The role of shear dilation in the stability of boreholes in unconsolidated sands

Abstract

Boreholes are commonly used in the shallow and deep subsurface for the extraction or injection of fluids, e.g. for drinking water production, geothermal energy extraction or fluidic waste disposal. Many boreholes have been drilled successfully in unconsolidated sediments, however the common approach to evaluate borehole stability predicts failure when boreholes are drilled according to standard practice for groundwater wells, i.e. with little overpressure (< 40 kPa) in unconsolidated sand. To explore this mismatch, this study investigates the role of shear dilation in explaining the existence of stable boreholes in unconsolidated cohesionless sand through experimental and numerical analyses. Firstly, consolidated drained triaxial tests along different stress paths are performed on proxy and cored sand specimens. The results demonstrate the strong and immediate dilative behaviour of sand subjected to lateral unloading. Subsequently, a calibrated SANISAND material model is used in the numerical analysis of a cavity contraction problem. The results indicate that the amount of deformation and extent of a hoop stress reduction zone depend on the initial state of the sand. The early onset of irreversible deformation observed in both the experimental and numerical findings suggests that the standard practice for determining the minimum borehole overpressure is too conservative for unconsolidated sands. It is concluded that the initial state of the sand and associated shear dilation plays a prominent role in the stability problem of boreholes drilled with overpressure p_w ∼ 10 kPa.