FRACTAL ANALYSIS OF PORE STRUCTURE HETEROGENEITY AND ITS IMPACT ON RESERVOIR PHYSICAL PROPERTY IN LOW PERMEABILITY SANDSTONE RESERVOIR: A CASE STUDY OF THE CHANG 6 RESERVOIR IN THE ANSAI OILFIELD, CHINA

It is critical to clarify important roles of pore structure and microscopic reservoir heterogeneity in the evaluation and development of low to ultra-low permeability reservoirs. As exclusively individual experiments are insufficient in effectively characterizing the full-range microscopic reservoir heterogeneity and pore throat structure, this study comprehensively employed a series of methods on samples collected from low to ultra-low permeability Triassic Chang 6 Reservoir in the Ansai Oilfield, China, including pore-casted thin section observation, SEM image analysis, constant-rate mercury intrusion test, NMR experiment, and fractal characteristic analysis. Specifically, the feasibility of using NMR T-2 spectrum to characterize the full-range pore throat parameters was explored in details. Chang 6 Reservoir is featured by its low porosity (averaging 12.91%) and low permeability (averaging 2.69 mD). Residual intergranular pores and dissolved pores are the main pore types, and some pore throats are seen with a certain amount of microcrystalline minerals and micro fractures, which increase the microscopic reservoir heterogeneity. The throat radius significantly varies from sample to sample, with the average pore radius range of 0.450-3.351 mu m. Reservoir physical properties are significantly controlled by the average throat radius and the mainstream throat radius. NMR T-2 spectra of all nine samples have obvious bimodal characteristics, and the peak heights are poorly related to the reservoir physical properties. Fractal dimensions determined using the tube-shaped pore model are lower than those using the spherical pore model, with respective average fractal dimensions of 2.2688 and 2.8578. Through comparison with pore throat parameters, it is found that fractal dimensions determined using the tube-shaped pore model are obviously negatively correlated with the displacement pressure and pore throat radius ratio, while positively correlated with the average throat radius. For low to ultra-low permeability reservoirs, fractal dimensions determined using the tube-shaped pore model can not only reflect the pore throat complexity, but also effectively characterize reservoir pore throat structure, which can help deepen understanding of the pore structure of low to ultra-low permeability reservoirs.