Experimental study on the CH4/CO2 competitive adsorption behaviors of typical shale minerals in low pressure reservoirs

Abstract

With the large-scale exploitation of shale gas resources, there are more and more shale gas wells with pressure exhaustion and even abandonment in low pressure reservoirs. However, these shale gas wells are rich in resources and still have great exploitation potential and utilization value. On this basis, the basic physical parameters of typical shale minerals (montmorillonite, calcite, and illite), such as pore structure, were investigated through carrying out low temperature nitrogen test based on field research and mineral composition tests. Meanwhile, by performing the volumetric method adsorption experiment, the competitive adsorption behavioral characteristics of minerals under the conditions of temperature at 70–130℃, pressure at 0–6 MPa and different gas mixture ratios of CH₄/CO₂ were studied. In addition, the key influencing factors and rules of the competitive adsorption of typical shale minerals under the high temperature and low pressure conditions were revealed. The results showed that under the experimental conditions, the lower temperature and the higher pressure led to the larger adsorption capacities of three minerals, and the growth rate of adsorption capacity gradually slowed down with the increase in pressure. To be specific, the order of minerals adsorption capacity montmorillonite > calcite > illite, proved that the larger mineral specific surface area resulted in the larger pore volume, the better pore structure development, and the superior adsorption performance. At the optimal adsorption effect (70℃, 6 MPa, 75 %CO₂ + 25 %CH₄), the adsorption capacity of montmorillonite was 0.381 mmol/g, while that of illite was 0.085 mmol/g, and that of calcite was 0.164 mmol/g. Because carbon dioxide occupied most of the adsorption sites and spaces on the surfaces of the three minerals during their competitive adsorption, carbon dioxide had stronger competitive adsorption performance. The results in this study prove to a certain extent that under the low pressure and high temperature conditions, increasing the carbon dioxide concentration is more conducive to shale gas displacement, so as to improve the shale gas recovery rate.