Mr George Lockett                            GEO0004

Written evidence submitted by Mr George Lockett

The North Sea Geothermal Power Project

Abstract:

High-Pressure High-Temperature HPHT wells in the North Sea operate just under 200 degrees centigrade and make perfect candidates for conversion to Geothermal Power Stations. When it is time to decommission concrete oil or gas rigs, they have a useful life of 300 years and deserve to find an alternate use. Cold seawater at 5 degrees centigrade has an advantage over land-based cooling towers, which only reduce the temperature to 27 degrees centigrade—especially when using Carbon Capture and Storage CO2 as the working fluid of the Geothermal Energy System. In addition, floating Power Stations can produce electrical power, cabled to shore or a nearby wind farm to increase the baseload content as Geothermal Energy is 24/7.

Here is one example of the use of Geothermal Energy in the offshore industries for Secondary Oil Recovery:

Geothermal Supercritical Carbon Dioxide (CO₂) Enhanced Oil Recovery© for Offshore and On-shore Applications by George E. Lockett—28th December 2012.

Carbon dioxide has long been known to help with the extraction of light oils; by using a geothermal reservoir below the oilfield we intend to extend its use to Heavy Oils and Tars. 

Oilfields only have up to about 20% of their oil in liquid flowing form. To extract the remaining 80% it is normal to employ some form of Enhanced or Secondary Oil Recovery - EOR or SOR, either as water flooding, thermal – injecting steam, CO₂, chemical or solvent injection or a combination of two or more methods.

Geothermal supercritical CO₂ enhanced oil recovery uses Carbon Capture and Storage (CCS) CO₂ and heat naturally occurring beneath the oilfield to turn the CO₂ into supercritical CO₂, which is at much higher temperatures and pressures after passing through the geothermal reservoir beneath the oilfield.

A typical offshore platform/flow configuration would have a production well drilled, say, to 3,000m and have an oil flow temperature of say 70 degC at wellhead. Two additional wells would be drilled down to between 6,000m—where temperatures would be about 170 degC—and 10,000m, where temperatures may rise to as much as 350 degC, depending on the geothermal gradient.

Typical Offshore Geothermal Gradients may be in the region of 25 to 35 degC per kilometre of depth depending on location. A higher temperature at the geothermal field would increase the supercritical effect of the CO₂ when it reaches the heavy oil in the oilfield.

At the bottom of the two geothermal wells, high pressure fracturing would be done from each well to create a connecting reservoir (1/2 cubic kilometre or more) of geothermally heated hot fractured rocks, say in this example at 250 degC. One of the wells would become the CCS injection well, where in an offshore situation CCS CO₂ would arrive in storage barges and be connected to high pressure pumps, to pump the CO₂ down the injection well and across the geothermal field.

The second well would then have been perforated at the oilfield level and fractured, then had a plug put in the well bore above these perforations, to allow the rising supercritical CO₂ to reach up into the heavy oilfield at a point opposite the production well to allow as much of the field as possible to be swept by the SCCO₂. 

The high temperature high pressure CO₂ can flood the oilfield over quite a large area, reducing the viscosity of the heavy oil by around 80% and raising its temperature to improve its laminar flow rate.

Five types of forces control the displacement of heavy oil by CO₂: viscous, capillary, gravitational, diffusive, and inertial forces.[i] Depending on the rock and reservoir-fluid properties, a few of these forces may be neglected. CO₂ at 2,600 psi (17.9MPa) is nearly four times as dense at 75 degC as at 350 degC.[ii]  This reduction in density of the SCCO₂ at a higher temperature will help to make the CO₂ miscible with the heavy oils and tars in the heavy oilfield.

Geothermal Supercritical CO₂ Enhanced Oil Recovery is a continuous process; it has many advantages over other forms of enhanced or secondary oil recovery that work on a batch process, using the same well as production. This system has a long field life of up to 30 years and has many advantages over using water as the working fluid.

Advantages:

• The Carbon Capture and Storage CO₂ would mainly remain in the reservoir, thus becoming a long-term storage location in the oilfield when the oil has been completely removed.
• CO₂ does not have a chemical reaction with the geology in the same way as water does, at high temperatures. Thus there is less movement of salts and other contaminants from the reactivity of rocks in the formation as the fluids pass.
• Secondly, the changes in density with temperature and pressure of supercritical CO₂ suggest that the use of pumps for moving CO₂ could be partially or completely eliminated by formation of a thermosiphon.[iii]
• Not a batch process, so continuous extraction of oil from the production well(s).

Disadvantages:

• One has to drill two deeper wells to reach the Geothermal Field for the higher temperatures. However there is lots of evidence, in Iceland and many other countries around the world, that the Geothermal wells will continue working for many decades.
• At present unproven technology for Enhanced Oil Recovery. However, examples of Supercritical CO₂ use in Geothermal can be seen: “A Hot Dry Rock Geothermal Energy Concept Utilizing Supercritical CO₂ Instead of Water.”—Brown at Los Alamos National Laboratories, USA.[iv]
• Use of CO₂ in Enhanced Recovery of Heavy Oil is already proven to be effective—Supercritical CO₂ may be better and more effective due to high pressure and temperature.

Raising the temperature of the CO₂ through the Geothermal Hot Rock Reservoir should help achieve the necessary miscibility with the heaver oils at higher temperatures—immiscibility being a known issue with heaver oils that have not been heated with high temperature high pressure SCCO₂ in this geothermal EOR system above, as in this extract:

“Oil displacement by CO₂ injection relies on the behaviour between CO₂ and crude. This interaction depends on the oil’s weight, and the reservoir characteristics. In high pressure applications with lighter oils, CO₂ is miscible with the oil (in all proportions forms a single phase liquid), with resultant swelling of the oil, and reduction in viscosity, and possibly also with a reduction in the surface tension with the reservoir rock. All these effects serve to improve the flow of oil to the production wells.

“In the case of low pressure reservoirs or heavy oils, CO₂ (potentially along with alternating water injection) will form an immiscible fluid, or will only partially mix with the oil. Some oil swelling may occur, and oil viscosity can still be significantly reduced. However, in immiscible CO₂ flooding the main function of the CO₂ is to raise and maintain reservoir pressure. CO₂ immiscible flooding is considered where the reservoir permeability is too low for water flooding, or where the geochemistry or other geological conditions are unfavourable for water flooding.

“During these CO₂-EOR applications, more than 50 per cent and up to 67 per cent of injected CO₂ will return to the surface with the extracted oil, requiring separation and reinjection into the well to prevent release into the atmosphere and to reduce operating cost of obtaining additional CO₂.

“The effectiveness of CO₂-EOR is dictated by reservoir characteristics, such as temperature, pressure, height, angle and permeability. For example, injection depth must be generally greater than 600m and well pressure over 10MPa into light weight oil to achieve the desirable miscible flood, described above. These factors along with the well’s stage of production must be considered when selecting a reservoir for CO₂-EOR.”[v]

By combining the Geothermal SCC₂ System below the oilfield with the SCC₂ Enhanced Oil Recovery System we created a combined cycle, which is able to raise the temperature of the heavy oil in the oilfield and make it miscible with the low density SCC₂ that is being injected at high pressure due to the high geothermal preheating which it has passed through before reaching the oilfield.

To find out more about Geothermal Supercritical CO₂ Enhanced Oil Recovery©, or look at the feasibility of a specific application, please contact the author George E. Lockett.

June 2022

REFERENCES: