In energy markets, in particular, natural gas (NG), many contracts incorporate flexibility in the volume to be delivered. These contracts are known as swing options or take-or-pay contracts. Subject to restrictions, such contracts allow the option holder to exercise the right to receive greater or smaller amounts of NG contracted in accordance with market price, economic indicators and demand. Through swing options it is possible to value the flexibilities built into a contract for NG. Swing options are part of family called exotic options, which have unique distinguishing characteristics in comparison to standard options. One of the key aspects in the evaluation of options is to determine how they behave as a result of the uncertainties of the contract. In this work, the price of NG was the main source of uncertainty and was considered following the stochastic two-factor model of Schwartz and Smith (2000) with quarterly seasonality. Commodities in general are not traded in the spot market, but rather traded in futures markets. Therefore, to estimate the spot prices of NG, using the prices of futures contracts traded on NYMEX Henry Hub, it was necessary to implement the Kalman filter method, which relates the unobservable variables in the future prices of various maturities. As the primary focus, we analyzed the value of contractual terms, i.e. swing options that help to hedge in a market subject to uncertainties. The pricing of the option was made through the binomial tree model bi-variable in discrete time developed by Hahn and Dyer (2011) for the model of Schwartz and Smith (2000). The value of the swing option was positive in both cases analyzed, showing that this option has value and therefore should be carefully considered for inclusion in contracts of natural gas. The characteristics of the analysis were the same as specified in Jaillet et al. (2004).