The geological conditions are very complex at the Second Coal Mine in Fuyuan County, Yunnan: the upper thick layer of limestone of the Yongning Town group is broken with falling stones and serious leaks, and cavities are also developed; while the lower coal stratum consisting mostly of water sensitive formation, such as mudstone, muddy siltstone, carbonaceous mudstone, is susceptible to hydrate expansion, contraction, and collapse, increasing the difficulty in borehole wall protection. Taking these factors into account, a solidfree drilling fluid with LG plant colloid is used to drill through a 6m thick coal without collapse against circulation loss, leading to good protection, and economical, safe, and fast completion of the well.