Mechanism of heavy oil recovery by cyclic superheated steam stimulation

•Superheated steam has superior properties in recovery heavy oil and was put into practice.•Steam with high superheat degree has low heat transfer coefficients.•Chemical changes among reservoir rocks and liquids improve reservoir permeability.•Mathematical models for superheated steam flowing down wellbores were established.•Superheated steam heating process in oil reservoirs were classified by four stages.

At the conclusion of several cycles of conventional saturated steam huff and puff in heavy oil reservoirs, the heating radius are typically only 10–20 m as it went through successive saturated steam huff and puff. The heating region cannot be enlarged by continuing saturated steam stimulation any more. However, superheated steam huff and puff as an additional recovery method significantly increased the heating radius by about 10 m after saturated steam huff and puff was completed. Conventional saturated steam stimulation theory is not applicable for superheated steam. In this study, several physical simulation tests were performed and reservoirs from conventional saturated steam played different roles in heavy oil . There exists a temperature gradient on the heat transfer surface, in which chemical reactions would happen such as formation water, heavy oil, and formation minerals under the favorable circumstances of high temperature of superheated steam. These chemical reactions not only result in some changes of composition and the irreversible reduction of oil viscosity, but also change the microscopic pore structure of rocks to improve the permeability of superheated steam heating area; changing the wettability of the reservoir rock and increasing the displacement efficiency of superheated steam flooding, which bring about significant improvements of heavy oil development effects. Due to the release of latent heat and the uniform temperature of saturated steam, temperature in the saturated steam heating area is kept constant everywhere. These mechanisms do not occur in ordinary saturated steam huff and puff. This work analyses the superior properties of superheated steam and bring forward the superiority of superheated steam huff and puff to effectively develop heavy oil reservoirs in recovery mechanisms, including mathematichal model establishing, physical and numerical simulation studies, and current pilot test effects.