Geothermal Applicability of Strain-Based Casing Design Guidelines from Extreme Temperature Oil and Gas Wells
Authors: Miller S. M. , Dall'Acqua D. 
Published: October 2023

Recent advancement and evolution of geothermal recovery technologies highlights an opportunity to revisit the strain-based mechanical design bases typically used for cemented tubular systems in these wells. A vast body of relevant knowledge has been developed over decades in the oil and gas and earlier geothermal industries. However, clear guidelines for applying this knowledge to geothermal systems, particularly where temperature changes are sufficient to cause tubular yielding, do not exist. This paper aims to accelerate knowledge transfer of established post-yield tubular system performance expertise gained in high-temperature enhanced oil recovery (EOR) applications and demonstrate suitability for geothermal applications.

There is no publicly-available general tubular design reference that accommodates the wide range of possible thermal well operating conditions. Primary thermal casing loads relate to thermally-induced axial mechanical tubular strain, which is directly related to the cementing temperature and to subsequent temperature changes. EOR wells are generally cemented in cold reservoirs, and high operating temperatures result in compressive mechanical strain beyond yield, which is clearly reflected in design guidelines like DACC IRP 3 and ISO/TS 12835. As geothermal well cementing temperatures can vary considerably as a function of depth and operating mode, operating load paths may induce axial tensile or compressive strain beyond yield on the tubular system, for which NZS 2403:2015 requires use of strain-based plastic design. This paper presents an initial development of generalized thermomechanical casing responses for a broad range of typical geothermal operating conditions.

A framework for geothermal casing designers to consider is defined herein by delineating three casing design categories: elastic, near-yield, and post-yield design. Depending on the installation and operating conditions and inherent uncertainties, these categories provide a range of options for casing material geometry, and connection selection. Whereas elastic design bases are well-characterized using widely available tubular design software, local or global plasticity renders stress-based design factors less relevant, and the principle of superposition (i.e., load-path-independence) no longer applies. The integrity of tubulars and connections thus becomes much more dependent on the near-yield and post-yield properties of the material and the deformation tolerance of the tubular structure under the given loading path.

Tubular design for high-temperature geothermal wells can be challenging, particularly when local or global yielding is induced by large temperature changes relative to cementing temperature (for instance, if the well is quenched). Post-yield design bases that promote deformation-tolerant tubular structures are well-established in the oil and gas literature and can be readily transposed to high-temperature geothermal operation. Connection evaluation protocols like API 5C5 and ISO/TS 12835 may not fully characterize the relevant connection responses for all geothermal loading paths, and initial areas for improvement in the evaluation protocols are suggested. Finally, thermomechanical loading paths identified here are an important material selection input in harsh environments where stress-corrosion-cracking is possible.