Modeling of Laser Spallation Drilling of Rocks for Gas and Oil Well Drilling

Abstract

Abstract High power lasers can weaken, spall, melt and vaporize natural 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, a potential application in gas and oil well drilling having a rock removal rate higher than that of conventional rotary drilling as well as that of flame-jet spallation.Research is also focused on using laser rock spallation to make perforation channels with improved permeability of the perforated rocks.However, laser rock spallation is a very complex phenomenon that depends on many factors.Fundamental understanding of this complex phenomenon is crucial to the success of its application to the petroleum industry. In this paper, we propose a combined approach to this complex problem, that is establishing models for each of the physical phenomena based on the finite difference method (FDM), then combining them into one numerical procedure using the Combined Unified Procedure (C-CUP) method.With this approach, the transient temperature and stress distributions in dry or water-saturated rocks exposed to a laser beam have been calculated.The spallation boundary and rock removal efficiency have been determined for different laser conditions.The modeling results provide a better understanding of laser rock spallation phenomenon and most importantly, guidelines for selecting processing parameters for fast rock removal. 1. Introduction Laser rock spallation is a rock removal process that utilizes laser-induced thermal stress to fracture the rock into small fragments before melting of the rock occurs.High intensity laser energy, applied on a rock that normally has very low thermal conductivity, concentrates locally on the rock surface area and causes the local temperature to increase instantaneously. This results in a local thermal stress in subsurface that is enough to spall the rock.Previous test data shows that it is the laser rock spallation that removes reservoir rocks most energy efficiently among all laser rock removal mechanisms [1]. The advantages of the laser spallation rock removal are three-folds:rock is removed by spallation, so it is most energy efficient,the process is easy on beam fiber-optical cable delivery due to low required laser power for each spalling beam, andsmall rock debris or fragments are readily flushed out by standard well flushing method. In order to take advantage of the laser spallation, recent research and development work on applications of advanced high power lasers to drilling and completion of gas and oil wells mainly focuses on two fronts. The first is to develop a multi laser beam rock spallation technique to drill large and deep holes in rocks such as gas and oil wells with a rock removal rate higher than that of conventional rotary drilling as well as flame-jet spallation. In this approach, each laser beam spalls a hole as big as the beam spot and half beam size deep. Multiple such beams are overlapped to remove a layer of large rock area. Layer by layer, a large and deep hole is drilled [2, 3]. The second front is to develop a laser rock perforation technique for gas and oil well completion applications. Perforation of gas and oil wells requires creating a hole into a composite structure of steel casing, cement, and rock formation. Current explosive charge perforation method, while capable of creating the holes, significantly reduces permeability of the rock, and is reaching its technical limits. On the other hand, lab tests demonstrate that laser beams not only cut rocks efficiently, but also significantly increase the permeability of spalling-drilled rock [4]. An innovative laser perforation system will allow the gas and oil industry to rejuvenate injection and production rates quickly and easily.