The temperature effects of high power lasers on rock strength and mechanical properties are presented. It was found that the high temperature induced by lasers drilling through rock enhances porosity and permeability and reduces strength. High temperature causes microcracks, vaporizes cementation, and dehydrates clays.
Microcracks were monitored using thin sections and SEM analyses. Representative samples were cut for thin section analyses before and after lasing. For the SEM test, a small fragment of the sample was removed and coated with gold for better electron conductivity, thus allowing fracture imaging in three dimensions.
Phase changes also affect rock properties such as permeability, porosity and strength. The effect depends on the temperature distribution and gradient as well as the mineralogy. At each temperature, different reactions take place. For example, at 550°C smectite clay will collapse and dehydrate. This mechanism results in increasing void space in the rock sample. At the same temperature, quartz grains will expand by 1.75%1 and when the grains cool, microcracks are created, which also increases the void space. This increase in void space may or may not change permeability, but it does decrease the acoustic velocities that in turn change the calculated porosity and elastic moduli.
Results show in all cases, the Young's modulus, shear modulus, bulk modulus, and combined modulus near the lased portion, were reduced when compared to the unlased portion.
In sandstones, the decrease was 50% or more. A bulk modulus decreased of 50% causes the compressibility near the wellbore to increase by 50%. Depending on the lithology, Poisson's ratio decreased or increased slightly. For limestones and shales, the differences between the moduli of lased and unlased samples were less significant than in sandstone.