Analysis on borehole stability of gas drilling with complete stress–strain and thermal stress theory

In conventional mud drilling, sufficient drilling fluid density is required to maintain wellbore stability. However, in gas drilling, the support ability of gas in wellbore is very weak, and the wellbore stability is usually good. This shows that the borehole instability mechanism of gas drilling is different from that of mud drilling. In gas drilling, the borehole wall rock still has a certain bearing capacity after reaching the peak strength. In this paper, the stability of gas drilling wellbore is studied by using the complete stress–strain model. The deformation of rock around borehole in gas drilling can be divided into elastic zone, plastic zone, and broken zone. The radii of plastic zone and broken zone were used to evaluate the stability of borehole wall. This method has a good general applicability to the case of no thermal stress in borehole wall rock. However, due to throttling and cooling as gas flows out of the bit nozzle during gas drilling, thermal stress occurs when the cryogenic flow in the annulus meets the hot formation. Therefore, thermal stress is introduced into the complete stress–strain calculation model. The radii of plastic zone and broken zone are calculated according to the actual drilling data, and compared with the measured borehole diameter. The results show that the model is reasonable and effective. Analysis shows that in gas drilling process, the borehole temperature is much lower than the original formation temperature near the bottom of the borehole, and the borehole temperature is higher than the formation temperature in the upper part of the borehole. Thermal stress has a significant effect on borehole stability. The thermal stress in the bottom borehole makes the rock shrink and offset the expansion of the original in situ stress on the surrounding rock, thus enhancing the stability of the borehole wall. The surrounding rock in the top borehole expands under the action of thermal stress, enhancing the expansion of the surrounding rock toward the borehole and aggravating the instability of the borehole wall. The model can be used to better explain the wellbore instability mechanism of gas drilling, and it is more intuitive to use plastic zone radius and broken zone to characterize wellbore stability of gas drilling.