Casing connections in thermal wells, such as SAGD and CSS wells, experience extreme loads due to exposure to high temperatures up to 200ºC-350ºC, stresses exceeding the elastic limit, and cyclic plastic deformation. To-date, no standard procedure has been adopted by the industry to qualify casing connections for such conditions. In particular, the existing evaluation standard ISO13679/API5C5 exclude temperatures above 180ºC and tubular loads beyond pipe body yield. Proprietary procedures have been used to qualify connections for individual thermal operations, but none of those has been accepted as an industry standard.
This paper introduces a new protocol for evaluating casing connections for thermal well applications: Thermal Well Casing Connection Evaluation Protocol (TWCCEP) founded on long-standing work in the thermal-well arena. TWCCEPT has been developed through a multi-client project, sponsored by operators and connection manufacturers involved in thermal-well operations in Canada: EnCana, Husky Energy, Evraz (formerly Ipsco), Nexen, Pengrowth, Petro-Canada, Shell, TenarisHydril, and Total. Recently, International Organization for Standardization (ISO) Technical Committee 67 Sub Committee 5 registered a new work item to consider adopted TWCCEP as an international standard.
This paper refers to the TWCCEPT version available at the time of submitting the paper manuscript. TWCCEP employs both analytical and experimental procedures to assess performance of a candidate connection under conditions typical of service in thermally-stimulated wells. The objective of the analytical component is to assess sensitivities of the candidate connection to selected design variables, and identify worst-case combinations of those variables for subsequent configuration of specimens for physical testing. The purpose of the physical testing is to verify performance of the connection specimens under assembly-and-loading conditions simulating the thermal-well service.
In addition to the protocol overview, this paper illustrates how engineering analysis, numerical simulation, and reduced-scale physical testing were used in the protocol development to examine impacts of various design and loading variables on connection strength and sealability, and how those results were utilized to formulate the analysis-and-test matrix prescribed in the TWCCEP evaluation procedure.
Adoption and consistent use of TWCCEP is expected to increase operational reliability and decrease failure potential of casing strings in thermal wells. Learnings from the protocol development will also help define requirements for connection re-qualification in cases when one or more of the design variables change (i.e., in product line qualification).
Thermal well service conditions
Loading conditions in extreme-temperature wells, such as Steam Assisted Gravity Drainage (SAGD) and Cyclic Stream Stimulation (CSS), are severe. Maximum operating temperatures in those wells currently reach into the interval between 200ºC and 350ºC. Large temperature variations occur due to production techniques and well interventions, leading to cyclic heating and cooling. When a restrained tubular, such as a cemented casing string, is subject to a large temperature increases during heating, constrained thermal expansion generates mechanical forces in the pipe. Those strain-induced forces are of sufficient magnitude to yield the pipe, even if it is made of a high-grade material. Theoretically, a high-yield pipe material could be chosen to avoid yielding, but typically such choices are not practical due to reduced resistance to environmental cracking and high cost. In consequence, average stresses in the pipe-connection system exceed the full-pipe-body yield stress, and the system deforms plastically. In addition, strain localization in weaker sections of the pipe-connection system can lead to local plastic strains higher than the average strain, which compounds the degree of the local plastic deformation.