An Experimental Study on the Displacement Characteristics of Discontinuous Depletion Process in Foamy Oil Reservoirs

Depletion process or the so-called cold production from some heavy oil reservoirs in Venezuela and Canada presents foamy oil behavior. The formation and displacement mechanism of foamy oil have been extensively studied. Maintaining a certain pressure drop rate is a prerequisite for the formation of stable foamy oil flow in situ and for favorable cold production performance in such reservoirs. However, for discontinuous depletion process in such reservoirs, the occurrence state and displacement characteristics of foamy oil flow are still not clear. A specific 1D long core displacement experimental device with micro-visualization function was designed. Three 1D core pressure depletion experiments were conducted using a typical heavy oil sample collected from the Heavy Oil Belt in Venezuela, and the displacement characteristics and oil/gas occurrence state during the continuous and discontinuous depletion processes for foamy oil were compared. In addition, the influence of pressure drop rate on production performance and the microscopic characteristics of foamy oil flow after the resumption of discontinuous depletion process was investigated. The experimental results show that small dispersed gas bubbles tend to coalesce into large bubbles and form free gas at the shut-in stage during the discontinuous depletion process. Compared to the case of continuous depletion process, after the discontinuous depletion process was resumed, the foamy oil displacement efficiency became lower, the peak oil production rate decreased, the pseudo bubble-point pressure increased, and the recovery factor became lower. The practice of increasing the pressure drop rate properly can reduce the pseudo bubble-point pressure and improve the foamy oil displacement efficiency to a certain extent. The study provides a theoretical basis for improving the performance of the discontinuous depletion process in foamy oil reservoirs.