Investigation of the Electrical Stability Test for Oil Muds
Authors: Ali A. , Schmidt D.D. , Harvey J. 
Published: December 2007

This paper reports the principal of operation of the electrical stability (ES) tester for oil muds, the construction of a research instrument that produces more reproducible results than the ES tester, and a series of ES tests on oil muds of varying and controlled composition.

Several conclusions and recommendations are reported. Recommendations are made for improvement of the present ES tester, which contains several design deficiencies. The effects of emulsifier concentration, hot rolling, oil/water (O/W) ratio, mud density and other composition variables on ES values have been documented. Recommendations are made for the proper operation of the current ES tester and for proper interpretation of ES tester data.

Introduction

Oil-based muds provide a high degree of borehole stability, minimize washouts and remain stable under high temperature conditions. These muds are finding increasingly greater applications in drilling troublesome shales, hot holes, sour and sweet gas environments and highly deviated wells where other mud systems perform poorly or are inappropriate.

Invert emulsion oil muds are water-in-oil (W/O) emulsions which are stabilized by emulsifiers. The water phase is a calcium chloride brine that comprises about 5% to 40% of total liquid volume of the mud. The calcium chloride concentration is adjusted to control the water activity of the mud to minimize water exchange between the mud and the formations being drilled. The solids in an invert emulsion mud are oil-wet so that they reside largely in the oil-continuous phase.

The brine emulsion droplets in oil muds are thermodynamically unstable and tend to coalesce into much larger drops. In addition, the brine can water-wet the solids in the mud. Water-wet barite settles in the pits. The reduced mud weight can result in wellbore collapse and the invasion of formation fluids into the wellbore. Water-wetting and coalescence also cause the oil mud to invert or "flip" from oil-continuous to water-continuous. A water-continuous oil mud system is generally too viscous to be pumped and cart result in the loss of a well.

Emulsifiers are added to an oil mud to reduce interfacial tension, to stabilize the brine droplets against coalescence and to oil-wet the weighting solids and drill solids in order to keep the system oil-continuous. The addition of emulsifiers generally stabilizes an oil mud. Maintaining a large excess of emulsifier has no apparent adverse effect on the muds and is a safe way to run an oil mud if one is concerned solely with oil mud stability. The excess emulsifiers associate as micelles and are available to adsorb on fresh surfaces introduced into a circulating mud. These surfaces can come from drill solids, from addition of weighting agent or from influxes of water from formation water flows or surface water contamination.

However, in drilling a well, operators are concerned both with overall well costs and with mud costs. Consequently, good treatment criteria are needed for oil muds. How much emulsifier and which type should be added to keep the mud in good condition without overtreating? Good treatment practices can result in lower mud costs which, in turn, could lead to increased application of oil muds in mare routine wells.

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