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

Abstract

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 °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 °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.

Influence of Sand Production Damage in Unconsolidated Sandstone Reservoirs on Pore Structure Characteristics and Oil Recovery at the Microscopic Scale

“…NMR technology can reflect the occurrence of fluid in pore throats of different sizes. − Therefore, we used NMR technology to quantitatively evaluate the sand production characteristics and microscopic mechanisms of unconsolidated sandstone reservoirs during water flooding. As shown in Figure , the initial saturated crude oil in the pore throat with a radius of 10 1 –10 2 ms is represented by ( S i ), the resaturated oil in this area after water flooding is represented by S o , and the damage degree of sand production in unconsolidated sandstone can be calculated as follows b = prefix− S i − S normalo S i × 100 % where b is the damage degree of sand production (%), S i is the T 2 spectrum frequency area of the secondary saturated oil, and S o is the T 2 spectrum frequency area of the initial saturated oil.…”

Advances in Liquid Nitrogen Fracturing for Unconventional Oil and Gas Development: A Review

“…In the 1970s, Tenneco Oil Company proposed injecting cryogenic fluid into coal and rock fractures to increase natural gas production. − Industrial field tests were successfully conducted by B. W. McDaniel in 1997 and Steve R. Grundman in 1998 and proved that repeated fracturing using LN 2 can increase the production of unconventional oil and gas. − Between the late 1990s and the early 21st century, unconventional oil and gas production met demand from the energy market, and research on LN 2 fracturing stagnated. − In recent years, the share of unconventional oil and gas in global oil and gas production has continued to increase. As a result, LN 2 technology, fracturing mechanism, and influencing factors have once again become the focus of many research studies. − Liquid nitrogen fracturing has many advantages over traditional fracturing technologies.…”

Deformation-Failure Characteristics of Coal with Liquid CO2 Cryogenic-Freezing Process: An Experimental and Digital Study

“…and variation ratio of its surface or internal area, especially coal permeability and mechanical characteristic response. Additionally, deep analysis using mathematical models or software to comparatively investigate the evolution laws of coal microstructures and permeability coefficients that are quantitatively described by a threedimensional (3D) visualization method is still required [29,30]; even the visualization and meticulous quantification of the deformation-failure characteristics of coal microstructures before and after LCO 2 cryogenic freezing has become one of the hotspots in related research fields [31,32].…”

“…In this paper, the X-ray computed tomography (X-ray CT) method is utilized to observe the morphological images of the coal microstructure before and after LCO 2 cryogenic freezing [32,33]. Then, we used digital software to reconstruct the 3D visualization and percolation theoretical models to investigate the variations in basic structural parameters (scale parameters and permeability) among the different metamorphic degree coal samples, Furthermore, the macro-mechanical and fractal properties are analyzed to further describe the damaged degree of coal under the corresponding experimental conditions.…”