Geothermal energy is a promising renewable energy source, but it comes with its own set of challenges. Two significant issues in geothermal well operations are corrosion and scaling of steel casings. These problems can severely reduce the lifespan of the wells, lead to wellbore integrity issues, and potentially cause environmental contamination. In addition, corrosion degrades the casing surface and causes excessive pressure losses. To prevent major damage, costly workover operations are inevitable, becoming a substantial financial burden and may ultimately jeopardize the project economics of steel-based geothermal well designs.
The Challenge of Corrosion and Scaling in Geothermal Wells
Geothermal wells using conventional steel casings face significant challenges due to corrosion and scaling. Corrosion, particularly, is a major problem. A study by the Dutch State Supervision of Mines (SodM) examined 38 geothermal wells and found that many experienced significant wall thickness reduction, with some even leaking. The study concluded that the anticipated design lifetime of steel-casing-based designs cannot be reached and that the probability of leakage was also significantly increased over time.
These issues underscore the need for an alternative material that can withstand the harsh conditions of geothermal wells. Fiberglass, with its excellent corrosion resistance, presents a promising solution.
Ensuring Well Integrity: HAZID Study and Material Compatibility
The GRE-GEO project aims to develop a fiberglass casing system that can better withstand the conditions in geothermal wells. As part of this initiative, DRG has conducted a Hazard Identification (HAZID) study to ensure the material’s suitability, focusing on well-integrity topics such as ESD, wear, and chemical compatibility.
A HAZID study identifies potential risks associated with using fiberglass casings in geothermal wells and outlines measures to mitigate these risks. Conducting a hazard identification and associated risk assessment is essential as part of the Well Integrity Management System (WIMS). The GRE-GEO project supports operators by providing guidance, focusing on the unique risks of fiberglass casings.
Initiating the HAZID study early aids in selecting the required material qualification tests. The study also identifies GRE-specific risks during handling and installation, using past HAZID studies for steel-casing designs as a foundation, extending these studies to include GRE-specific risks.
Existing studies often are primarily focused on the installation- and operation phases of the geothermal well lifecycle. Abandoning related risks should also be part of the Well Integrity Management System. GRE-GEO has included this phase in the HAZID study as well. By addressing risks across the well’s lifecycle, the study ensures comprehensive risk management.
The findings guide the development of installation tools, guidelines, and industry standards for GRE casings, crucial for safe and effective implementation in geothermal wells.
Corrosion
One of the main reasons to select GRE for geothermal well casings is the excellent corrosion resistance compared to conventional steel casing. GRE is therefore seen as an attractive alternative to counter corrosion problems related to high salinity production fluids in geothermal wells.
Electrostatic Discharge (ESD)
From the HAZID study, ESD has been flagged as a potential risk as discharge can form an ignition source in the explosion area of the well. Four areas of concern were raised where ESD could form a potential risk:
- Pumping of drilling fluids.
- Pumping of cement slurry.
- Flow of production fluids.
- Handling of fiberglass pipes during installation.
Studies have shown that typical drilling and production fluids have high conductivity, meaning accumulated charge dissipates almost instantaneously, reducing the risk of ESD. However, handling fiberglass in explosive environments requires precautions, such as using anti-static coatings or controlling humidity at the worksite. Mitigation actions to minimize these risks were defined and will be part of the guidelines prepared by the GRE-GEO consortium.
The main findings of the ESD study are:
- Drilling operations: OBM or WBM-based drilling fluids were found to be highly conductive and show relaxation times in the order of magnitude of picoseconds. This means that accumulated charge in the liquid almost instantaneously dissipates and therefore will not form an ESD risk. However, not necessary earthing via the drill shaft would contribute to the dissipation of charge.
- Cementing operations: The same holds for cementing operations. Also, cements have a high conductivity and accumulated charge will dissipate almost instantaneously.
- Production: Production fluids in general are also highly conductive fluids (high salinity) and thus have very short relaxation times as well and charge will not accumulate in these fluids. In addition, in combination with downstream earthed conductors (valves, etc) accumulated charge would dissipate via these conductors to earth.
- Verify conductivity of fluids used during installation, cementing, and production: As indicated above, typical OBM, WBM, cement slurries and production fluids have very high conductivity and will not accumulate electrostatic charge in the fluids. It is recommended to verify the conductivity of the fluids used throughout the well’s lifecycle.
- Handling of GRE during installation: The handling of GRE in the explosion area seems to be a potential risk for ESD. Although these risks are equal to ESD risks created by human bodies, precautions will be required to fulfill the requirements provided by industry standards. Examples are e.g. anti-static coatings/sprays, conductive additives to be added to the resin, relative humidity control at the work area, or ionizing the air at the working area.
Chemical Compatibility
Throughout the lifecycle of the well, casings will be exposed to:
- Production water
- Well-intervention fluids (descaling chemicals like HCl, HF)
- Sulfide-reducing bacteria (SRB)
- Associated gases like CO2, H2S
- Exposure to radioisotopes that are present in the production fluids
An extensive literature study has been carried out to investigate the compatibility of GRE with these fluids. Based on long-term experience with GRE in several industry applications, it can be concluded that the chemical compatibility of GRE is generally good, with well-defined limitations in terms of exposure concentrations, temperatures, and times. This information will determine guidelines for the safe operation of GRE casings in geothermal wells. Where needed, exposure testing will be part of the test program.
Wear Resistance
Like steel casings, fiberglass casings must withstand wear from abrasion during installation and intervention operations. Wear testing is part of the GRE-GEO test qualification program, ensuring the material’s durability in real-world conditions.
Towards an Industry Standards
One of the GRE-GEO project’s main objectives is to develop an industry standard for fiberglass casings in geothermal wells. This involves extensive testing and validation to construct a design envelope for fiberglass-based well designs. The project aims to apply the developed fiberglass system in a demonstration well, showcasing its viability as an alternative to steel casings.
Conclusion
The GRE-GEO project represents a significant advancement in geothermal well design. By addressing the challenges of corrosion and scaling with innovative fiberglass casings, the project aims to improve well integrity and extend the lifespan of geothermal wells. The studies conducted by DRG within the GRE-GEO project provide a solid foundation for developing industry standards and guidelines for using fiberglass casings in geothermal applications.
The shift from steel to fiberglass casings could be a game-changer for the geothermal industry, offering a more durable and cost-effective solution for sustainable energy production. As the project progresses, the results from the demonstration well will be crucial in validating the efficacy of fiberglass casings and potentially setting a new standard for geothermal well design.
Author: Lukas Karolis Bajarūnas, Engineer
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