| Title | Geothermal well design: extending feasibility under challenging geomechanical and thermal loads |
|---|---|
| Authors | Raffaello Zambetti, Pietro Quattri, Paolo Novelli, Roberto Martini, Stefano Tarquini, Santiago Grittini |
| Year | 2023 |
| Conference | World Geothermal Congress |
| Keywords | geomechanics, thermal loads, well design, strain based design, working stress design |
| Abstract | The aim of this paper is the presentation of the analysis of a casing design in a geothermal well in the Larderello field where the drilling of the third well from the same pad was planned. In this case applying normal casing design a residual risk is present, in fact the shallower sections of the well are considered with not high temperature of the formations. Just because in the same pad other very productive wells were present since many years, also the warming effect of the fluid production of this wells in the shallower sections have to be considered in case the distance between wells is very low. In order to consider also this effect in the casing design verifying the role of the influence of the induced thermal load, it was decided to apply a different methodology. This paper presents a methodology which complements “working stress design” with “thermal load strain-based design”. This approach allows to better define loads typical of high enthalpy geothermal wells, extending projects feasibility and optimizing well design. The analysis explores the main complexities of well design in such environment, including annular pressure buildup (extremely important in case of very high temperature environment), materials plastic regime, thermal cycling effects and critical buckling lengths. When cementing formations with large fractures, with the aim to have a fully cemented casing string due to mitigate thermal loads effects, there is a high risk of low quality casing cementing, with possible problems of APB and buckling. Moreover, geothermal wells are critical in case of thermal cycles because normally operates with high temperature and a fast quenching effect on the casing may introduce high axial thermal loads under which the string may enter plastic regime introducing residual stresses. Another relevant phenomenon investigated is “cold collapse”, which is the reduction of collapse resistance when tubulars are subjected to high axial loads. In conclusion, a case study in which a production well in the same pad produced an anomaly in the temperature profiles of a nearby well compared to the geothermal gradient is presented. Design modifications and best practices are defined by applying this design methodology to the well. |