Thermal expansionof casings in geothermal wells and possible mitigation of resultant axial strain

Abstract

Thermal expansion of casings is one of the major structural concern for high temperature geothermal wells. Unlike casings in the well, pipes on the surface are designed with bends that are included for mitigating thermal expansion. Casings in geothermal wells on the other hand are straight steel pipes that reach 700-1500 m length in typical high temperature wells and wellbore temperature changes of 200-350°C are common. The casings are structurally constrained by concrete and when the well reaches its working temperature, as a result of thermal expansion, stresses and strains reach beyond the yield strength in compression which in turn leads to plastic strain in the casings. In order to utilize superheated and even supercritical geothermal fluids by drilling deeper geothermal wells cased down to 2000 m or more, urge rises for solutions mitigating thermal expansion. Possible solutions are limited to the axial direction of the well since bends are not an option. Casing segments are joined by threaded couplings and as the connections are designed for the oil industry, they must remain pressure tight. Currently connections are designed to have similar strength as the pipe body but have no means of reducing the axial strain that builds up with thermal expansion. In this paper, possible solution to thermal expansion of casings in geothermal wells will be discussed.