# 18 Adding a New Component into PVTsim

Introduction

PVTsim has a comprehensive list of pure components covering not only oil and gas components, but also water, hydrate inhibitors and salts. Simulations may still be needed for mixtures with components that are not in the pure component database that comes with a PVTsim installation. In such cases, the user has the option to add the missing component to the PVTsim pure component database.

In this technical note, dimethyl ether (DME) is used as an example of a new component to be added to PVTsim. DME is a potential candidate for enhanced water-flooding. It has a reasonable solubility in water, but more affinity for oil. When water with DME contacts a reservoir oil, a major fraction of the DME will transfer from the water phase to the oil phase. This will make the oil swell and reduce the viscosity of the oil. This viscosity reduction provides an additional contribution to the enhanced oil recovery achieved with water-flooding. The performance of DME as a viscosity reducing agent depends on the DME partitioning between the brine and oil phases. A recent paper from Ratnakar et al. (2017) has all the necessary parameters required to enter DME as a new component in PVTsim.

Model parameters needed by PVTsim

PVTsim requires the below component properties, which usually can be found in the open literature.

Component name
Molecular weight (MW)
Liquid density
Boiling Point Temperature (T_b)
Critical Temperature (T_c)
Critical Pressure (P_c)
Acentric Factor (ω)

However, not only pure component properties are required for a new component to be added to PVTsim. To capture the phase behavior of multi-component mixtures, model parameters are needed that describe the interactions between the new component and the other components in the PVTsim pure component database.

PVTsim supports two different mixing rules for the a-parameter of a cubic equation of state. The simplest one is the classical mixing rule, which in addition to the above pure component parameters requires a binary interaction parameter (k_ij), which parameter can possibly be made temperature dependent.

In the presence of brine (water + salt), the classical mixing rule is no longer adequate to describe the more complex polar-nonpolar interactions. PVTsim uses the Huron-Vidal (HV) mixing rule as the default polar component model. It is based on a Non-Random Two-Liquid (NRTL) G^E model. To apply the HV mixing rule, a number of interaction parameters are required, which must be determined from experimental data. The HV mixing rule is attractive by only requiring HV specific interaction parameters for the component pairs that cannot be described using the classical mixing rule. This reduces the number of interaction parameters that need to be determined.

Component parameters from Ratnakar et al.

Pure Component Properties

Ratnakar et al. represented brine as a mixture of H₂O and NaCl and used the pure component EoS parameters for DME, H₂O, and NaCl shown in Table 1.

***Table 1*** *EoS parameters for DME, H₂O, NaCl. Data from Ratnakar et al.*

Choice of Mixing Rule

The choice of mixing rule by Ratnakar et al. is shown in Table 2. The HV mixing rule is used for DME-H₂O while Classic is used for other component pairs with DME.

***Table 2*** *Mixing rule to be applied for DME as suggested by Ratnakar et al. (yellow color).*

Binary Interaction Parameters (k_ij) for DME

DME is miscible with hydrocarbon components and as shown in Table 2 the Classic mixing rule was applied for DME-hydrocarbon pairs. Ratnakar et al. tuned the k_ij’s to match data found in the literature and got the k_ij-values in Table 3.

A temperature dependent k_ij was determined for DME-NaCl using data for the DME solubility in salt water (10 weight% NaCl) at temperatures of 303 K and 353 K. The derived temperature dependent expression for the k_ij for DME-NaCl is shown in Table 3.

***Table 3*** *Binary interaction parameters (k_ij) for DME from Ratnakar et al.*

PVTsim uses the below expression for temperature dependent k_ij’s:

This is slightly different from the expression for DME-NaCl in Table 3. To comply with the expression used in PVTsim, the kij_0 value must be adjusted as shown below (with a kij’-value of 0.00247).

HV-NRTL Parameters

Ratnakar et al. determined HV parameters for DME-H₂O to match experimental data for the solubility of DME in water of various salinities (0-17 weight% NaCl in H₂O) and at varying pressure and temperature conditions. The HV parameters are shown in Table 4. All other component HV parameters used are the default parameters in PVTsim.

***Table 4*** *HV parameters for DME-H₂O from Ratnakar et al.*

Adding the new component DME to PVTsim

The model parameters by Ratnakar et al. are determined for use with the 1978 modification of the volume corrected Peng-Robinson equation. In PVTsim Nova the EoS was set to PR 78 Peneloux. The parameters in Table 1 were entered under Pure Component ® Add Pure Component as shown in Figure 1.

***Figure 1*** *Add new pure component menu in PVTsim. DME is added as an Organic Defined component.*

Once the new component is defined, the critical properties can be entered in the main Pure Components menu shown in Figure 2.

***Figure 2*** *Pure Components menu in PVTsim with EoS parameters entered.*

Under the Interaction Parameters tab in the Pure Components menu, the mixing rules in Table 2 are selected and the parameters in Tables 3 and 4 entered as seen from Figure 3.

***Figure 3*** *Summary of data entered into PVTsim under Pure Components – Interaction Parameters tab.*

PVTsim simulations for mixtures with DME

DME-H₂O System

A 50/50 mol% DME/H₂O mixture was entered in PVTsim and the solubility of DME in H₂O was simulated. As can be seen from Figure 4, the simulated data agrees well with the experimental data.

***Figure 4*** *Experimental and simulated solubility of DME in pure H₂O.*

DME-H₂O-NaCl System

The left hand plot in Figure 5 shows the simulated solublilty of DME in a NaCl + H₂O mixture (10 weight% NaCl) at three different temperatures and varying pressures. The right hand plot shows the DME solublilty in brine of three different salinites at 323 K. A good match is seen between experimental and simulated data.

***Figure 5*** *DME solubility in 10 weight% H₂O+NaCl (left) and DME solublilty at 323 K in brine of different salinities (right) .*

DME-H₂O-NaCl-Oil System

Ratnaker et al. used a reservoir fluid (Oil A) to investigate the partitioning of DME between brine and oil. An EoS model was developed that provided a good match of the PVT data shown in Table 5 and Figure 6.

***Table 5*** *Match of saturation pressure, GOR, and STO density using the tuned EoS model for Oil A.*

***Figure 6*** *Match of relative volume, live density, and viscosity using the tuned EoS model for Oil A.*

The DME partitioning coefficient is defined as the ratio of mole fractions of DME in the oil and the brine phase. For a series of different DME concentrations, the DME partitioning coefficient was measured for Oil A in contact with a 3.5 weight% NaCl brine solution at a temperature of 343 K and pressures in the interval 10,000 -14,000 kPa. As shown in Figure 7 a good match was seen between simulated and experimental DME partitioning data.

***Figure 7*** *Experimental and simulated DME partitioning coefficients using new component parameters. Experiments were carried out at 343 K and 10,000-14,000 kPa. Simulations were done at 343K and 12,000 kPa.*

Summary

Adding a new component to the PVTsim pure component database requires more parameters than just pure component properties (T_c, P_c, and acentric factor). The interactions between the new component and existing components require selection of the appropriate mixing rule and corresponding binary interaction parameters. In this technical note, DME was used as an example of a new component to be added to PVTsim. Ratnakar et al. have published all the data required to input DME. They found that the HV-NRTL mixing rule was appropriate for DME-H₂O interactions, along with the optimized parameters determined from experimental data. For DME-NaCl interactions, a classical mixing rule with a temperature dependent k_ij was used, while for DME-hydrocarbon interactions an optimized k_ij was found to be sufficient. This technical note has shown how to enter DME with the parameters from the paper by Ratnakar at al. and key simulation results in the paper have been reproduced.

Reference

Ratnakar, R., Dindoruk, B. and Wilson L.C., Phase behavior experiments and PVT modeling of DME-brine-crude oil mixtures based on Huron-Vidal mixing rules for EOR applications, Fluid Phase Equilibria 434, 2017, pp. 49-62.