Enhanced Oil Recovery and in Situ Upgrading of Heavy Oil by Supercritical Water Injection

Heavy oil accounts for two-thirds of the world oil resources but contributes only one-seventh of the world oil production due to its high oil viscosity and heavy distillates. Steam injection has been widely used for heavy oil recovery by heating up the reservoir to reduce oil viscosity. However, severe carbon loss to coking causes low recovery efficiency and high energy consumption. Here, we report supercritical water injection for heavy oil recovery. Supercritical water is expected to be both a heat carrier and an organic solvent, thereby not only reducing oil viscosity but also dissolving heavy distillates to avoid coking. To test its feasibility, core experiments were first conducted to simulate the recovery process. Results showed that supercritical water flooding improved oil recovery by 17% and reduced heat consumption by 34% versus classsical steam flooding. Further, to clarify its recovery mechanism, a visualization technique and a quantitative method were developed for regulating phase behaviors and upgrading reactions between heavy oil and supercritical water. Results showed that supercritical water has good miscibility with heavy oil, and it is the key to both enhanced oil recovery and in situ upgrading. High miscibility means formation of supercritical water clusters around organic macromolecules, which makes asphaltene difficult to aggregate and polymerize to form coke but easy to decompose to form maltene and recover. Overall, supercritical water injection has made great advances in enhanced oil recovery, energy saving, and in situ upgrading for heavy oil recovery. The work provides a sound basis for its application in oilfields.