ABSTRACT:

The existing pilot hydrate recovery projects have not reached the commercial recovery level due to the low gas production, though there is a large amount of natural gas hydrate (NGH) reserve around the world. Therefore, it is significant to investigate how to increase the gas production. This paper attempts to prove that hydraulic fracturing is an effective way to the enhancement of gas production by developing a numerical dissociation model with the software Tough+ Hydrate. Heat and mass transfer as well as phase change (solid to gas) in the liquid, gas and hydrate bearing sediments are considered. The evolving behavior over time such as the gas production rate, hydrate saturation and pressure, are captured and investigated for the reservoirs, which shows that the rate of hydrate dissociation in reservoir and the efficiency of gas production in the wellbore in the case with one fracture in the first nine months are significantly greater than those without fractures. With one fracture, hydrates dissociate simultaneously in the upper and lower parts of the wellbore, while they dissociate mainly in the lower part of the wellbore in the early period (the first 8 months) without fractures. In addition, fractures promote the expansion of the dissociation front of the upper and lower boundary of the hydrate formation, and their fusion with the fractures increases the gas production rate of the wellbore. In addition, the design of three fractures is suggested to apply to the real hydrate recovery projects for the greatest effectiveness.

1. Introduction

Natural gas hydrate is abundant and widely distributed, about 7.5×1018 m3 in global reserve (Kvenvolden and Lorenson, 2001). Also, the existence of natural gas hydrates (NGHs) in the South China Sea has been confirmed geologically and geochemically, and the estimated natural gas reserve is around 6.5×1013 m3 (Wu et al., 2011). Among them, the hydrate reserve in Shenhu area is about 19.4×1010 m3 (Liu et al., 2018). If a proper recovery method is used to commercialize the NGH, the pressure of energy supply and demand will be greatly reduced.

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