Quantifying Controls on Threshold Pressure during CO₂ Injection in Tight Gas Reservoir Rocks

The simultaneous flow of gas and water is controlled by a threshold pressure gradient (TPG) effect during CO2 injection of tight gas reservoirs. The TPG effect is dynamic because it varies with both the effective stress and the water saturation. The sensitivity of TPG to effective stress and mobile water is affected by the pore-throat microstructure. In this paper, we report the results of dynamic TPG tests on six cores with similar permeability. The influence of the pore-throat microstructure on the sensitivity of the TPG to stress and to mobile water was also quantitatively studied using a fractal method, and the distribution of the threshold pressure and corresponding gas production loss were calculated during CO2 injection in tight gas reservoirs. The test results show that TPG decreases logarithmically with the increase of the pore fluid pressure during CO2 injection, a change of 0.1–50 MPa in the pore fluid pressure corresponding to a 1.8–3.5 times increase of the TPG variation. The TPG increases exponentially by 3.5–6.7 times from irreducible water saturation to a mobile water saturation of 30%. The fractal dimension (D) of the heterogeneity of the rock pore-throat microstructure has a linear relationship with both the stress sensitivity coefficient (λ) and mobile water sensitivity coefficient (η), with the larger values of λ and η being associated with more heterogeneous pore-throat microstructures. The reservoir threshold pressure showed a significant nonlinear distribution in near-well reservoirs at low bottom-hole flow pressures of the production well during CO2 injection. The calculated gas well production loss as a result of a dynamic threshold pressure is 7–16% higher than that of the fixed threshold pressure, and the difference is larger at low pressures.