Microscopic Mechanism of CO2 Flooding in Extra-low Permeability Reservoirs
ZHANG Yingzhi1,2, YANG Zhengming3, TANG Ligen3, ZHANG Shuo3, LIU Xuewei3
1. College of Petroleum Engineering, China University of Petroleum, Beijing 100029, China;2. Daqing Oilfield Company Limited, PetroChina, Daqing 163712, Heilongjiang Province, China;3. PetroChina Research Institute of Petroleum Exploration & Development-Langfang, Langfang 065007, Hebei Province, China
Abstract:By means of NMR, oil gas chromatography, slim tube experiments, and extra-low permeability core physical simulation experiments, the produced oil and residual oil acquiring from slim tube experiments both in CO2 immiscible flooding and CO2 miscible flooding are analyzed. Combined with the NMR results obtained from the physical simulation experiments for extra-low permeability core, the microscopic mechanism of CO2 flooding in low permeability reservoirs is discovered. It includes three aspects: The components before C18 are mainly extracted by CO2 immiscible flooding, while the CO2 miscible flooding not only could extract the components before C18, but also could extract components C19-C25; low-carbon molecules are initially extracted, with the ongoing flooding, higher-carbon molecules are gradually extracted; Compared with crude oil and immiscible flooding residual oil, the components of miscible flooding residual oil consist of smaller proportion of light components and larger proportion of intermediate and heavy components, causing lager residual oil viscosity; The residual oil viscosity changes and the asphaltene precipitates caused by miscible and immiscible gas flooding make a part of pore fluid immobile and the graph of NMR shifted left.