Gas Hydrates: Evaluating Their Technical Producibility

It is estimated that the amount of natural gas trapped in hydrates around the world is roughly two orders of magnitude more than the recoverable gas in conventional reservoirs. Activities are underway in relation to a number of hydrate accumulations: those at the Mallik field, Northwest Territories, Canada; at the Nankai trough, offshore southeast Japan; at Milne Point in Alaska, USA and offshore India. This talk reviews some of these activities. It also discusses the methods proposed for gas production from hydrates and those actually employed. Furthermore, we examine the impact of the top hydrates in improving the productivity and life of these gas reservoirs. We use a recently developed simulator to model a hydrate reservoir where the hydrate-bearing layer overlies a free gas zone. The results indicate that considerable amount of gas can be generated and produced by the depressurization method. A number of sensitivity and mechanistic studies indicate that rate of gas production by the depressurization method is governed by the sensible heat of the hydrate-bearing zone plus heat conduction from the cap- and base-rocks along with fluid flow through the reservoir. Simple models have been developed that resemble the complex behavior of these reservoirs. These simple solutions have been used for development of inverse-solution techniques that have led to interpretation techniques of well-testing of gas-hydrate reservoirs. This talk will present the development and application of these well-testing techniques.