Air stored in cased wells?

CAES or storage of energy by using compressed air is a fairly simple and proven technology. In principle air is compressed using excess energy, preferably renewable energy.??When the air is compressed, it warms up. The associated heat can be captured using a heat exchanger and stored. The compressed air is subsequently stored in a closed subsurface system. When the compressed air is released at surface it expands and fuels a turbine to generate electricity. When the air expands it cools off and the heat captured from the compression can be used to heat the gas to avoid freezing conditions at the turbine.??

Current systems exist in Germany and in the US and can reach an efficiency between 42 and 54%. With heat exchangers and more efficient compression facilities and turbines this industry is aiming for a 70% efficiency around 2025.??

In order to store significant amounts of energy a sizable high pressure storage unit is needed. Both in Germany as well as in the US man-made underground salt caverns are being used to store thousands of m3 of compressed air at pressures between 750 and 1000 psi.

These units can generate some 100-300 GWh over a period of a few hours to a day. Such large energy storage capacity and long-storage duration are the key advantages of CAES systems. Potential risks such as leakage of compressed air from caverns or chemical reactions between air and rock have not been reported but remain a concern. The main challenge is scaling up these facilities to the size needed to support an economy running on renewable energy.???

Whereas existing facilities and planned new facilities by companies like Hydrostor and Corre Energy also look at storage in man-made caverns, CleanTech Geomechanics Inc. (CTG) is considering to use cased wellbores for their storage (Cased-Wellbore Compressed Air Storage or CWCAS) as an advanced CAES process. They have branded their process "WellStore". Although the stored volumes are much, much smaller than conventional CAES, CTG believes that by raising the pressure some 10-fold up to 7250 Psi (500 Bar) they can store sufficient volumes of compressed air in 4 wellbores to generate 40-60 MWhr and thereby being cost competitive with a similar capacity of battery storage whilst being more environmentally friendly. CWCAS is a low volume but Higher Pressure/High Temperature (HP/HT) process. This feature is unique for CAES and allows for much improved versatility for certain energy storage applications. Site independence, scalability, and long-storage duration are some key advantages of the CWCAS system, and particularly attractive for grid firming, off-grid and behind-the-meter applications with renewable power systems (i.e., solar and wind). Furthermore, CWCAS has a low cradle-to-grave environmental impact and a involves small surface footprint?

Although the capacity of CWCAS system is a factor 100 smaller than that of the conventional CAES systems, I do believe that there is a niche for this technology. First of all, geology in many places is not suitable for making man-made caverns to store compressed air. Thick salt layers in the shallow subsurface are ideal locations but these do not occur in many places. Porous clastic or carbonate reservoirs can’t be used as they mostly lack the perfect sealing capacity of salt combined with the ease of making caverns. Where perfectly sealing clastic reservoirs are described, they are often filled with hydrocarbons that should not get into contact with the oxygen in the compressed air.?

CWCAS uses wellbores to store the compressed air and these wellbores appear the largest CAPEX components of this technique. These cased wellbores ideally should have a 30cm diameter down to some 1500m, cemented at the top in a conductor. This set-up is not uncommon is shale gas fields where most wells have a short lifespan after which they will need to be abandoned. Next to wells with an ideal casing size and strong cement bond a high-pressure Christmas tree is needed for storing compressed air. With a wide selection of wells to choose from as well as with sufficient upscaling possibilities, abandoned shale gas areas provide both the surface and subsurface infra structure and grid connectivity needed for a successful CWCAS project.?

?CWCAS is also more scalable than conventional CAES. For instance, additional cased-wellbores can be constructed if additional storage capacity is required for a CWCAS facility in operation. The expansion can be completed relatively rapidly and easily. It is also anticipated that the regulatory approval process will be less onerous for CWCAS projects vs conventional CAES.?

In summary CWCAS is a very interesting technology to store small volumes of compressed air at very high pressures in closed well-bore systems. Economically these systems make sense in areas connected to an electricity grid, where (renewable) energy is available and where large scale CAES can’t be considered due to the absence of suitable geology (salt).?Due to its high pressure storage capacity, this process could also be considered for hydrogen storage.