Characterising Curing Cement Slurries by Permeability, Tensile Strength and Shrinkage
Authors: Backe K.R. , Lile O.B. , Lyomov S.K. , Elvebakk H. , Skalle P. 
Published: December 2007

Abstract
This work was carried out to obtain more knowledge about the transition period of curing oil well cements. The results show that the curing characteristics are a function of temperature and that there is a correlation between shrinkage and cement content. The paper also introduces a new mechanism for gas migration and discusses how the studied parameters can be used to predict gas migration.

Introduction
The setting process of cement slurries in oil wells is very complex. Many parameters contribute to the final result, such as gelling, shrinkage, temperature, pressure, filter loss, cement structure and strength build-up, slurry permeability, entry pressure, capillary pressure, mud and mud cake, formation properties, well history, and possibly other parameters as well. Some of these parameters are next to impossible to characterise. Others are simple to measure in a laboratory set-up, but may not reflect downhole conditions. The cement setting process has been investigated extensively, but there are still many factors not fully understood. We have tried to extend the knowledge by monitoring temperature evolution, hydrostatic pressure, permeability, tensile strength and total chemical shrinkage during hydration. We will in turn discuss the importance of these parameters. The cement hydration is an exothermic reaction which can he observed as a temperature increase. Temperature is easy to measure, and the shape and peak of the temperature curve give valuable information on the hydration process, i.e. hydration onset and rate. The hydrostatic pressure is important as gas flow into the cement will be initiated when the pressure of the cement column falls below that of a gas bearing formation. This pressure drop is due to cement shrinkage at the same time as the shear strength develops, enabling the cement to hang onto the wellbore and casing.
But what mechanism will govern inflow of gas when the cement pressure has dropped far enough? We think the capillary entry pressure of the cement pore structure is important. When exposing gas to the water saturated cement, the non-wetting gas phase has to overcome the entry pressure of the cement pore system due to the interfacial tension between cement pore fluid and gas. The entry pressure is high when the pores are small while the permeability on the other hand is low, and entry pressure is in general inversely proportional to permeability. After having overcome the entry pressure, the relative permeability and the differential pressure between the formation gas and the cement column controls how much cement pore water that will be displaced by gas. This is a complicated process where the permeability and pressure change continuously.
Considering how important the permeability is for governing flow into and through the cement pores, surprisingly little work have been carried out on permeability during setting. Sutton and Ravi states that low fluid loss slurries exhibit a permeability of less than 100 md at a static gel strength of 200 lb/100ft2 and that it approaches 5 md at 500 lb/100ft2. Plee et al. have studied the permeability of bentonite-cement slurries and found a typical value of 50–100 md for fresh slurries and that decreasing permeability correlates linearly with increasing surface area. Only Apple by and Wilson have monitored the permeability at several points of time up till and past final set. Their results show, an initial permeability about 1 d falling down to around 1 md at the temperature peak.
When the cement slurry is in the process of loosing its hydraulic (liquid) properties, the strength of the cement matrix is still low. The pressure difference between the formation gas and the hydrostatic pressure of the cement slurry may overcome the strength of the matrix. In this situation it is the compressional strength of a confined cement which is of importance and this strength parameter is the highest one.
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