Research on Pore Structure Characteristics and Rational Production Scheme of the Changling Shale Oil, Southern Songliao Basin

This research first investigated the types and sizes of pores of the Changling shale in the Songnan area (southern Songliao Basin) via scanning electron microscopy (SEM) and reveals that the Changling shale oil reservoir has various storage space types and wide pore size distribution range. Nuclear magnetic resonance (NMR) experiments show that the pores of the shale oil reservoir are mostly characterized by bimodal and tri-model pore size distribution. The bimodal distribution indicates that the reservoir contains micron-sized inorganic macro pores with nano-sized pores, and the tri-model distribution suggests bedding seams or structural fractures. The Changling shale has microscopic heterogeneity featuring the coexistence of nano-sized organic pores and inorganic micron-sized pores and fractures. Dissolved gas driving experiments show that controlling the drawdown pressure can greatly improve oil recovery, and the smaller the drawdown pressure is, the higher the ultimate oil recovery grows. Also, the differences in oil recovery with different pressure drop rates are dependent on reservoir permeability. The smaller the permeability is, the larger the differences become, representing greater effects of the drawdown pressure. This is because in pure shale, pores and throats are small, nano pores are extensively distributed, and the capillary pressure is large, which delays the coalescence of bubbles and gives full play to the oil displacement by dissolved gas. Smaller permeability is associated with greater capillary pressure, and the above effects are stronger. The production process of shale oil can be divided into three stages, namely those of the elastic drive, discrete bubbles and continuous gas phase. A smaller pressure drop rate prolongs the discrete bubble stage, which is in favor of crude oil recovery. Tests also show that the movability of fluids in pores below 20 nm is low, and the oil existing in organic nano pores is difficult to flow. The application of a pressure-controlled production scheme for pure shale oil is favorable for improving the EUR of wells. The findings of this research provide an important theoretical basis for the formulation of a rational production scheme for shale oil.