Threshold pressure gradient characteristics and drainage radius determination of low-permeability reservoirs

In view of the limited insights into the threshold pressure gradients (TPGs) of low-permeability reservoirs and inadequate methods for determining their drainage radii, we present a detailed analysis of the variation characteristics and mechanisms of the TPG in combining experiments with numerical simulations, and propose a method for calculating the drainage radii of low-permeability reservoirs which considering the stress conditions of actual reservoirs. The results indicate that the TPG of a reservoir is not a fixed value. Instead, it changes with the net confining pressure, exhibiting a strong positive correlation. The curve showing the relationship between the TPG and the net confining pressure follows a three-stage pattern. In the first stage, plastic deformations occur, primarily involving the closure of microfractures and the sharp contraction of large pore throats. This stage has a significant impact on the seepage capacity of reservoirs, with the TPG increasing by more than five times. The second stage is characterized by pseudoplastic deformations, principally reflected in the contraction of large pore throats and microfractures, with the TPG increasing by over 40 %. The third stage shows elastic deformations, characterized primarily by the contraction of small pore throats. This stage has a minimal impact on the seepage capacity, with the TPG increasing by only 17 %. Notably, the stress state of rocks in most actual low-permeability reservoirs corresponds to this elastic deformation stage. Based on the dynamic changes in the TPG and the presence of pressure drop funnels in actual reservoirs, we determine the changes in the net overburden pressure and then calculate the threshold pressure along a certain seepage distance in reservoirs through integration. Assuming that the threshold pressure equals the driving pressure difference, we derive the seepage distance (i.e., the driving drainage radius) through inversion. Finally, the feasibility of the proposed method for determining the drainage radius is verified through practical application. This study enhances the understanding of the characteristics of low-permeability reservoirs while also providing a significant theoretical basis and technical support for the assessment and exploitation scheme optimization of low-permeability reservoirs.