Geomechanical property changes with hydrous pyrolysis in Mahogany oil shale in Green River Formation

Abstract

The formation characteristics of organic-rich shales have proven to be affected by mineralogical composition, total organic content, water concentration, and bedding-plane orientation along with other parameters. However, it is not known with good accuracy what types of influence thermal maturation has on the immature organic matter on geomechanical properties of the formations. In order to quantify the behavior of the oil shale samples under thermal maturation, it is imperative to understand its elastic properties. Thus, it becomes vital to understand the shale’s elastic properties via geomechanical analysis of the selected core samples. In this experimental study, vertical core samples of Mahogany shale in Green River formation from the Anvil Points mine site were selected. These samples possess immature organic content consisting of Type-I kerogen. This study consists of an experimental investigation to comprehend if thermal maturation of immature organic matter has an impact on the geomechanical properties of shale formation. Hydrous pyrolysis experiments were conducted to represent the four different stages of petroleum formation: temperature ranges that represent the bitumen generation (300°C), bitumen generation and initial oil generation (330°C), oil generation (360°C) and the oil cracking to gas (390°C). The samples were tested in these four different temperature states were analyzed later for the changes in their geomechanical properties. The geomechanical properties are evaluated in terms of dynamic elastic properties and resistivity. Unfortunately, the pyrolyzed samples had sustained a heavy loss of their mechanical strength and integrity due to the expansion because of the samples undergoing thermal maturation resulting in their infringement. The weakening prevented us from analyzing the changes in static properties. However, investigation of the changes in dynamic elastic properties and resistivity was possible. The samples were evaluated for dynamic elastic properties before and after pyrolysis to note the variations that it had undergone during the process of thermal maturation that was artificially induced by hydrous pyrolysis. In this research study, we analyze the thermal maturation effects on the geomechanical properties of the oil shale plugs via the analysis of elastic properties and resistivity measurements. The post analysis of the thermally matured core plugs, we found a prominent decrease in the elastic properties such as the Young’s, bulk and shear modulus. We conclude that thermal maturation of organic-rich samples results in mechanical weakening of the sample. Correspondingly, resistivity data were a function of organic richness and temperature. An analytical comparison of the elastic properties of formations across the globe for a comprehensive evaluation of the differences amongst them is also carried out. The results of the study indicate that thermal maturation tends to decrease the mechanical integrity and strength of the rock. The generation of petroleum products due to the maturation of organic products results in pore volume expansion, which causes fracture development and most often the failure of the sample. The behavior of resistivity tends to be dependent on the amount of organic matter expelled during the thermal maturation as well as the amount of organic matter withheld in the core plug, which relatively makes them temperature dependent, as the organic matter expelled is a function of the temperature.