Multifractal analysis of pore structure of Middle Bakken formation using low temperature N-2 adsorption and NMR measurements

摘要

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 N-2 adsorption and NMR T-2 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 N-2 adsorption samples are 3.31 m(2)/g, 0.008 cm(3)/g, and 19.57 nm, respectively. Six NMR T(2 )distributions are bimodal and the signals of left dominant peaks are mainly distributed between 0.3 ms and 10 ms. The T-2 logarithmic mean value ranges from 2.18 ms to 2.95 ms with an average value of 2.57 ms. Both N-2 adsorption isotherms and T-2 distribution demonstrated the multifractal nature of different pore sizes. The key multifractal parameter, singularity strength range (Aa) calculated from N-2 adsorption and T-2 distribution are respectively distributed in scopes of 0.72-1.54 and 1.63-2.89, indicating that the heterogeneity of pore space from T-2 distribution is stronger than that from N-2 adsorption. It was observed that the specific surface, micro pore volume, and average pore size are correlated well with Delta alpha. There are no any correlations between Delta alpha and porosity and T-21m, but the cumulative porosity and width of right secondary peaks have an impact on the Delta alpha, which means that the heterogeneity difference between the two methods is attributed by larger pore and micro fractures.