Effect of Pore-Throat Heterogeneity on Gas–Water Flow in Tight Gas Reservoirs: From Micro- to Centimeter Scale

Abstract

The practice of gas reservoir development shows that tight sandstone gas wells show an ultralow water flowback rate, even if the displacement differential pressure is continuously increased. Scholars have studied the influence of physical properties, pore structure, and fluid properties on flowback efficiency, but the influence of heterogeneity on water phase flowback is still not sufficient, especially comprehensive research across different scales. To deal with this, we investigate the effect of heterogeneity on fluid flow capability through centimeter core-scale experiments and micrometer pore-scale simulations. First, on the basis of the bounds of different pore-throat structure testing methods, the full-scale pore size distribution (PSD) and throat size distribution (TSD) structures are characterized by splicing. Next, we extract the high-pressure mercury injection (HPMI) sorting coefficient and the pixel variation coefficient of pore segmentation in scanning electron microscopy (SEM) images as the characterization parameters of heterogeneity. Direct experimental evidence shows that the stronger the rock heterogeneity, the larger the sorting coefficient and variation coefficient and the weaker the fluid mobility. It is difficult to improve the gas phase seepage ability even with a continuously increasing displacement pressure differential. Finally, we explored whether the reason for this phenomenon is mainly controlled by heterogeneity. We carried out microfluidic simulations, and the simulation results provide a good complementary explanation for this phenomenon. The preferential flow pathway effect induced by strong heterogeneity makes it impossible to affect the fluid flow state by only changing the displacement pressure differential, and it is difficult for the displacement phase to establish a new flow channel. The findings at different scales provide unique insights into the combined effects of pore heterogeneity and displacement pressure differentials on fluid flow.