Determination of Petrophysical Properties of Carbonate Rocks by NMR Relaxometry

Abstract

Abstract This paper presents the results of the petrophysical analysis carried out on plugs from two oil-producing carbonate reservoirs using the technique of NMR relaxometry (NMRR). Transverse relaxation time (T2) measurements were carried out on a 2MHz spectrometer. Whereas porosity determination is straightforward, permeability and producible fluid estimation by NMRR is rendered difficult by the often heterogeneous, vuggy and fractured nature of carbonate formations. Yet, the availability of good estimators for K and Swi is important to maximise the benefits of both NMR measurements on plugs as well as the NMR log. In this work, permeability estimators of the type K = A * * T2gC were generated by "calibration" with traditionally measured permeability data. Likewise, starting from conventionally measured Swi values, "cut-off" times were calculated on the continuous distribution of transverse relaxation times curves below which the immobile fluid is assumed to lie. Excellent correlations were found between NMR-determined and conventionally-measured values of porosity. Unlike for homogeneous sandstones, no universal K estimator has yet been found for carbonate reservoirs. In fact, not only was a different set of A, B and C parameters found for each of the two reservoirs studied, but permeability estimation was improved by further subdividing each well into its lithofacies. Cut-off times for Swi determination were found to be 120 and 140 ms respectively, comparable to the 100 ms reported in the literature for carbonate rocks. Introduction NMR has recently re-emerged of as a tool for the extraction of petrophysical information both from core plugs and downhole (via the NMR log). Briefly, nuclear magnetic resonance studies the interaction of nuclear magnetic moments and applied external magnetic fields. When nuclei possessing magnetic moments are placed in a magnetic field, B0, the magnetic moments have a slight tendency to align themselves with B0, generating a bulk magnetisation, M0, and to precess about the axis of B0. If Mo is disturbed from this equilibrium alignment by application of a second, oscillating magnetic field, B1, orthogonal to B0, it will return to equilibrium by two processes, longitudinal relaxation, T1 and transverse relaxation, T2. The dependence of the NMR relaxation of fluids in porous media on the geometry of the porous network itself allows the determination of parameters such as porosity, permeability, Swi and pore size in a nondestructive and non-invasive manner. In this paper, we present some of the work that is becoming part of the routine procedure for the petrophysical analysis of core plugs in our laboratories. NMR analysis is rapid with respect to conventional methods, a single relaxation decay curve being acquired for each core plug. But apart from porosity, determination of these petrophysical parameters requires calibration by conventional methods. Once this calibration has been achieved, however, it can be applied not only to all core plugs unsuitable for conventional analysis, but also to the rock "in-situ" during the NMR log resulting in porosity, permeability and irreducible fluid determination while logging. Porosity determination by NMRR is rapid with respect to conventional methods. Permeability estimation has been applied with success to sandstone reservoirs. It has been our experience to find permeability estimation in carbonates problematic, there sometimes being little or no correlation between relaxation time and the K-controlling factor, pore-throat size. Division of a formation into its lithofacies often improves permeability estimation on core plugs. The possibility to estimate irreducible water saturation by NMRR using cut-off times means that Swi can be determined for any number of plugs starting from 4 - 8 traditionally measured data values. Application of these cut-off values during the NMR log acquisition provides a continuous producible fluid profile. The final part of this paper presents the results of a pore-size determination by NMR. P. 331