Experimental study on typical characteristics of borehole breathing under different pressure and rock type conditions

Borehole breathing observed while drilling naturally fractured formations was often mistakenly interpreted as the influx of formation fluid or the loss of drilling fluids. Amongst several mechanisms that were caused for borehole ballooning, the opening and closing of finite fractures played a major role in naturally fractured formations. In this study, a novel experimental setup was designed to simulate the borehole breathing under different conditions. In this experiment, the typical characteristics of borehole breathing event under different pressure conditions and rock types were studied. Additionally, the influence of three important parameters on the cumulative loss of mud caused by borehole breathing was also investigated in detail. The results show that mud circulation pressure, fracture opening pressure and rock type had important influences on the maximum loss of mud and the speed at which the borehole breathing event occurs of borehole breathing, and were the key parameters for predicting the cumulative loss of mud. Based on analysis of the results, a new parameter, breathing coefficient ab which can be used to evaluate the borehole breathing event, was proposed, and its magnitude was proportional to the severity of the borehole breathing. This study provides theoretical support for the study of borehole breathing.