Fluid Density and Gravitational Variations in Deep Boreholes and Their Effect on Fluid Potential

Abstract

ABSTRACTAs depth below the water table increases, there can be significant spatial variations in fluid density. Fluid density is a function of fluid temperature, total dissolved solids and gases content, fluid compressibility, and the force of gravity. Variations in fluid density can affect the applicability of measured water levels to represent fluid pressure at depth and the existence of a scalar fluid potential. Where ground water is sufficiently heterogeneous, fluid pressure and fluid density must be known spatially to properly determine the three‐dimensional impelling force per unit mass E =— [gk + (1/p) ▽P]. Assuming that the water‐level elevation in deep boreholes represents a fluid potential may result in significant errors in determining flow directions and quantities under conditions of a variable density fluid and low hydraulic gradients. Boreholes constructed to investigate the feasibility of deep geologic disposal of nuclear waste have penetrated to depths greater than 1,500 m below the Earth's surface. In these deep boreholes, fluid density variations may need to be considered as a part of the hydrologic analysis. Fluid density variations with depth, predicted as general cases based on simple models, indicate the relative importance of temperature, compressibility, and gravity variations at three potential high‐level nuclear waste repository locations. Fluid density generally decreases with depth for sites where geothermal gradients are greater than 20°C/km, and thermal expansion can offset the effects of fluid compressibility.