Pressure Observations and Water Densities in Aquifers and Their Relation to Problems in Gas Storage
Authors: Bond D.C. , Cartwright Keros 
Published: December 2007

Given here are criteria for flow in aquifers that contain water of varying density. In general, if we know the potential between two points we must also know the density of the water at all points along a given flow path between the two points in order to determine if water is flowing along the path. path. Introduction

M.K. Hubbert's classical papers on groundwater flow were published in 1940 and 1953. Since then many people have applied his concepts to a variety of people have applied his concepts to a variety of problems including entrapment of oil, underground gas problems including entrapment of oil, underground gas storage, and groundwater flow. Although Hubbert's original ideas have been extended into many areas of investigation, one important area still needs some study and clarification. This is in the treatment of problems in the flow of underground waters whose density varies from point to point because of changes in the concentrations of point because of changes in the concentrations of dissolved solids. McNeal touches upon this question. In a personal communication he gave a procedure for correcting potentiometric surface maps in basins where water potentiometric surface maps in basins where water density varies; a pressure correction, Delta p(s), is subtracted from recorded pressures.

(1)

where p(sw) and p(fw) are specific gravities of salt and fresh water, respectively, and delta Z is the vertical distance over which each type of water exists. Hitchon says, "Flow in variable density water systems, such as exist in most sedimentary basins, may be empirically represented by using a standard density. He describes a method that involves dividing the area into discrete density regions. Using a best-value density, hydraulic head maps are made for each region and flow lines are constructed within each region. If there is no opposed flow between any two regions of differing densities, it may be concluded that the flow paths may be empirically represented by using a standard density throughout the system. Hanshaw and Hill say, "One can attempt to correct all the potentiometric data for salinity variations, but this is a difficult task. Not only must one correct for the density of water in the aquifer at the well site, but one must also consider the integrated density of all water in that aquifer which is at higher points on the potentiometric surface". potentiometric surface". None of these authors give the basis for their statements about density corrections. The derivation of their pressure corrections needs to be outlined. Terms such as "integrated density" need to be defined. And the implications of these density effects, especially with respect to possibilities for flow, need to be clarified. Hubbert discusses this problem briefly. In particular, he shows how to handle the case of a basin in particular, he shows how to handle the case of a basin in which the salinity of the water increases with depth. Pressure and density measurements are taken in a row Pressure and density measurements are taken in a row of wells extending downdip from the flank of the basin. A well in the middle of the row is used as a reference well; the density of the water in this well is used as a reference density. At each of the wells, the potential of the water having the reference density is calculated on the basis of this density and the locally measured pressure. The plot of potentials calculated in this pressure. The plot of potentials calculated in this manner vs distance shows whether the water is static (minimum in curve) or flowing (direction of flow is tangent to curve).

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