Magnetic resonance relaxation-tomography to assess fractures induced in Vugular Carbonate Cores

摘要

A method is presented to study fractures induced in full-size carbonate cores by means of Relaxation-Tomography (RT). This method combines nuclear magnetic resonance (NMR) relaxation and imaging techniques, and consists of obtaining, in addition to the usual H nuclei density (porosity) maps, images as relaxation time maps. By combining voxel-by-voxel signal intensity from the porosity map and relaxation time from the relaxation time map, one get the distribution of the signal intensity as a function of the relaxation time for selected internal regions of interest (ROI). The distribution of the signal intensity vs relaxation time gives information on the distribution of water in different structures inside the sample and therefore on the structure itself. One can compute the fraction of water in the ROI, relative to the total amount of water in the ROI, having relaxation time in a selected range. Also, average relaxation time, as the geometric mean, can be computed. In a previous paper (SPE 49294), we applied the method to quantify (following an operational NMR definition) matrix and vugular porosity in carbonate cores. In this paper we improve the computation of the relaxation time by introducing non-linear fitting of the data and extending the application to the study of fractures induced in carbonates. Sveral internal images from undisturbed vugular and microporous full-size carbonate cores saturated with water were obtained as porosity and longitudianl relaxation time (T sub 1) maps. The T sub 1 histograms from selected ROI's were obtained. Then, fractures were induced in the cores by repeated cycles of freezing and heating. The RT investigation was repeated on the treated cores after each step and the distributions of the relaxation times compared. The changes in the distributions suggest a twofold effect: the forming of new microfractures (increase of local surface-to-volume-ratio) and the expansion of pre-existing fractures (decrease of surface-to-volume ratio).