interesting ..........

i think in this case use the volumetrics till you have more pressure data
ask for RFT for the new wells for example
maybe you can use the available data so far
you can juist try several pressure decline curves till you get resonable number (recovery mechanism , how it should look
if you have aquifer lets say)
you can play around till you get a number you can go with


hope this can give you a start point to go with

Ahmed Al-Alawi