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 (Tb)
- Critical Temperature (Tc)
- Critical Pressure (Pc)
- 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 (kij), 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) GE 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.
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.
Binary Interaction Parameters (kij) 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 kij’s to match data found in the literature and got the kij-values in Table 3.
A temperature dependent kij 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 kij for DME-NaCl is shown in Table 3.
PVTsim uses the below expression for temperature dependent kij’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).
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.
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.
Once the new component is defined, the critical properties can be entered in the main Pure Components menu shown in Figure 2.
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.
PVTsim simulations for mixtures with DME
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.
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.
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.
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.
Adding a new component to the PVTsim pure component database requires more parameters than just pure component properties (Tc, Pc, 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 kij was used, while for DME-hydrocarbon interactions an optimized kij 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.
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.