Methods to mitigate well failures in high-temperature geothermal wells

Well heads at Hellisheidi power plant of Reykjavik Energy (source: flickr/thinkgeonergy, creative commons)
Alexander Richter 10 May 2020

The EU Horizon 2020 funded GeConnect project aims at increasing the reliability of the downhole construction of geothermal wells in high-temperature settings.

The cost of siting and drilling geothermal wells for co-generation of electricity and district heating system is often about 40-50% of the total investment cost of the energy production. The high cost of drilling wells demands high production capacity and longevity to generate a solid business case for the geothermal power plant.

Studies have shown that the single most occurring failure mechanism for high-temperature geothermal wells is mechanical overload of the casing string in the well due to constrained thermal expansion that can result in collapse or tensile failure of the casing. In addition, for medium enthalpy geothermal wells, it is known that temperature and pressure cycles above 100 degrees Celsius during drilling and completion, operation and shut-in phases have the potential to severely deteriorate the integrity of the cemented annulus.

The concept of new flexible couplings is to compensate for such axial thermal expansion of geothermal casing strings during production and shut-in. The concept has been designed and prototypes have been produced within the framework of Horizon 2020 EU-funded research projects GeoWell and DEEPEGS and two patents have already been obtained. Prototypes were tested for functionality, in total 8 prototypes in full size of 9? and 13? casing diameter in two third-party laboratories. Sliding function and repeatability of sliding were tested. Structural strength was tested by determining the ultimate tensile strength.

Within the ongoing GEOTHERMICA project GeConnect, the aim is to take this concept a step closer to a realistic operation scenario. A full-scale prototype (figure, see below) of the flexible coupling will be tested at real working conditions as well as shut down in a controlled manner and quenching that can be performed as an emergency in operation. Together with the new flexible coupling, cement sheath integrity and the cement-metal boundary will be evaluated by simulating thermal cycling loads at moderate (<100 degrees C) to high temperatures (~300 degrees C). For this, a 9 5/8″ flexible coupling will be attached to a 12 m casing pipe and cemented into a 13 3/8″ outer casing. Geothermal steam from an operational high-temperature well will be flushed through the 9 5/8″ casing to approximate downhole conditions. The following testing procedure is planned:

  1. Heat to 100 degrees C at atmospheric pressure
  2. Cool with water
  3. Heat to maximum temperature
  4. Cool to ambient conditions
  5. Cyclic loading by repeating C. and D.
  6. Quench with cold water

The set-up will be equipped with cutting edge electrical and fiber-optic sensing devices to monitor and validate the operating principle as well as the integrity of the cemented annulus. Possible implications and risks associated with using the new flexible couplings will be assessed through structural modelling of the surface test, structural modelling of a geothermal well equipped with flexible couplings and by performing a quantitative risk assessment analysis.

Structural analysis will be used to evaluate the performance of flexible connections and to demonstrate the benefits to well integrity of reducing thermal axial stress and strain in casings by introducing the new flexible coupling.

Partners from ÍSOR, TNO, GFZ, Landsvirkjun, HS Orka and ON Power are looking forward to the planned field testing in Iceland in October 2020.

Source: Sent in by email, GeConnect project website