Objectives: The aim of WP4 is to identify and evaluate possible implications and risks associated with connecting casing strings with the new flexible couplings to the structural integrity and tightness of high temperature geothermal wells. The large-scale demonstration test described in WP3 will be modelled to investigate the effects of flexible coupling on the evolution of axial stresses in connected casing strings and evaluate cement integrity under thermal cycling loads. A model of the whole well with multiple casings connected with the new flexible coupling will be developed to analyze the performance of the well system under thermal cycling at moderate and high temperatures. Risks associated with the use of flexible couplings will be evaluated by performing a quantitative risk assessment analysis.
- Task 4.1 Structural analysis of surface test (TNO, GFZ, ISOR) A finite element model will be developed that efficiently simulates the behavior of flexible coupling, annular cement and well material interfaces observed in the surface test (WP3). Large-scale testing is very costly and the number of thermal cycles during the test is limited. It is therefore essential to develop calibrated numerical models, which are able to predict the actual structural behavior. Calibrated models can be used with confidence in further numerical investigations to obtain additional insights into the structural response of model setup not tested experimentally and to carry out parametric studies. Nonlinear finite element models of the surface test will predict evolution of axial stress, hoop stress and strain in the casing, coupling and cemented annulus, when exposed to temperatures from 100 °C to 300 °C. In particular, the effect of casing movement on the integrity of annular cement in the vicinity of coupling will be investigated. Model setups with and without flexible coupling can be considered to determine a threshold temperature above which flexible couplings are needed. Model results will be used to evaluate improvements in the integrity of geothermal wells equipped with flexible couplings and to investigate specific risks associated with the use of couplings identified in risk assessment analysis (Task 4.3).
- Task 4.2 Structural integrity of geothermal wells with flexible couplings (TNO, ISOR) A structural model of the whole well with multiple casings connected with the new flexible coupling will be developed. Results of surface test (WP3) and structural modeling of surface test (Task 4.1) will be used to develop a simplified structural model of the geothermal well. The aim of modelling in this task is to simulate the effects of thermal cycling on displacement, stress and strain along the whole well. The main failure mode to be analyzed is casing buckling and plastic straining. The model needs to capture thermal expansion in casing segments while allowing movement of segment on one side of the flexible coupling. Analysis will evaluate the performance of flexible connections in a geothermal well and demonstrate the benefits of using flexible couplings to significantly reduce thermal axial stress and strain in casings and therefore practically eliminate the risk of casing failures as a result of thermal straining.
- Task 4.3 Risk assessment of using flexible couplings (TNO, GFZ, ISOR) The risk assessment approach will be based on the bowtie concept linking possible causes (threats) to undesired event(s) and resulting consequences. This task will concentrate on the mechanisms of failure of the well during construction and exploitation and less on the consequences of failure for the technical performance, safety and environment. Possible well failure modes will be identified based on expert elicitation; attention will be directed in particular to the use of the novel flexible couplings and their effect on the overall technical, safety and environmental performance of the well system (casing, couplings, cement and near-well rocks). Individual threats or threats in concatenation will be evaluated and prioritized in terms of their probability and impact. Expert judgement, supported with outcomes of the simulation work in GeConnect (Tasks 4.1 and 4.2) and of other projects (e.g. GeoWell) will be used here. The most critical threats or combination of threats will be translated in scenarios (description of possible state or evolution of the well barriers leading to failure), the results of which will be visualized in a risk register and risk matrix. For each scenario potential risk reduction measures, either of preventative or corrective nature, will be defined and added to the register. Models will be defined for selected scenarios which than will be executed (in Task 4.3 or in Task 4.1/4.2). The model output will be evaluated in their effect on the previously assigned risk and the risk register and matrix will be adjusted accordingly. The outcome will be used to adjust the design of the flexible coupling. The work proposed in this task will centre around two workshops, one at the start of the activities directed to the risk identification and the second one at the end of task 4.3 evaluating the outcomes of the modelling exercise and the consequences for the design of the flexible coupling.