Don't think of minesweepers as "carving a path through the minefield"; they remove a specified number of mines, then become overwhelmed by the remaining mines in the sector.
Minesweeper pilots have one of the crappiest jobs in the galaxy when asked to go out alone...
They are inefficient in this way, and somewhat costly to build. Unless early in the game you should, as a rule of thumb, assume the field has 100 mines.