Abstract

In this study, we have investigated a gas production system from methane hydrate layers using hot water injection using horizontal wells including a production well and multiple injection wells. Experiments on the gas production were performed using scaled two-dimensional physical models to investigate fluid flow characteristics and production performance. Furthermore, experiments on an improved production system by hot water injection using multiple injectors have been carried out. It has been shown that gas production rate can be improved by hot water injection using the multiple horizontal injectors compared with that with a single horizontal injector for thick hydrate layers of over 15 meters.

The numerical simulations have been done on the field gas production performances by hot water injection with multiple injectors which are drilled in a hydrate layer of 15 meters in thickness. The cumulative gas production for three years with 1000 ton/day of hot water injection was evaluated at the end of three years as 8?106 std m3. The system using multiple injectors is appeared to be more economical procedure than that with the single injector.

Furthermore, a synthetic gas production system has been presented in considering of a gas turbine combined thermal cycle which can provide dissociation heat of methane hydrate by waste heat of its exhaust gas.

Introduction

Recent studies confirm that conserved methane hydrate 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, decompression or temperature increase out of the equilibrium zone is necessary, while heat should be supplied for continuous dissociation. Accordingly, new gas production systems that supply heat continuously 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 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.

In this paper, a hot water injection method using horizontal wells consisting of a producer and multiple injectors is proposed. Hot water is injected into the hot water chamber formed in the hydrate layer from a lower horizontal injector. Gas and water are continuously produced from the upper horizontal producer. Physical models and numerical simulations have been carried out to determine the production performance of the present thermal recovery system and numerical modeling with STARS ™.

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