Historically Bore Hole Enlargement (BHE) operations have been restricted to softer formations. However, when including thermoporoelasticity as part of deriving Mechanical Specific Energy (MSE) for BHE more informed decisions can be made for BHE for a specific formation in the sense of optimizing reamer-pilot size ratio. This thesis focuses on the development of a thermoporoelastic model of rock mechanics for quantifying the stress distribution around the wellbore and the Apparent Rock Strength (ARS) after drilling a pilot hole. This is intrinsically linked to fluid and heat diffusion due to the effects of the drilling fluid. Indeed, ARS of the rock in the Depth of Cut (DOC) zone beneath the reamer can be determined by using Mohr-Coulomb theory. Additionally, the MSE is analytically estimated in different rock formations which is named as Analytical Mechanical Specific Energy (AMSE), for varying permeability values, in the presence of non-hydrostatic in-situ stress. Following these parameters, a set of laboratory drilling tests were carried out on sandstone formation. The rock samples were drilled and reamed, and the MSE calculated by using measured drilling parameters. Prior to the test, the rock samples were either pressurized by circulating highly pressurized mud; or heated up in an oven. The rock underwent confining and overburden pressure, a circulation of high pressure, and it was exposed to high or low temperature mud. The effects of stress alteration, pore pressure, temperature, time, distance between the pilot and a hole enlargement tool on the rock weakening around the wellbore and the performance of the hole enlargement operation, were studied. MSE was calculated and compared for different test conditions to determine the hole enlargement performance. To conclude, as the pilot hole is created, stress alteration will occur around the wellbore and the rock will weaken due to stress alteration, mud diffusion and heat diffusion. This simulation can help to estimate the optimum reamer/pilot size ratio, as well as the positioning of the reamer in order to take advantage of rock weakening around the wellbore. Considering that so far in the market there is no evidence of a specific model to predict rock strength below the reamers, this research and study shows its degree of novelty since it proposes a model to fill this gap by estimating AMSE applying thermoporoelastic approach. The model can be fine-tuned and used as a reference application in the petroleum industry to facilitate decision making and project cost analysis.