Chemical++Inhibition

=﻿Chemical Inhibition=

Crystal Growth Inhibition
In crystal growth inhibition (also known as crystal growth retardation), the intention is to impede the crystal growth of scaling. In the case of barium sulphate, this specific type of inhibition is said to occur when the inhibitor molecules adsorb at the active growth sites of an existing barium sulphate crystal, resulting in the blockage and prevention of further growth.

The most common inhibitors used that follow the crystal growth inhibition mechanism are small phosphonate groups, such as **DETPMP** (a penta phosphonate) which is mostly used in down-hole scaling control. These inhibitors work for a wide range of pH's and temperatures, but researchers have found it to be the most effective in static (bottle testing) conditions with a higher pH and temperature, and low supersaturation. The inhibition efficiency of phosphonates are also effected by the barium sulphate saturation ratio and the presence of calcium and magnesium ions. However, different types of phosphonates tend to be more effected by one factor than the other.

One difference between crystal growth inhibition and nucleation inhibition is that the inhibition mechanism is dependent on lattice matching. Lattice matching is a critical parameter for the performance of phosphonates. It allows the inhibitors to adsorb onto the active growth site without introducing a change in crystal structure.

The Presence of Divalent Cations in Barium sulphate Precipitation:
Calcium ions have proved to hinder the precipitation of barium sulphate. The effectiveness with the use of any type of inhibitor has been improved with the incorporation of calcium ions into the lattice of the growth site. The above figure illustrates the mechanism of the barite crystal growth mechanism for **DETPMP**. On the left, no scale inhibitors are present and show the Ca interacting with the normal BaSO4 Lattice. The right figure shows the presence of the scale inhibitor and the growth of BaSO4 with the incorporated Ca-SI (SI = scale inhibitor). The relatively high level of Ca ions compared to the scale inhibitor allows more interaction with the growing barium sulphate crystal. In solutions with very low Ca levels, the lattice does not distort significantly. Also, note that Ca-SI is the only active species interacting with the growing barite crystal. Magnesium ions have shown to decrease the inhibition efficiency in the case of this particular phosphonate inhibitor.