Abstract: The self-absorption water ability of shale formation can be reduced because of foam structure,but it can't stop the trend of shale hydration which increases over time and finally causes the wellbore instability.Therefore,adopting appropriate evaluation methods to seek the excellent shale inhibitors becomes the focus of people's attention.Through analyzing the wellbore instability process caused by foam fluid,the performance evaluation of foam fluid should consider two aspects,shale self-absorption water ability and shale inhibition.The process of shale self-absorption water is the necessary prerequisite to shale hydration,and the shale self-absorption water test contributes to optimize foam drilling fluids which have good sealing capacity or slow the self-absorption water speed.Bentonite inhibition,hot rolling dispersion test and bulk hardness test are best choice used in shale inhibition evaluation,and help to select the foam drilling fluid that has strong and permanent shale inhibition in shale hydration.