The objective of this work is to utilize high power laser technology (HPL) for two downhole applications in carbonate formations including perforation and fracture initiation. This is critical because current technologies for perforation cause compaction and reduce the permeability around the perforated tunnel. Furthermore, initiating fractures in tight carbonate formations requires high breakthrough pressures, which leads to additional damages and costs. HPL technology is a contactless and waterless alternative for these applications in any kind of rock; earlier laboratory results showed that the technology improves permeability, increases porosity, enhances the mechanical structure of the rock, and reduces damage to the neighboring formation. This work investigates some of the physical and chemical changes induced by high power laser on carbonate rocks.
A high power laser system is used to perforate and heat different types of carbonate rocks. These include some limestone and dolomitic samples of different shape and size. The rocks are characterized before and after the process using a combined multi-physics and multi-scale approaches. Small core plugs and cuttings are used to study the effect at micro-level. Larger samples are used to characterize the effect of the overall structure of the rock. The chemical transformations are captured using differential thermal analysis (DTA) and X-ray diffraction and fluorescence (XRD/XRF). The laser interaction is recorded in real-time using high-speed infra-red thermography.
Experimental results illustrate how the high power laser can create long and wide perforation tunnels in carbonate rocks. Micro-level observations evince that the permeability increase due to the creation of micro-fractures and changes in interconnectivity between the pores. Large-scale measurements show that high power laser can controllably weaken the rock within the heat-affected zone, enhance porosity, and increase permeability. These changes can improve the fracture initiation process in these rocks. The analysis of the laser-rock interaction also shows that the laser induces a calcination process that dissociates the calcium carbonate into CO2, lime, and other compounds. This may have a positive effect on the overall energy balance of the process.
This work demonstrates that high power laser can be used to enhance the perforation and fracture initiation processes in carbonate rocks. The technology can controllably create tunnels of different sizes and lower the compressive strength of the rock; consequently, it reduces the pressure needed for fracture initiation. The multi-scale and multi-physics analyses presented in this paper provide a comprehensive overview of the process and its implications for downhole applications.