I. Introduction
As the global reserves are reallocated to satisfy the demand of economically growing countries [1] and the regional or national gas reserves decrease, the security of gas supply in Europe is threatened. In 2009, the share of gas in the total energy consumption of 27 EU members was recorded as 24%. Between 1999 and 2009, gas consumption increased by 9% while the production decreased by 25%, and this situation led import dependency to shift from 47.9% to 64.2% in the same decade [2]. This trend in the past is expected to continue, as the forecasts show an 18.7% increase in the demand and a 57% decrease in the EU production from 2008 to 2035 [1]. This growing consumption against falling production indicates an approaching threat for security of supply.