Study of Thermal and Mechanical Stresses Variations on Rock Caused by Laser Drilling in High Confining Pressure With Finite Element Method

Dini, Abdollah (Tarbiat Modares University) | Ahmadi, Morteza (Tarbiat Modares University)

OnePetro 

Abstract

Laser perforating is a new approach to the generation of uniform holes in oil and gas reservoir wells at a selected pitch to improve the permeability of rocks. Laser drilling in rocks is a very complex phenomenon that its performance depends on many factors. Since it is not possible to consider all of these factors in the laboratory, numerical modelling is used. In this study, a finite element code (FEM) has been taken to model the thermal and mechanical stresses induced by ND: YAG laser drilling in the hydrocarbon reservoir rock samples. For this purpose, the software ABAQUS was used to analyze the thermal and mechanical stresses induced by laser. It is found that Numerical models show good agreement with the actual observation of holes drilled by the laser. During laser drilling, the rock temperature quickly increases in a few seconds and immediately reduces, thus instantaneous heating and cooling process cause thermal stresses around the hole. Also, the maximum value of thermo-mechanical stress exceeds the strength of the limestone rock and consequently, the formation of cracks and fractures in the wall of the hole are unavoidable.

1. Introduction

Laser perforating is a new scientific way to the creation of uniform holes in petroleum reservoir wells to increase the permeability of rocks (Ahmadi et al., 2011). Thermal stress generated by differential thermal expansion of minerals and high-temperature gradient, cause to break the bonds between the grains. In this range of temperature, physical and chemical changes occur that are associated with the process of spallation. A primary physical change associated with this process is due to the thermal expansion of the grains of the rock. For example, the expansion of quartz and plagioclase grains in sandstone lead to a sudden temperature increase in it (Gahan et al., 2004).

As closely-packed grains in the matrix expand with a rapid rise in temperature, they develop thermal stress fractures and cracks within the grains, as well as break the cementation of adjacent grains. As a result, an affected grain will begin to break free from one another (Salehi et al., 2007). Laser effects appear in two steps in rocks, firstly, the creation of a hole in the rock and secondary include melting, evaporation, laser beam gases and micro fractures.