Thermal Growth Assessments
High temperature reservoirs can lead to thermal loads on casing strings, causing expansion and wellhead growth and possible pressure increases in casing annuli. Thermal expansion can also lead to unwanted loading on casing hangers under certain conditions and may necessitate additional lockdown sleeves.
Thermal assessment of a well can be split into three main tasks:
- Cross sectional thermal analysis
- Annular pressure build-up analysis
- Multi-string load sharing analysis
Cross sectional thermal analysis
2D steady-state thermal analysis is carried out to determine temperature distributions at all stages of well installation, including uncontrolled blowout. Well bore temperature is applied to the inner string in each condition with undisturbed environmental temperature applied to soil at the model boundary. Thermal conductivity properties are carefully selected for each material to simulate heat transfer from the well bore out to the conductor. Figure 1 shows the 2D model with casing strings, and annuli with mud and cement, as well as soil. It is important to model an appropriate thickness of soil to ensure accurate boundary conditions at which the ambient temperature will be applied. Figure 2 shows the resulting thermal profile output.
Figure 1: ANSYS model
Figure 2: 2D thermal profile through conductor and casing
Annular pressure build-up analysis
If an annulus becomes trapped, either through over cementing or the formation of a barite plug in the mud, increased temperature causes pressure to build-up which could exceed casing or casing hanger structural limits. A 2D Axisymmetric model of each annulus is built in ANSYS containing hydrostatic fluid elements representing the mud. Temperature change in the annulus and casing strings is then applied to the model to determine pressure increase due to thermal loading and compared to casing burst or collapse capacities.
Multi-string load sharing analysis
Multi-string models are built in ANSYS for each blowout condition and production scenario using 3D pipe elements. Modelling is performed at each stage of well installation to ensure wellhead and casing hanger loads are determined at all stages. Temperature and pressure profiles, including end loads on casing hangers, can then be applied to the 3D pipe model to determine wellhead growth/squat, casing and casing hanger loads.
AS Mosley can then recommend whether loads on casing strings and hangers are acceptable and suggest mitigation if necessary.
For more information on Thermal Growth Assessments, please contact us.