Stainless steel coiled tubing (CT) was used for the first time in Spain in 1997, to convey a burner as part of an underground Coal Gasification feasibility project by Underground Gasification Europe (UGE). Minimizing the risk of combustion associated with contact between pure oxygen and carbon steeldictated the requirement of stainless steel coiled tubing for this project. Additionally, stainless steel CT was required to minimize the corrosive aggressiveness of pure oxygen.
The 1.75" 316 L stainless steel coiled tubing used for this project, was milled in a continuous length in Switzerland and tested for its low cycle fatigue properties in Houston.
Two stainless steel control lines were installed inside the coiled tubing to initiate the burning process, together with a thermocouple to monitor the downhole temperature during the gasification. This paper discusses the role that stainless steel coiled tubing played in this underground coal gasification project as well as the development and testing of all the components.
Coal has been a known source of energy for centuries. Although high in energy content, coal usage has been in steady decline since the mid 1960's.
The main disadvantages of coal have always been that mining is a labor intensive and potentially dangerous occupation and that the residues (ash and other components) have to be disposed of after the carbon has burned from the coal. In the seventies, methods were developed to extract the energy from the coal, through an above ground process. This process still left developers with the task to mine the coal, but coal did not have to be shipped to consumers anymore, which simplified residue disposal.
The concept of gasifying the coal underground therefore is very attractive. This would enable the 'energy content' of the coal to be brought to surface, while leaving the residue underground. In addition to disposal advantages, it could also be used to exploit coal deposits which are not accessible by conventional methods, like the known coal reserves below the North Sea.