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Description
Vesicular volcanic reservoirs, characterized by well-developed micro-fracture and vug combinations, exhibit favorable connectivity and high hydrocarbon enrichment. However, drilling in such formations frequently encounters downhole incidents such as lost circulation and pipe sticking, with high and severe loss rates, complex leakage mechanisms, and low success rates in primary plugging operations. To address these challenges, this study employs three-dimensional X-ray micro-computed tomography (micro-CT) to reconstruct digital rock cores, combined with scanning electron microscopy (SEM), to clarify the occurrence of fine-to-microscopic fractures and vugs as well as pore characteristics in the volcanic rocks of the Feng-1 member in the Mabei area. The distribution patterns of vug clusters under varying vesicle contents are delineated, and the leakage mechanisms dominated by pore structures at fine-to-microscopic scales are revealed. Digital core analysis clearly shows that the vesicular volcanic reservoirs in the Feng-1 member of Mabei have an average pore volume proportion of 26.7%. The vesicles exhibit irregular elliptical structures with significant size variations, ranging from 4 mm to 30 mm within a single standard core plug. The distribution of vesicular structures is highly heterogeneous, with pore volume proportions varying from 3% to 33%. In low-vesicle-content samples, pores are isolated and distributed along boundaries. In medium-vesicle-content samples, pores are uniformly distributed, with micro-fractures and vesicles forming an oriented three-dimensional fracture network. In high-vesicle-content samples, large pores dominate, with a high proportion of isolated pores, and permeability is constrained by the absence of micro-fractures. SEM results indicate that polygonal dissolution pores account for 43% of the micro-pores, while elongated intergranular pores constitute 52%. On average, approximately 24 micro-pores with diameters greater than 50 nm are developed per 1 μm². Dissolution pores are often filled with organic matter, with some containing inorganic cement. Micro-fractures extend in a lightning-like pattern from low-vesicle-density zones to high-vesicle-density zones. The leakage type is primarily a pore–fracture composite loss. Both vesicle content and density jointly regulate the loss volume and rate. The connectivity between micrometer-scale micro-fractures and dissolution pores significantly enhances the complexity of leakage pathways, while cementation inhibits leakage by blocking throats.
| Country | China |
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