If a plant operating under this subchapter commingles casinghead gas with sweet gas or sweet gas with sour gas, the commission shall ascertain:
(1) the quantity of residue gas required to be used for fuel purposes in the efficient operation of the plant; and
(2) the quantity of residue gas required to be returned by the operator of the plant to the leases to which the plant is connected for use as fuel in the operation of the leases.