QUANTITATIVE EVALUATION OF PORE STRUCTURES AND FULL-SCALE PORE SIZE OF LACUSTRINE SHALE RESERVOIR

Shale mainly develops nano-scale pores, so a single measurement experiment is difficult to effectively describe the full-scale pores of shale. In this paper, taking Shahezi Formation shale in Xujiaweizi area of Songliao Basin as an example, the full-size pore size distribution of lacustrine shale has been quantitatively characterized by a combination of high-pressure mercury injection, low-temperature N-2, and CO2 adsorptions. The research results show that the pore types of lacustrine shale in the study area include organic pores and inorganic pores. The diameter of organic pores is generally 0.01 to 1.00 gm, but it has poor pore connectivity. Inorganic pores mainly include micro-fractures, micro-channels and intercrystalline pores. Micro-fractures and micro-pores are connected to each other and constitute the main seepage migration channel in shale. Three-dimensional CT scans arc used to restore the three-dimensional spatial distribution of minerals, pores and micro-fractures in the shale. The connectivity between pores can be characterized by coordination numbers. The high-pressure mercury intrusion test results show that the average porosity of the target shale is 1.512% and the average total pore volume is 0.0035 cm(3)/g. This feature reflects the overall low total pore volume of the shale. According to the capillary pressure curve, displacement pressure and pore size distribution, the shale reservoir is divided into Type I, Type II and Type III reservoirs. The nitrogen adsorption curve of the target layer is characterized by type IV, and the pore morphology is a slit type pore. The relationship between pore parameters and depth is not obvious. The carbon dioxide adsorption results show that a large number of microporescale pores are developed in lacustrine shale. The pore size distribution has three peaks, which are mainly between 0.32 similar to 0.42 nm, 0.43 similar to 0.73 nm and 0.74 similar to 0.98 nm. Finally, a combination of mercury intrusion method, cryogenic liquid nitrogen adsorption method and CO2 adsorption method are used to quantitatively characterize the full-scale pore size of shale. This research can provide scientific guidance for the evaluation of similar lacustrine shale reservoir space.