An Experimental Screening Study for Mud Selection in Permafrost
Volume 1, Number 4
Umiat field is located in the North Slope, Alaska where most oil reserves are within the permafrost. The formation temperature is around -6.67°C (20°F) and thus frozen filtrate that forms an impermeable barrier to oil is a crucial issue in formation damage. Its impact is even more pronounced in horizontal wells due to extended contact time and large contact area. Thus, selection of a proper mud type with particular properties is of utmost importance for successful drilling operations. The present paper summarizes a comprehensive experimental investigation of various drilling fluids to eventually select, pro-pose or design a non-damaging fluid system that is appropriate for permafrost drilling operations. Specially formulated, free-solid mud systems including water-based mud, water-based mud (WBM), brine mud, native crude oil-based mud, oil-based mud (OBM) and synthetic oil mud were evaluated. The evaluation included rheological properties, filtration properties, clay swelling properties and potential freezing of fluid filtrate. The results show that, even though clay swelling is not a major contributor to formation damage due to rock mineralogy, WBM drastically damages the formation due to freezing of water filtrate. On the contrary brine, in spite of its large filtrate volume, shows a moderate impact on formation damage. Its depressed freezing point makes it suitable for permafrost drilling operations. How-ever, it may not be preferable due to its ice disintegration effects. OBM and synthetic oil mud yield minimal damage and show excellent rheological and filtration properties. Their depressed freezing points favour their use. Finally, a specific formulation of drilling fluid is recommended to minimize formation damage, to alleviate production impairment problems, to effectively reduce well completion cost and, eventually, to increase well productivity from such shallow frozen oil fields.
© 2012 Canadian Energy Technology & Innovation (CETI) Group
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