Microfluidic Method to Investigate the Precipitation of Calcium/Magnesium Phosphonate Scale-Inhibitor Complexes
Authors: Vazquez Oscar , Dudek Marcin , Beteta Alan , Øye Gisle 
Published: May 2022

Abstract
Phosphonate scale inhibitors (SI) are one of the most effective chemicals deployed in oilfield reservoirs for the prevention of mineral scale deposition. To prevent scale deposition in the near wellbore region, a certain volume of this chemical, at a certain concentration is bull-headed into the reservoir, which is commonly known as a squeeze treatment. Typically, the chemical slug is injected as an acidic solution. Once it is in contact with the formation brine, the pH of the solution increases to neutral pH, and the chemical becomes actively charged, leading to a number of reactions with divalent ions, such as Ca2+ and Mg2+. The formed SI complex salts attach to the surface rock, resulting in the retention of the SI in the reservoir formation. When the well is brought back on production, the SI complex dissolves, releasing the phosphonate into the flowing brine. These types of chemicals are commonly known as threshold chemicals, since they can prevent scale deposition if the concentration is above a threshold concentration, usually from 1 to 20 ppm. The treatment lifetime depends on the level of retention, which subsequently depends on the complexation with divalent ions present in the injected and formation brines.
The goal of this manuscript was to investigate the effect of calcium concentration and calcium/magnesium ratio in precipitation of the SI-divalent cation complex using a microfluidic chip. The microfluidic setup consists of a glass chip, where a solution containing CaCl2 and/or MgCl2; and another one containing a commonly used scale inhibitor, namely a diethylenetriamine pentakis methylene phosphonic acid (DETPMP), a pentaphosphonate, or a Polyphosphino Carboxylic Acid (PPCA), a common polymeric species, are combined in a stratified flow. The mixing occurs in the interphase between both solutions because of diffusion across the water-water boundary, allowing the controlled deposition of SI complex. The process is performed with the use of a motorized and programmable microscopic stage, which enables the automatic capturing of channel snapshots over the duration of experiment. The amount of precipitate was evaluated by measuring the coverage of precipitate in the chip by image processing, specifically by calculating the grey scale histogram of the precipitation.
In the presence of DETPMP, increasing [Ca2+] results in increased amount of precipitate. When the salinity (TDS) of the 2000 ppm Ca2+ solution was increased with 1500 ppm Na+, there was a negligible variation in the detected precipitate, however when the same salinity was achieved through addition of Mg2+ there was a significant reduction amount of precipitate, which is envisaged to affect growth of the precipitate. Bulk precipitation experiments results showed that in the presence of Mg, the mass of precipitate was almost half in the presence of Ca only, the samples were filtered at 0.2 micron and weighted. These results are consistent with the ESEM analysis, where the morphology of the precipitate was significant different, in the presence of Mg, the precipitate consisted in small crystals smaller than 1 micron, in comparison to precipitate structure of up to 250 microns in the presence of Ca ions only.