Inhibition

This section will apply the concepts outlined in the Nucleation and Crystal Growth sections. Common inhibitors used in the industry that follow both nucleation and crystal growth mechanisms will be discussed, as well as the factors effecting the performance of these inhibitors.

The conventional scale inhibitors used for oilfield scale prevention are DETPMP (diethylenetriamine penta methylphosphoric acid), PPCA (phosphinopolycarboxylic acid), and PVS (polyvinyl sulphonate). It is important to note that these species will be discussed accordingly to the mechanism it predominately follows, although it is possible for them to follow **both** nucleation and crystal growth inhibition mechanisms.

Nucleation Inhibition
Nucleation inhibition occurs when formed scale proto-crystal are disrupted or redissolved by inhibitor molecules. For homogenous crystallization, this disruption effects the thermodynamic stability of the growing nucleons.

Species that generally follow this mechanism are small polymeric species. Such polymer inhibitors include PVS, and S-PMA (sulphonated polymaleic acid co-polymer). The inhibition efficiency these species exhibit tend to improve at a lower range of pH and temperatures (high supersaturation ratio) because of their relatively low pKa values compared to those inhibitors that follow a crystal growth mechanism. This improvement has also shown to be related to the slower kinetics of barium sulphate precipitation at lower temperatures.

Some research indicates that nucleation inhibitors may work by modifying the actual scale crystal structure in such a way that drives the crystal thermodynamically towards redissolution rather than towards a spontaneous growth phase. During the nucleation process, a crystal must exceed a critical ratio of surface area to volume in order to remain in existence. If a nucleation inhibitor is able to change the shape of the proto-crystal in such a way that this ratio of surface are to volume becomes much larger, then effectively it has created an additional energy barrier that must be exceeded in order for that proto-crystal to exist and grow.

The series of micro-photographs below are taken from a study by A.M. Prithard et. al. into the effects of nucleation inhibitors on crystal structure. The effect of this nucleation inhibitor on the crystal's structure is quite evident here:



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.

The presence of divalent cations have continuously been proven to have less effect on nucleation inhibition mechanisms. Although Ca ions still improve the inhibition efficiency of such nucleation inhibitors like PVS, it is relatively small compared to crystal growth inhibitors like DETPMP. In contrast to having a "poisonous" effect on DETPMP, the presence of Mg ions have shown to have no effect on PVS.

Selection of Species to Inhibit Further Barite Growth:
Depending on the environment and conditions in which the barite scale is under, inhibitors with the highest inhibition efficiency under these conditions will be selected accordingly. For example: In colder deep water conditions, it is more reasonable to use an inhibitor that follows nucleation inhibition since they work most effectively at lower temperatures.

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