There are many technological challenges that must be overcome for downhole service and completion tools to operate successfully in the ultra-HP/HT environment.

The first challenge is the shortness of effective design verification codes because ultra-HP/HT is new territory to anyone in the field. API 5C3 provides some design verification formulae to calculate collapse, burst, axial buckling, etc., but there are no compelling cases that qualify those formulae to be used to predict design failure in ultra-HP/HT environments. It is recommended that current design verification codes using API 5C3 need to be re-evaluated for systems beyond 15,000 psi and 169°C (300°F).

The second challenge is that there are fewer options for elastic sealing and metallic material for ultra-HP/HT well completions. Fluoroelastomers such as Aflas and Viton are normally chosen because of superior chemical resistance and flexibility at temperatures up to 232°C (450°F). Fluoroelastomers are not applicable as seal elements at temperatures exceeding 232°C; the elements become pasty, making them extremely difficult to contain with an anti-extrusion system (especially in as-rolled casing). For Baker Hughes, perfluoroelastomer (FFKM) was chosen as the elastomer seal element because it has the highest temperature rating – more than 232°C – compared to other elastomers currently available on the market, and it is stable up to 288°C (550°F). Nickel alloy C276 (UNS N10276) was chosen as the seal element carrier because it excels in all aspects in terms of mechanical material properties and chemical stability in ultra-HP/HT harsh corrosive environments.

Component design

  • Axial buckling. Most components in the completion tools are long tubing strings, and often those tubing strings encounter the axial loading more or less during the operation cycles. A tubing component design with protection against axial buckling has to be satisfied. Numerical simulation is commonly used to identify critical compressive buckling load for tubing with complicated features.
  • Thermal-mechanical coupling. Thermal-mechanical coupling has to be conducted by linear elastic analysis or elastic-plastic analysis. This procedure serves two purposes: How much does the tubing size change because of temperature variance from ambient temperature to 280°C (536°F)?; and are expanded components interfering with surrounding components and resulting in malfunction?
  • Collapse and burst under combined loadings. A new approach that integrates design of experiments (DOE) and stochastic study is used for collapse and burst under combined loading. The DOE provides an efficient methodology to study the effects on collapse or burst ratings due to geometric imperfections such as ovality, eccentricity, size variance caused by thermal expansion or manufacturing, and thickness tolerance. Stochastic study gives explicit quantitative evaluation of the confidence level of the pressure ratings when imperfections and manufacture tolerance are explicitly taken into consideration for calculation.

Subsystem design. The most critical subsystems in an ultra-HP/HT completion and production tool are the slip system and the seal system.

  • Slip system analysis. A reliable slip system is critical to a packer or bridge plug-type completion tool. First, a slip system will expand against the casing during and after setting. A good grip on the casing will prevent the seal system from damage. Second, a slip system provides hanging support to the tailpipe below the production packer. A 3-D contact model is set up to determine: Can this slip system be set by the specified setting force?; and is it strong enough to resist the specified hanging capacity?
  • Seal system analysis and optimization. The seal system is an essential part of any ultra-HP/HT completion system. To reduce cost and time, elastic-plastic and hyper-elastic finite element analysis (FEA) were relied upon greatly for design verification and optimization. To be considered as a viable design, the following criteria are to be met: The equivalent plastic strain in the metal should not exceed the maximum allowable plastic strain, and the maximum elastic strain in the seal should not exceed the maximum allowable strain; and the seal has to set and then withstand specified differential pressure from above and below with combined tensile and compressive loading from the boost load acting on the tool without packing element extrusion.

System integrity verification. A few analyses have to be done to verify the system integrity and determine any potential risks during deployment and production.

  • Dogleg analysis. Wells have different well profiles; therefore, it is necessary to have a final design verification analysis to make sure the tool can pass the most severe dogleg area without excessive torque, bending, tension, or compressive force.
  • Noise, vibration, and harshness (NVH) analysis. To enhance reliability, it is recommended to run NVH analysis. During production, there is noise generated from above or below the production tools. For example, an electrical submersible pump is sometimes installed below the production packer, and its generated noise could cause resonance if the natural frequencies of the tool are close to the electrical submersible pump’s operating frequency.

Design verification and optimization were embedded in multilevels (component, subsystem, and system). In each level’s thermal-mechanical coupling, elastic analysis, elastic-plastic, and hyper-elastic analysis were conducted extensively. Time and cost were greatly reduced by implementing this design verification and optimization process involving intensive structural simulation.

Test validation

Two products that went through the described design verification and optimization processes were an ISO 14310 V0-rated 280°C (500°F) 25,000-psi permanent bridge plug and an ISO 14310 V3-rated 263°C (470°F) 20,000-psi permanent production packer for as-rolled casing.

The 280°C 25,000-psi permanent bridge plug underwent two types of tests. First, it was tested in maximum casing inside diameter with nitrogen as the medium. The maximum temperature was 280°C. Maximum temperature swing and multiple pressure reversals were applied on the bridge plug. It passed multiple V0 qualification tests per ISO 14310/API11D1.

Second, it was tested in as-rolled casing with water as the medium. The maximum temperature was 280°C. Maximum temperature swing and multiple pressure reversals were applied on the bridge plug. It passed multiple V0 qualification tests per ISO 14310/API11D1.

The 263°C 20,000-psi permanent production packer also was tested in as-rolled casing with water as the medium. The maximum temperature was 263°C with temperature cycles, multiple pressure reversals, and combined tension or compressive loading per ISO14310/ API11D1. It passed modified V3 validation.

Results

The successful launch of the the permanent production packer and bridge plug is attributed to the following:

  • New metallic and polymeric materials are used to mitigate the ever-increasing temperature, pressure, and associated negative impacts on the reliability of the well completion;
  • The state-of-the-art FEA, which integrates design of experiments, thermal mechanical coupling, large-scale 3-D contact, and elastic-plastic analysis, is an indispensable tool for design verification and optimization;
  • Design verification and optimization are conducted in a systematic way that starts from component levels and works up to the assembly system level; and
  • Test facilities able to test up to 371°C (700°F) and 30,000 psi are keys to validation of the ultra-HP/HT tools.