Ultra-HPHT wells are challenging the limits of tubular performance, in terms of metallurgy and threaded connections. But higher pressures and temperatures are only part of the challenge. Carbon steel, hot-rolled chromium-based stainless steel, and cold-hardened materials are all options that must be considered to optimize economics in ultra-HPHT applications.
Historically, HPHT wells have been defined as operations with pressures in the 10,000-psi-to-15,000-psi range and temperatures in excess of 300°F. But with many operators drilling or planning wells in excess of 20,000 psi and temperatures in excess of 400°F, previous materials and connections no longer meet the requirements to ensure effective well completions and HSE concerns.
The simple solution is to use thicker, heavier tubulars, but there are additional considerations. The effects of temperature degradation on physical material properties, aggressive wellbore environments, and economic considerations add constraints to material selections. Furthermore, once the tubulars are determined, appropriate threaded connections are required to provide ratings up to, or equivalent to, the pipe body, along with validated performance and field installation reliability.
The use of cold hardened alloys provides the resistance to downhole environments for H2S and CO2, but they are expensive and have long lead times. They also are more greatly affected by temperature and anisotropy than hot finished products. One advantage of cold hardened alloys is the availability in higher yield strengths above 140 KSI. In many cases, the use of higher chromium (Cr) hot finished materials and specialized carbon steels can provide appropriate performances at reduced costs, and with less degradation at higher temperatures.