Why Cement Fluid Loss Additives Are Necessary

Abstract

ABSTRACT With respect to the fluid loss behaviour of a cement slurry basically two stages need be considered: (i) a dynamic one corresponding to the placement and then (ii) a static one, the waiting on cement. During the first period the slurry flow is eroding the filter cake as it is growing, thus rapidly a steady state is reached where the filtration occurs through a cake of constant thickness; at the same time, since the slurry is losing water but no solid particle, its density is increasing in line with the fluid loss rate. During the second period, referred to as "static", the cake grows due to the absence of flow. It may possibly grow up to a point where it locally but completely fills the annulus: bridging takes place and the hydrostatic pressure is no longer transmitted to the deeper zones. Dynamic:A maximum acceptable value of the total volume of fluid lost during the cement placement can be easily calculated from an upper density limit. Since basic slurry properties, like thickening time and rheology, are greatly dependent on density (at least in the low water content domain), they may be used to define this limit. To convert the total amount of fluid loss into an API fluid loss value, three parameters are needed: the permeable formation area, the mudcake thickness and its permeability. Static: A maximum acceptable value of the thickness of the cake built-up during the waiting on cement period is deduced from the annular gap. The time taken by the filter cake to completely fill up the annulus, i.e., the bridging time, depends on the cement and mud cake properties (mudcake thickness and permeability and cement cake permeability). Therefore, by comparing thickening time and bridging time, a maximum value for the cement cake permeability can be deduced which is then expressed as an API fluid loss value. From typical mudcake resistances it can be estimated that, both in dynamic and in static conditions, the fluid loss could, in some conditions, have to be reduced to an API value one order of magnitude lower than what is generally considered as a fair control of fluid loss. Some examples are given. However, still very little is known about the effect of spacers, washes, mechanical aids and cement itself on the mudcake state and more data have to be gathered on mudcake thickness and permeability under various conditions, before definite fluid loss limits can be asserted.