Weight of the cement grout slurry, and impact of the heat of hydration of the
grout on the outside of a partially water filled casing;
Extreme drawdown inside the casing caused by overpumping;
Forces associated with well development that produce large differential
pressures on the casing; and
Forces associated with improper installation procedures where unusual force is
used either to counteract a borehole that is not straight or to overcome buoyant
Of these stresses, only external hydrostatic pressure can be predicted and calculated
with accuracy; the others can be avoided by common sense and good practice. To provide a
sufficient margin against possible collapse by all normally anticipated external loadings, a
casing should be selected so that resistance to collapse is more than required to withstand
external hydrostatic pressure alone. According to Purdin (1980), steps to minimize the
possibility of collapse include:
Drilling a straight, clean borehole;
Uniformly distributing the filter pack materials at a slow, even rate;
Avoiding the use of quick setting (high temperature) cements for thermoplastic
casing installation;
Adding sand to cement to lower the heat of hydration; and
Controlling negative pressures inside the well during development.
Nielsen and Schalla (1991) provide a discussion on the physical strength of various
well casing and screen materials. Table 9 provides a summary of comparative strengths of
well casing materials.
Chemical Resistance Characteristics
Monitoring well casing and screen materials should maintain their structural integrity
and durability in the environment in which they are used over their operating life.
Monitoring well casings and screens should be resistant to chemical and microbiological
corrosion and degradation in contaminated and uncontaminated waters. Metallic casing and
screen materials are subject to corrosion, and thermoplastic casing and screen materials are
subject to chemical degradation by solvents. The extent to which these processes occur
depends on water quality within the formation and changing chemical conditions such as
November 1992
6 22






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