Coiled-Tubing Drilling with Supercritical Carbon Dioxide

Abstract

Abstract High-pressure fluid jet-assisted drilling is attractive for drilling horizontal holes because of the potential for high penetration rate in hard formations using low-cost, small-diameter tools. Unfortunately, the threshold pressure for water-jet erosion of most sedimentary rocks is much higher than the pressure capacity of conventional coiled tubing. Recent experiments have demonstrated that supercritical carbon dioxide (SC-CO2) jets will cut hard shale, marble and granite at much lower pressure than water. SC-CO2 was also shown to enhance mechanical drilling rates. The use of SC-CO2 to drill ultra-short- radius lateral well completions is evaluated here. SC-CO2 is a dense, low-viscosity fluid with excellent hole cleaning capabilities. Conventional under-balanced drilling equipment can be used to control formation pressures while drilling with SC-CO2. SC-CO2 is a non-damaging fluid that should provide enhanced production from completions relative to holes drilled with water-based fluids. The use of SC-CO2 for jet assisted drilling would allow fast drilling of small-diameter, ultra-short radius laterals using low-cost, small diameter coiled tubing equipment. Introduction Lateral drainage holes can result in large increases in productivity relative to a vertical wellbore. Two approaches for short-radius lateral drilling are currently available:Guided rotary drilling with a flexible drill string and decoupled downhole guide mechanism1.Coiled tubing drilling (CTD) with a downhole orienter and bent-housing motor Both of these approaches have an effective minimum turn radius of over 10-m as shown in Figure 1. Reducing the size of tubing and tools can further reduce the turn radius and cost of drilling. Ultra-short-radius (under 10-m) drilling would allow the entire lateral to be drilled within the target formation, increasing productive drilling time and eliminating secondary costs for casing. Small-diameter lateral wells can be significantly more cost effective for gas gathering than large wells (see Appendix). High-pressure jet-assist can reduce the size of drilling tools by reducing the thrust and torque required to penetrate the rock. Smaller motors can be operated on smaller, lower-cost CT and will fit through a shorter curve radius. Jet-assist also has the potential to increase drilling rates in hard, slow-drilling formations. Many jet-drilling systems have not performed well because developers have underestimated the pressure required to cut hard formations under submerged conditions. As discussed below, the pressure available from conventional CT is not high enough to cut most formations with water-based drilling fluids. Recent work has shown that supercritical carbon dioxide (SC-CO2) will cut rock at lower pressure than water1. The application of SC-CO2 for jet-assisted drilling of ultra-short lateral completions is discussed here. High-pressure CO2 can be supplied through coiled tubing to drill short-radius laterals. SC-CO2 jet-drilling tools would fit through production tubing and be powered by a lightweight coiled tubing unit. A CO2 circulation model is employed to show how formation pressures can be balanced or underbalanced with surface pressure control equipment. The model provides CO2 consumption rates, power requirements and annular flow velocities. Specialized equipment requirements include a high-pressure pump, vane motor and surface pressure control equipment. CO2 will reduce the pH of produced water and the potential for corrosion is discussed.