Where royalties are paid in lieu of drilling off-set wells on forty-acre units adjacent to forty-acre units already in production, the forty-acre units adjacent to the forty-acre unit receiving the royalty payments shall immediately after the signing of the agreement or contract become off-set units, and drilling operations shall begin on each adjacent forty-acre off-sets, to the forty-acre unit receiving royalty payments in lieu of drilling the well in not less than ninety (90) days thereafter.