The detailed laboratory studies have been carried out with the aim at restricting the spontaneous imbibition and penetration of drilling fluids in tight sand gas and BCGA reservoirs. The measurements were extended to natural cores having permeability between 1 and 0.001 mD. It was found that because of the unusual pore structure and water-wet character of the cores extremely high capillary forces predominantly determine the flow and spontaneous imbibition phenomena in natural cores obtained from unconventional gas reservoirs. Based on the experimental results, it was clearly indicated that the water is a natural blocking phase, causing serious formation damaging hard to cure. Consequently, the fluid penetration from drilling fluids can be modified only by surface (interfacial) tension lowering and wettability alteration (from strongly water-wet to intermediate or oil-wet state). In addition, proper selection the cake forming components may significantly decrease the liquid uptake in tight, low permeable sandstones. The laboratory studies proved that using special surfactants, organic water miscible solvents and special high molecular weight additives the spontaneous imbibition and penetration of liquid phase of the water-based drilling mud can significantly be influenced in unconventional gas reservoirs mitigating thus the formation damage often encountered in tight sands and BCGA.