Abstract

A gas production system from methane hydrate layer by hot water injection using a pair of dual-horizontal wells has been proposed. Experiments with physical and numerical reservoir models have been carried out in order to simulate gas production characteristics with the system. In the experiments, the reservoir models consisting with ice of NaHCO3 aqueous solution formed in glass-bees porous medium were used to express the dissociation heat of methane hydrate by melting one of ice. Gas production at dissociation front of methane hydrate was simulated by gas generation with a chemical reaction of NaHCO3 included in the ice and HCl mixed in hot water injected at ice melting front. In the system, a dissociated region including the dual horizontal wells filled with hot water, named as hot water chamber, was generated to produce gas continuously. The gas production rate has the maximum peak just after breakthrough of injected water between dual horizontal wells, then it declined and gas was produced by almost constant rate. We have successfully developed the numerical model, and matched the history of physical gas production. Moreover, numerical simulations of gas production by the hot water injection into a Nankai Trough sediment layer model using a pair of dual horizontal wells 500m in length were carried out for a methane hydrate reservoir of 20 m in layer thickness, 46% of average methane hydrate saturation, 100 and 25 md in horizontal and vertical absolute permeabilities, respectively. The cumulative gas production is simulated as 5×106 std-m3 for initial two years. Furthermore, a new gas production scheme, which uses four pairs of dual horizontal wells in radian arrangement in a methane hydrate sediment layer with area of 1km×1km located at Nankai Trough, has been presented and evaluated with the numerical simulation as the cumulative gas production for 15 years is 1.3×108 std-m3.

Introduction

Reservoir characterization of methane hydrate bearing turbidite channel in the eastern Nankai Trough, Japan, has been proceeded to develop gas production method. The sediments are of the sand and mud alternation layers, which show heterogeneity characteristics especially on permeability(Suzuki et al., 2009). On the other hand, recent studies confirm that conserved methane hydrate (MH) deposits in sedimentation layers at a depth of more than several hundred meters from the bottom of the sea floor can be utilized as novel natural gas resources. In situ hydrate decomposition into water and gas is required to produce methane gas economically from these layers, since methane hydrate is a type of non-mobile solid energy resource. To trigger methane hydrate decomposition (see Masuda et al., 2002), decompression or temperature increase out of the equilibrium zone is necessary, while dissociation heat should be supplied for continuous gas production. Accordingly, new gas production systems that continuously supply the heat into the methane hydrate layers have been reported. The conventional methods of gas production to date include depressurization, inhibitor injection, and thermal recovery. Gas production by the hot water injection system is advantageous compared to depressurization (Kmath et al., 1991) and inhibitor injection in that the dissociation speed of methane hydrate and gas production rate may be relatively low due to a lower heat supply rate. On the other hand, the hot water injection method using a vertical wells requires drilling with high density into the targeted area (Masuda et al., 2002), since methane hydrates are formed in sand layers with very low permeability.

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