AbstractsEarth & Environmental Science

Proper completion of a well is essential to the protection of ground-water resources. Selecting a proper material for the annular seal should be based on the geologic conditions at the well site. Previous studies have not adequately linked annular seal material selection with geologic variability. This study created unique geological-based criteria for the selection of annular seal materials. The shear strength with which the annular seal materials held the PVC well pipe was tested, as were the expansive properties of annular seal materials. Both bentonite and cement seals were tested. Unidirectional swell and reserve expansion of bentonite was also examined. Bentonite pellets were found to have a greater reserve (potential) expansion than bentonite chips. However, when emplaced dry and subsequently hydrated, bentonite chips showed greater unidirectional expansion than pellets, because the slaking of pellets caused them to quickly fall apart and seal the remaining bentonite from the water supply. When emplaced through a standing column of water, pellets showed greater unidirectional expansion, because they adhered to the sides of the container, ensuring a larger volume of water to a greater surface area of bentonite. Pumpable bentonite grouts were found to have shear strengths of 0. I psi or lower. Chips were found to have strengths of approximately 0.7 psi and pellets around 2 psi. However, when pellets were emplaced through water, they achieved shear strengths of only 0.8 psi. All bentonite samples had good recoverability. Cement-based samples had initial shear strengths of 4.7 psi or greater. After failure these values dropped significantly, they are therefore considered to have poor recoverability. 'Me performance of various annular seal materials was related to rock strength, hole condition, and hydrologic condition to devise a material selection technique. Pumpable grouts are best suited for pressurized (artesian) well conditions because their weight counters the upward pressure of water; semi-rigid bentonite seals (chips and pellets) are best suited for wet hole conditions because when hydrated these materials form a low permeability seal with moderately high shear strength; and cement base seals are best suited for dry strong rock environments.