 10% (BWOC) and silica flour 30% (BWOC) in the composition revealed a material with high fracture energy and compressive strength, which resulted in an increase from ~ 36 J to ~ 52 J and ~ 28 MPa to ~ 37 MPa, respectively. Curing at 300 ºC resulted in significant fiber consumption by pozzolanic reactions that could not be prevented by the addition of silica flour. Therefore, ball-milled basalt fibers can be a cost-efficient and environmental-friendly solution, to improve the mechanical properties of oil well cement slurries used below the retrogression temperature.