Test system and method for determining fluid migration front based on pulse pressure-enhanced distributed strain

A test method for determining a fluid migration front based on pulse pressure-enhanced distributed strain, wherein a test system adopted in the test method comprises a CO2 displacement brine unit, a pulse pressure injection unit, a confining pressure control unit, and a distributed optical fiber strain monitoring unit;

• in the CO2 displacement brine unit, a brine tank is connected to a four-way joint sequentially through a second valve, a second injection pump and a third valve, a first CO2 gas cylinder is connected to the four-way joint through a first injection pump and a first pressure sensor, and the four-way joint is then connected to a pore pressure injection hole of a core holder through a fourth valve and a second pressure sensor to communicate with a core;
• in the pulse pressure injection unit, a second CO2 gas cylinder is connected to the four-way joint sequentially through a third injection pump, a third pressure sensor and a fifth valve;
• in the confining pressure control unit, a deionized water bottle is connected to confining pressure injection holes of the core holder sequentially through a fourth injection pump, a seventh valve and a fourth pressure sensor to communicate with a confining pressure interlayer;
• and
• the distributed optical fiber strain monitoring unit comprises a distributed optical fiber, an optical fiber demodulator and a computer module, the distributed optical fiber being spirally wound and fixed onto the core;
• the test method comprises the following steps:
• S1, core preparation stage;
• S2, confining pressure control;
• S3, CO2 displacement brine stage:
• injecting brine in the brine tank into the core through the pore pressure injection hole by the second injection pump to ensure that pores inside the core are completely saturated;
• then, injecting CO2 in the first CO2 gas cylinder into the core using the first injection pump to perform a brine displacement experiment so as to ensure that a pore pressure does not exceed the confining pressure during the entire displacement process;
• and collecting the displaced brine into a gas-water collection tank;
• S4, injection of pulse pressure:
• with the fifth valve closed, adjusting a pulse pressure to a preset pulse pressure higher than the pore pressure but lower than the confining pressure by the third injection pump, and controlling the pulse pressure to be injected into the core of the core holder by opening and closing the fifth valve;
• S5, optical fiber strain signal acquisition stage:
• monitoring and recording distributed optical fiber strain data during pulse pressure injection in real time, with particular attention given to strain changes at a CO2/brine interface;
• extracting high-resolution strain data by the optical fiber demodulator, and analyzing and processing the data by the computer module;
• and visualizing the data, and generating a strain distribution map and a strain-time changing curve to identify the strain changes caused by the pulse pressure and determine a position of a CO2/brine migration front.