Calcium- Silicate-Cement Based Blends with Natural Zeolites - Self-Healing Performance under Conditions of High-Temperature Geothermal Wells

摘要

Geothermal cements are subjects to thermal and mechanical shocks, high temperatures and aggressive environments that can cause cement damage and compromise well integrity, contaminate underground waters and, if not repaired in time, lead to the well collapse. Repairs of the damaged areas are problematic for subterranean constructions. Self-repairing cements are especially attractive for such applications. This work focuses on calcium-silicate and calcium-aluminate-based cement blends with natural zeolites for enhanced compressive-, bond-strength recoveries and fractures sealing after imposed damage. Three blends of 1) Ordinary Portland cement (OPC) clinkeratural zeolite ferrierite (FER) (commercial name: FlexCem); 2) OPC and natural zeolite clinoptilolite (CLIN); and 3) calcium-aluminum cement-fly ash F-sodium metasilicate with CLIN were tested after initial 1 d-curing at 270 or 300°C followed by imposed compressive damage and additional 5 or 10 d periods under the initial curing conditions for recovery of mechanical properties or cement-carbon steel bond strength by compressive or lap-shear bond strength measurements. The phase compositions and their transformations were studied with x-ray diffraction, differential thermogravimetric analyses, Fourier transform infrared analyses and scanning electron microscopy coupled with energy dispersive x-ray spectroscopy. Zeolites decomposed under high-temperature conditions with release of hydrolysates that along with the hydrating cements participated in the formation of new phases contributing to strength and bond recoveries. These phases included crystalline magnesium and aluminum-containing silicates, calcium and carbonated calcium silicates and amorphous hydrates. The main sealing phases included crystalline and amorphous silica, high-temperature-stable zeolites and talc mineral. Several of the tested formulations showed compressive strength recoveries of more than 100% after repeated damage under the test conditions.