Study of pore-fracture filling sequences in tight reservoirs based on multi-stage pressure injection experiments with Wood’s alloy
LIU Jipeng, XIAO Hanmin, JIANG Zhenxue, TANG Xianglu, XU Mingshuai
1 State Key Laboratory of Petroleum Resources and Engineering, China University of Petroleum-Beijing, Beijing 102249, China 2 Unconventional Petroleum Research Institute, China University of Petroleum-Beijing, Beijing 102249, China 3 Research Institute of Exploration and Development, PetroChina, Beijing 100083, China
The development of micro-nano-scale pores and microfractures in the Fuyu tight sandstone reservoirs of the Cretaceous Quantou Formation in the northern part of the Songliao Basin, as well as the complex structure of the pores and fractures, leads to the unclear sequence of tight oil and gas filling in different types of reservoir spaces during the formation period, which increases the difficulty of exploration and development of tight oil and gas. In this paper, a quantitative study is conducted on the filling characteristics of tight oil under different filling pressures in tight sandstone reservoirs, and a microscopic evaluation method of tight oil filling patterns based on pore-microfracture combinations is proposed by combining large field-of-view splicing swept electron microscopy and mineral surface scanning technology. The study shows that the Fuyu tight sandstone reservoirs of the Quantou Formation in the area can be classified into three main types of pore-fracture combinations: intergranular pore-clay mineral shrink fractures, intergranular pore-brittle mineral intergranular fractures and clay mineral intergranular pore-shrink fractures combinations. By injecting Wood’s alloy with high-temperature rheological properties into the core at different pressures, it can be observed that the injection of the Wood’s alloy in the different pore-fracture combinations shows orderliness with the increase of injection pressure, and the filling priority of the intergranular pores and fractures is obviously higher than that of the clay mineral-related pores and fractures, and the injection efficiency of Wood’s alloy shows an obvious parabolic pattern of rapid increase first, followed by a slow increase that tends to be stable eventually. Based on the properties of Wood’s alloy and crude oil, the injection pressure was equivalently transformed to simulate the microscopic filling process of tight oil under stratigraphic conditions, and accordingly the pore and fracture sequence filling model of a tight reservoir was established. Under the condition of high source storage pressure difference, the tight oil can be filled into the nanoscale intergranular pore-shrink fractures of clay minerals, and with the decrease of source storage pressure difference, the tight oil is gradually difficult to be filled into the nanoscale pore and fractures. When the source storage pressure difference is reduced to the lowest, the tight oil can only enter the micron-sized intergranular pores and the well-connected mould pores through the wide intergranular connecting throat by means of osmosis. This study combines the fluid injection and visualization techniques for the multi-scale evaluation of reservoir space in tight reservoirs, which is of great significance for the evaluation of microscopic sweet spots and efficient development of tight reservoirs.
