Effects of micrite microtextures on the elastic and petrophysical properties of carbonate reservoirs

Abstract

Apparent differences in sedimentation and diagenesis exist between carbonate reservoirs in different areas and affect their petrophysical and elastic properties. To elucidate the relevant mechanism, we study and analyze the characteristics of rock microstructure and elastic properties of carbonates and their variation regularity using 89 carbonate samples from the different areas The results show that the overall variation regularities of the physical and elastic properties of the carbonate rocks are controlled by the microtextures of the microcrystalline calcite, whereas the traditional classification of rock- and pore- structures is no longer applicable. The micrite microtextures can be divided, with respect to their morphological features, into porous micrite, compact micrite, and tight micrite. As the micrites evolves from the first to the last type, crystal boundaries are observed with increasingly close coalescence, the micritic intercrystalline porosity and pore-throat radius gradually decrease; meanwhile, the rigidity of the calcite microcrystalline particle boundary and elastic homogeneity are enhanced. As a result, the seismic elastic characteristics, such as permeability and velocity of samples, show a general trend of decreasing with the increase of porosity. For low-porosity rock samples (φ 5%) dominated by tight micrite, the micritic pores have limited contributions to porosity and permeability and the micrite elastic properties are similar to those of the rock matrix. In such cases, the macroscopic physical and elastic properties are more susceptible to the formation of cracks and dissolution pores, but these features are controlled by the pore structure. The pore aspect ratio can be used as a good indication of pore types. The bulk modulus aspect ratio for dissolution pores is greater than 0.2, whereas that of the intergranular pores ranges from 0.1 to 0.2. The porous and compact micrites are observed to have a bulk modulus aspect ratio less than 0.1, whereas the ratio of the tight micrite approaches 0.2.