Mesopore Fractal Characteristics in Low-Permeability Sandstone Affected by High Temperature Using NMR

Abstract

Effectively evaluating the pore structure in sandstone proves to be a difficult task in the field of oil and gas seepage in unconventional reservoirs. Fractal theory demonstrates great potential as a feasible evaluation method. In this paper, we perform transverse relaxation time \(T_{2}\) spectrum tests of low-permeability sandstones subjected to high temperatures via low-field nuclear magnetic resonance (NMR). Introducing fractal theory, statistical analysis and quantitative characterization of pore structure from aspects of pore size distribution and fluid migration are carried out. Five types of pore characteristic fractal dimensions are calculated and the response relationship between fractal characteristics and physical parameters is established. The results demonstrate that the fractal dimension of the adsorption pores does not have fractal characteristics, while that of the seepage pores exhibits obvious fractal characteristics, as well as a strong negative correlation with temperature. This indicates the improvement of adsorption pore uniformity with increasing temperature and its gradual transition to an ordered state. The total pores and pores (corresponding to bound and movable water) exhibit fractal characteristics yet they are not as obvious as those of the seepage pores. An ideal negative correlation is formed with the fractal dimension of seepage pores and the porosity and permeability, indicating that the seepage pores following a high-temperature treatment have a decisive influence on the migration and storage capacity of the reservoir rock, while the extensive amount of developed adsorption pores have a poor connectivity with each other and their contribution to the physical quality of the rock is limited. The effect of high temperature leads to the development of seepage pores, accompanied by a significant increase in the complexity of large-scale pores, which intensifies the complexity and non-uniformity of the pore structure.