Well Casing Challenges
Extreme well casing loading conditions can occur in several production operating scenarios including:
Multi-stage Hydraulic Fracturing:
The pressure and temperature cycles that occur during hydraulic fracturing can impose high cyclic stresses on the well casing and connections. In addition, fractures from neighbouring wells can impact closely spaced wells, resulting in additional loads on the well casing.
Thermal Enhanced Oil Recovery:
When producing heavy oil or bitumen, steam injection strategies like Cyclic Steam Stimulation (CSS) or Steam Assisted Gravity Drainage (SAGD) impose huge thermally induced stress cycles on the well casing.
Additionally, high temperature creep and stress relaxation in the casing can combine with the thermal cycles to cause the casing to yield both during heating and cooling phases of each cycle.
This can result in low cycle fatigue that can cause threaded connection failures within only 4 or 5 thermal cycles.
Compacting Reservoirs:
Pressure depletion in the oil or gas reservoir can cause the compaction or collapse of weak reservoir formations such as diatomites, chalks or weak sandstones. This compresses the well casing in the reservoir interval.
As the reservoir compacts, the overlying formations subside. The well casing may experience shear stress.
Tensile stresses may also affect the well casing in the overlying formations as some formations subside more than others.
Sand Production:
Massive sand production may cause subsidence, leaving a void in the reservoir that cannot support the overlying formations.
This occurs in primary production from heavy oil wells using Cold Heavy Oil Production with Sand (CHOPS).
Operators have also observed massive sand production in offshore wells where they encounter shallow water flow zones.
Sand also poses challenge in high productivity wells where the sand control system fails or is improperly designed.
Seismic Activity:
Global tectonic activity or local changes in formation stresses from fluid injection or withdrawal can cause movement along planes of weakness in the reservoir and overlying formations.
This may result in localized shearing of the well casing, causing large doglegs that limit well access or cause threaded connections to leak or fail.
Permafrost:
Wells through permafrost may experience high loads as the warm produced fluids melt the overlying layers of frozen soil, causing them to compact or swell. Formations composed of loose sand with high ice contents are particularly susceptible to thaw-induced compaction.
C-FER Yourself
Helping Improve Well Design
Testing to Industry Standards
- Conducting full-scale casing connection tests following industry standards:
- API – RP 5C5 Recommended Practice for Evaluation Procedures for Casing and Tubing Connections
- ISO 13679:2019 Petroleum and natural gas industries, Procedures for testing casing and tubing connections
- ISO/TS 12835:2022 Qualification of casing connections for thermal wells
Developing New Protocols
- Developing new protocols and standards for full-scale qualification testing of premium connections for specialized application such as:
- Thermal and geothermal
- Multi-stage hydraulic fracturing
- Underground storage of CO2 and hydrogen
Assessing Structural Integrity
- Modeling the stresses and strains imposed on the body and threaded connections of the well casing to assist in specifying the casing design for specific applications
- Modeling and testing the performance of sand screens and slotted liners for SAGD wells
Analyzing Designs
- Analyzing subsidence induced casing damage in primary heavy oil operations
- Analyzing well casing design requirements considering reservoir and overburden formation movements caused by compaction of chalk reservoirs
- Designing wells through permafrost considering the effects of interacting thaw zones on multi-well pads
- Evaluating the impact of fractures intersecting neighbouring wells in multi-stage hydraulic fracturing operations
Role of Modelling to Address Challenges
C-FER uses modeling to understand the stresses and strains imposed on the well casing body and threaded connections. We use the information to select appropriate casing material, wall thickness and threaded connection type to ensure the integrity of the wellbore. This modeling considers two key aspects of the well performance:
- Structural Integrity – to ensure the well provides an open pathway to access the reservoir and allow the installation of the required downhole equipment
- Pressure Integrity – to ensure that the casing provides a competent and reliable seal to prevent fluid from leaking into, or out of, the wellbore to the surrounding formations
Analysis Considerations for Well Design
Most analyses must also consider the interactions between the well casing, cement and surrounding formations to determine what loads and displacements might be imposed on the well casing during the life of the well. The analysis must also consider how these loads and displacements that may crack the well cement and compromise the seal between the well casing and formation, leading to gas or liquid migration to overlying formations and to the ground surface.
Well Design Stresses and Strains
In some cases, the stresses and strains in the well casing during production operations can cause the casing to yield, making traditional design methods inadequate to ensure well integrity over the operation life. Pressure or temperature changes in the well can complicate casing design, including the specification of threaded casing connections. As operating stresses and strains cycle, resulting in fatigue or even low-cycle fatigue failures.
When stresses exceed the yield strength of the casing material or cause yielding in the casing connection, a strain-based design approach is required to evaluate the post-yield performance of the casing and connections. The approach ensures the casing maintains its structural integrity even though it has yielded. This often involves the counter-intuitive approach of selecting lower yield strength casing materials that exhibit better post-yield stress-strain behaviour.
Approaching Challenges with Reliability-based Design
These designs also incorporate a reliability-based design approach that considers all the uncertainties associated with the casing and connection performance and the expected loading conditions.
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Contributor
Brian Wagg, MSc, PEng.
Director, Testing Services
With over 35 years with C-FER, Brian Wagg’s current focus is on applying C-FER’s analytical and full-scale testing capabilities to projects in clean energy including CCUS, hydrogen, geothermal and small modular nuclear reactors.
