(a) If a lawful daily demand exists for sour gas from gas wells for purposes of utilization permitted by law, other than the manufacture of carbon black, the additional demand shall be added to the daily demand for carbon black manufacture, and that sum shall constitute the daily volume of sour gas from gas wells that may be withdrawn from the common reservoir for utilization.
(b) The commission shall prorate the daily volume provided for in Subsection (a) of this section among the sour gas wells in the area on the basis set forth in Section 87.052 of this code.