Browsing by Subject "Alkali-activated materials"

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An evaluation of the self-healing capabilities of fly ash-based geopolymers
(2020-12-05) Ross, John Henry; Juenger, Maria C. G.; Oort, Eric van
From a drilling perspective, forces in the downhole environment of the wellbore such as tectonic, thermal, and operationally induced stresses can often cause damage to the cement sheath, leading to a loss of zonal isolation due to small fractures that form in the cement sheath along the length of the well. A self-healing cementing material offers the ability to close small microcracks that inevitably form due to the aforementioned stresses in the wellbore, thereby maintaining hydraulic isolation and preventing continued crack propagation with associated gas migration to surface. One such material previously shown to self-heal is a geopolymer. Geopolymers are alkali-activated cementing materials that have been demonstrated to achieve autogenous self-healing through compressive strength regain. Geopolymers are made from aluminosilicate precursors, such as fly ash, and a hydroxide activator and are cured at elevated temperatures. They can be used as alternatives to traditional portland cement. The goal of this work was to confirm autogenous self-healing of a Class F fly ash-based geopolymer material and evaluate the extent of self-healing. Self-healing was evaluated through unconfined strength regain and permeability recovery using unconfined compression and pressure-transmission testing. Samples were stressed after an initial period of curing and then allowed to heal, before being retested for a final time to determine the extent of self-healing. Initial tests used compressive loading to pre-load the samples to 30, 50, and 70% of their compressive strength at 7 days. Samples were allowed to heal for 21 days before being re-tested to failure. Results unfortunately showed too high of an uncertainty to draw any conclusion about the occurrence of autogenous self-healing. A thermal shock procedure was then developed using liquid nitrogen as an alternative way of micro-cracking the samples. Micro-computed tomography confirmed that the procedure caused micro-cracking in a 25 mm (1 in.) diameter core sample. A baseline pressure transmission test for geopolymers showed that the average permeability of 28-day old specimens was 0.26µD. Pressure transmission testing was then used in a study of self-healing of geopolymers. Results showed that after an initial permeability increase caused by thermal shock damage at 7 days, permeability values for three different samples decreased after a healing period, suggesting that self-healing had indeed occurred. It is the first time such permeability re-healing has been observed and quantified using an appropriate experimental technique. The results of this study are particularly important in the quest for permanent well abandonment and decommissioning solutions that can be relied upon for long periods of time (e.g. hundreds of years and longer).
Early-age shrinkage of alkali-activated Class F fly ash and portland cement for long-term oil well zonal isolation control
(2019-05) Olvera, Raul; Juenger, Maria C. G.; Oort, Eric van; Panchmatia, Parth
Volumetric changes due to shrinkage occur during the hydration/polymerization process of ordinary portland cement (OPC) and alkali-activated Class F fly ash systems. If not accounted for in the design of structures, especially those at elevated temperature and pressure conditions, shrinkage can develop internal stresses, which could generate micro-cracking. Specifically in oil and gas wells, shrinkage can compromise the hydraulic annular and result in loss of zonal isolation. This thesis compares the early-age shrinkage behavior of Class H OPC, sodium hydroxide-activated Class F fly ash (geopolymer), and geopolymer-hybrid (geopolymers incorporating drilling mud) slurries with up to 20% (by volume) synthetic based mud (SBM) contamination cured at different temperatures and pressure conditions. A study into the effectiveness of zinc- and aluminum-based expansive agents for shrinkage mitigation is also presented. Shrinkage was recorded for specimens cured at temperatures between 23 - 80°C and up to 2000 psi confining pressure. The results from testing indicate that A) temperature plays a major role in the shrinkage development of OPC and geopolymer slurries; B) pressure significantly increases OPC slurry shrinkage, but only minimally for geopolymers; C) addition of SBM increases shrinkage for OPC and geopolymer slurries; D) expansive agents can potentially counteract shrinkage in both OPC and geopolymer slurries. In addition, issues with shrinkage measurement at elevated temperature and pressure with current ASTM and API shrinkage tests are presented, along with a novel shrinkage measurement method avoiding these issues.