An Analysis and the Control of Lost Circulation

Abstract

Abstract During the drilling of wells, fractures which are created or widened bydrilling fluid pressure are suspected of being a frequent cause of lostcirculation. A study of the variables which are believed by the authors to berelated to fracturing led to the premise that the presence of an effective lostcirculation material in a drilling mud stream would serve to plug smallfractures at the moment they are encountered or created and thereby eliminate apossible cause of lost circulation by preventing fluid pressure from wideningand extending the fractures. A large number of lost circulation materials were classified and tested insimulated fractures and in shallow wells. Granular type materials with agradation of particle sizes were found to be the most effective for plugging.Within the limits of pump ability, the concentration in mud of each lostcirculation material tested was found to be a controlling factor in determiningthe maximum size fracture which could be sealed. A new type centrifugal sorterwas given preliminary tests and found to be capable of concentrating cuttingsfor rejection and reclaiming lost circulation materials. Introduction The control and prevention of lost circulation of drilling fluids is aproblem frequently encountered during the drilling of oil and gas wells. Lost circulation or lost returns may be defined as the loss to formationvoids of the drilling fluids or cement slurries used in the process of rotarydrilling and well completion. The loss may vary from a gradual lowering of mudlevel in the pits to a complete loss of returns. Not to be confused with lostcirculation is the continuous reduction in mud volume caused by the loss of thefluid phase of muds as filtrate and the solid phase as filter cake during thecontinuous filtration process which occurs on permeable formation surfaces. T.P. 3096