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In a previous study using a single wellbore production system, it was demonstrated that a combination of depressurization and wellbore heating is more efficient than depressurization alone, where the endothermic dissociation process rapidly consumes the specific heat of the formation, leading to a sharp decrease in the dissociation rate. This study extends the work on gas production and explores the feasibility of a novel dual wellbore production scheme, where heating and depressurization are conducted on separate wellbores. The drawback with combining heating and depressurization on a single wellbore is that the produced fluids are flowing in an opposite direction to the heat from the wellbore, and this forced convection may slow the dissociation process. Gas production tests are carried out using the dual wellbore system with different combinations of pressure and temperature at the depressurization and heating wellbores, respectively. The ensuing experimental results showed that both increased depressurization and heating can lead to optimized gas production. A production scheme with a higher depressurization compared to a lower depressurization at the same wellbore heating is generally more energy-efficient, while a higher wellbore temperature at the same depressurization resulted in more gas produced but no improvement in efficiency. Although a dual wellbore scheme has been an established practice in the petroleum industry, this is likely to be the first employed in the hydrate gas production tests.

Natural occurring methane hydrate (MH) is found abundantly in sediments and rocks under stable thermodynamic conditions. The majority of this resource is located under the permafrost layer and in the shallow depths of the seafloor in the deepwater regions. Over the past two decades, a variety of efforts were dedicated in laboratory researches and field production tests using different methods to examine the best production strategy that is economically viable for commercialization. In order to produce gas, the solid methane hydrate in pores of sediment or rock will need to be dissociated either by reducing the pore pressure, increasing the temperature, injection of inhibitors, or undergo gas exchange using carbon dioxide. It is possible to have a combination of these options in the gas production. Currently, the depressurization method is deemed the most efficient way to produce gas. The key controlling factor in dissociating, exchanging or producing gas from methane hydrate is the flow conductivity through the pores of the hydrate-bearing layer. Larger contact exposure area between solid methane hydrate to the fluid pore pressure enables more dissociation to occur using the methods above. In this aspect, the creation of artificial fracture in hydrate-bearing layers is thought to promote a better dissociation process. This idea has surfaced with numerous efforts from the research community to explore its feasibility. There are multiple technical challenges and uncertainties to address if methane hydrate in sand can be fractured artificially. These challenges and the recent progressions in identifying/determining its fracture properties are discussed with some future considerations required to move towards the prospect of introducing artificial fractures for gas production purposes.

In this paper, a method is proposed to determine the fracture toughness of brittle and heterogeneous material through hydraulic fracturing. The intent of the proposed method is to deploy it on materials whose are not easily determined by standard test. One example of such a material is hydrate-bearing sand, which is unstable under atmospheric pressure and temperature. The pressure-time record, injection flow rate and total volume injection are necessary information to determine the fracture toughness of the material using the classical linear elastic fracture mechanics approach. To verify the method, frozen sand is used as a substitute material and its fracture toughness found using the standard test. Frozen sand has similar composition as natural hydrate-bearing sediment except for the absence of methane gas and high pressure. Then, this method is applied on the hydrate-bearing sand.Keywords hydraulic fracturing · fracture toughness · brittle materials · frozen sand · hydrate-bearing sand