Investigation of Pore-Fracture Charging Sequences in Tight Reservoirs Based on Multi-Stage Pressure Injection Experiments with Wood's Alloy

The tight sandstone reservoir features the development of micro- and nano-scale pores and micro-fractures, contributing to a complex pore-throat structure. This complexity results in an indistinct charging sequence of oil and gas in different types of storage spaces during the accumulation period, thereby escalating the challenges associated with the exploration and development of tight oil and gas. Focusing on the Fuyu oil reservoir in the northern Songliao Basin, this study integrates large-field-of-view stitched scanning electron microscopy with mineral surface scanning techniques. Innovatively, a micro-nano scale comprehensive reservoir evaluation method, incorporating both pores and micro-fractures, is proposed. Within the study area, three predominant pore-fracture combination types are identified: intergranular pore-clay mineral shrinkage fractures, intergranular pore-brittle mineral intergranular fractures, and intragranular pore-clay mineral shrinkage fractures. Building upon this, Wood's alloy, exhibiting high-temperature rheological properties, is injected into rock cores under various pressure conditions. It is observed that with increasing injection pressure, the alloy injection process demonstrates a structured order, with a clear preference for charging intergranular pore-fractures over clay mineral-related pores. Furthermore, the alloy injection efficiency curve exhibits a distinctive parabolic shape. Based on the characteristic properties of Wood's alloy and crude oil, the injection pressure is equivalently transformed, reconstructing the micro-scale charging process of tight oil under reservoir conditions. Consequently, a sequential charging model for tight reservoirs is established, encompassing micro-nano-scale intergranular pore-fractures, nano-scale clay mineral intragranular pores, and shrinkage fractures. This model considers parameters such as source-reservoir pressure difference, storage space type, fluid properties, etc. From both qualitative and quantitative perspectives, it clarifies the microscopic accumulation sequence of tight reservoirs. This research focuses on the development of a new multi-scale evaluation method for tight reservoir storage spaces, combining fluid injection with visualization technology. The findings are crucial for the microscopic sweet spot evaluation and efficient development of tight reservoirs.