By Scott Myers
Like oil and gas wells, geothermal wells are designed around the life of the well with the goals of containment of well pressure, and prevention of uncontrolled flow of reservoir fluids to the environment or groundwater. Monitoring, inspection and maintenance programs are put in place to accomplish this for both producing and injection wells. Beyond these high-level objectives, however, some key differences between oil and gas and geothermal wells merit deeper consideration of certain factors:
- Temperatures; are, in most cases, more extreme than in a ‘typical’ oil & gas well, or in most district heating applications. Hence greater consideration to thermal expansion of both downhole and surface equipment, e.g., downhole completions, wellheads & hookups, valves and surface pipework.
- Downhole tubulars & metallurgies; when downhole temperature changes occur in wider variation, cemented casing & connections are subjected greater thermally-induced stresses. Depending on the thermal loading path, this can be an important consideration for tubular system selection. In addition, the tubular metallurgies must be designed around (i) exposure to reservoir fluids, (ii) corrosion resistance and (iii) long-term loss of wall thickness and (iv) potentially reduced performance at elevated temperatures.
- Surface components; often subject to bending side-loads, and thermal expansion in addition to the usual pressure cycling found in most oil and gas wells.
- Fluids & pressures; as opposed to an oil or gas well, the fluid typically present in geothermal wells will be water or steam, and are often very hot, acidic and corrosive. Pressures are generally but not always lower, but should be given additional scrutiny in consideration of the temperature and fluid types.
Of course the consequences of having a well go down may be more critical, since both power generation and district heating are more directly connected to the production source.
So then, what if we want to re-purpose an old oil or gas well to a geothermal, as many 'energy transitioners' are thinking of? In short, we start with what we have at hand in terms of existing barriers, completions, and their current condition considering age & prior use. We then compare what we have with the considerations mentioned above. Will these sustain the demands of the proposed geothermal system, in terms of temperatures, metallurgies, corrosion, completion design, and thermal expansion? And if so, for how long? All must be considered in both technical and economic determination.