Deep wells are an essential component of the global energy infrastructure. However, well integrity
issues — the failure of wells to effectively seal and isolate subsurface formations — are widespread
and can lead to severe environmental consequences such as aquifer contamination and greenhouse
gas emissions. Avoiding leakage by maintaining well integrity is therefore critical throughout
the entire lifecycle of wells. In this context, the cement-filled annulus between steel casings
represents a key barrier system, as the cement–steel compound constitutes a common weakness.
The complexity of the processes governing well integrity suggests the need for a multi-scale
approach. Accordingly, this study links material property characterization at the sub-millimeter
scale, meter-scale cyclic loading experiments, and field-scale operational practices to improve the
understanding of well integrity.
Understanding the mechanisms responsible for leakage is a prerequisite for well integrity management.
These mechanisms were therefore investigated experimentally, with a focus on the
cement–steel compound in wells used for underground gas storage. ... mehrThese wells experience
cyclic loading due to repeated gas injection and extraction and may therefore be particularly
susceptible to integrity degradation. To investigate this, a total of 256 large-scale autoclave
experiments were conducted on five 2-meter-long cement-filled steel casings designed to represent
the casing-cement interface in wells. These specimens were subjected to cyclic pressure and
thermal loading simulating underground gas storage operations. The system permeability of
the cement-filled casing and its evolution with successive loading cycles and over time were
determined from pressure changes associated with gas flow through the casing. These large-scale
experiments were complemented by small-scale laboratory investigations on cement samples to
characterize properties such as matrix permeability, porosity, and mineralogy before and after
cyclic loading.
To quantitatively interpret the experiments, a numerical finite-difference model was developed to
simulate gas flow through an annular gap between cement and steel and to derive effective gap
apertures after each loading cycle. The model accounts for Hagen–Poiseuille gas flow as well as
pressure–temperature coupling. This coupling is important because adiabatic gas compression
and expansion during flow cause temperature changes in the gas. The resulting temperature
difference between the gas and the surrounding autoclave steel induces heat transfer between gas
and steel under approximately isochoric conditions, which superimposes the adiabatic temperature
evolution driven by gas flow. This heat transfer in turn affects the gas pressure, thereby changing
pressure gradients and consequently influencing gas flow.
The results demonstrate that interpreting gas flow and pressure evolution requires consideration
of pressure–temperature interactions, which are often neglected. Furthermore, repeated pressure
loading resulted in increased system permeabilities, most likely due to progressive debonding
at the cement–steel interface and the development of an annular gap. Gas flow through the
cement matrix can be ruled out due to its low matrix permeability. Thermal loading caused
temporary permeability reductions, which are interpreted as condensation of water within the gap.
Consequently, maintaining a fluid column in the annulus and selecting fluids with appropriate
density and viscosity may contribute to improved sealing performance. Generally, the modeled annular gap apertures stabilized between approximately 25–45 μm after a limited number of
loading cycles, indicating that the gap aperture approaches a quasi-stable maximum.
The numerical model is further applied to diagnostic well integrity testing scenarios, suggesting
that annular gaps generated by cyclic loading alone are unlikely to produce major leaks as defined
by the American Petroleum Institute (2006). These findings indicate that, under conditions of
high-quality cementation, the likelihood of major leakage is low, although minor leaks may still
pose long-term risks, particularly for abandoned wells that are not monitored. Ultimately, the
use of hybrid sealing concepts that combine cement with additional barrier materials, such as
bentonite, rock plugs, bitumen, or metallic alloys, could further enhance long-term well integrity.
Beyond insights into leakage mechanisms gained from laboratory and modeling investigations,
maintaining well integrity also requires appropriate diagnostic procedures and regulatory frameworks
for the detection and management of leakage in the field. Because downhole integrity
is difficult to assess directly, regulations often focus on manifestations of leaks at the surface.
A typical manifestation of leakage within an annulus is pressure build-up at the wellhead that
reoccurs after pressure release. This phenomenon is known as sustained casing pressure (SCP).
If the leaking annulus is open to the atmosphere, pressure cannot build up and instead gas
continuously flows to the atmosphere, which is referred to as casing vent flow (CVF).
Consequently, SCP and CVF can be used to infer the integrity condition of a well from pressure
build-up or gas flow measurements at the wellhead and therefore often form the basis of
regulatory requirements. To evaluate how different countries address these phenomena, well
integrity regulations as well as industry guidelines in major gas-producing countries are reviewed,
including the United States, Russia, China, Canada, Norway, and Australia. The analysis
reveals substantial differences in regulatory approaches. While the United States, Canada, and
Russia mainly apply direct and prescriptive requirements, Norway and Australia rely on indirect,
risk-based frameworks that place greater responsibility on operators. A common shortcoming of
prescriptive regulations is the lack of comprehensive workflows that span the entire process from
the detection of pressure or flow to effective remediation. A minimum workflow for SCP and
CVF management is therefore proposed, beginning with monitoring, i.e., the measurement of
pressure build-up or flow in annuli. If pressure build-up or flow is detected, diagnostic testing
must be performed to determine whether leakage is present or if, for example, the pressure is
induced by temperature variations. If leakage is confirmed, appropriate remediation measures
must be implemented. Monitoring and follow-up actions should continue throughout the postabandonment
phase. In addition, well integrity data should be thoroughly documented and made
publicly available whenever possible.
By adopting a holistic multi-scale approach, this study contributes to a more comprehensive
understanding of well integrity in deep wells.
