The energy transition has caused an increase in interest in Geothermal wells for industrial and domestic heating purposes. Geothermal energy is already being used in the agriculture industry for the heating of greenhouses. Water from these reservoirs often contain corrosive components like carbon dioxide (CO2) and chlorides, which in combination with elevated temperatures can be very corrosive for the carbon steel tubing typically used in geothermal wells. Hydrogen sulfide (H2S) and oxygen when present in low concentrations can lead to high corrosion rates. In some situations the lifetime is reduced to a few years. Corrosion inhibition can be used to increase the lifetime of carbon steel well-tubing. Environmentally this solution is not the best option as the risk of contamination of the water reservoir by the chemical corrosion inhibitors. Corrosion resistant steels, like the economical 13 Cr MSS provide an alternative to the use of corrosion inhibitors. The practice shows however that more exotic, and expensive, corrosion resistant steels are required to provide the necessary corrosion resistance.
An affordable and inherently corrosion resistant alternative for steel is Fiber Reinforced Plastic piping (FRP). This alternative for well tubing is gaining interest in the geothermal industry. The combination of resin and glass that is in use presently allows for application in deep thermal wells up to temperatures between 100 and 120°C.
For more than 3 decades Dynaflow Research Group has been involved in the successful application of Fiber Reinforced Plastic piping in cooling water systems, water injection systems, firewater systems and for the transport of crude oil and chemicals. The experience that Dynaflow has gained with the material translates well to answer questions related to the design of components and of systems of geothermal wells.
DRG has been involved in the design of Geothermal well tubing and a new EU sponsored project development program for FRP tubing in geothermal wells including the development of a casing standard.
The design of FRP piping for geothermal tubing comes with specific challenges related to loading conditions that differ from standard process piping. Dynaflow’s experience has been applied successfully to tackle the more complex thermal, hydrostatic and mechanical loads acting on the tubing present in a geothermal well.
During startup and interruptions during operation of a well, complex combinations of pressure – and temperature variations will occur. These variations depend on depth and operating conditions during the different phases of the operating cycle.
When in service, temperature gradients develop in the tubing in both axial and radial direction. Also, geothermal specific loads like self-weight (vertical piping) lead to additional axial loads and length changes of the tubing. The so called “Piston Effect” and “Ballooning” induced by temperature and pressure variations (e.g. during pump start-up events) have a significant effect on the mechanical loads and induced deformations of the tubing. Besides, thermal expansion and contraction play a crucial role. The forces developed in the tubing are among others determined by the temperature variations and selection of the well-sealing technology used at the bottom of the well. The loading conditions using a free movable Polished Bore Receptacle (PBR) for example is significantly different from that of a rigid Packer Seal design. The displacements of the tubing as a result of expansion and contraction during the different phases of the operating cycle of the geothermal well is of importance for the design of the PBR receptacle (required axial movement-amplitude of the stem of the PBR). In case of a “Packer Seal” design the loads that are induced by the thermal expansion (or contraction) can lead to buckling of the tubing and therefore also need to be part of the design calculations.
All above described aspects need to be covered by the design because, in the end, they all affect the integrity and the lifetime of the system. The information from the modelling is also of importance for the tubing design. The tubing in a geothermal well often requires a higher axial strength of pipe and connectors and a higher buckling resistance compared to standard FRP pipe. The pipe manufacturer has to modify the pipe design to fulfill these additional requirements.
Glass reinforced plastics show a regression behavior, which means that the failure strength reduces with time. This regression behavior depends on the type of FRP, the temperature and the mechanical loads acting on the tubing material. To guarantee a certain design lifetime, the stresses in the FRP tubing must not exceed the maximum allowable design values. As part of the qualification, the pipe manufacturer is required to demonstrate that the regression properties stay within the limits as dictated by the industry standards. The knowledge and experience of Dynaflow with the qualification and design of FRP systems in the oil and gas industry and close relations with many pipe manufacturers is very valuable also for the qualification of well tubing.
The application of FRP material as a tubing material is an economically interesting option that reduces the costs associated with corrosion and will significantly increase the lifetime of the tubing in a geothermal well.
Currently carbon steel is used for Geothermal well casings. Corrosion is a general threat and may lead to mechanical failure of the casing or to leakage of water in the formation. Sometimes corrosion inhibitors are used to reduce corrosion, but these chemicals often are not environmental friendly and in the unfortunate event of a corrosion-failure of the well-casing this may lead to e.g. leak paths (operational issue) and contamination of intermediate water reservoirs that are passed by the well hole. Fiber Reinforced Plastic (FRP) casings are expected to provide a corrosion resistant, economic and technical alternative to carbon steel.
Other than for well tubing, Fiber Reinforced Plastics (FRP) materials are currently not used for well casings. From a design point of view, the (critical) loads acting on well casings are typical to casings and differ in type/magnitude compared to that of a ‘standard’ FRP pipe or FRP well tubing. Therefore the design and glass fiber build-up of the FRP well-casing need to be tailored to give the casing the necessary strength under the specific load scenarios the casing(s) will see during both the installation and operation phase.
The European Geothermica project proposal “Glass Fiber Reinforced Epoxy Casing Systems for Geothermal Application” (GRE-GEO) has been awarded and will commence the third quarter of 2020. Our partners in this project are experienced companies and market leaders in their respective fields.
The project objective is to develop a cost-reducing single barrier, corrosive resistant, large diameter, glass reinforced epoxy casing system designed for geothermal applications, including well guidelines and tools for installation. The application of Glass Fiber Reinforced Epoxy casing for geothermal applications aims to establish a corrosion resistant alternative to decrease development and production costs of geothermal energy while avoiding extra investments in double barrier casing systems or additional costs for inhibitor solutions of conventional steel casings.
Existing (international) standards currently do not cover Glass Fiber Reinforced composite pipe (FRP) as material for well casings and therefore the preparation of a new standard is essential and will be a main achievement of the project.
The project is a logical extension of Dynaflows’ portfolio and an important step in sustainable applications using fiberglass systems.
The European Geothermica is a joint effort by the EU Member States and Associated Countries and targets innovation of geothermal energy supply and integrates into Europe’s energy system of the future. Europe aims to increase the share of renewable energy for direct heating and cooling, industrial processes, power generation and energy storage. More information can be found at the following website: http://www.geothermica.eu/.