Investigation on immiscible N2 WAG and SWAG after water flooding in the paleo-subterranean river of fractured-vuggy reservoirs

Abstract

The complex spatial distribution of caves and fractures results in the low oil recovery after water flooding in paleo-subterranean river of fractured-vuggy reservoirs. This paper tries to solve the problem of how to adopt water-alternating-gas (WAG) and simultaneous-water-and-gas (SWAG) to enhance oil recovery in paleo-subterranean river reservoirs. Aiming at this objective, we constructed one physical experimental and numerical model according to the geological section in Tahe Oilfield. The research on water flooding, WAG, and SWAG were carried out through experiments and simulations. The results show that there were four kinds of remaining oil after water flooding in the paleo-subterranean river karst reservoirs, i.e., blind end remaining oil, shielded remaining oil, attic remaining oil, and corner remaining oil. WAG can extract the shielded remaining oil, attic remaining oil, and part of the corner remaining oil efficiently, while SWAG can only work on the attic remaining oil. In addition, too low injection rate lacked driving force support and restricted the expansion of the swept zone. Too small gas-water ratio and slug size can’t play a good role in gas displacement. This paper is the first systematic study of the paleo-subterranean river reservoirs, and the results can provide a basis for production practice.