Table 3 Statistical summary of the available data using violin plots, pair plots, and heat Maps.
From: Machine learning analysis of CO2 and methane adsorption in tight reservoir rocks
Plot Type | Feature/Relationship | Details and Results |
|---|---|---|
Violin Plots | CO2 Percentage | Data is mostly concentrated at lower percentages, with some spread in higher ranges. This indicates that most samples have low CO2 concentrations, but a few outliers suggest variability in the gas composition among different samples. |
TOC Percentage | TOC values are predominantly below 5%, reflecting the natural heterogeneity of organic matter in shale samples. Higher TOC levels could potentially influence adsorption capacity due to their impact on microporosity and adsorption sites. | |
Moisture | The distribution of moisture shows a wide range with significant variability. Samples with higher moisture content might have reduced gas adsorption due to competitive water adsorption at adsorption sites. | |
Temperature | Temperatures range from 20 to 160 °C, covering a wide spectrum of conditions. This broad range reflects the varying geothermal gradients and reservoir conditions, which significantly influence gas adsorption and desorption behaviors. | |
Pressure | Pressure data is mainly concentrated above 10 MPa, highlighting the high-pressure conditions typical of gas storage in shale reservoirs. Such pressures are critical for assessing the adsorption and phase behavior of gases under reservoir-like conditions. | |
CH4 and CO2 Adsorption | Adsorption values for both gases are generally low, indicating limited adsorption capacity in some shale samples, possibly due to low TOC or less-developed pore structures. | |
Pair Plots | CO2 Adsorption and Pressure | A clear positive trend; as pressure increases, CO2 adsorption rises. This indicates that pressure is a key driver in enhancing CO2 storage capacity in shale by increasing gas density and facilitating gas adsorption within nanopores. |
CH4 Adsorption and Pressure | Negative trend; higher pressure reduces CH4 adsorption. This may result from competitive adsorption with CO2 or changes in gas phase behavior at elevated pressures, leading to preferential adsorption of CO2 over CH4. | |
Heatmap (Pearson Correlation) | CO2 Percentage and CO2 Adsorption | Strong positive correlation (0.58) suggests that higher CO2 concentrations in the injected gas significantly enhance CO2 adsorption. This relationship emphasizes the role of partial pressure in determining adsorption efficiency. |
CH4 Adsorption and CO2 Adsorption | Noticeable negative correlation (−0.16) indicates competitive adsorption between CO2 and CH4. As CO2 adsorption increases, CH4 adsorption decreases, likely due to competition for limited adsorption sites. | |
CO2 Percentage and TOC | Strong positive correlation (0.61) highlights the role of TOC in influencing gas composition and its interaction with shale, potentially by providing additional microporous sites for CO2 adsorption. | |
TOC and CO2 Adsorption | Moderate positive correlation (0.34) indicates that TOC enhances CO2 adsorption. This is likely due to the presence of organic matter with higher affinity for CO2, increasing the overall adsorption capacity. | |
TOC and CH4 Adsorption | Weak negative correlation (−0.10) suggests TOC has a negligible or slightly adverse effect on CH4 adsorption. This might result from differences in the molecular interaction of CH4 and CO2 with organic matter. | |
Pressure and CO2 Adsorption | Weak positive correlation (0.18) shows that higher pressure moderately facilitates CO2 adsorption. This aligns with the observed density-dependent adsorption behavior of CO2 in shale reservoirs. | |
Pressure and CH4 Adsorption | Weak negative correlation (−0.17) indicates that increasing pressure may slightly hinder CH4 adsorption, possibly due to the dominance of CO2 at higher pressures. | |
Temperature and CO2 Adsorption | Weak negative correlation (−0.11) suggests that higher temperatures might reduce CO2 adsorption, likely due to increased gas desorption rates and reduced adsorption affinity at elevated temperatures. | |
Temperature and CH4 Adsorption | Moderate positive correlation (0.26) indicates that CH4 adsorption may slightly increase with temperature, possibly due to changes in gas mobility and shale properties, though the effect is not strong. |
