Acid imbibition and ion diffusion characteristics during fracturing and acidizing of volcanic rock reservoirs: A case study of Carboniferous volcanic rocks in Junggar Basin, NW China

Four types of volcanic rock samples, i.e. breccia, andesite, tuff, and dacite, selected from the Carboniferous in the Junggar Basin were characterized through experiments such as X-ray diffraction (XRD), scanning electron microscopy (SEM) and nuclear magnetic resonance (NMR) for identifying the acid imbibition and ion diffusion behaviors during fracture acidizing in volcanic rock reservoirs. The results demonstrate that the invaded acid dissolves the minerals and alters the pore structure in the reservoir. Volcanic rocks of different lithologies exhibit substantial variations in their acidification and dissolution effects. Breccia and andesite, which contain abundant calcite and other soluble minerals, show markedly improved pore connectivity after acidizing. In addition, pronounced differences are observed between the acid-induced dissolution responses of oil-rich and water-rich pores within volcanic rocks. In water-rich pores, acid-induced dissolution is dominated by H + diffusion, whereas in oil-rich pores, imbibition-driven dissolution is the primary mechanism. The hydrated hydrogen-ion network formed in water-rich pores enhances H + diffusion, facilitating uniform dissolution across pore scales. As a result, the pore structure becomes more homogenized, leading to a reduction in fractal dimension. In oil-rich pores, acid imbibition driven by capillary pressure is the predominant mechanism, enabling small pores to be dissolved preferentially, followed by medium to large pores. Consequently, the overall extent of acid erosion remains limited, and pore heterogeneity persists at a high level. Both the acid-imbibition and ion-diffusion processes exhibit a three-stage evolution: linear–transitional–stable. In the linear stage, the acid imbibition and H + diffusion distances scale proportionally with the square root of time. In the transitional stage, the H + diffusion rate decreases due to pore-throat blockage induced by the hydration and precipitation of clay minerals. Concurrently, acid imbibition and mineral dissolution enhance the fluid flow capacity, partially offsetting the attenuation of capillary pressure, and sustaining the increase in imbibition rate. In the stable stage, both acid imbibition and ion diffusion approach equilibrium.