Determination of optimal conversion time from air drilling to mist drilling in Xushen Gasfield

When air drilling is adopted, mud rings tend to be formed from debris, hindering the circulation in annulus after formation water breakthrough. To deal with these problems, it is necessary to determine the conversion time from air drilling to mist drilling. Therefore, the critical formation water production and the optimal conversion time were investigated and tested. A series of ground experiments were carried out on the dried rock samples taken from different horizons during air drilling, and they are air atomized water experiment, debris water absorption experiment, core water absorption experiment and morphological change experiment after debris water absorption. And accordingly, the morphological change after the debris were absorbed with water was understood. It is indicated that debris agglomeration is the dangerous state of air drilling after water breakthrough. Finally, a case study was performed on an open hole interval of 1 000 m long where air drilling was carried out. Experimental data analysis results show that the air drilling shall be converted into mist drilling as soon as the formation water production rate reaches 0.51 m³/h after free water occurs in clearance pipelines. Field tests indicate that the conversion time from air drilling to mist drilling determined by means of experiments can be used as the effective guidance for safe air drilling in Xushen Gasfield.