Well Construction, Capping, and Abandonment Standards
Construction
The purpose of every construction standard is to keep the annulus sealed so that water cannot move between aquifers or travel down the outside of the casing. Key concepts: surface seal (grout or bentonite from the surface down to a required depth), annular seal continuing to the top of the producing zone, casing of adequate wall thickness and diameter, screen sized to the formation, and well development to clear fines. Casing must extend above grade a sufficient height, be capped, and be protected from surface drainage.
Capping
An inactive but not abandoned well must be capped - a secure, watertight cover that prevents entry of surface water, debris, and animals while preserving the well. Capping is not abandonment.
Abandonment
Abandonment is a construction activity requiring the same care as drilling. The well is filled so that it cannot act as a conduit: typically a plug or seal at the bottom, grout or approved fill placed by tremie or other approved method from the bottom up, and a surface seal. Casing may be removed or perforated to ensure the seal contacts the formation. The driller files an abandonment report.
Traps
(1) Thinking a steel plate over the casing equals abandonment. (2) Assuming a well that has not been used for years is automatically abandoned - legal abandonment requires the prescribed procedure and report. (3) Grouting from the top down, which bridges and leaves voids. (4) Forgetting to seal the annular space where an artesian flow could occur. (5) Ignoring artesian conditions that could cause uncontrolled flow during abandonment.
Rule of thumb
Cap it if you intend to keep it; seal it bottom-up if you intend to lose it.
Worked example
A 300-foot irrigation well is to be retired. The driller sets a plug near the bottom, tremies grout upward in lifts, perforates the casing at selected intervals so grout contacts the formation, completes a surface seal, and files an abandonment report. A cap alone would fail.