Permeability Prediction for Low Porosity Rocks by Mobile NMR

Abstract

Estimating permeability from NMR well logs or mobile NMR core scanner data is an attractive method as the measurements can be performed directly in the formation or on fresh cores right after drilling. Furthermore, the method is fast and non-destructive. Compared to T 1 relaxation times, commonly measured T 2 distributions are influenced by external and internal magnetic field gradients. We performed two-dimensional T 1 and T 2 relaxation experiments on samples of Rhaetian sandstone, a rock with low porosity and small pore radii, using a mobile NMR core scanner which operates within a nearly homogeneous static magnetic field. Because small pore sizes are associated with high internal magnetic field gradients, standard methods from NMR logging in the oil industry cannot be applied for accurate permeability prediction. Therefore, a new model theory was developed, which describes the pore radius dependence of the surface relaxivity q 2 by both an analytical and a more practical empirical equation. Using corrected q 2 values, permeability can be predicted accurately from the logarithmic mean of the T 2 distribution and the physically based Kozeny-Carman equation. Additional core plug measurements of structural parameters such as porosity, permeability, specific inner surface area and pore radius distributions supported the NMR results.

Combination of MRI and SEM to Assess Changes in the Chemical Properties and Permeability of Porous Media due to Barite Precipitation

“…We used both models to estimate the ratio of permeability of unreacted column (pure celestine) and reacted column (celestine and barite) to compare the results with the experimental data taken from Figure 6b; Table 2 gives a summary. Although, the absolute permeability values derived from [28] differ by a factor of 2-4, the experimental ratio of k unreacted /k reacted = 30 is very satisfactorily confirmed by the NMR-based results. The classical mean 1/T 2,eff model of Schlumberger-Doll yields a ratio of 12 [35], whereas the model of Pape [28] gives a closer value (20) to the experimentally observed ratio.…”

“…The empirical parameter a is specific for the mineralogy of the rock. Later, further elaborated models have been developed, here only the model of Pape et al is mentioned [28], which introduces a tortuosity parameter T hydr and a porosity specific factor c φ :…”

“…In Equation (5) r 2 corr is a corrected average pore radius, and T 2LM is the log-mean transverse relaxation time [28]. This allows the estimation of hydraulic conductivities for series of similar rocks with equal specific factors from NMR relaxometry, if the average pore radius is obtained from independent measurements such as X-ray CT, electron micrographs, or intrusion porosimetry.…”

“…The most widespread classical model of Kenyon relates k to the 2nd power of the average effective relaxation time and the 4th power of porosity. Alternatively, Pape et al have developed another model, where the permeability scales linearly with the relaxation time and quadratic with the mean pore radius and the porosity [28]. We used both models to estimate the ratio of permeability of unreacted column (pure celestine) and reacted column (celestine and barite) to compare the results with the experimental data taken from Figure 6b; Table 2 gives a summary.…”

“…Although NMRR is a widely used technique to determine rock properties such as porosity and pore connectivity, its application to monitor the evolution of dissolution and precipitation processes in porous media is to our knowledge not well explored [27]. In addition, we test the conventional NMR porosity-permeability relationships, which are used to calculate the permeability from relaxometry measurements and verify their applicability for evolving porous media due to mineral dissolution/ precipitation [28]. The experimental model system used in our study follow Poonoosamy et al, selecting the boundary conditions that led to different barite growth mechanisms in a single experiment [16].…”