Textural and pore size analysis of carbonates from integrated core and nuclear magnetic resonance logging: An Arbuckle study

Abstract

The T2 relaxation times recorded by nuclear magnetic resonance (NMR) logging are measures of the ratio of the internal surface area to volume of the formation pore system. Although standard porosity logs are restricted to estimating the volume, the NMR log partitions the pore space as a spectrum of pore sizes. These logs have great potential to elucidate carbonate sequences, which can have single, double, or triple porosity systems and whose pores have a wide variety of sizes and shapes. Continuous coring and NMR logging was made of the Cambro-Ordovician Arbuckle saline aquifer in a proposed CO2 injection well in southern Kansas. The large data set gave a rare opportunity to compare the core textural descriptions to NMR T2 relaxation time signatures over an extensive interval. Geochemical logs provided useful elemental information to assess the potential role of paramagnetic components that affect surface relaxivity. Principal component analysis of the T2 relaxation time subdivided the spectrum into five distinctive pore-size classes. When the T2 distribution was allocated between grainstones, packstones, and mudstones, the interparticle porosity component of the spectrum takes a bimodal form that marks a distinction between grain-supported and mud-supported texture. This discrimination was also reflected by the computed gamma-ray log, which recorded contributions from potassium and thorium and therefore assessed clay content reflected by fast relaxation times. A megaporosity class was equated with T2 relaxation times summed from 1024 to 2048 ms bins, and the volumetric curve compared favorably with variation over a range of vug sizes observed in the core. The complementary link between grain textures and pore textures was fruitful in the development of geomodels that integrates geologic core observations with petrophysical log measurements.

Fractal Characterization of Tight Oil Reservoir Pore Structure Using Nuclear Magnetic Resonance and Mercury Intrusion Porosimetry

“…17 With the development of unconventional oil and gas, NMR is gradually applied to the study of the pore characteristics of unconventional reservoirs. [18][19][20] Yao et al applied the NMR to study the pore characteristics of coals and the strong correlation between transverse relaxation time T 2 and pore structures of coals were found; they used the T 2cutoff point to divide NMR T 2 spectrum and pore distribution models were established combining with MIP. 21 They also compared MIP, constant-rate-controlled mercury porosimetry, micro-CT and NMR for analyzing pore size distribution and found that NMR can better evaluate pore size distribution of coal under nondestructive conditions.…”

Evaluating the potential of carbonate sub-facies classification using NMR longitudinal over transverse relaxation time ratio

“…3). The discrepancy at low relaxation time, on the other hand, is caused by long echo time used in NMR log, which is 1.2 ms (Coates et al, 1999;Doveton and Watney, 2015). With a long echo time, signals from micropores which have relaxation times below 1.2 ms are not collected.…”

“…Combining with other logs, NMR log has been applied to zonation and lithology classification (or rock typing) for the last two decades. Previous studies use NMR log to identify the lithology by analyzing the partitioning of the pore size distribution (Doveton and Watney, 2015) and the variation of log mean relaxation time (T 2ml ) (Skalinski et al, 2006;Rastegarnia et al, 2017). However, there are other information from NMR logs are not fully exploited in rock typing, such as the longitudinal (T 1 ) to transverse (T 2 ) relaxation time ratio-T 1 /T 2 .…”

Rock fabric controls on pore evolution and porosity–permeability trends in oolitic grainstone reservoirs and reservoir analogs

“…From these T2 distributions, certain pore attributes can be extracted, including modal time (T2 Mode; cf. Doveton and Watney, 2014), mean time (logarithmic), curve peakedness (T2 Kurtosis), total porosity, and macro-/micro-porosity.…”