Keywords: 
• potassium and calcium based cation /
• anti-collapse /
• anti-pollution /
• borehole stability

Abstract: The high mud making mudstone is long in Qingxi Oilfield,as well as hydration swelling easily emerges,which results in shrinkage and collapse of holes and collapse of hard and brittle formations in the Cretaceous system.In particular,it is difficult to tackle the problems that the drilling fluid may be polluted and the well may collapse during the transformation of conventional drilling fluid after air drilling.Based on the analysis of typical easily collapsed zones of the oilfield by conducting experiments on X-ray diffraction,expansion ratio,and decentralized recovery,the corresponding anti-collapse mechanism is obtained.Potassium and calcium based cationic drilling fluid is developed on the basis of the study on the atomization of air drilling and the compatibility of foam formulation.The formulation is optimized by carrying out recovery test and test on resistibility to salt-gypsum contamination.Field application shows that potassium and calcium based cationic drilling fluid features in good compatibility,strong anti-pollution capacity,high capacity to prevent from collapse,and excellent fluidity,and can fulfill the requirements of drilling construction and reservoir protection of Qingxi Oilfield.It can be further popularized.