Microwave-assisted heating on sandstone subjected to liquid nitrogen freeze-thaw: Pore structures and temperature characteristics

Liquid nitrogen (LN 2 ) and microwave are the alternative methods for reservoir fracturing, which are rarely combined. To investigate the combined effects, sandstone is frozen with LN 2 before microwave heating (MI), and nuclear magnetic resonance (NMR), ultrasonic wave, and infrared thermal imaging (ITI) are used to understand the pore structures, moisture change, and surface temperature of the sandstone samples. With the heating time, the average surface temperature of the combining-treatment samples firstly increases from the room temperature (25 °C) to 144.7 °C (65 s) fast, and then increases slowly to 176.6 °C (95 s). For the individual MI, the temperature increases to 146.7 °C at 65 s. As 30 min of LN 2 freezing, the samples perform well in removing pore water during heating. The NMR results show that after LN 2 freezing, the seepage pores and total pores increase by 2.93% and 4.11%, respectively, and the pore connectivity enhances. However, the individual MI performs weak in enhancing the pore structures, forming a high vapor pressure (0.428 MPa at 65 s) and causing burst after 65 s. Due to the improved pore connectivity, the vapor pressure (0.378 MPa) and temperature are small at 65 s, and burst can be avoided. After freezing, the wave velocity decreases by 13.48% and the damage variable reaches 0.251. The velocity attenuation rates and damage variable gradually increase with heating time; under the same duration, the two variables of the combining treatments are greater than that of the individual treatments. This can prove a reference for gas production in sandstone reservoirs.