| THEORY & FORMULAE |
Determination of the Profitability Indicators and Cash Flow
Streams of an oilfield development project,
given the following input: time
profiles of oil-production rates, oil-prices and capital outlay.
Let us assume a time-series of oil rates (q1, q2,…qt…), 
oil price (p1, p2,…pt…),
capital expenditure (C1, C2,…Ct…),
  over a planning horizon T years
where qt = oil rate in year t,
     pt = oil price in year t,
     Ct = capital cost in year t; t = 1,2,3…T.
And also given:
     σ = discount rate,
     r = royalty rate,
     α = profit tax rate,
     θ = annual operating cost factor,
     φ = annual capital recovery limit.
The Cash Flow Stream consists of the following derived series:
Oil Revenue REVt in year t = qt * pt;
Royalty Payment ROYt = REVt * r
Annual Operating Cost is assumed to depend only on cumulative capital cost to date;
For illustration, consider a 5-year plan, with $200m capex in year-1 and $100m in year-3, and an
operating cost factor of 10%. The capex series is (200,0,100,0,0), the cumulative capex
is (200,200,300,300,300). Therefore the opex series is (20,20,30,30,30).
Operating Cost OPCt =
    
A generic profit tax model, consisting of the main elements in most of today's oil taxation regimes, is adopted. Taxable income is derived from revenue less all costs, including capital recovery. An annual limit is set on capital recovery, such that it does not exceed φ % of capital expenditure to date.
Capital Recovery CRVt in year t =
    
Thus, Profit Tax PFTt in year t =
    
Net Cash Flow NCFt = REVt - ROYt - Ct - OPCt - PFTt
The Profitability Indicators are computed from the generalized relationship:
    
where Net Present Value NPV = γ, when λ = T and σ = discount rate;
     Internal Rate Of Return IRR = σ, when λ = T and γ = 0;
     PayOut Time = λ, when γ = 0 and σ = 0;
Variants of the above primary profitability indictors can be readily derived. For example,
Investment Efficiency IE = NPV/[Investment]
    
Finally, Growth Rate Of Return