Permeability estimation from NMR diffusion measurements in reservoir rocks.

It is well known that in restricted geometries, such as in porous media, the apparent diffusion coefficient (D) of the fluid depends on the observation time. From the time dependence of D, interesting information can be derived to characterise geometrical features of the porous media that are relevant in oil industry applications. In particular, the permeability can be related to the surface-to-volume ratio (S/V), estimated from the short time behaviour of D(t), and to the connectivity of the pore space, which is probed by the long time behaviour of D(t). The stimulated spin-echo pulse sequence, with pulsed magnetic field gradients, has been used to measure the diffusion coefficients on various homogeneous and heterogeneous sandstone samples. It is shown that the petrophysical parameters obtained by our measurements are in good agreement with those yielded by conventional laboratory techniques (gas permeability and electrical conductivity). Although the diffusing time is limited by T1, eventually preventing an observation of the real asymptotic behaviour, and the surface-to-volume ratio measured by nuclear magnetic resonance is different from the value obtained by BET because of the different length scales probed, the measurement remains reliable and low-time consuming.

Developments in core analysis by NMR measurements.

For a large suite of consolidated sandstone samples low in shale content we have measured the permeability k, irreducible water saturation Swi, porosity phi, electrical-resistivity formation factor F, porosity by NMR, geometric-mean relaxation times T1g, and stretched-exponential relaxation times T1s. We find that T1g (or T1s) is the decisive parameter for the estimation of k or Swi of porous sandstones by other than direct measurements of these quantities. The additional use of phi or F brings appreciable, but not decisive, improvement. We show isovalue maps of the error factor delta, which show substantial regions of near-minimum values of delta and show basic compatibility of our estimators for permeability with different published estimators. The exponents of T1g (or T1s) in our power-law estimators and those of various published estimators for k are not very far from 2.0 if either or both of phi and F are also used in the estimators.

MRI as a tool for the study of waterflooding processes in heterogeneous cores.

NMR imaging has shown itself to be an important tool for improving analysis of flow behaviour during waterflooding in heterogeneous cores. Waterflooding is a widely employed technique in enhancing oil and gas recovery. However the success of such a process could be considerably reduced by instability of the displacing front with negative effects on production efficiency. MRI can be easily applied in evaluating the flow advancement because of its ability to distinguish different phases during dynamic experiments. In our study we have evaluated the shape of the displacing front during water injection in highly heterogeneous reservoir carbonates. The effect of petrographical heterogeneities which strongly characterize the rocks, revealed a marked influence on flow behaviour. Viscosity increase by polymer addition, in spite of a more favourable mobility ratio, resulted in a poorer performance because of higher channelling effects. The results of selected simulation experiments are discussed.

Water-air saturation changes in restricted geometries studied by proton relaxation.

The results are reported of a systemic T1 and T2 investigation of natural (sandstones) and artificial (microporous porcelain) porous media, after each step of a water desaturation process by centrifugation in air. The analysis of the relaxation curves permitted distinguishing well the different behaviour of the natural samples as compared to the artificial ones, which can be explained by the different pore structures. In both kinds of samples the evolution of the relaxation time distributions yielded a clear picture of the changes of the water distribution in the pore framework following the displacement process, until irreducible water saturation was attained. The results are compatible with the assumption of a fixed amount of surface area contributing to the relaxation of decreasing amounts of fluid as SW is reduced.

Capillary water determination in core plugs: a combined study based on imaging techniques and relaxation analysis.

NMR Spectroscopy and Imaging (MRI) have been used in determining the presence and the distribution of residual water inside a dolomite core saturated with oil. 1H relaxation analysis revealed the presence of different "classes" of water within the core as a function of drying temperature. The results appear to be consistent with the peculiar porosity distribution of the rock, as shown by MRI and X-ray CT analysis. The latter technique provided details of the rock matrix complementary to the information acquired by the NMR method. This kind of approach, based on the application of different techniques, was found to be very useful for an accurate evaluation of petrophysical properties of rocks, a task of relevant interest for an oil company.