Combined Magnetic Resonance Relaxation and Imaging for Quantitative Determination of Matrix and Vugular Porosity in Carbonate Cores

Abstract

Abstract A method is presented to characterize the internal structure of full-size carbonate cores by combining nuclear magnetic resonance relaxation and imaging techniques (Relaxation-Tomography, RT). In addition to usual imaging techniques, a method for obtaining images as relaxation time maps was developed. An operational definition of matrix porosity based on the RT technique is presented, which has the advantage of taking into account the pore connectivity and which therefore seems to be useful in describing the flow properties of the rock. Several internal images were obtained as H nuclei density and relaxation time maps on a set of vugular and microporous carbonate cores saturated with water. From RT maps the histograms of relaxation times of selected internal regions of interest can be obtained. Reference measurements on smaller samples were performed by means of a specialized NMR relaxometer, and the reliability of the distributions of relaxation times from RT maps was confirmed. The matrix porosity was determined by combining voxel by voxel information from porosity and relaxation time maps. This investigation was performed on some meters of cores in all. In the samples examined, all from the same outcropping reservoir, the matrix porosity averages about 30% of the total porosity, with a threshold for the matrix pore space of the order of a micron or a few microns. The results indicate this approach to be a very promising technique for obtaining, among other things, basic structural information on undisturbed full-size cores. The advantages are twofold. On one hand, this technique furnishes a cost- and time-saving method of work. On the other, there are technical advantages of specificity to the phenomena involved and also the ability to select isolated regions of interest within a core. This method can be useful when it is necessary or convenient to investigate some internal parts of the cores, without any cut or disturbance. Another promising opportunity lies in the possibility of enhancing the knowledge of petrophysical properties by avoiding the effects of the mud-invaded zone of the core. P. 751