In respect of the coring difficulties, such as high circulation loss, low stability, encountered in observatory borehole drilling in loose backfilled rubbles with many cavities and high osmosis at the TALEGHAN Reservoir project in IRAN, a solid-free, non-dispersible. Drilling fluid is concocted with the KL vegetable colloid as the main additive, which, combined with sealing agents, has achieved good results at site. The composition of the KL colloid, properties of the drilling fluid and the sealing effectiveness are described, providing some experience for drilling in similar conditions.