Modeling Laser-Spallation Rock Drilling

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 95746, "Modeling of Laser Spallation Drilling of Rocks for Gas- and Oilwell Drilling," by Z. Xu, SPE, Y. Yamashita, and C.B. Reed, Argonne Natl. Laboratory, prepared for the 2005 SPE Annual Technical Conference and Exhibition, Dallas, 9–12 October. High-power lasers can weaken, spall, melt, and vaporize Earth materials, with thermal spallation being the most energy-efficient rock-removal mechanism. The most interesting focus of recent laser rock-drilling research is on developing a laser rock-spallation technique to drill large and deep holes in rocks. Laser spallation has a rock-removal rate higher than conventional rotary drilling and flame-jet spallation. Research also is focused on the use of laser rock spallation to make perforation channels with improved permeability of the perforated rocks. Introduction Laser rock spallation is a rock-removal process that uses laser-induced thermal stress to fracture the rock into small fragments before it melts. High-intensity-laser energy, focused on a rock that has very low thermal conductivity, causes the local rock temperature to increase instantaneously. This results in a local thermal stress that spalls the rock. Previous test data show that laser rock spallation is the most energy efficient among all laser rock-removal mechanisms. Recent research and development concentrating on use of advanced high-power lasers to drill and complete oil and gas wells has focused on two fronts. The first was to develop a multibeam laser rock-spallation technique to drill large and deep holes in rocks with a rock-removal rate higher than that of conventional rotary drilling and flame-jet spallation. In this approach, each laser beam spalls a hole as big as the beam spot and half the beam diameter deep. Multiple beams are overlapped to remove a layer of rock. Layer by layer, a large and deep hole is drilled. The second focus was to develop a laser rock-perforation technique for oil- and gas-well completion applications. Perforating oil and gas wells requires creating a hole through a composite structure of steel casing, cement, and formation rock. Current explosive-charge perforation methods, while capable of creating the holes, significantly reduce the rock permeability. Laboratory tests demonstrate that laser beams not only cut rocks efficiently, but also increase the permeability of spalling-drilled rock significantly. An innovative laser perforating system will allow the oil and gas industry to improve injection and production rates.