No abstract

Summary Experimental data have shown that the solubility of water in the oleic (L) phase (xwL) can be significant at elevated temperatures. However, xwL was not properly considered in prior studies of steam-assisted gravity drainage (SAGD) and expanding-solvent (ES)-SAGD. The main objective of this research is to present a detailed study of compositional mechanisms in SAGD and ES-SAGD simulation by considering xwL. The phase-behavior models used in this research are carefully created on the basis of experimental studies presented in the literature. Mechanistic simulation studies are then conducted for SAGD and ES-SAGD. Coinjectants used in ES-SAGD simulations range from propane through n-decane. Results show that xwL enhances bitumen production in both SAGD and ES-SAGD, mainly because xwL results in reduction of L-phase viscosity. The enhancement is more significant when the chamber-edge temperature is higher, because xwL increases with temperature. The enhancement of bitumen production observed in the case studies is 7.66% for SAGD, 4.08% for n-C6-SAGD, and 4.85% for n-C8-SAGD for a fixed period of operation at 35 bar. It is important to consider xwL in SAGD and ES-SAGD simulations, because the performance of ES-SAGD relative to SAGD tends to be overestimated without considering xwL. A guideline is presented to leverage xwL to improve bitumen production in ES-SAGD. As discussed in our prior research, solvent becomes effective in diluting bitumen and reducing the steam requirement only when it sufficiently accumulates near the chamber edge. New results show that water can act as a diluting agent until solvent sufficiently accumulates near the chamber edge.

No abstract

Experimental results in the literature show that the water solubility in the oleic (L) phase can be high at reservoir conditions in thermal oil recovery processes; e.g., 24 mol% in the water/n-eicosane binary system at 41 bars and 523 K. It becomes even more significant as the L phase becomes more aromatic, which is the case with heavy oil and bitumen. Efficient and accurate representation of multiphase behavior, which consists of the L, vapor (V), and aqueous (W) phases, is crucial in reliable numerical simulation of steam injection processes. This research presents multiphase behavior predictions for water/n-alkane mixtures by use of the Peng-Robinson equation of state (PR EOS) with the van der Waals mixing rules. Binary interaction parameters (BIPs) are first optimized for water with n-alkanes in terms of three-phase curves including upper critical endpoints (UCEPs), where the V phase and the less dense liquid phase merge in the presence of the denser liquid phase. A new correlation is then developed on the basis of the optimized BIP values. Thermodynamic predictions from the PR EOS with the new BIP correlation are given for various mixtures and compared with experimental data available in the literature. Results show that the PR EOS with the BIP correlation yields reasonable predictions for multiphase behavior of water/n-alkane mixtures. It gives the transition of binary phase behavior between types IIIa and IIIb that is consistent with experimental results. It also can reproduce asymptotic behavior of three-phase curves and the water solubilities in the L phase (xwL) that has been observed as n-alkane becomes heavier. When applied to water-containing reservoir fluids, the PR EOS with the BIP correlation results in a systematic underprediction of xwL. This is expected considering that reservoir oil consists of various types of hydrocarbons and that the affinity towards water is lowest for n-alkanes and highest for aromatics. Accurate xwL predictions can be obtained by systematically reducing the BIP values from the correlation. The correlation developed may serve as the upper limit of BIPs for water with pseudo components that are required for characterizing water-containing reservoir fluids.

Expanding-solvent steam-assisted gravity drainage (ES-SAGD) is a widely-investigated alternative to SAGD, considering its potential to reduce thermal losses while enhancing bitumen recovery. However, most prior studies on ES-SAGD were limited to homogeneous reservoirs. This research presents a mechanistic analysis of ES-SAGD in heterogeneous reservoirs in terms of cumulative steam-oil ratio (SOR) as a function of steam-chamber size. Simulation case studies for SAGD and ES-SAGD with normal hexane are conducted for geostatistical realizations of two types of heterogeneous Athabasca-bitumen reservoirs. For the first type, shale barriers are oriented horizontally relative to the top and basal planes of the reservoir. For the second type, they are inclined and more representative of the middle McMurray member. The solubility of water in the oleic phase at elevated temperatures is properly modeled to ensure reliable comparison between SAGD and ES-SAGD. Results show that ES-SAGD is less sensitive to heterogeneity than SAGD in terms of cumulative SOR for simulations at 35 bars and 2 mol% solvent-injection concentration for a reservoir thickness of 20 m. On average, the reduction in SOR due to steam-solvent coinjection is simulated to be greater under heterogeneity. The margin of SOR reduction is greater in reservoirs with inclined shale barriers than in those with horizontal shale barriers. Analysis of simulation results indicates that the injected solvent tends to accumulate more significantly under heterogeneity, which enhances the mechanisms of ES-SAGD, such as dilution of bitumen by solvent and reduced thermal losses to the overburden. Tortuous hydraulic paths and slower gravity drainage under heterogeneity enhance the mixing between solvent and bitumen in the transverse direction along the edge of a steam chamber. Then, a larger amount of the accumulated solvent tends to facilitate lower temperatures near the chamber edge. Lower chamber-edge temperatures combined with restricted access to the overburden under heterogeneity alter the chamber geometry such that the contact area for overburden heat losses is further reduced.