Print this pageGeological Criteria for Selection of Monitor Well Completion Methods
MATHEWSON, CHRISTOPHER C., and ERIC JOZEF TEN WOLDE
Geological criteria for the selection of annular seal materials is based on the shear strength between the PVC pipe and the seal material, and recoverability and expansion properties of the material. Pumpable bentonite grouts have shear strengths of 0.1 psi or lower. Chips have shear strengths of approximately 0.7 psi and pellets around 2 psi. However, when pellets were placed through water, they achieved shear strengths of only 0.8 psi. All bentonite samples had good recoverability, regaining their original shear strength once displacements stop. Cement-based samples had initial shear strengths of 4.7 psi or greater, however, after failure these values dropped significantly. Cements have poor recoverability. Bentonite pellets were found to have a greater reserve expansion than bentonite chips, however, when placed dry and subsequently hydrated, bentonite chips showed greater expansion than pellets, because rapid slaking prevented hydration of the remaining bentonite. When placed in standing water, pellets showed greater expansion, because they adhered to the sides of the container, ensuring a larger volume of water to a greater surface area of bentonite. Pumpable grouts are best suited for pressurized (artesian) well conditions or in conditions where subsidence is anticipated, semi-rigid bentonite seals (chips and pellets) are best suited for wet hole conditions or in environments, such as karst limestone, where some vertical movement may occur and cement base seals are best suited for dry strong-rock environments.