Study of thermal and mechanical stresses variations on rock caused by laser drilling in high confining pressure with Finite Element Method

Summary
Modelling and simulation have increased in the study of rock engineering in the past decades. Numerical simulation is an important tool for analysis that is impossible to do in the laboratory. The main reasons for using numerical modeling in laser drilling which can be mentioned are lack of access to high confining pressure in the existing laboratory condition, increase in the cost of drilling in large diameter, differentiation mechanical and thermal stresses and strain caused by the laser in the laboratory, as well as important secondary effects in laser drilling on rock. In this study, a Finite Element Method (FEM) is used for understanding the thermal and mechanical stresses caused by the ND: YAG laser drilling in the reservoir rocks. For this purpose, ABAQUS code was employed to analyse the thermal and mechanical stresses. A numerical model of a core rock was simulated and thermal properties of a reservoir rock such as thermal conductivity, heat capacity and density were imported into the code. By entering drilling rate to the model, tests carried out in the lab were simulated and after conformity with them, high confining pressures were imposed.
 
Introduction
Laser perforating is a new scientific approach to the generation of uniform holes in oil and gas reservoir wells at a selected pitch to improve the permeability of rocks. Thermal stress generated by differential thermal expansion of minerals and high-temperature gradient, breaks the bonds between the grains. In this range of temperature, physical and chemical changes occur that are associated with the process of spallation. Hence, Laser rack spallation is a complicated phenomenon depending on many factors. Using experimental studies to comprehend this phenomenon is merely expensive and time-consuming. Since there are not enough techniques to appraise some of the factors, they should be studied in the laboratory. On the other hand, numerical modelling provides simulative action of the factors which are difficult to be considered by experimental research. Many researchers have been examined the laser cutting process. In previous studies, researchers focused mainly on some factors to improve the cutting quality and assess the thermal stress levels around the cutting section. However, these studies were limited mainly laser cutting of rock and has not been examined the thermal and mechanical stress analysis in detail.
 
Methodology and Approaches
The finite element model has been used to simulate the thermal and mechanical stresses induced by laser drilling ND: YAG. For this investigation, some samples have been taken from a hydrocarbon reservoir in Ahvaz. For this purpose, ABAQUS software was used and thermal properties of reservoir rocks, such as thermal conductivity, density and heat capacity were considered.
 
Results and Conclusions
Results of the numerical study show that the model has good agreement with the lab tests. Around the drilled holes, thermal stress induced by the laser is lower than the mechanical stress caused by confining pressure. And the distribution of the stress concentration has a high correlation with the amount of confining pressure.