The effect of micrite content and macroporosity on the transport and elastic properties of carbonates [electronic resource]
Keywords: Geophysics.

Carbonate rocks are major sedimentary rocks playing an important role both as water and hydrocarbon reservoirs. Thus, understanding the functional relationships between remotely probed, geophysical parameters and sedimentology-related properties is essential for better characterization of reservoir resources. Carbonates, however, are well known for their complex dual-particle size and dual-porosity microstructures, which create significant scatter in fundamental rock physics relationships such as the porosity-permeability and porosity-velocity relationship. The main factor controlling carbonate heterogeneity is the complex post-depositional diagenesis, which superimposes on the original microstructures inherited from the depositional environment. The micrite and macroporosity content in carbonate rocks are sedimentology-related parameters that vary because of both the energy of the depositional environment and the extent of leaching/washing that the rock experiences upon diagenesis. Thus, the primary focus of this thesis is to investigate how sedimentology-related parameters such as micrite content and macroporosity affect the variability of transport (porosity and permeability) and elastic (P- and S-wave velocities) properties. By understanding the interplay between these factors, the final objective of this research is to better inform modeling by providing quantifiable, textural parameters, which can improve the interpretation of transport and elastic properties of carbonate reservoirs. Previous attempts to investigate the role of these sedimentology-related parameters remained mainly qualitative and not systematic. As a consequence, results in the literature still appear inconclusive. Overall, this has led to poor agreement in the geophysics literature about the effect of micrite and macroporosity on the transport and elastic properties of carbonates. In this dissertation, we conduct a comprehensive study starting with controlled analogs serving as a proof of concept for the analysis and then extending the investigation to natural carbonates. The novelty lays on the fact that we prepare analog samples in the laboratory using natural calcite grains and micrite with the goal of studying the role of the content of micrite. In addition, we introduce controlled volumes of acetone-soluble solid matter (camphor) into the microstructures at the expense of the micrite aggregates. Once dissolved, the mold functions as macropores. We perform a series of experimental measurements to obtain porosity, permeability, and acoustic velocities under both bench-top conditions and as a function of confining pressure. We then investigate the correlation between the measured properties and the sedimentology-related parameters, and then attempt to quantitatively model the observed trends. Finally, we extend the investigation to carbonate reservoir rocks in order to test the hypothesis and methodology developed from the work on analog samples. Toward this goal, we study 15 samples from Tengiz Field, an isolated carbonate platform, where the sedimentology-related parameters are estimated based on image analysis of micro-CT scans and thin sections of the samples. The results obtained from the analogs and natural samples showed consistent trends regarding the effect of micirte content and macroporosity on the transport and elastic properties. With regard to the effect of micrite content and macroporosity on the transport properties, samples with higher micrite content and lower macroporosity exhibit lower permeability at any given porosity. Results show that the fraction of macropores in the samples is strongly correlated with the measured permeability since such pores do contribute more significantly to fluid flow compared to micropores. We used the varying micrite-to-grains ratio and its effect on the porosity-permeability relationship to inform the Kozeny-Carman relation suitable for a pack of spheres. Our analysis showed that micrite affects the porosity-permeability relationship of carbonates by reducing the effective particle size and increasing the percolation porosity. Additionally, the coefficient of determination (R2) between porosity and permeability was found to increase significantly when incorporating the micrite content and macroporosity into the analysis. This study shows that knowledge of both micrite content and macroporosity is of paramount importance to interpret and model porosity-permeability relationships in carbonates. Our results regarding the effect of micrite content on elastic properties show that the sensitivity of acoustic velocity to pressure decreases as the micrite content increases. This suggests a stiffer pore structure in micrite rich samples compared to that in grain-supported samples. Such conclusion is supported by a) observations from SEM images showing rounder pores in micrite-supported samples compared to grain-supported samples characterized by microcracks at grain contacts, and b) smaller change in length (i.e., strain) measured under pressure for the micrite-rich samples, compared to grain-supported samples. Unlike micrite content, the fraction of macroporosity shows no strong correlation with acoustic velocities, at a given porosity, or with the sensitivity of velocity to pressure. This is contrary to what is frequently reported in literature suggesting that the fraction of macroporosity correlates with an increasing velocity at a given porosity. In our case, the effect of subrounded macropores (stiff pores) is in general compensated by a decrease in the content of micrite (stiff component) and thus, the overall acoustic velocity remains uncorrelated with the fraction of macropores.