Analysis of Geological Characteristics and Reservoir Potential Formation Damage Factors of Shallow Low-Temperature Low-Pressure Low-Permeability Sandstone Reservoir

Abstract

The C-S reservoir in the YQ district of Ordos basin, China, is located at a relatively shallow depth (240–720 m), with an original pressure coefficient of approximately 0.85 for the oil layer. Calculations indicate that the initial pressure of the oil layer ranges from 4.1 to 6.0 MPa, averaging 4.75 MPa, with an average temperature of 30°C. The reservoir is classified as shallow, low-pressure, low-temperature sandstone. This research examines the C-S tight sandstone oil reservoir located in the Ordos Basin, providing an in-depth analysis of its mineral and rock composition along with its porosity and permeability characteristics. Through the analysis of the microscopic geological features of the reservoir, significant geological factors that may contribute to reservoir degradation are identified. Research shows that the C-S reservoir has an average porosity of 8.39%, average permeability of 0.54 × 10⁻³ μm², a micro-thin-necked pore type, and a median pore radius of 1.9060 μm. The reservoir exhibits strong heterogeneity, characterized by low porosity and permeability. Laboratory experiments revealed sensitivity characteristics including weak sensitivity to velocity and water, as well as moderate sensitivity to acid and salt. Water-phase seal test results show that the self-absorption rate decreases to less than 0.1 g/h within about 12 h, leading to significant water-phase seal formation damage due to high water saturation (above 45%) within a short time. The research suggests that limited fluid passageways in the reservoir result in insufficient in situ energy for fluid migration and increased viscosity, which complicates the process of returning fractured reservoirs to their original state after digitalization.