The 8# coal seam in the Benxi Formation of the southeastern margin of the Ordos Basin is a deep coal seam with abundant coalbed methane resources. Calcite veins are commonly developed within the 8# coal seam, and their formation processes and mechanisms have significant implications for the enrichment of deep coalbed methane. Genesis of the calcite veins was analyzed to reveal the impact of the calcite veins formation on coalbed methane accumulation, with an integrated application of petrographic study by thin section, cathodoluminescence analysis, carbon-oxygen isotope analysis, and homogeneous temperature measurements of fluid inclusions. The research findings indicate that the calcite veins in the 8# coal seam can be classified into three stages: C1, C2, and C3. The diagenetic fluids of C1 primarily originated from contemporaneous seawater. The fluids responsible for the formation of C2 primarily consist of organic fluids enriched in biogenic gas, whereas the fluids contributing to the formation of C3 are primarily associated with liquid hydrocarbons originated form decarboxylation of organic matter. Furthermore, the development of both C2 and C3 is influenced by deep hydrothermal fluids resulting from tectonic heating events during the Early Cretaceous. By combining analysis of the hydrocarbon accumulation history and burial history in the study area, it has been established that C2 formation occurred during the Late Triassic to Early Jurassic, while C3 formation took place during the Late Jurassic to Early Cretaceous. The exploration and production practices in the study area have firmly established the crucial significance of the formation and evolution of calcite veins within the 8# coal seam for the migration and accumulation of coalbed methane. The research outcomes provide valuable insights for the exploration of deep coalbed methane enrichment areas.

Carbonate rocks typically exhibit strong heterogeneity and multi-bedding characteristics, which exert a significant impact on acidification performance. In this study, based on the dual-scale continuum model and the open-source software FMOT, we developed an acidification simulation program suitable for multi-layered rocks, and focused on investigating the influence laws of two representative characteristics—heterogeneity intensity and interlayer permeability difference—on acidification performance. The results show that heterogeneity intensity significantly affects the development morphology of wormholes in high- and low-permeability rocks: under low-heterogeneity intensity conditions, the wormhole morphology in both layers is close to a straight line; under high-heterogeneity intensity conditions, the wormhole morphology exhibits a dendritic shape. When the interlayer permeability difference is 40 mD, the acid breakthrough times of rocks under three heterogeneity intensities (Low, Medium, High) are [476 s, 463 s, 450 s], indicating that the higher the rock heterogeneity intensity, the faster the acid breakthrough speed. In terms of porosity increment, under this condition, the porosity increments of high-permeability rocks are [0.0952, 0.0921, 0.0868], while those of low-permeability rocks are [0.0321, 0.0308, 0.0297]. It is evident that the acidizing effect of high-permeability rocks is significantly superior to that of low-permeability rocks. A further analysis of the influence of interlayer permeability difference shows that reducing the interlayer permeability difference significantly slows down the acid breakthrough speed of rocks, while weakening the acidizing effect of high-permeability rocks and enhancing that of low-permeability rocks; conversely, increasing the interlayer permeability difference accelerates the acid breakthrough of rocks, strengthens the acidizing effect of high-permeability rocks, and inhibits that of low-permeability rocks.

Carbonate reservoirs are typically characterized by well-developed natural fractures, and complex fracture-network geometries can significantly affect the effectiveness and efficiency of acidizing. In this study, a numerical simulation program for acidizing reactive transport in fractured carbonate reservoirs was developed based on a dual-continuum theory coupled with an Embedded Discrete Fracture Model. By decoupling complex fracture characteristics, we systematically investigated the controlling mechanisms of fracture dip angle and feature length ratio (FLR) on wormhole evolution and acidizing efficiency. The results show that fractures with small dip angles favor the development of the main wormhole trunk, whereas fractures with large dip angles redirect wormhole growth and promote the formation of complex branches. Under low FLR conditions, wormhole propagation is primarily governed by matrix heterogeneity, while a higher FLR markedly enhances the influence of fracture dip angle on overall wormhole development. The response of acid breakthrough volume (${\mathrm{PV}}_{\mathrm{BT}}$) to dip angle is strongly regulated by FLR. When FLR < 0.2, ${\mathrm{PV}}_{\mathrm{BT}}$ first increases and then decreases with dip angle, reaching a maximum at 45°, whereas for FLR > 0.2, ${\mathrm{PV}}_{\mathrm{BT}}$ increases approximately linearly with dip angle, indicating a substantial reduction in breakthrough efficiency. This study provides a theoretical basis for optimizing acidizing treatments in heterogeneous fractured carbonate reservoirs.