NMR Measurements in Carbonate Rocks: Problems and an Approach to a Solution

Abstract

Carbonate rocks are well known for their complex petrophysical behavior where, in contrast to siliciclastic rocks, different parameters, including porosity and permeability, usually are not directly related. This behavior is the result of thorough reorganization of porosity during diagenesis, and it turns prediction of reservoir quality of carbonate rocks into a challenge. The study presented here deals with the problem of utilizing NMR techniques in prediction of petrophysical properties in carbonates.We employ a visual porosity classification as a priori knowledge for better interpreting NMR data for prediction purposes. This allows for choice of suitable T 2 cutoff values to differentiate movable from bound fluids adapted for the specific carbonate rock, thus resulting in better interpretation of NMR data. The approach of using a genetic pore type classification for adapting the conventional method for T 2 cutoff determination, which originally was developed for siliciclastic rocks, is promising. Similarly, for permeability determination on the basis of NMR measurements, the classification of carbonate rocks based on porosity types also shows potential. The approach implemented here has the promise to provide a basis of standardized interpretation of NMR data from carbonate rocks.

Nanoscale Pore Structure Characterization and Permeability of Mudrocks and Fine-Grained Sandstones in Coal Reservoirs by Scanning Electron Microscopy, Mercury Intrusion Porosimetry, and Low-Field Nuclear Magnetic Resonance

“…Except for one fine-grained sandstone sample NT-S which has a relatively high value of 20.03 ms, all other samples are less than 5 ms with a median value of 2.68 ms. By comparison, these values are found to be significantly lower than those of other conventional reservoir rocks such as sandstones (standard 2 = 33 ms) and carbonates (standard 2 = 92 ms) [43,57]. It is already well understood that different types of rocks have varied 2 values originated from their differences in many aspects such as pore types, pore structures, and mineral compositions which also have close and complex interconnections among them [43]. In this study, such a difference in 2 values between our investigated mudrock and fine-grained sandstone samples and other conventional reservoir rock types may primarily attributed to their different lithologies and high contents of clay minerals in our investigated samples.…”

“…As listed in Table 5, the 2 values of all the investigated rock samples range from 1.55 to 20.03 ms. Except for one fine-grained sandstone sample NT-S which has a relatively high value of 20.03 ms, all other samples are less than 5 ms with a median value of 2.68 ms. By comparison, these values are found to be significantly lower than those of other conventional reservoir rocks such as sandstones (standard 2 = 33 ms) and carbonates (standard 2 = 92 ms) [43,57]. It is already well understood that different types of rocks have varied 2 values originated from their differences in many aspects such as pore types, pore structures, and mineral compositions which also have close and complex interconnections among them [43].…”

“…Based on NMR measurements, a few kinds of permeability estimation models have been proposed and further developed by a lot of previous researchers in the past few decades [38,40,43,44,57]. Among them, the mean 2 model (also called the SDR model) which is based on the SchlumbergerDoll-Research (SDR) equation [43,60] and the free-fluid model (also called the Timur-Coates model) which is based on the Timur-Coates (TC) equation [40,44] are commonly used to estimate permeability of conventional reservoir rocks such as sandstone and carbonate [57,58].…”

“…It features as a fast, convenient, and nondestructive tool for characterizing complex porous media, particularly the petroleum reservoir rocks such as carbonate and sandstone [37,38,43]. Besides pore distribution information, NMR can also distinguish adsorption pores and seepage pores based on 2 distributions at both the full water-saturated and irreducible-watersaturated conditions.…”

Experimental Analysis of Pore and Permeability Characteristics of Coal by Low-Field NMR

“…where k SDR is the NMR permeability calculated by SDR model, mD; and T 2,log mean is the geometric mean of transverse relaxation time, ms. Westphal et al [32] used the SDR model to predict the permeability of the limestone reservoir, but the calculation effect in the coal reservoir was poor. Based on the NMR porosity of movable water, Yao et al [11] proposed a PP model to calculate the permeability of coal reservoir.…”