New design approach on combination of liquid-gas flow rates of gas based fluid

During Underbalanced Drilling(UBD),the highest bottom hole pressure during normal drilling condition affects the degree of formation damage,the lowest bottom hole pressure during pipe connection controls severity of the wellbore damage.And bottom hole pressure is controlled by the combination of the liquid and gas flow rates on the ground.In order to minimize both the formation damage and wellbore damage problems,it is highly desirable to have an easy-to-use approach to generate balanced design of combinations of liquid and gas flow rates under given drilling conditions.Based on the four phases fluids dynimics model,the design method could convert security window of bottom hole pressure to the security window of combination of the liquid and gas flow rates on the ground.The method adopted Excel Sheet to protract the window,and could choose the optimum combination.In the liquid-gas rate window,formation fluid pressure limits the upper bound of the flowing bottom hole pressure and wellbore collapse pressure was the lower bound of the circulation-breaking bottom hole pressure.The window was closed by fluid’s cutting carrying capacity and wellbore washout criteria.And to verify the new approach,an illustrative example(SN4003) was presented in this paper to demonstrate how to use the new design method.this successful UBD field case was reviewed and compared with the optimum design results.Good consistency was indicated.