Significance of Defects in Annular Well Seals

Abstract

The hydraulic significance of defects encountered in annular seals around well casings has been evaluated with numerical ground-water flow and transport models. Contaminant transport was modeled through intact and defective seals surrounding a monitoring well penetrating two permeable formations (aquifers) separated by a less permeable formation (aquitard). The lower aquifer was assumed to be clean initially, whereas the upper aquifer was assumed to be contaminated. The findings indicate that seals should be no more permeable than the least permeable formation through which the well penetrates. Also, seals need to be no longer than the less permeable formations through which a well penetrates, provided the hydraulic conductivity of the seal is equal to or less than that of the less permeable formaton. Occluded defects in the seal within a less permeable formation are hydraulically insignificant provided the less permeable formation is sealed at its upper and lower boundaries.