Hole cleaning is still among the most important problems to handle in drilling operations and one of the most studied phenomena within the petroleum industry. Insufficient hole cleansing can lead to stuck-pipe, excessive pump pressures and equivalent circulating density (ECD), pack-offs, lost circulation, etc. all of which again can lead to loss of sections or complete wells. Due to the number of parameters influencing the hole cleaning operation and the complex mechanisms involved, the phenomenon has not yet been fully understood.

The issue of transportation of cuttings along with drilling fluids is essentially a two-phase flow. Several other industries are dealing with similar issues when transporting solids using air (fluid) as the transporting medium. Although the transporting mediums are different, flow regimes and basic transporting mechanisms similar to hole cleaning can be observed also in pneumatic conveying of particulate materials. While horizontal and vertical pneumatic transport are quite often used, very rarely people try to use inclined pneumatic conveying for the very reason that it involves much higher pressure drop as compared to both horizontal & vertical conveying and is associated with the problems of particles dropping out of suspension unless a very high fluid velocity is used. Through an extensive laboratory study a simple model1,2,3 has been developed for pressure drop calculation in pneumatic conveying based on classical Darcy's equation with some modifications. The predicted pressure values match well with the test data.

This article has tried to look at the possibility of a unified approach for two-phase flow. An attempt has been made to relate the knowledge developed in the field of two-phase pneumatic conveying systems to the challenges associated with the hole cleaning mechanism. The paper also describes some of the work that has been performed for studying hole cleaning and the associated pressure loss prediction.

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