Effect of Temperature on VAPEX Performance

Incorporating some heat injection along with solvent injection appears to be the most viable option for improving the oil-drainage rate of vapour-assisted petroleum extraction (VAPEX) in extraheavy-oil formations. This study was intended to quantify the maximum possible increase in VAPEX drainage rate that can be obtained by heating the formation to a target temperature. The experimental phase of this study involved conducting VAPEX experiments in a large high-pressure physical model, packed with 250-darcy sand, using propane as the solvent. The physical model was preheated to 40, 50 and 60 C, and propane was injected at the same test temperature but different injection pressures to observe how injection pressure affects oil-drainage rate at elevated temperatures. In the experiments at elevated temperatures, but without increasing the injection pressure, higher rate of oil production was achieved in the early stages of the process. However, a stabilized rate of oil production did not show pronounced improvement caused by a lower solubility of propane in the oil at higher temperatures. Increasing injection pressure along with increasing the test temperatures was successful in accelerating the oil production. The oil used in these experiments was found to become mobile with the increase in temperature even without solvent dissolution. As a result, the total rate of oil production appeared to be controlled by two mechanisms: (1) by solvent dissolution and oil mobilization at the boundaries of the vapour chamber and (2) by pure free-fall gravity drainage beyond the vapour chamber wherever gravity head was sufficient to push the mobile oil toward the production well. The results of this these tests define the upper limit of oil rates achievable with heated solvent injection. They can also be used to assess the applicability of VAPEX to warm reservoirs naturally (e.g., in Venezuela) and reservoirs with mobile oil in place.