The goal of this project is to demonstrate a novel, oxyfuel downhole steam generator (DSG) that will efficiently recover gas from methane hydrate deposits while simultaneously reducing emissions and also having the potential for carbon dioxide (CO2) sequestration. Success will lead to commercialization of a downhole combustor for natural gas production.
Precision Combustion Inc. (PCI), North Haven, Connecticut, 06473-3106
In-place estimates of methane hydrates in the arctic sandstones beneath existing facilities are on the order of 10s of trillion cubic feet (tcf). In-place estimates beneath the U.S. continental margin are even larger, on the order of 1,000s of tcf. Recent drilling efforts have confirmed the presence of gas hydrates in these areas, and limited testing has been conducted. However, these gas hydrates are extremely difficult to exploit because they are immobile in the subsurface formations that contain them; thus, they must be disassociated in place, allowing the natural gas and water comprising them to be released. The most commonly proposed methods for producing natural gas from hydrate-bearing sediments are depressurization and thermal stimulation. Test well results indicate that depressurization alone may not be sufficient to allow for commercial recovery of this resource, and a downhole heat source may be required to maintain production rates. However, downhole heat generation has significant challenges.
While the potential benefits of downhole combustion are widely recognized, the technical challenges have been daunting. Attaining high combustion intensity in a very small volume at downhole conditions involves achieving a high heat rate at very high pressure while overcoming (1) combustion stability challenges at very high pressure (beyond industrial combustion experience), (2) tight stoichiometry requirements (excess oxygen (O2) at high temperature is highly corrosive in addition to being a hazard in the formation/produced natural gas, while excess fuel creates coking that destroy operability), (3) strict requirements for reliability and failure control in a remote location, and (4) strict requirements for control. Direct-and indirect-fired downhole steam generation has been explored dating back to the late 1970s and early 1980s, ultimately with unsatisfactory results.
The proposed oxyfuel combustor, built using PCI’s advanced technology for high-pressure combustion in gas turbines, offers a means to overcome the key barriers blocking technical success of a high intensity, downhole steam generator.
The proposed DSG consists of a direct fired oxyfuel combustor where the heat is generated with O2/steam/natural gas (NG) combustion, followed by water injection downstream of the flame zone leading to a 100% evaporation efficiency to provide the necessary steam. This innovative process technology involves injecting a modest amount of oxygen (instead of air) through one channel and natural gas through another channel, with combustion occurring in the combustor. The oxygen may be diluted with steam for ease of handling and safety.
Instead of steam, CO2 can also be used based on availability; however, the impact on the quality of produced natural gas (through dilution) will need to be considered, but overall production may be enhanced through exothermic formation of CO2 hydrates. A backside water-cooled liner will be used in the primary zone combustor to protect the casing/packer material from increases in temperature.
The DSG can be placed in a horizontal, vertical, or angled orientation based on the wellbore constraints.
Successful development of the downhole steam generator would provide a device and system design with three major benefits:
The PCI research team presented the project results at a closeout meeting on March 17, 2011. The team reported that all technical objectives were successfully met:
PCI has completed the set up of their in-house test facility and completed the assembly and installation of hardware necessary to demonstrate the performance of the subscale combustor system.
The DSG subscale module specifications and sizing of the oxyfuel system has been completed.
PCI has completed a energy savings analysis to determine the optimum cost guidelines for implementation in a production well.
With the assistance of consultant Art Johnson, PCI has completed the identification and development of geological settings data from field drilling programs. The data (such as hydrate formation details, pressures and temperatures, material limitations if any etc.) will provide the comprehensive knowledge for the integration/optimization of the DSG for in-house subscale testing and Phase II full scale testing at the high pressure Rocky Mountain Oilfield Testing Center (RMOTC).
PCI’s proposal for a Phase II follow-on effort, involving the fabrication of a full scale combustor for testing at the RMOTC, was not among those selected during the FY11 SBIR award process.