Abstract
Abstract
The effect of hydrate inhibitors on oilfield scale formation has been studied. A self-consistent activity model is presented to model the effect of methanol on carbonate equilibrium, calcite, barite, gypsum, celestite, and halite solubility in a gas/methanol/water/salt solution. The model uses Pitzer theory to model the effect of salt and Born equation to model the effect of methanol. Excellent agreements between the model prediction versus both experimental and literature results are observed. The model has been incroporated into ScaleSoftPitzer®, a scale prediction software specifically written for the oil and gas industries. From ScaleSoftPitzer® predictions, scaling problems can be worsened with as little as 5% (by volume) of methanol in a typical oil and gas production system. Ethylene glycol has a much lesser impact on oilfield scale formation than methanol. The efficiency of common phosphonate inhibitors in the methanol and ethylene glycol containing brines will also be discussed.
Introduction
In the oil and gas industries, methanol and ethylene glycol are often used to inhibit gas hydrate formation during production. Gas hydrate is a crystalline solid consisting of gas molecule surrounded by a cage of water molecules, which forms at certain high pressure and low temperature regimes. Gas hydrate formation is particularly troublesome for offshore gas wells where the producing temperature is low due to both adiabatic expansion of gas and seawater cooling. Once gas hydrate forms, it can plug up the well and prevent gas production. One economic solution to prevent hydrate formation is to inject a large quantity of methanol or ethylene glycol. However, methanol or ethylene glycol may cause adverse scaling problems in the associated brine solution, which often contains high concentrations of dissolved minerals. There is little research on the solubility of mineral salt in methanol/water/salt or ethylene glycol/water/salt solutions. In the following, the solubility of mineral salts in these mixed solvent systems are studied. Inhibition of scale formation in the presence of methanol or ethylene glycol will also be discussed. Lastly, the impact of methanol under realistic oil field condition are predicted by ScaleSoftPitzer®, a scale prediction software specifically written for the oil and gas industries.
Self-Consistent Activity Coefficients in Alcohol/water/salt
In principle, the free energy of transfer from a solution of one dielectric constant to another dielectric constant (?Gal/w = ?Gtr) can be calculated via the Born equation and this in turn is the medium-effect activity coefficient, ?N:Equation 1
A discussion of the Born equation can be found in any of several texts on the molecular nature of ions in mixed alcohol/water solutions in that the water is selectively concentrated around the ions1,2. The Born equation is strictly approximate at all alcohol concentrations, but at greater than 50 to 80 %, w/w alcohol, the error can increase substantially1,3. In this paper, we have simply used the Born equation as a model template to suggest a functional form to our semi-empirical approach. The reasonableness of this assumption lies in how well it is able to represent the solution chemistry of species of interest as a function of salt and methanol. In fact, it has been shown to be an excellent assumption over the entire range that we have tested, to date (essentially, up to about 6 m salt, 85% w/w methanol and at 4 25°C).