When drilling challenging formations such as very thick highly fractured sour reservoirs or carbonate/karstformations,a lost-circulation zone can be encountered.This causes mud to be lost and gas kick to take place,making the drilling process uncontrollable.Blocking or plugging wide fractures is impossible in many cases,which results in severe safety issues associated with toxic gases.This study investigates an application of mud cap drilling by injecting foam mixture into the annulus forwell control in such harsh conditions.An annular fluid column with foam mixture can be used to preventkicks and push the toxic gas back into the formation down along the annulus.This foam-assisted mud capdrilling process has been proved to reduce non-productive time and fluid expenses.This study presents how to model and simulate the process with accurate foam characteristics when foamsare used to suppress gas kicks under certain well and fluid conditions.More specifically,this study dealswith three scenarios:Base Scenario with a relatively short response time such that the injected foams do notcontact the formation gas,and Scenario 1 and 2 with a relatively long response time such that the injectedfoams interact with the gas,with and without foam coalescence respectively,at the foam/gas interface.Theresults show how mud-cap drilling parameters(such as pressure,foam density(or,equivalent mud weight),foam velocity,and foam quality)change at different operating conditions and scenarios.Non-Newtonianfoam rheology,depending on bubble size and bubble size distribution as modeled by Wang et al.(2017),lies at the heart of this simulation study.
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