THE RESEARCH OF FEATURE OF FORMATION FLUID INFLUX DURING UNDERBALACED DRILLING

Underbalanced drilling has been more and more utilized to minimize problems associated with invasive formation damage which often greatly reduce the productivity of oil and gas reservoirs.However,in many underbalanced drilling operations,additional benefits are not seen due to the difficulty of safely controlling the bottom hole pressure difference.Damage due to poorly designed and/or executed underbalanced drilling programs can rival or even greatly exceed that which may occur using a well_designed conventional overbalanced drilling program.A method for calculating the bottom hole flowing pressure during underbalanced drilling has been developed which takes into account all important factors contributing to this process.This transient,one_dimensional multi_phase flow method is closer to the true than conventional ones which consider the constant formation thickness as an assumption during drilling.Taking into account the changes of the formation thickness in calculating the BHFP,in addition author put forward the original viewpoint that keeping constant gas invasive velocity is safer than the traditional one that keeps the constant bottom hole pressure.We can safely draw the conclusion that it is possible to calculate and control the volume of gas influx if the method discussed in this paper is used during underbalanced drilling.