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

forces.

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

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