I have found an article on oil well decline curves.Constant...

I have found an article on oil well decline curves.
Constant factor or exponential seems to be the commonest and this is what I used.

http://www.petrobjects.com/downloads/Petroleum%20Reservoirs%20Estimation%20Methods/Reserve%20Estimation%20Methods_03_DeclineCurve.pdf

Forget the complicated formulae
An initial rate of 75000bbl/day = 27.375million bblpa
Using 0.8245 as the factor by which rates decline each year, the sum for 9 years is 123million bbl. In the 9th year the rate would be 5.85million bblpa. HDR would receive approx. 5.20 million bbl in year 1 declining to 1.11million bbl in year 9.

Presumably the operating cost/bbl increases as the well declines and at some point in time cost = realisation and the well is abandoned.