An inherent problem with natural gas production or transmission is the formation of gas hydrates, which can lead to safety hazards to production and transportation systems and to substantial economic risks. Therefore, an understanding of the inception of hydrate formation is necessary to overcoming hydrate problems. The aim of the first step of this study is to develop a simple-to-use correlation for predicting hydrate-forming conditions of sweet natural gases. This simple correlation estimates hydrate formation pressure of sweet natural gases for pressures up to 40,000 kPa and temperatures between 260 K and 298 K as well as molecular weights in the range of 16 to 29. In the next step, novel empirical correlations are developed to predict the required MEG weight percent in the rich solution and the flow-rate for desired depression of the gas hydrate formation temperature. These correlations are generated for a natural gas with relative density of 0.6 at pressures of 3, 5, 7, and 9 MPa, which are applicable to wet gas temperatures of 20, 30, 40, and 50 °C. In order to extend the application of these correlations to wide ranges of natural gas mixtures with specific gravities of up to 0.8, two generalized correction factors are also provided. The accuracy of this simple method is compared with the simulation results obtained by commercial software which showed excellent agreement. In all cases the error percent was approximately 2% and 5% for predicting hydrate formation temperature depression and MEG injection rate, respectively.
The combination of water molecules and guest gas molecules under favorable conditions, usually at low temperatures and elevated pressures, can lead to the formation of hydrates. The most common guest molecules are methane, ethane, propane, isobutane, normal butane, nitrogen, carbon dioxide and hydrogen sulfide, of which methane occurs most abundantly in natural hydrates. It should be noted that normal-butane does form a hydrate, but is very unstable. However, it will form a stabilized hydrate in the presence of small "help" gases such as methane or nitrogen. It has been assumed that normal paraffin molecules larger than normal-butane are nonhydrate formers [1, 2]
Although gas hydrates may be of potential benefit both as an important source of hydrocarbon energy and as a means of storing and transmitting natural gas, they represent a severe operational problem as the hydrate crystals deposit on pipe walls and accumulate as large plugs, resulting in blocked pipelines and over pressuring and eventually leading to shut down of production facilities. Acceleration of these plugs when driven by a pressure gradient (e.g., single-sided depressurization after hydrate formation) can also cause considerable damage to production facilities, and therefore create a severe safety and environmental hazard [3]. The removal of hydrate plugs in hydrocarbon production/transmission systems poses safety concerns and can be time consuming and expensive [4]. For this reason, the hydrate formation in gas transmission pipelines should be prevented effectively and economically to guarantee the pipelines operate normally.
