A new rheological approach helps formulation of gas impermeable cement slurries

The prime tasks of cementing an oil well are to keep the casing in place, t o prevent corrosion from formation water, and, most importantly, to avoid t he flow of fluids on the outside of the casing. Sometimes gas migration in cement slurries occurs when gas present in the formation exerts pressure ag ainst the cement slurry column, causing the formation of microfractures in the setting matrix, expecially during the transition time of the slurry. Ma ny studies have been conducted in the last two decades to help in understan ding which properties the cement slurry must have to avoid the migration of such fluids. In this paper we propose a new rheological approach to evalua te the capacity of a cement slurry to stop the migration of fluids from the formation in the wells where the risk of gas migration is relatively high, especially during the transition time. Our rheological approach is based o n small amplitude oscillatory measurements. We collected information on eit her kinetics of gelation of cements for oil well usage or the strength of t heir structures. This fundamental approach has been useful to formulate dif ferent cement slurries that have been used with success in several field tr ials where the risk of gas migration was relatively high. (C) 1999 Elsevier Science Ltd. All rights reserved.