Experimental Study on Changes in Pore Throat Systems Owing to Liquid CO2 Cooling in Shale Oil Reservoirs

The interactions between CO2 and reservoirs under low-temperature cooling conditions can change the pore throat characteristics of the corresponding rocks. In this work, the interaction mechanisms between liquid CO2 cooling and pore throat systems of shale were researched using nuclear magnetic resonance (NMR) technology. Five typical shale core samples were selected and subjected to liquid CO2 cooling experiments under different conditions. Furthermore, the change ranges of the pores were quantitatively calculated based on the NMR T-2 spectra. The results indicate that the overall pore throat system of the shale changes after liquid CO2 cooling. The factors influencing the degree of change are the pore throat structure, liquid CO2 cooling temperature, and cooling time. The degree of change for different pores are related to the relative contents of the pores in the initial state. When the liquid CO2 cooling experiments comprise one, two, and three iterations, the average overall increase in the NMR T-2 spectrum are 2.56, 5.81, and 8.97%, respectively, and the average increase of the smaller pores are 0.77, 0.92, and 1.84%, respectively. The overall pore throat system changes greatly with an increase in the liquid CO2 cooling times, and the degree of change in the smaller pores increases. When the liquid CO2 cooling temperature of core sample A2 ranges from -30 to -20 degrees C, the increase in the T-2 spectrum is between 3.57 and 2.20%. When the liquid CO2 cooling temperature of core sample B2 is increased from -30 to -20 to -10 degrees C, the increases in the T-2 spectrum are 2.97, 2.90, and 2.79%, respectively. As the temperature of the liquid CO2 increases, the change in the overall pore throat system is weakened. The smaller pores change more obviously after liquid CO2 cooling within a certain low-temperature range. The degree of change of the larger pores is less affected by temperature.

Automated summary

The interaction between CO2 and reservoirs at low temperatures can alter the pore throat characteristics of rocks. By using NMR technology, this study explored the impact of liquid CO2 cooling on shale pore throat systems. Factors such as pore throat structure, temperature, and cooling time influence the degree of change in the pore throat system, with smaller pores being more significantly affected.