PH Measurement for Low Soluble Salts Monitoring in Monoethylene Glycol(MEG) Regeneration System

Abstract

Abstract MEG (Monoethylene Glycol) is usually used as a gas hydrate inhibitor, and it is essential to regenerate and re-inject this into well because great amount of MEG is consumed to control the gas hydrate problems. MEG is recovered as rich MEG from the topside. Rich MEG is a water solution, which is about 50 wt% of MEG with rich water. This is regenerated as lean MEG through MEG regeneration process. Lean MEG is high concentration of MEG. In the production of subsea oil and gas, formation water is also produced. Formation water contains salts1. This causes problem in MEG regeneration process. If problem arises, the whole process is stopped2. And it can result in tremendous loss. It is important to know the salts equilibrium under MEG regeneration conditions in order to prevent problems related to deposition and scale precipitation. That is, MEG regeneration process is to separate water and salts from the mixed solution, which is composed of MEG, water, and salts. Using the difference in vapor pressure to separate water is called re-concentration and to separate salts is called reclamation process. As it shows from figure 1 and figure 2, density of MEG solution increases along with increase of NaCl and MEG concentration. NaCl concentration was calculated from the conductivity. If MEG effect and salts effect are separated, assumption of MEG concentration is possible inversely. Sandengen introduced this method in his paper3. But there is a limit to monitor the MEG system with only these results. Ions which are included within the formation water are Sodium (Na+), Potassium (K+), Calcium (Ca2+), Magnesium (Mg2+), Barium (Ba2+), Strontium (Sr2+), Iron (Fe2+), Chloride (Cl-), Sulfate (SO42-), Alkalinity as HCO3-, and Bromide (Br-) etc.1. Salts can be separated into low soluble salts and high soluble salts. High soluble salts are dissolved in water well like NaCl and KCl. Low soluble salts cannot be dissolved in water well like CaCO3 and MgSO44. There are two methods in MEG regeneration. One is the full stream concept, and the other is slip stream concept5. Many studies were conducted to explain this process. In the full stream concept, high soluble salts and low soluble salts are not sorted. The reclamation process flashes the feed solution. In the slip stream concept, water is separated in the re-concentration process first, and then from part of it, high soluble salts are removed using flash. Core of this concept is to tolerate some of high soluble salts. On the other hand, since low soluble salts are at high risk of scale problems, it is removed in the pre-treatment process beforehand. Enough studies on low soluble salts need to be conducted. Only measurements of density and conductivity are not enough, and they have limits.