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Carbon Energy Storage - an efficient storage of renewable energy for later peak demand       see Video

Many legacy oil and gas fields in ductile reservoir formations have significant unrecovered hydrocarbons. Gravity drainage can significantly enhance oil recovery above that from primary recovery of ~35% to yield up to 80% recovery in highly permeable reservoirs. However, gravity drainage is a slow process and can not operate in a reservoir of low vertical permeability. Using GeoSierra's Azi-Frac technology, these legacy anelastic ductile reservoirs can be enhanced by the installation of high permeable vertical inclusions, thus enabling recovery of the light oil by gravity drainage. However, on its own merits such a enhanced recovery scheme may be only marginal economical; however, combined with the geological storage of carbon dioxide acting as the gas cap to enable gravity drainage and also assisting oil mobility, the system is much more attractive economically. Many of these legacy oil fields have depleted gas reservoirs above them, so combining the system with a peak electricity on demand energy storage scheme, that is, by closed cycling the supercritical carbon dioxide between the upper and lower reservoirs in off-peak and peak periods, the combined system becomes extremely attractive, both economically and environmentally, especially since these legacy have significant volumes of unrecovered light oil.

The legacy oil and gas fields are now transformed from a potential environmental liability into a valuable asset. As hydrocarbons are produced from the fields, additional carbon dioxide can be stored and also the energy storage system's capacity is also increased. There are a significant number of these legacy fields throughout the USA, which are in close proximity to infrastructure, where there is an urgent need for geological storage of carbon dioxide and energy storage of renewables for peak electrical energy demand.

GeoSierra's carbon energy storage system in anelastic ductile turbidite reservoir legacy oil and gas fields is in final feasibility assessment study for field pilot deployment mid-2017 in Kern County, CA. Revenue for the Carbon Energy Storage system comprises of carbon storage, peak electrical energy supply and produced hydrocarbonsNet operating costs are comparable to the operating cost of conventional pumped hydro, but with a significant lower capital cost per MW stored. Net operating cost is operating cost minus carbon storage and produced hydrocarbon revenue.To utilize carbon dioxide as the subsurface working fluid requires a meaningful carbon tax on emitters; otherwise the carbon dioxide is simply released to the atmosphere and not captured and stored. If such a carbon tax is not in place, then the carbon dioxide is not available for use and the pilot will be delayed until climate change initiatives force a meaningful tax on carbon emissions.


Carbon Energy Storage - in legacy oil & gas fields

The two reservoir formations, a shallow depleted gas reservoir and a deeper oil reservoir  with respective cap rocks are hydrocarbon bearing formations that have been considered depleted or legacy fields. Both of the reservoir formations may have moderate permeability, however their vertical permeabilities are too low for gravity drainage to be an effective recovery method. Therefore, there are considerable reserves of oil contained in the deeper reservoir. The deeper reservoir contains significant quantities of oil of relatively low viscosity and low density hydrocarbons that have a moderate miscible pressure with supercritical carbon dioxide. The cap rocks of both reservoirs are geological traps for the hydrocarbons and have stored the hydrocarbons over geological time under considerable pressure. These cap rocks are suitable for the long term storage and containment of carbon dioxide.

The well system is particularly useful for the injection and the injection/ withdrawal of carbon dioxide into and from the two reservoirs. Multiple vertical high permeable propped planes to enhanced injection and withdrawal of fluids into and from the formations are installed in both reservoirs. These high permeable planes enable the enhanced recovery of hydrocarbons by gravity drainage from the deeper reservoir into a sump and produced to the surface. As hydrocarbons are produced, additional carbon dioxide can be injected into the reservoirs.

Carbon dioxide is injected into both reservoirs. At off-peak periods, the carbon dioxide in the shallower reservoir is pumped at a higher energy state and injected into the deeper reservoir by a pump turbine driven by electrical power. Based on the expected efficiency, the production and injection flow rates of supercritical carbon dioxide are 140 metric tons per hour to achieve a peak electric power capacity of 10MWe for the pilot.The electrical power drive of the pump turbine is a variable speed generator thus providing a refinement in balancing electrical loads on a power grid. The carbon dioxide in the deeper reservoir is in a supercritical state and at a pressure greater than the miscible pressure thus greatly assisting gravity drainage.

At peak periods, the carbon dioxide is released from the deeper reservoir and flows to the shallower reservoir, being at a lower energy state and thus drives the pump turbine to produced peak demand electricity.  The energy storage cycle is a closed system with no atmospheric emissions. The continuous operation of the energy storage system enhances the recovery of hydrocarbons from the deeper reservoir, and thus allows for greater volumes of carbon dioxide to be stored in the system.

The oil in the deeper reservoir will thus be mobilized by the miscible carbon dioxide and flow under gravity from the formation through the highly permeable inclusions towards the wells and enter the sumps and pumped to surface via a PCP (progressive cavity pump), ESP (electrical submersible pump), gas lift or natural lift, depending on operating temperatures, pressures and depth, via a production tubing in the wells. The level of hydrocarbon fluids in the well will be maintained above the production tubing to ensure carbon dioxide is not produced from the formation up the production tubing.


Carbon Energy Storage with Oil Recovery


Kern County Legacy Oil & Gas Fields

Kern County oil and gas fields have been extremely prolific over the past century, and the county contains 4 super giant fields each produced over 1Bbbls of oil, with Midway-Sunset being the largest, which has produced over 3Bbbls of oil. The county also contains 22 giant oil fields, each produced over 100MMbbls of oil. Many of the oil and gas fields in Kern County are either at the end of their recovery life, abandoned or pumping at extremely low rates. Significant hydrocarbons have been recovered from these fields over the past 100 years; however, there are significant reserves remaining, that can be recovered by gravity drainage in the Carbon Energy Storage system described above.

For example, a moderate size oil field is the Paloma oil field, shown opposite. The Paloma field is a large anticline structure some 12 miles long and 4 miles wide, from which 61.5MMbbls of oil and 433Bcft of gas have been recovered from the Upper Stevens formation with minor amounts recovered from the Lower Stevens formation. The Stevens formations are turbidite reservoirs, with the Lower Stevens of much lower permeability that the Upper Stevens. The shallow depleted gas reservoir is at a depth of 5,000'.

Numerous attempts to conventionally hydraulic fracture these turbidite reservoirs have had zero success, as horizontal features were induced at depths ranging from 10,000' to 12,000'. The turbidite reservoirs are anelastic ductile formations that require an initiation procedure to generate a self propagating inclusion, as is conducted using GeoSierra's Azi-Frac technology.

Recoverable reserves from gravity drainage in the Paloma oil field are 80MMbbls of oil and 60Bcft of gas. Such a field operating as a carbon energy storage scheme, could recovery those hydrocarbons over it's life, contain and store 98 million metric tons of carbon dioxide, store off-peak renewable energy and provide significant peak electrical power generation. The Lower Stevens formation is at a temperature of 130°C. Based on the expected efficiency of a combined electric power generation cycle, 140 metric tons per hour of supercritical carbon dioxide would need to be produced and re-injected per hour for a peak power generation capacity of 10MWe.


    Kern County Oil & Gas Fields for Carbon Energy Storage

Combined Power Generation Cycle

The optimum power generation scheme is a combined upstream supercritical carbon dioxide turbine with a downstream organic Rankine cycle as shown opposite to utilize the excess pressure head available. Due to the different efficiencies of the two cycles, the peak power generation capacity is approximately 70% of the electrical power generation is from the supercritical carbon dioxide turbine and 30% from the organic Rankine cycle. To achieve a peak electric power generation of 10MWe, 140 metric tons per hour of supercritical carbon dioxide need to be produced at depth and injected into the shallower reservoir.

As the peak generating capacity of the system is increased above the 10MWe pilot, the power generation capacity of the downstream organic Rankine cycle will drop percentage wise compared to the upstream cycle. The organic Rankine cycle's power generation capacity depends on the produced fluid temperature, and if this drops then power output of the downstream organic Rankine cycle drops. At full peak power generation capacity, it may be more optimum to operate as a single cycle system, being the supercritical carbon dioxide turbine.






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