A Study of Vibratory Screening of Drilling Fluids

Abstract

This work presents results of an experimental and theoretical study ofvibratory screening of bentonite/water drilling fluids containing amedium-fine fracturing sand to simulate granular drilled solids. Satisfactory solids conveyance off the screen can be obtained for flat decks if the vibrator is placed to yield proper phasing between the normal and tangential acceleration components at all points on the vibrating deck. Introduction Drilling fluids returning from a well during drilling are passed serially through a number of solid/liquid separation devices to maximize removal of drilledsolids from the fluid before reuse. The vibratingscreen or shale shaker, the first device to process thefluid, removes the largest particles. Othersolids-separation equipment in the surface mud system can operate efficiently only if the shale shaker is performing properly. Consequently, it often is considered the most important solids-control device on a drilling rig. Most, if not all, of the various shaker designs now available have evolved from vibrating screens used in other industries, primarily the mining industry. Because of its obvious importance in such industries, vibratory screening has been the subject of numerous studies and investigations. On the other hand, very little investigative work has been published on vibratory screening of drilling fluids. The essential difference between this type of screening and that in other industries is that most of the material processed by a drilling fluid screen is liquid, rather than solid. For example, a well-maintained drilling fluid could have less than 10% tool solids by volume. Consequently, much of the existingtechnology for vibratory screening of dry solids, or even very wet slurries, does not apply toscreening of drilling fluids. The importance of this problem has been recognized by the IADC, which recently sponsored a conference on vibrating screen separators. Because the screen cloth itself is critical in determining the amount and size of solids removed by a shale shaker, the API has published a recommended practice for designating shale shaker screen cloth. More recently, Cagle and Wilder reported results of an experimental comparison between two commercially available shakers, primarily to evaluate a new design in screening cloth. Beyond this study, however, comprehensive investigations of the problem apparently have not been published. JPT P. 1889＾