Multifractal analysis of pore structure of Middle Bakken formation using low temperature N2 adsorption and NMR measurements

Abstract

Understanding the complexity and heterogeneity of pore structure can improve the comprehensive characterization for complex unconventional reservoirs and better exploit those resources. In this paper, multifractal analysis was conducted on low temperature N2 adsorption and NMR T2 distribution for the pore structure investigation in Middle Bakken formation in Williston Basin, USA. Results show that the averages of specific surface area, total pore volume, the average pore size of five N2 adsorption samples are 3.31 m2/g, 0.008 cm3/g, and 19.57 nm, respectively. Six NMR T2 distributions are bimodal and the signals of left dominant peaks are mainly distributed between 0.3 ms and 10 ms. The T2 logarithmic mean value ranges from 2.18 ms to 2.95 ms with an average value of 2.57 ms. Both N2 adsorption isotherms and T2 distribution demonstrated the multifractal nature of different pore sizes. The key multifractal parameter, singularity strength range (Δα) calculated from N2 adsorption and T2 distribution are respectively distributed in scopes of 0.72–1.54 and 1.63–2.89, indicating that the heterogeneity of pore space from T2 distribution is stronger than that from N2 adsorption. It was observed that the specific surface, micro pore volume, and average pore size are correlated well with Δα. There are no any correlations between Δα and porosity and T2lm, but the cumulative porosity and width of right secondary peaks have an impact on the Δα, which means that the heterogeneity difference between the two methods is attributed by larger pore and micro fractures.