A gas accumulator for the storage of biogas, comprising three gas-impermeable membranes, the first of which at least partially defines a gas accumulation chamber and the third of which is attached to the second by means of a gas-impermeable seal, such that the second and third membranes together define a gas-impermeable pressurisation chamber, as well as comprising means for pressurising said pressurisation chamber and means for anchoring the gas accumulator to a support surface, characterised in that the anchoring means is configured to anchor only the second membrane to the support surface.