Monotonical mechanical behavior of thermo-pressure-activated geopolymeric composites in the indirect tensile test using digital image correlation

Abstract

There is an increasing interest in alternatives to Ordinary Portland Cement (OPC) for plug and abandonment of oil wells, aiming to obtain more reliable and durable well barriers, with reduced CO2 footprint. Geopolymers can have lower permeability and higher chemical resistance to downhole fluids, compared to OPC-based systems. However, there are concerns about their mechanical behavior, in particular their brittle cracking in tension, which may lead to continuous flow paths for escape of fluids. Fibers can improve this behavior by arresting the growth of cracks, reducing their width, and distributing the strain over a larger area. This work aims to study the tensile behavior of a neat metakaolin-based geopolymer system formulated for well applications, before and after addition of 2%v./v. polypropylene fiber or 2%v./v. wollastonite. The indirect tensile test (ITS) was performed in 11.3 mm diameter cylindrical samples cured under high pressure to minimize macroscopic defects. We used digital image correlation (DIC) to monitor the samples at the millimeter scale, aiming to quantify the efficiency of fibers as crack arrestors. We demonstrated a procedure to obtain reliable and quantitative strain and stress fields for the ITS using DIC, enabling the investigation of local stress heterogeneity. First, a penalized spline-based smoothing scheme is used to compensate random errors and to regularize the strain field. Then, two different approaches are used to determine the average elastic properties and calculate the stress field. A three-dimensional finite element model was used to compare the theoretical strain distribution in the sample to the observable strains at the surface. The experimental results were closer to the simulation than to the analytical solution, which is more representative of the behavior inside the specimen. Hot spots of stress concentration were analyzed to obtain the monotonic stress-strain behavior of the material until failure, showing that local stresses were 33% larger that the macroscopic strength measured in the ITS. The neat geopolymer had higher stiffness and tensile strength, but the samples displayed surface delamination in compression, near the force application points. Wollastonite prevented this superficial phenomenon, but the samples had lower tensile strength. Polypropylene, on the other hand, was very effective in arresting crack opening, allowing significant post-peak deformation. These polypropylene-reinforced geopolymer systems are more suitable for downhole applications, since they can withstand higher strains before the formation of continuous cracks and subsequent loss of hydraulic isolation. The methodology proposed in this paper enabled us to observe and quantify the performance of the reinforcement.