Pore Structure Evaluation of Bioclastic Limestone Using NMR and HPMI Measurements

Abstract

Pore structure evaluation is a crucial segment of revealing the reservoir characteristics and percolation mechanism. Considering the diversity of origins, types and combinations in reservoir space, the effective evaluation method of bioclastic limestone pore structure had not been built yet, which greatly restricted the comprehension of storage-permeability mechanism, as well as the subsequent development strategies. Therefore, this study systematically analyzed the corresponding relationship between the fractal characteristics of capillary pressure curves and pore connectivity. The T₂ relaxation criterion of different pore diameter components were determined reasonably according to the features of nuclear magnetic resonance T₂ spectrum. Combined with a fuzzy clustering algorithm, a new logging classification method was established using proportions of different pore components as sensitive parameters. The results showed that the capillary force curves of bioclastic limestone reservoir mainly exhibited two kinds of form: “convex” and “concave”. The former showed better storage-percolation characteristics; while the same characteristics of the latter were closely related to inflection points, degrading by the location of respective point from right to left; In addition, the relationship between the pore throat radius r and nuclear magnetic relaxation time T₂ could be classified into four stages obviously. With the pore throat radius of 0.15, 1, 5 μm and T₂ relaxation time 30, 90 and 200 ms, the pore structure of bioclastic limestone was effectively divided into four categories. On this basis, the calculation precision of permeability would be significantly improved.