A large number of studies are underway to evaluate the possible role of gas hydrates as a potential energy resource. While one class of such studies involves characterization and quantification of hydrates in geological setting suitable for exploitation, another class of studies involves development and use of mathematical models to estimate hydrate recovery under different operating conditions. Quantification of the hydrate resource using geological and geophysical techniques, while continuously improving is subject to very large uncertainties. A meaningful development plan should incorporate the associated uncertainty, requiring large number of simulation runs. This is impractical using currently available models, as (i) most of the available numerical models are not commercially available and (ii) setting up and running of these complex models is quite time-consuming. Furthermore, the accuracy associated with these models is not necessary, because the input parameters could vary over a wide range. Recently, the authors have developed an analytical model for prediction of gas production from hydrate capped gas reservoir. In this paper we present of this model and then use it along with Monte Carlo simulation to capture the effect of uncertainty in hydrate and reservoir parameters on the performance of the hydrate capped reservoir. The usefulness of these analytical models for fast engineering calculations, particularly those that are conducted at the beginning of the life of reservoirs (and are based on very limited information that do not warrant numerical simulation) is shown using a hypothetical case studies.
The demand for energy has stimulated the development of the unconventional gas resources (such as gas hydrates) which are occur in enormous quantities around the world. Gas hydrates tend to form in two geologic settings:
on land in permafrost regions, and
in the ocean sediments of continental margins.
During the last decade a number of projects have been conducted for detection and characterization of the hydrate-bearing formations, drilling, logging, coring, production testing and mathematical modeling of hydrate reservoirs, with the eventual objective of assessing commercial producibility of gas from hydrate resources.
One type of hydrate reservoirs that are thought to be most amenable to production are those that overly free-gas bearing sands (i.e. hydrate-capped gas reservoir). Makogon9 reports that in the Messoyakha reservoir, where part of the reservoir lies below the base of the hydrate stability zone (and contains free gas), production of the lower conventional free gas led to the decomposition of the overlying hydrates by depressurization. Another example is the Eileen accumulation in Alaska25, where production testing using the depressurization method is underway.
