Dynamic threshold pressure gradient characteristics of CO2 injection in coal-measure tight sandstone reservoirs

Abstract

The threshold pressure gradient (TPG) of gas-liquid seepage in tight sandstone varies with the rock effective stress, that is, the dynamic threshold pressure gradient effect, its dynamic variation characteristics are controlled by the pore-throat structure. In order to explore the characteristics of dynamic TPG of reservoirs in the process of multi-layer co-injection CO2 development in coal-measure tight sandstone reservoirs, the TPG tests were conducted on cores with different physical properties at different effective stress. The influence of rock pore-throat structure and permeability on the dynamic TPG was quantitatively studied based on the fractal analysis method, the calculation method of dynamic TPG was established. The dynamic and fixed threshold pressures and the corresponding gas productivity loss in reservoirs during CO2 injection were calculated. The experimental results show that the effective stress of tight rocks increased from 1 MPa to 29 MPa, and TPG showed a logarithmic increasing trend with effective stress, with an increase range of 31.2%−120.1%. The TPG value depended on the rock permeability at low stress and showed a power-decreasing trend with permeability. The fractal dimension of the pore-throat structure controlled the dynamic TPG stress sensitivity. The fractal dimension of the rock pore-throat structure and the TPG sensitivity coefficient showed a power-increasing trend. The larger injection pressure and smaller spacing between injection and production wells could reduce the additional pressure consumption caused by dynamic threshold pressure, and reduce the interlayer interference during CO2 co-injection. The calculated minimum effective injection pressure of the reservoir was 32.8%−69.6% lower according to the fixed TPG. The more complex the pore-throat structure of the reservoir, the greater the deviation of the estimated value. When the injection pressure increased to 29 MPa, the dynamic threshold pressure decreased by 3.4−7.4 MPa, and the additional pressure loss caused by the dynamic threshold pressure was 1.0−5.8 MPa. The differences in the sweep range of injected CO2 under the influence of dynamic and fixed TPG under the same injection pressure were 40.5%−51.3%. The reservoir productivity loss caused by TPG at maximum injection pressure was 24.6%−65.3%, which was 4.1%−21.1% higher than the fixed TPG. The more complex the pore-throat structure of reservoir rock was, and the lower the injection pressure, the greater the difference in productivity loss caused by dynamic threshold pressure. In the development of multi-layer CO2 co-injection in the coal-measure tight sandstone reservoir, the reservoir pressure should be prevented from weakening to a lower reservoir pressure value. A larger injection pressure and a smaller injection-production well spacing should be adopted to reduce the additional production capacity loss caused by the dynamic TPG effect, at the same time, the interference between layers should be mitigated to improve the multi-layer CO2 co-injection development effect.

Keywords

• dynamic threshold pressure gradient
• coal-measure tight sandstone
• co2 injection
• pore-throat structure
• productivity loss