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Although thermal heavy oil recovery methods are extensively used, no unified and standardized basis exists for selecting materials and configuring intermediate (production) casing/connection systems for these extreme-service applications. Thermal intermediate casing systems must accommodate a wide variety of mechanical and environmental loads sustained during well construction, thermal service at temperatures exceeding 200°C, and well abandonment. Numerous operator- and field-specific designs have been used with good success and only a few isolated challenges, but industry's use of its operating experience to calibrate tubular design bases for future wells has been limited.
This paper identifies the benefits and components of a unified casing system design basis for thermal wells, aimed to be technically comprehensive, inclusive of the available elements of industry's collective knowledge and experience, and adaptable to technological advancements. The technical element of the unified basis broadly relates to the engineering foundation used to make three primary design selections: material, pipe body, and connections. For each design selection, the paper provides an overview of the associated technological challenges and the current state of the industry in addressing those challenges, including the commonly-adopted design approaches. Key performance considerations include integrity during well construction, connection thermal service structural integrity, pipe thermal service integrity and deformation tolerance, connection sealability, and casing system environmental cracking resistance. Where applicable, the paper identifies interdependencies that exist between design selections (for instance, the impact of pipe material selection on the thermally-induced axial load that must be borne by the tubular and connection), and discusses mechanisms for accounting for those added complexities in the design.
Ultimately, the intent of this paper is to provide a framework for referencing existing technical knowledge and for considering further development and field benchmarking work that will reduce the technological uncertainty and increase simplicity in thermal casing system designs. Industry will benefit from a unified engineering approach that offers operators sufficient flexibility to accommodate application requirements and prior experience.
Integrity of high-temperature post-yield (HTPY) wells demands use of casing connections that demonstrate suitable structural strength, sealability and galling resistance. Since 2010, Thermal Well Casing Connection Evaluation Protocol (TWCCEP, published as ISO 12835) has been used by operators and connection manufacturers to assess connection performance under representative field conditions. While technically rigorous, TWCCEP programs are costly and time-consuming, and full-scope TWCCEP evaluations on multiple size/weight/material combinations are challenging to achieve.
This paper describes a product line evaluation (PLE) approach that enables effective qualification of several members of a connection family for HTPY applications with substantial savings of effort, cost and time. Some PLE strategies were used in the past for conventional stress-based-design applications, but not for strain-based-design applications – until recently, when a new HTPY PLE approach was proposed via an industry-sponsored project. This new HTPY PLE method relies on existence of a Reference Connection that has been qualified with a prior evaluation and/or field use. Another member of the same family (Candidate Connection) is then qualified based on existing data for the Reference Connection, and comparative evaluations of the Reference and Candidate Connections.
The paper illustrates basic HTPY PLE concepts, and analytical-experimental tools and processes used for the comparative evaluations. The HTPY PLE method incorporates several assessment steps, which PLE users select according to their field application severity, risk tolerance, and desired level of confidence in the assessment results. The paper emphasizes significance of design and manufacturing variables that are relevant for the HTPY PLE method, some of which are not commonly monitored by connection users – such as circumferential non-uniformity (seal waviness). In a case study example, the paper shows how the HTPY PLE can be used to qualify a connection needed for an optimized well completion, and the resultant cost-time savings. Upon validation, the HTPY PLE method is expected to enhance effectiveness of thermal well integrity and reduce connection failure potential.
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.