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Energy Return on Energy Invested for the Production of Methane from Hydrates by Electrical Heating and by Hot Water Injection
1  Universidad del Turabo, Gurabo (Puerto Rico)

Abstract: In this paper we expand our previous publications on the production of methane from methane hydrate (MH) submarine reservoirs via electrical heating and by hot water injection. Initially we calculated the primary energy balance (energy out / applied energy) for the possible production of methane from submarine deposits. In the case of low frequency electrical heaters located in the MH reservoir we determined that for optimal heaters lengths and location the EROI was 5/3 (1-2). We then considered the methane production via hot water injection – the scheme suggested in Japan for production in the Nankay trough (3). The reservoir considered was 500 meters long with a radius of 100 meters and an initial temperature of 2 C. A pipe located at the center of the reservoir carries hot water entering at different initial temperatures. In order to solve this problem we first modeled the heating via a standard second order finite difference heat transfer scheme in cylindrical coordinates. Since this scheme proved to be numerically unstable, we assumed as a first approximation that the temperature distribution along the length of the pipe was linear and the temperature at any point in the reservoir was determined using an enthalpy finite difference scheme. This scheme considered the change of phase of the solid methane hydrate into water and methane gas when the temperature of each volume element is greater than a melting temperature of 20 C. The energy produced is taken to be of the order of 6.1x109 joules for each cubic meter of methane hydrate, which dissociates into 160 cubic meters of gas at STP conditions (published data indicates a methane heat of combustion of 3.868 x 107 joules/m3 , in close agreement with reported methane energy content of 1000 BTU per cubic foot). The results obtained for an initial water temperature of 200 C, indicate an EROI [(Energy out) / (Primary energy in)] which varied from 25 as the production is started, to 2.5 after 50 years of production (4). The primary input energy 1) Callarotti R.C., Energy efficiency in the electrical heating of methane hydrate reservoirs. SPE paper 137585. In Proceedings of the Canadian Unconventional Resources and International Petroleum Conference, CURIPC 10, Calgary, Alberta, Canada, 19–21 October 2010. Ed., Society of Petroleum Engineers: Houston, TX, USA 2) Callarotti R.C., Energy return on energy invested (EROI) for the electrical heating of methane hydrate reservoirs, Sustainability, 2011, 3, 2105-2114; doi:10.3390/su3112105 3) Yamakawa, T. , Ono S., Iwamoto A., Sugai Y., and Sasaki K.; A Gas Production System From Methane Hydrate Layers By Hot Water Injection And BHP Control With Radial Horizontal Wells. SPE paper 137801. In Proceedings of the Canadian Unconventional Resources and International Petroleum Conference, CURIPC 10, Calgary, Alberta, Canada, 19–21 October 2010; Society of Petroleum Engineers: Houston, TX, USA. 4) Callarotti R.C., Energy efficiency in the heating of methane hydrate reservoirs by hot water injection", Heat Transfer 2012, 12th international conference on simulation and experiments on Heat Transfer and their applications, Split (Croatia), June 27-29, 2012 was determined as the sum of the kinetic energy of the water flow into a pipe of 1 m diameter with a 1 m/sec velocity and the thermal energy input to the reservoir. We now present results for the complete heat exchange problem for applied hot water, where the heat transfer is determined by solving the correct discretized equations both inside and outside the pipe, by application of Gauss theorem. This approach stabilized the numerical results in both regions (inside and outside the pipe) and we were able to obtain stable numerical solutions. The correct EROI is determined to be of the order of 30 at the beginning of the operation, and of the order 7 after 30 years of operation. We will also give a brief description of the problem of MH plug removal in ocean oil producing pipes, via the application of microwave energy from the surface. The calculation of the primary energy balance for MH proposed production schemes has been the motivation of our work. If these partial EROI had turned out to be less than 1, the discussion concerning the use of methane hydrates as a new source of non-renewable energy would have been irrelevant.
Keywords: methane hydrates, EROI, sustainability,heat transfer