Fluid-Movement Measurements Through Eccentric Annuli Produce Unique Results

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 109563, "Fluid-Movement Measurements Through Eccentric Annuli: Unique Results Uncovered," by Larry Moran, SPE, ConocoPhillips Co., and Mark Savery, SPE, Halliburton, prepared for the 2007 SPE Annual Technical Conference and Exhibition, Anaheim, California, 11–14 November. The paper has not been peer reviewed. Better understanding of fluid movement through eccentric annuli over a wide range of casing standoffs and flow rates is essential for proper cement placement. A full suite of physical testing was performed to find differences in velocities and flow rates on the wide side vs. the narrow side of an annulus. Nine models were built, each taller than 6.5 ft and each with unique annular geometries. The annular sizes chosen are common to primary-cementing operations. The flow area was divided at the top of each model to capture wide- and narrow-side flow variations. The weight of recovered fluid vs. time was recorded and used to determine flow rates and velocities on both sides. Introduction In oil- and gas-well completions, cementing operations are used to ensure zonal isolation. On many production liners, the wellbore geometry is such that very slow flow rates during cementing are required to prevent fracturing/losses. In addition, liner-top packers and tiebacks give annular clearances that are quite small. These narrow clearances cause excessive backpressure with normal flow rates, often requiring the use of slow-flow-rate cementing to prevent losses. The other option is pumping cement at a high rate with losses, often with uneconomical and undesired outcome. The most likely causes of slow-flow-rate cementing failure were poor centralization and poor compatibility between muds, spacers, and cements. Poor centralization causes channeling, and poor compatibility causes viscous interfaces leading to channeling. Today, centralizers are better than in the past and higher degrees of centralization are often attempted. Spacers also are better today. Another significant change is cement slurries that are often much thicker than anything pumped years ago. Accurate modeling of these flow phenomena can help uncover the conditions required to achieve a good cement bond around the entire casing string. Experimental The goal was to build a set of large-scale apparati that allowed a complete study of steady-state-flow response of various fluids in eccentric annuli. Nine pipe-in-pipe eccentric annular models were built using standard casing materials. Each model was unique in its combination of geometry and inner-pipe standoff.