THE EFFECT OF TEMPERATURE ON THE MECHANICAL AND PETROPHYSICAL PROPERTIES OF SEDIMENTARY ROCKS FROM LEBANON

Abstract

Assessing the geomechanical and petrophysical properties of rocks under in-situ environmental conditions is required to enhance industrial processes such as oil and gas drilling, carbon storage, nuclear waste storage, and building underground facilities. Studies have shown that temperature alters these properties differently at various temperature ranges. Hence, this work aims at assessing the effect of temperature on the Unconfined Compressive Strength (UCS), Young’s modulus, Poisson’s ratio, compressional wave velocity, porosity, and permeability of sandstone and carbonate rocks collected from southern Lebanon. Sandstone and limestone samples were collected from four locations in the areas of Bisri and Jezzine. Cylindrical core samples of diameter 44 mm and length 88 mm were drilled to assess both the static properties and the compressional velocity, while samples of one-inch diameter were cored to measure the rock’s petrophysical properties. First, rock types were identified via thin-sectioning and SEM imaging and analyses. Then, stress-strain curves, UCS, Young’s modulus, Poisson’s ratio and rock compressional velocity were obtained via uni-axial compression test and ultrasonic measurements. Permeability and porosity were measured using a gas-permeameter and a Helium-porosimeter, respectively. Properties are calculated for temperatures that range between 25oC and 900oC. The SEM and the thin sectioning analyses confirm that the collected rocks are mainly bioturbated limestone, bioturbated siliceous limestone, bioclastic intraclastic sandstone and quartz arenite. The effect of temperature on the static properties is different for sandstone and carbonate rocks. For carbonates, the variation of the UCS, Young’s modulus and Poisson’s ratio with temperature is not continuous due to the complex structure of the rocks, whereas for sandstone samples, these properties decrease slightly between 25 and 100oC, and then increase for higher temperatures. For all rocks, the compressional velocity decreases when temperature increases because of the generation of thermal cracks and expansion of intragranular pores. However, the decrease of compressional velocity for carbonate rocks is noticeable for temperatures above 300oC whereas that of sandstone sample is continous. Furthermore, we observe that permeability decreases between 25oC and 85oC because of the narrowing of pre-existing intergranular micro-cracks caused by the thermal expansion of the porous solid matrix. Beyond 85oC, the permeability increases with temperature as thermal cracks form creating interconnected pathways. The porosity of rocks, however, increases continuously as temperature increases because porosity is controlled by both the inter- and intra-granular pores and the presence of cracks which expand as temperature increases.