In 2016, the Paris Agreement came into force, requiring the 196 participating countries to limit global warming to below 2 and preferably 1.5 deg. C above pre-industrial levels. To achieve this, it was recognised that global greenhouse gas emissions would have to reach “net zero” by the middle of the century. The UK was the first major economy to pass this into law in 2019, committing to achieve net zero by 2050.

The UK government published their “Ten Point Plan” in 2020, followed by their more detailed “Net Zero Strategy” a year later, just before hosting COP26 in Glasgow in November 2021. The proposals focus on reducing reliance on fossil fuels, with a transition to low-carbon energy, which will include renewables, hydrogen, and nuclear power generation.

In our latest blog post, we discuss how carbon capture and storage can help support the move towards net zero.

Achieving net zero

Whilst low-carbon power has become more available and affordable, it is expected that being emissions-free by 2050 is unlikely. Several technological and economic factors contribute to this challenge, particularly for the full transition of high-energy, high-emissions industries such as cement manufacture and steel works. Additionally, petroleum continues to provide feedstocks or consumables for many other industries, including plastic manufacture, solvent manufacture, and the various other industries which use these materials (pharmaceuticals, food industry, etc.).

Given that some CO2 is still expected to be emitted by 2050, albeit significantly lower volumes than now, these must instead be countered to achieve net-zero. Natural carbon sinks, such as forests and peatlands, provide an obvious and natural method for carbon sequestration. However, to sequester sufficient carbon within the short time frame set out, new technologies such as carbon capture and storage (CCS) are proposed.

So what is CCS?

CCS is a process whereby CO2 emitted from industrial processes is captured and stored permanently and securely in subsurface formations, preventing their release into the atmosphere.

CO2 is first captured from the flue gases resulting from combustion and is then separated from the flue gases using one of a range of methods, such as adsorption, absorption, membrane separation.

The higher purity CO2 stream is compressed to above the critical point (73.8 bar, 31 ˚C), taking the CO2 from the gas phase to the supercritical phase. In this state, the CO2 stream is densified like a liquid, but maintains viscosity similar to gases, making it easier to transport. The CO2 is then transported to the storage site, typically through a pipeline, and injected into the storage reservoir via an injection well.

The storage reservoirs are generally expected to be saline aquifers or depleted oil and gas reservoirs. The formations must possess sufficient permeability and capacity for CO2 injection, and an overlying caprock of adequate strength and integrity to store the CO2 indefinitely.

Challenges of CCS

There are several challenges which CCS must overcome to make it a viable and safe solution that reduces emissions.

For example, during CO2 transportation and storage, the system must be carefully designed to avoid significant drops in pressure which could result in phase transition from supercritical/liquid phase to gas phase. The transition results not only in gas expansion, but also Joule Thomson cooling, which can cause temperature drops below 0 ˚C. Such significant and instantaneous temperature drops can have severe effects on material integrity, such as thermal fracturing, and flow assurance issues, due to ice formation.

Another challenge is ensuring the long-term integrity of the CO2 storage reservoir, which is critical for the success of CCS. The greatest risks to CO2 store integrity are the wells which penetrate the cap rock and the CO2 store. These may be wells used to inject CO2 during CCS, or legacy wells from historical oil and gas operations.

Failure of barriers within the well, including annular cement, casing or abandonment barriers, could cause leakage of CO2 to the surface. It is therefore critical that any wells penetrating a CO2 store are adequately abandoned and any issues well understood and mitigated prior to CO2 injection.

The UK’s Oil and Gas Authority has acknowledged this in their OGA Strategy, which now requires that operators of wells must consider the potential reuse of field infrastructure, including the reservoir, for CCS repurposing. As a result, should a reservoir be suitable for repurposing, any wells in it must be adequately abandoned so that they will not leak if the reservoir is repurposed for CCS.

How might this affect well abandonment?

Currently, well abandonment in the UK is conducted with reference to Oil and Gas UK’s Well Decommissioning Guidelines – Issue 6. Typically, a cement plug is placed in the wellbore, creating a rock-to-rock seal with an adjacent cap rock. The cement may be placed across an open hole section, or in a cased hole section, with verified annular cement behind the casing.

Wells must be abandoned taking into consideration the current and potential future downhole conditions. Aspects which require particular focus for CCS include:

  • Downhole fluids - CO2 and other components in the captured CO2 stream (e.g. H2S, SOx, NOx) generate acidic solutions which may degrade well barrier elements in an abandoned well.

  • Pressure - Injection of CO2 into a formation may increase pressures above original.

  • Temperature - Thermal cycling caused by periodic injection of CO2 may affect material properties.

These additional considerations mean that standard P&A methods for oil and gas wells may not be adequate for wells in CO2 stores.

What can be done?

Due to the effects of thermal and pressure cycling, it is firstly critical to ensure that the materials which may form part of the barrier are still intact and their interfaces remain bonded. The presence of cracks, fractures or microannuli in the wellbore will provide conduits through which fluids could pass and worsen due to material degradation. Annular cement verification and, where necessary, remediation is therefore critical prior to abandonment.

The compatibility of the barrier materials with the downhole fluids should also be considered. Whilst cement degradation or casing corrosion may be limited and therefore may not necessarily result in barrier failure, best practice would be to use CO2-resistant barrier materials which will not degrade. In some cases, for example where a plug would ordinarily be set in a cased hole, this may require remedial work such as section milling to remove CO2-susceptible cement and casing from the well prior to plug placement.

An alternative or additional method to mitigate against degradation of wellbore materials is to displace the corrosive fluids away from the wellbore. This could be achieved using Aubin’s Xclude. Xclude could be pumped into the near wellbore formation, displacing the CO2 plume, and instead precipitating calcium sulfate scale, which reduces the permeability of the rock. Once in place, Xclude would effectively create a protective “shield” around the wellbore, preventing influx of corrosive fluids towards CO2-susceptible wellbore materials.