A Design Basis for Geothermal Well Tubulars Subjected to Annular Pressure Buildup from Fluids Trapped in Cement

In geothermal and thermal service wells, the most common source of annular pressure buildup (APB) is trapped fluid pockets in the cement between two tubulars. Such pockets are created due to poor displacement of the drilling fluid during primary cementing. If the fluid pocket is in a cemented section open to formation, the pressure is relieved by leak-off and the inner tubular is safe. If these pockets occur in a pipe-in-pipe cemented section (for example, cemented tieback), they become trapped with no path for relief, and quickly develop pressure when heated during production. The collapse load on the inner string as a result can deform (or fail) the string. Failures from APB-induced collapse of inner strings have been implicated in enough number of wells that this is a major concern of geothermal well designers. A common recourse is careful planning and implementation of the cement operation. The Code of Practice for Deep Geothermal Wells NZS 2403:2015 also suggests a design basis wherein the collapse strength of the inner string is at least 1.2 times the burst strength of the outer string. The implication is that if this design basis is adopted, the outer string bursts (or connections leak), thus providing relief of APB before the inner string collapses. While the rationale appears reasonable, there is no quantitative basis for this recommendation. Besides, the elastic-based API ratings used in this approach are not representative of the limit states of the tubulars. In the paper, we present a quantitative basis of design for APB loads such that the outer string bursts and relieves APB before the inner string collapses, and investigate the practicability of such a design in typical geothermal wells. An arbitrary fluid pocket is assumed to be present in the cement adjacent to the inner string, surrounded by cement. As pressure increases with temperature, the response of the cement to this local increase in pressure is modeled as a Griffith crack. The initiation and propagation of the crack as a function of increasing pressure are followed until either the inner tubular collapses, or the fluid finds access to the outer tubular and the load is high enough to rupture the outer tubular. The collapse and burst limits in this analysis are based on the limit states described in API TR 5C3 (ISO TR 10400). A number of sensitivities (depth location of fluid pocket, radial location of fluid pocket in the annular gap, cement properties) are studied to characterize the problem. Based on this analysis, the required collapse and burst capacities of the strings and the properties of cement to assure that the outer string bursts (or connections leak) before the inner string collapses are determined. The impact of this method on typical geothermal well architectures is illustrated through examples, and the current New Zealand Standard recommendation is examined in this context. The authors hope that this work provides an approach to design geothermal wells such that the integrity of the production string is maintained when faced with APB from inadvertent fluid pockets in cemented zones.