Abstract
Stable gas gravity drainage is considered an effective method to enhance oil recovery, especially suitable for deep buried, large dip angle, and thick oil reservoirs. The influence of reservoir heterogeneity on controlling the gas–oil interface and sweep characteristics of injected gas is particularly important to design reservoir development schemes. In this study, according to the interlayer characteristics of Donghe carboniferous oil reservoirs in the Tarim Basin, NW China, 2D visual physical models are established, in which the matrix permeability is 68.1 mD and average pore throat radius is 60 nm. Then, hydrocarbon gas gravity drainage simulation experiments are carried out systematically, and a high-speed camera is used to record the process of gas–oil flow and interface movement. In this experiment, the miscible zone of crude oil and hydrocarbon gas is observed for the first time. The interlayer has an obvious shielding influence, which can destroy the stability of the gas–oil interface and miscible zone, change the movement direction of the gas–oil interface, and reduce the final oil recovery after gravity drainage. The remaining oil mainly is distributed near the interlayers. The higher displacement pressure leads to increased stability of the gas–oil displacement front and later gas breakthrough, which leads to higher oil recovery. The lower gas injection rate contributes to a slower front velocity and wider miscible zone, which could delay gas breakthrough. For the immiscible gas gravity drainage, there is a critical gas injection rate, with which the oil recovery factor is the highest.
Highlights
Low-permeability reservoirs account for about 46% of oil and gas resources in China (HU et al, 2018)
In practice, the sweep characteristics of gas drainage and remaining oil distribution are affected by interlayers and fractures in oil reservoirs
To reveal the influence of barriers, interlayers, and fractures on the sweep characteristics of immiscible nitrogen gravity drainage, the homogeneous model and the double vertical fracture model were used in the experiment
Summary
Low-permeability reservoirs account for about 46% of oil and gas resources in China (HU et al, 2018). Due to the poor physical properties, fracture development, and strong heterogeneity, the water flooding efficiency in low-permeability oil reservoirs is low. It is crucial to exploit the remaining oil of low-permeability oil reservoirs with advanced technologies. Several studies have shown that the oil recovery factor of low-permeability sandstone reservoirs can be improved by gas injection after water flooding (Liu, 2012). Gas injection flooding mainly includes nitrogen flooding, air flooding, hydrocarbon gas flooding, and CO2 flooding (Feng et al, 2019). Janssen et al (2018) analyzed several immiscible nitrogen injection schemes and found that compared with the two-phase flow, the residual oil saturation after immiscible nitrogen flooding was Gas injection flooding mainly includes nitrogen flooding, air flooding, hydrocarbon gas flooding, and CO2 flooding (Feng et al, 2019). Janssen et al (2018) analyzed several immiscible nitrogen injection schemes and found that compared with the two-phase flow, the residual oil saturation after immiscible nitrogen flooding was
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