Chapter 4 - Multifractal Analysis of Pore Structure of Tight Oil Reservoirs Using Low-Field NMR Measurements

Abstract

Fractal and multifractal analysis are commonly used to evaluate pore structure and its heterogeneity. Compared with fractal analysis, multifractal analysis can determine the globe and local properties of pores in rocks. Low-field NMR measurement in laboratory and field wells is an important method for studying pore structures and other petrophysical parameters of reservoirs. In this chapter, the multifractal analysis based on the NMR T2 distributions is applied to investigate the pore structure and heterogeneity of the unconventional tight oil reservoirs of the Permian Lucaogou Formation of Jimusaer Sag, Junggar Basin. Porosity, permeability, mineralogical compositions, and NMR experiments were conducted on samples. Two parameters, T35 and T50, are newly defined as relaxation times corresponding to the 35% and 50% saturation on the T2 reverse accumulative curve, respectively. It is observed that the reservoir quality index (RQI), and flow zone indicator (FZI) values were extremely low. The NMR T2 distributions of water-saturated samples has a much shorter relaxation component with little or no long relaxation component. The T2lm values are in the range of 2.01ms to 42.63ms with an average of 14.09ms, indicating a complex microscopic pore structure. The singularity spectrum, generalized dimension and related parameters demonstrate a multifractal nature of T2 distribution of the studied samples. The values of the important multifractal parameter, singularity strength range, are high, varying from 2.405 to 4.253 with an average of 3.495. This reveals that the pore structures of the samples are strongly heterogeneous. The clay content has an obvious influence on the multifractal parameters, and the FZI and newly defined T35 are found to be two good indicators for heterogeneity of pore structure. The effectiveness and reliability of the multifractal analysis on low-field NMR measurements is further verified through comparison with conventional sandstone reservoirs.