Paste filling or Back filling is a method used to dispose of mine tailings underground, to backfill underground mine stopes, or to achieve both simultaneously.
One of the key difficulties in sustaining an effective paste fill operation is to maintain a serviceable conduit in which the paste material can travel from the above ground processing plant to the specified void or stope below surface without contamination. This is most often achieved by cementing a large diameter conductor casing from surface to underground within a borehole, then suspending a replaceable wear casing inside through which the paste material can travel.
In many cases we are contacted by a new client who is experiencing water diluted paste underground. Upon conducting a desktop review and downhole camera inspection survey, we quickly identify that there is no sacrificial internal lining installed and the main bore casing has been worn away exposing underground aquifers that are introducing excess water and diluting the paste.
A common occurrence in these replaceable wear casing strings when managed poorly is the premature failure of the replaceable casing and subsequent cementing of the string into the main borehole. This can make retrieval for replacement extremely difficult or impossible, forcing the need for a smaller diameter casing to be installed within the old string, or a new hole to be drilled entirely.
The premature failure is caused by the highly abrasive paste freefalling at high velocity which produces slurry erosion in localised areas of the string. Once this erosion pattern is established, a channel will form and promote further erosion in the same region. This channel will scallop extremely rapidly until the casing breaches – typically this will result in the paste fill casing being cemented into the outer casing. If removal is possible, it will usually reveal that the majority of the replaceable casing material shows little wear other than at the localised point of failure.
Click here to see downhole camera inspection footage of worn casing due to poorly managed paste fill operations.