Advances in Carbonate Characterization Using Low Field NMR
Mai, A. and Kantzas, A.
Accepted, 54th Annual Technical Meeting of the Petroleum Society held in Calgary, June 10-12, 2003.
Western Canada contains significant deposits of oil and gas in carbonate formations. Carbonates have fairly complicated pore structures with various types of porosity, thus the characterization of carbonates still remains a daunting task. Conventional log analysis of carbonates often leads to incorrect descriptions of the reservoir properties. Low field nuclear magnetic resonance (NMR) is an emerging technology that shows great promise in rock characterization. Previous results in the literature give disparaging accounts of the applicability of NMR in carbonate rock characterization, but this work demonstrates that low field NMR can be a valuable tool even in these reservoirs.
The data set for this experimental work consists of a large collection of core samples from many different fields in Canada. NMR spectra interpretations have been compared to other core analysis methods. Definite correlations have been observed between the NMR spectra properties and the results from conventional core analysis, which verifies that NMR spectra can be used to characterize even complex pore structures. Unfortunately, there is too much scatter in these correlations for them to be accurate to within less than an order of magnitude. The trends observed were developed using all the data from different formations. In this work, the data were divided into their respective formations. Within a formation, the properties of the NMR spectra are compared to the conventional data to develop correlations to predict T2cutoff, irreducible water saturation (Swi), and permeability. These results show that if the general NMR correlations developed can be tuned to specific formations, NMR can become a very useful tool for characterizing carbonate reservoirs.