I have written before about cementing casing. That is the most common use of cement in wells, but cement will fix many other situations. For example, if you have perforated casing and the formation no longer produces enough, you must seal the open perforation with cement before you drill down to the next aquifer. Also, if you discover a split in the casing, you need to similarly seal it.
The technique is called a “squeeze.” This where you squeeze cement out through a problem area and seal the problem. The important thing is to put the cement, under control, in the right place. I want to emphasize “under control” in this process. It does not include backing up a ready-mix truck and just dumping. Control of the cement is vital to a successful squeeze.