A Feasibility Study of Using Geothermal Energy to Enhance Natural Gas Production from Offshore Gas Hydrate Reservoirs by CO? Swapping

Journal Article Recommendation: A Feasibility Study of Using Geothermal Energy to Enhance Natural Gas Production from Offshore Gas Hydrate Reservoirs by CO? Swapping

Author：Md Nahin Mahmood, Boyun Guo

Department of Petroleum Engineering, University of Louisiana at Lafayette, Lafayette, 70504, USA

Abstract

The energy industry faces a significant challenge in extracting natural gas from offshore natural gas hydrate (NGH) reservoirs, primarily due to the low productivity of wells and the high operational costs involved. The present study offers an assessment of the feasibility of utilizing geothermal energy to augment the production of natural gas from offshore gas hydrate reservoirs through the implementation of the methane-CO? swapping technique. The present study expands the research scope of the authors beyond their previous publication, which exclusively examined the generation of methane from marine gas hydrates. Specifically, the current investigation explores the feasibility of utilizing the void spaces created by the extracted methane in the hydrate reservoir for carbon dioxide storage.

Background

Methane gas hydrates are formed through the crystallization of ice and methane. Water molecules enclose methane molecules. Depressurization and heat decompose methane hydrate into natural gas and water. Offshore methane hydrate (fire ice) reserves exist worldwide. Organic-rich silt, permafrost, and marine subsurface produce methane hydrates. Scholarly sources believe this resource outnumbers all other fossil fuels, and methane hydrate reservoirs have great promise as a global economy-boosting natural gas supply.

Methods

Fig. 1 is a schematic of the wellbore system enabling heat transfer to a gas reservoir zone from a geothermal zone.The heat transfer from the heating wellbore to the gas hydrate reservoir is impacted by heat conduction and convection. In the gas hydrate reservoir, heat convection is controlled by the CO? injectivity of the heating wellbore and is challenging to predict. With the assumption that convection heat transfer is negligible, a conservative analysis of feasibility can be conducted. This analytical model considered the following assumptions:

- The reservoir is considered homogeneous and isotropic with constant density, specific heat, and thermal conductivity.

- The reservoir is infinitely large compared to the wellbore size.

Result

Assumptions inherent to mathematical modeling introduce uncertainty into the mathematical models described here. The heat transfer model does not consider heat convection, which may arise in the event of gas production during the heating phase. The heat that is lost to the byproduct gas causes the model to overstate the efficiency of the heat transfer. Heat transfer modeling could not account for reservoir temperature changes caused by depressurization . As a corollary, this should cause the efficiency of heat transport to be overestimated. The dissociated gas's pressure will increase because of heating, which should reduce the pace of hydrate dissociation. Second, under the starting condition assumed by the well productivity model, no free gas is present in the reservoir, which is a Class 1W hydrate reservoir. When applied to other types of reservoirs containing free gas initially, this should cause the model to overestimate productivity. The dissociation pressure of hydrates at the dissociated zone’s external flow boundary of the reservoir is known to be the driving pressure in the well productivity model. Since the boundary distance varies with time and is governed by heat transfer efficiency, it stands to reason that well productivity would similarly vary with the rate of fluid circulation.

Conclusion

1. Initially, the propagation of the heat-front within a gas hydrate reservoir follows the expected behavior of a radial heat-transfer system. However, over time, the pace of this propagation slows down, consistent with the predictions of the heat conduction model.?Based on the simulation outcomes, it can be inferred that the heat front within the gas hydrate reservoir under investigation will propagate towards its upper and lower boundaries, spanning a distance of 39 feet (equivalent to 12 meters) within a time frame of 0.35 years (or 4.26 months). Consequently, it can be inferred that the dissociation and consequent release of gas from all gas hydrates located within a 39-foot radius of the heated wellbore will occur within a period of 4.26 months.

2. Furthermore, the utilization of geothermal heating during the CO? exchanging process has the potential to significantly enhance the initial productivity of wells located in heated reservoirs. The non-linear relationship between the increase in fold and the elevation of reservoir temperature is evidenced by the rate of ascent. The anticipated outcome suggests that the increase will exceed a quintuple factor when the gas hydrate reservoir undergoes a temperature elevation from 6oC to 16oC.

3. The mathematical models utilized in this study neglected a crucial factor that could have potentially enhanced the productivity of the well. Specifically, the models failed to account for the heat convection resulting from the flow of CO? into the gas reservoir. The utilization of mathematical models in geothermal-stimulated reservoirs may result in over-prediction of well productivity due to the deviance that may arise from the underlying assumptions made. Future research should consider various factors such as heat convection, temperature reduction due to the presence of free gas in the hydrate reservoir, depressurization, and?gas pressure elevation due to heating.

4. Geothermal stimulation of the CH?-CO? swapping process is an effective and promising way for infusing CO? into gas hydrate reservoirs to permanently lock the CO? there in solid state. Research on the CO? mass transfer rate within gas hydrate reservoirs is desirable.

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