Advanced Pore and Rock Type Characterization Using NMR T2 Gamma Inversion in Carbonate Reservoirs

Abstract

Carbonate reservoir complexity imposes some challenges in formation evaluation and characterization. Grain, pore, and throat size distributions play major roles in rock typing to understand static and dynamic behaviors of carbonate reservoirs. Special core analysis techniques, such as MICP and digital core imaging, revealed that the presence of different types of pore structures can be classified based on sizes as micro, meso and macro pores. This paper explores a unique inversion technique using nuclear magnetic resonance (NMR) data to deliver fast, accurate, and continuous pore typing across logged intervals. The traditional NMR data processing technique consists of sequential steps that ultimately convert echoes from time domain into T2 domain using an exponential inversion, also known as Laplace transform. NMR-gamma inversion (NMR-GI) workflow is a mathematical approach to process the NMR data using probabilistic functions. The gamma inversion function has a form of a bell curve with the base in the logarithmic x-axis. Unlike exponential inversion technique, this inversion produces multiple components. Each component is located at a particular time and it is labeled with a specific number, which reflects the T2 time stamp of each component. The individual area under each component is translated into porosity units. The application of gamma inversion in a T2 spectrum results in subcomponents via deconvolution of the original spectrum. The display of the components makes it easy to visually analyze and interpret the porosimetry for pore size distribution and group the components for pore typing. A deeper look into the different components of the T2 spectrums enlarges the NMR measurement portfolio by displaying the porosimetry and pore size distribution. The inverted T2 results from both logging while drilling (LWD) and wireline tools in different fields are consistent with the reservoir geological and petrophysical models. The added value of this technique can be tangible for geophysical and geological (G&G) applications as well as completion design optimization. The NMR-GI technique can be further utilized and calibrated to improve reservoir understanding and optimize advanced core analysis by providing a quick continuous carbonate pore and rock type log.