Underground gas storage plays a critical role in the diversification of energy sources, potentially reducing CO2 emissions and reaching sustainability goals while assuring a reliable and flexible power source.
Natural gas is taking a larger share of the energy mix as countries reduce their use of coal for electricity generation. It is easy to see why. It can be stored underground and natural gas delivers nearly twice as much electricity than coal for the same amount of CO2 emissions.
Natural gas storage is used as a strategic reserve to level out peaks in demand. Based on seasonal variations, gas is injected when demand and price are low, then withdrawn again as needed Gas storage also provides short-term protection against unforeseen supply disruption by providing an immediate and reliable energy source. Unlike spot markets, storage offers the advantage of a physical asset located close to demand. Natural gas storage allows large natural gas consumers to hedge supply and price risk and, should price suddenly spike, capture the upside opportunity.
A natural gas storage system needs a working gas volume, injection capacity, withdrawal capacity and a volume of cushion gas. The working gas volume is the volume that can be injected and withdrawn during normal operations. The cushion gas volume is the gas that is required to remain in store to maintain reservoir pressure.
The injection and withdrawal capacity, which are largely dependent on the storage pressure, control the rate at which gas can be injected or withdrawn. If storage pressure is low, gas can be injected at high rates but withdrawn at low rates. Conversely, if storage pressure is high due to considerable working gas in store, injection rates will be low, but withdrawal rates will be high. There is a well-documented hysteresis in the field's ability to be repeatedly repressurised and depleted which has to be modelled.
The most common types of natural gas storage are in depleted gas reservoirs and salt caverns and, to a lesser extent, in aquifers. Where aquifers and depleted aquifers do have a much more useful storage capacity is in the area of CO2 long-term capture and storage.
Depleted gas reservoirs have advantages over other types of gas storage as any unproduced gas can be used as cushion gas, and the fields may also have very large working gas capacity. Depleted reservoirs are also likely to be linked to existing gas infrastructure, which reduces initial investment costs.
Salt caverns are located in thick halite formations. They are made by pumping fresh water down a borehole into the salt layer to dissolve the salt and circulate the saline solution to the surface. This process continues until the required size of cavern is reached. Hydrocarbon liquid or gas storage in salt caverns is well understood. For example, the US Strategic Petroleum Reserve (SPR), which is the world's largest supply of emergency crude oil, is stored in salt caverns at four sites along the Gulf of Mexico coastline.
Converting an aquifer to a natural gas reservoir requires long, slow injection cycles to displace the water.
Cushion gas in depleted fields can be 50% of the total storage volume, whereas in salt caverns, it may only be 25% of the total storage volume.
Aquifer storage of natural gas requires a very large percentage of cushion gas making this the least efficient option for storage and withdrawal of natural gas. However, since the intent for CO2 storage is that it stay stored, this is not an issue in that case.
RPS’ teams of geoscientists, engineers, environmental scientists and economists are uniquely placed to model the reservoir, quantify the injection and withdrawal capacity of depleted gas fields or salt caverns, model the hysteresis effects, identify environmental issues and assess the commerciality of any gas storage project.
RPS conducted a gas storage screening study of a large complex of producing gas fields in the United Kingdom Continental Shelf, including technical and commercial feasibility analysis.
Following an initial audit of the Operator's existing subsurface models, RPR reservoir engineers reservoir assessed the reservoir's performance, including sensitivities to a range of parameters such as well count, well location, well design, injection & production pressure and cushion gas volume. Existing infrastructure was also reviewed and required modifications identified and costed. Final decommissioning costs were also reviewed.
We assessed the economic viability using a simple pre-tax cash flow model using the difference in seasonal gas prices. We tested whether investor thresholds such as return on investment, payback period, net present value and internal rates of return were met.
RPS supported a Middle Eastern government ministry to review national infrastructure and options for gas storage. This included a review of existing subsurface reservoirs and a high-level assessment of their viability for gas storage, along with recommendations for next steps and further studies.
RPS has provided independent audit and validation for detailed modelling of existing gas storage facilities in onshore Europe to support contractual negotiations with clients and partners. study included modelling existing operations—including history matching—, independent estimation of the range of remaining cushion gas in place, and forecasting future operational behaviour of the asset.
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