Abstract: Fractures in the Bekhme carbonate reservoir, the main production layer of oilfield A in Iraqi Kurdistan, have caused a reduction in the formation pressure over the last few years. As a result, downhole failures such as drilling fluid loss and reservoir pollution are often encountered, which causes there to be a low rate of penetration (ROP), low levels of production, high water cut, etc. We studied a new under-balanced drilling technique of injecting nitrogen into the annulus with crude oil as the drilling fluid. Different from the traditional mode of gas charging, nitrogen was injected into the wellbore from the annulus and directly flow back to the wellhead along with drilling fluid in the wellbore. In this process, nitrogen only participated in partial circulation of drilling fluid, and the pressure of the drilling fluid column in the bottom hole was reduced after that in the upper wellborehole was lowered, thus realizing under-balanced drilling without affecting downhole measurement while drilling (MWD) and its message transmission. This technique has been successfully applied to 6 wells in the oilfield without any downhole failures such as lost circulation, well kicks and wellbore instability, enabling drilling while producing. Compared with conventional drilling techniques, the ROP is greatly improved and single-well production increases 1 to 3 times. Research and applications demonstrate that the under-balanced drilling technique of nitrogen injection into crude oil enables safe drilling in mature oilfields characterized by low formation pressure and narrow safe density window, and improves oil recovery and exploration.