| Title | Utilization of PVA Flakes in Promoting Self-Degradation of Temporary Cementitious Fracture Sealing Material |
|---|---|
| Authors | Sugama, Toshifumi; Pyatina, Tatiana |
| Year | 2014 |
| Conference | Geothermal Resources Council Transactions |
| Keywords | Temporary cementitious sealer; polyvinyl alcohol; self-degradation; slag; Class F fly ash; sodium metasilicate; gel; sol |
| Abstract | This paper describes the effect of flakes of polyvinyl alcohol (PVA) on the self-degradation of sodium metasilicate (SMS)-activated slag/Class C fly ash cement containing PVA fiber-bridging additive. The cement is used as temporary sealer for fractures in Enhanced Geothermal System (EGS) wells under a hydrothermal environment at ?150°C and at pressure of 1000 psi. The reactions between PVA and SMS dissolved in an aqueous medium at 85°C led to the formation of a colloidal hydroxylated silicatecross- linked PVA gel in the cement body. The cement with the incorporated gel had a compressive strength >2000 psi, so ensuring adequate plugging of fractures. Increasing the hydrothermal temperature to ?150°C triggered the transformation of the gel into a sol. This in-situ gel?sol phase transition played a pivotal role in promoting the cement?s self-degradation because of the extensive spreading of sol in the cement body, and its leaching from the cement. PVA fibers present in cement also were simultaneously decomposed. The magnitude of degradation depended primarily on the content of PVA flakes and hydrothermal temperature; presence of PVA and elevated temperature were responsible for the conversion of bulk cement into fine particles. Adding 7.5 wt% PVA to cement resulted in cement crumbling at 200oC into small particles of 4.75-0.6 mm size (<35 wt.% of cement) and fine particles, ranging from 0.3 to 0.05 mm a size (>65 wt% of cement). More than 85 wt.% of this crumbled cement were dissolved in 7.5 % HCl at 90°C after 48 hours. The major oxide compound of insoluble remnants was silica, and its dominant particle size was 0.05 mm. |