The main research topics are briefly described below.
Cementing
Cement samples from the IDDP-1 well in Iceland, which were exposed to temperatures up to 450°C, were analysed. The results showed that a) Portland cement mixtures including silica are appropriate for the use in geothermal applications and b) surplus water was found to have created serious bleeding problems and needs to be reduced. Numerical modelling complemented the experimental results.
Pressure build-up in cement sheaths was studied and information gathered on pressures at elevated temperatures. Water pockets can lead to critical pressure build-up, but high pressures can also be generated in the cement by surplus water. The studies resulted in an approach to develop “pumpable” cement with reduced water content without reducing its sealing properties.
A ductile intermediate layer between cement and casing that could take up strains generated by temperature changes was developed. Nanomaterials were evaluated for their potential to reduce friction in small-scale tests. The developed formulation has promising properties and a thin layer is able to reduce the friction forces by more than a factor of 10.
HTCC
This activity aimed at developing high-temperature composite casings (HTCC - Glass fibre reinforced polymers) and increase the current application temperature. A list of requirements and specifications for HTCC was established and a concept design made. Extensive research was done on casing connections which resulted in the design of Fiber Reinforced Polymer (FRP) threaded composite connection. Through an extensive lab assessment and some prototyping tests within the project it became clear that it would not be possible to reach the ambitious goal regarding temperature limit. The research indicates that the achievable service temperature is 85°C.
Flexible couplings
Plastic straining of casings due to large temperature differences regularly results in casing failures. A flexible coupling for casings that allows axial movement of casing segments was designed and patented. As a result, thermal stresses are controlled to be below the yield strength of the casing material. No axial plastic strains are therefore generated ensuring a safe operation throughout a wells lifetime. Several full-scale prototypes were built and tested at ambient temperature in laboratories. The concept has shown to be of high interest in the geothermal industry and most likely it will be tested in a geothermal well in the near future.
Casing materials
Casing material data at very high temperatures has not been readily available. Therefore, tensile tests were performed in a dedicated high temperature testing rig revealing its mechanical properties. The tests were performed at room- and elevated temperatures (250°C, 450°C, 550°C) for the proposed material candidates, K55, L80, T95 and Inconel 625. Additionally, corrosion tests were performed in an autoclave system specially designed for the purpose of high-temperature and high-pressure testing at well sites. In total, 15 samples were tested at 210°C and 450°C for a period of 21 days. The expanded knowledge will significantly expand the scientific basis for designing future HTGW.
Well monitoring
Work was done on methods to identify processes that affect the integrity of geothermal wells by developing and testing distributed fibre optic sensing technologies to simultaneously measure temperature, strain and acoustic noise within the cemented annulus. The combined application of fibre optic cable, casing hardware, data acquisition systems, signal filtering supported by the assessment of the well construction process and well logging information helped to gain confidence that strain and temperature readings can give a reliable picture of the downhole well integrity conditions in applications reaching temperatures up to 300°C. Successful trial tests were performed in different downhole conditions from low to high temperature ranges at different depths in Germany and Iceland, including the IDDP-2 scientific drilling project.
Risk assessment
The work on risk assessment has targeted well integrity in the production phase. The goal is to raise the standard of risk analysis tools for geothermal wells to a standard that is comparable to that of oil & gas wells. A thorough literature study as well as a survey was performed to map the status and availability of qualitative and quantitative risk assessment methods. The results show that well barriers are generally less focused in the geothermal industry. A list of barriers and associated failure modes in the geothermal production phase has been compiled using available guidelines, standards and industry input. The failure modes covered are representative of commonly occurring problems in geothermal wells, also those operating in high-temperature conditions. The risk assessment activity has resulted in a proposal for a risk management framework that can be used for deep geothermal wells.
