Recent numerical studies have provided strong indications that it is possible to produce large volumes of gas from natural hydrate deposits at high rates (in excess of 10 MMSCFD) for long times by depressurization-induced dissociation of hydrates. Of the various factors that can adversely affect the production potential of hydrates, low temperatures have one of the strongest negative impact. These can be caused by low initial temperatures, increasing stability of the hydrate (as defined by the deviation between the temperature of the deposit and the equilibrium temperature at the reservoir pressure), and by an advanced stage of dissociation (a strongly endothermic reaction) when substantial amounts of hydrates remain. The reasons for the production decline include a reduction in the rate of the hydrate dissociation at lower temperatures and the evolution of flow restrictions in the vicinity of the well caused by the formation of hydrate and/or ice in the vicinity of the wellbore. The latter is caused by continuous cooling, and is the reason why large amounts of gas that may have been released in the reservoir in the course of earlier dissociation cannot be easily recovered.
We investigated the possibility of alleviating the problem of low production at low temperatures in Class 2 and Class 3 deposits by means of co-production with gas from conventional reservoirs. Large-scale numerical simulations involving injection-production multi-well systems indicate that, by routing some of the pressurized hot conventional gas through Class 2 hydrate reservoirs at rates that do not exceed 50% of the total production rate, it is possible to achieve a significant increase in hydrate dissociation and gas production. This occurs mainly because of the enhanced relative permeability of the gas phase and the increased effective permeability of the hydrate interval, which facilitates gas flow and further dissociation. The thermal effect on dissociation appears to be limited. Co-production appears to confer no benefits to the performance of Class 3 deposits when the production well is kept at a constant pressure.