In order to study the reasons for borehole instability, we calculated the mud density window for horizontal wells in complex formations based on logging data. From the results of the mud density window calculation, we found that for mudstone shales it decreases as the inclination angle and the azimuth increase, while for coal it took on negative values. We found that collapse and expansion are conducive to maintaining borehole stability, while in coal seams a positive mud density window can be achieved only by collapse (a decrease in the borehole diameter). It was determined that we need to increase the mud density from 1.2 to 1.4 g/cm(3) and use drill pipes that are as small as possible for the longest possible period of time. In this case, the borehole trajectory should be determined according to the seam location and the mud density.