Measurement and Modelling of Phase Equilibrium of Oil - Water - Polar Chemicals

As the exploitable resources decrease, more sophisticated recovery methods are employed in the oilindustry to produce the remaining resources. A result of using more sophisticated recovery methodsis that oil field chemicals are more widely used, especially in the offshore oil production. Thesechemicals belong to different families like alcohols, glycols, alkanolamines, surfactants andpolymers. They have various functions, e.g., methanol and MEG are used as gas hydrate inhibitors,surfactants are used to lower interfacial tension between crude oil and microemulsion and polymersin a polymer-waterflooding process act primarily as thickeners.

The main purpose of this work, focusing on the phase equilibrium of complex systems containingthermodynamic gas hydrate inhibitors, is to give a solid contribution in bridging the existing gaps inwhat experimental data is concerned. This was achieved not just with the measurement of newexperimental data, but through the development of new experimental equipment for the study ofmulti-phase equilibrium. In addition to measurement of well-defined systems, LLE have beenmeasured for North Sea oils with MEG and water.
The work can be split up into two parts:
Experimental: VLE, LLE and VLLE
Modeling: Well-defined systems, oil systems

In the first part, an existing experimental set-up is described and the investigation of limitations andoptimizations needed for optimal use. A complete description of the equipment is made, and theresults obtained in the study of reference systems presented, confirming the quality of theequipment. The equipment is used for measurement of VLE for several systems of interest; methane+ water, methane + methanol, methane + methanol + water and methane + MEG.

Details dealing with the design, assembling and testing of new experimental equipment formeasuring VLLE are given in chapter 3. A general insight on the processes behind the developmentof new equipment is given, followed by the complete description of the set-up developed in this work. The results obtained in the study of reference systems are also presented, confirming the viquality of the equipment and its potential for the attainment of high quality data. Measurements were performed for VLLE of a multicomponent system consisting of methane + n-hexane +methanol + water.

In order to develop a thermodynamic model for the distribution of chemicals in oil-water systemsexperimental data are required, but such data with oil systems are very rare in the literature. In this project experimental work has been carried out at Statoil R&D and an experimental method hasbeen established and tested for such measurements. The mutual solubility of two North Sea oils,MEG and water has been measured in the temperature range of 303-323 K at atmospheric pressure.

In the second part of this work, the CPA EoS has been used for modeling hydrocarbon systemcontaining polar chemicals, such as water and gas hydrate inhibitor MEG or methanol. All the experimental data measured in this work have been investigated using CPA, with satisfactory results. A single temperature independent k_ij between the components present in the system, is usually enough to describe the solubility of all phases. Accurate predictions are made for VLLE of aquaternary system of methane + n-hexane + methanol + water, using the CPA EoS with binaryinteraction parameters taken from binary systems. Predictions are in good agreement with theexperimental data, even for very low solubility, such as n-hexane in aqueous phase. In conclusion,the CPA EoS predicts satisfactorily the multiphase equilibrium of multicomponent water – alcohol– aliphatic hydrocarbon systems, based solely on the binary interaction parameters taken frombinary systems, using the 2B association scheme for methanol and the 4C association scheme for water.

Finally, CPA has been extended to reservoir-fluid + MEG and reservoir-fluid + MEG + watersystems. The reservoir fluid consists of three condensates and four oils from fields in the North Sea.The mutual solubility of oil and MEG is satisfactorily correlated using correlations for estimating k_ij for all MEG-HC pairs. Similarly the mutual solubility of condensate/oil, MEG and water ispredicted satisfactorily using correlations for k_ij of all MEG-HC pairs and water-HC pairs, as afunction of molecular weight. The experimental trends in mutual solubility as a function oftemperature and MEG content in polar phase are predicted satisfactorily which are correct in orderof magnitude according to the industrial requirements.