Method and an apparatus for obtaining energy by expanding a gas at a wellhead

Abstract

A method and an apparatus () for reducing the pressure of a natural gas () at a wellhead of a natural gas field. The method provides prearranging an energy recovery expansion device (), which comprises a stator and a rotor mechanically connected with a power generator (), and a gas passageway between the rotor and the stator, and conveying the natural gas into the passageway of the energy recovery expansion device () to cause a rotation of the rotor and the expansion of the gas, the pressure being reduced from the extraction pressure (P) to the use pressure (P), which makes it possible to obtain an electric power (W) during the expansion, adapted to satisfy the energy requirements of the gas extraction unit, in particular of an offshore platform, without installing local electric generators. In order to allow a reliable operation of a recovery expansion device, such as a turbine, a means is provided for finely separating solid and liquid matter present in the gas, as well as a means () for conditioning the gas entering the expansion device, which may comprise a feeding means () of an additive into the gas (), said additive adapted to decrease the formation temperature of the solid, and, in an advantageous way in case of higher amount of water, a means for heating the gas such that the temperature is maintained above the formation temperature of the solid.