This paper presents the concept of the Storable Slag Slurry, a zonal isolation system which can be prepared in advance, stored and transported to the rigsite at anytime, without setting time concerns.
The compositions contain blast furnace slag to be used in water-based fluids as the hydraulic material, in the absence of set inhibitors. The slurry can be stored in liquid state from 3 days up to several months, at room temperature, depending on its composition.
There are two basic methods of preparing the Storable Slag Slurry. In both it is possible to avoid using the single batch (a so-called batch-mixer) saving money and to maintain good homogeneity of the slurry, and consequently, the uniform physical properties (density, compressive strength, etc.) in all its volume.
Introduction
Differently from ordinary cement slurries, the storable slurries designed to be used in well cementing operations may be prepared in advance, stored, moved and taken to the site of utilization at the most convenient time, without the need of immediate use.
The cementing operation requires that the slurry be displaced into the wellbore as soon as the hydraulic material is mixed to the water and additives, in order to prevent the set to occur in the tanks and in the cementing lines or still in the casing. Any of these situations constitute an operation drawback, Besides that, damages caused by loss of material, waste of time, and even ultimately loss of the well may occur. This way of conducting the cementing operation, by mixing hydraulic material to the water with additives, followed by the displacement of the slurry to the wellbore, causes a variation in the concentration of hydraulic material in the slurry, this in turn entraining a variation in physical properties (density, compressive strength, etc.) which possibly leads to a low quality cementing to be corrected by squeezing, with the consequent increased costs.
One way to avoid the oscillation in the concentration of hydraulic material is to use a mixer able to prepare the entire required amount for the cementing operation in one single batch (batch-mixer). However the slurry prepared using this type of mixer increases the cost of the cementing operation of oil and gas wells.
The Storable Slag Slurry, being a system that remains fluid during a long period at surface conditions, reduces the risks in the cementing operations.
This feature makes possible that the slurry may be prepared in advance, improving the quality as a result of the composition being uniform and also reducing the time of operation, since it is only necessary to displace the slurry to the wellbore.
The storable slurry, presented in this paper, consists in a slag based slurry. The use of blast furnace slag slurries in cementing operations was first described by Cowan et al1 Nahm et al2,3,4. Benge5,6 et al, Sabins et al7 and Silva et al8 evaluated the performance of slurries made with slag in bench apparatus and / or in large scale apparatus.
The Storable Slag Slurry is designed in such a way to avoid its set during a long time (three days to several months) at room temperature. In this system there is no need to use the set retarders to avoid the setting of the slurry at the surface.
System Concept
Slag is considered a latent hydraulic compound, since it needs activation to set. The required activation may be provided by thermal energy, chemical activators or through mechanical energy.9 The activation increases the solubility of the slag and/or favors the precipitation of the hydrated compounds.
The mechanical activation is obtained by milling the blast furnace slag: the higher is the surface area, the larger will be the dissolution of the slag. The particle reduction in size increases surface area, and therefore the contact area, consequently increasing the hydration rates.
Thermal activation also increases the solubility of the slag in solution.
Chemical activation occurs through the combination of two parameters: increase in the rate of dissolution of slag caused by an increase in pH, and anticipation of the precipitation of hydrated compounds through the increase in the ion concentration in solution or by altering the solubility of the slag compounds.
