Gas Production From Hydrate Reservoirs and Its Modeling
Authors: Pooladi-Darvish Mehran 
Published: June 2014

Distinguished Author Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in the topics discussed. Written by individuals recognized to be experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: to inform the general readership of recent advances in various areas of petroleum engineering.

Abstract
It is estimated that the amount of natural gas trapped in hydrates around the world is approximately two orders of magnitude larger than the recoverable gas in conventional reservoirs. This estimate has attracted governments, especially those with limited access to other sources of fossil fuels, as well as several oil and gas producing companies, to take on projects for drilling and testing hydrate reservoirs. Current objectives include devising methods for fast and safe drilling and testing and for improving characterization techniques, and pilot testing of production techniques such as depressurization and thermal stimulation. Activities are underway in relation to at least three onshore and offshore hydrate accumulations around the world. Running in parallel to these activities, progress is underway in microscopic characterization of hydrates to determine important fluid-flow, heat-transfer, thermodynamics, kinetics, and geomechanical properties. Also, research is in progress toward developing numerical simulators for hydrate reservoirs and acquiring experimental information required for accurate modeling of reservoir behavior.

Introduction
Gas hydrates are ice-like crystalline materials that contain water and gases with small molecules and which can occur at temperatures above the freezing point of water. The gas molecules are trapped in the cage-like structure of the surrounding water molecules, leading to a tight structure. One volume of hydrate could release 150 to 180 volumes of natural gas at standard conditions. The high concentration of natural gas puts the energy content of hydrate-bearing formations on a par with bitumen and heavy-oil reservoirs, and much higher than the energy content of other unconventional sources of gas, such as coalbed methane and tight gas.1
Interest in gas hydrates started in the early part of the 19th century, mostly as a curiosity in the laboratory, when chemists made hydrates of different gases. In the 1930s, it was suggested that blockage observed in some gas-transmission pipelines was not ice, but gas hydrate. Extensive research subsequently ensued, primarily for finding ways of avoiding or delaying hydrate formation through the use of thermodynamic and kinetic inhibitors, respectively. Several recent monographs and textbooks, including that of Sloan,2 review these developments.
In the 1960s, another aspect was raised when gas production from naturally occurring hydrate deposits was reported in the Messoyakha field in western Siberia where an interval saturated with gas hydrate overlies the gas-saturated formation. It is believed that hydrates contributed to the long-term production of gas, when gas was simply produced from the free-gas zone. The production of the underlying free gas led to depressurization and dissociation of the upper hydrates.3 It is estimated that approximately 36&#x0025; (5&#x00D7;109 m3) of the gas withdrawn from the field came from the gas hydrate. Both this field experience and recent modeling studies4 suggest that the production life of gas reservoirs, such as those in the Mackenzie delta of Canada that include hydrate-bearing formations at their top,5 can be significantly increased because of the contribution from the top hydrates.
Interest in gas hydrates has increased steadily, particularly in recent years, with many studies suggesting that the amount of carbon present in the form of hydrates represents twice the total carbon present in other fossil fuels on Earth. Drilling and testing efforts are underway in three hydrate accumulations: the Mallik field, Northwest Territories, Canada; Prudhoe Bay/Kuparuk River area, Alaska, U.S.; and the Nankai trough, offshore Japan. Some of these accumulations show the presence of free gas below the hydrates, suggesting that a depressurization technique may be successful.
This paper reviews the potential of gas hydrates as a source of natural gas and the current efforts in unlocking this resource. It discusses methods proposed for gas production from hydrates as well as those used. The second part of this paper presents the status of mathematical modeling of gas production from hydrate reservoirs as a tool for evaluating the potential of gas hydrates for natural-gas production. Areas are noted in which more progress is required in formulating the processes and in determining the related physical parameters. The role of analytical models for better understanding the effect of processes and parameters involved and as a validation tool for the numerical models is presented.

Hydrate Resource The Prize
Formation of gas hydrates requires the presence of hydrate-forming gases and water under appropriate pressures and temperatures. Fig. 1 shows the three-phase hydrate-equilibrium line and identifies the area above the curve as the hydrate-stability zone. As depicted in Fig. 1, low temperatures and/or high pressures are required for the stability of the hydrate structure. In natural environments, these conditions could occur offshore in shallow depths below the ocean floor and onshore beneath the permafrost. The geothermal gradient of the Earth increases the pressure requirement for the stability of the hydrate at a much greater rate than that provided by the available increased pressure from the hydrostatic gradient. Therefore, there is a depth interval where hydrates may be stable. Fig. 2 demonstrates hydrate stability with depth in permafrost and marine environments.6 In permafrost regions,7 in which surface temperatures are well below freezing, gas hydrates can be present at depths between 150 and 2000 m. Under offshore conditions, gas-hydrate stability conditions usually extend to depths 100 to 500 m below the ocean floor,8 although hydrates have been recovered from the ocean floor in some cases.