Cashflow calculations are an essential part of the economic evaluation. The flows of cash in and out during the lifetime of a prospect, through discovery, appraisal, development and abandonment have to be estimated per year (or month). From this the cumnulative cashflow is calculated in money of the day and in terms of present value ("PV"). A cashflow can be calculated "before tax" or "after tax".
The important data for a cashflow calculation (in the case of oil) can be listed as follows:
There are a number of firms that have developed software to do the cashflow calculations. The variety of conditions in different fiscal settings complicate the design of a "universal" program. A specific solution is to use a spreadsheet that can handle basically any complex evaluation. For exploration prospects a difficulty may arise to obtain data on drilling and development costs, in order to guesstimate the economic cutoff volume. There are firms that specialize in providing such data for which one has to pay, of course. For instance try www.spe.org, Industry Statistics.
Despite great variation in the way oilfields are developed, there are a number of worldwide rules that can be used to estimate a production profile. The ProdProf program uses the correlation between ultimate recovery of oil and plateau production level (r=0.93), as well as a rough correlation between the ultimate recovery and the build-up time to plateau. Plateau production gives a linear correlation on a log-log plot:
The logic relating ultimate recovery to the three production phases is as follows: The larger the field, the longer it will take to reach the plateau level. On a global scale this is only a weak correlation:
The plateau phase is usually so chosen that at the end of the plateau production half of the ultimate recovery is produced. This includes the volume produced during buildup. Thereafter, the field production declines at some percentage per year. This decline rate is not an arbitrary assumption, but based on the ratio of production over remaining reserves at the end of the plateau. In this way a smooth transition from plateau to decline is ensured. To read more about decline curves, see the paper by Höök et al. (2009). The operators are cought between a desire to produce maximum amounts of oil as soon as possible to increase the PV value of it and the requirement to keep the CAPEX as low as possible and protect the field from unwanted reservoir damage that would limit the U.R. Using this logic and a few assumptions a production profile can be constructed, hence a certain optimal production profile. Here is an example for a 300 mb field with a buildup period of 4.4 years, created with the above simple logic.
With more information better forecasts should be possible. The simple profile is probably good enough for exploration prospects.
For gas fields in the North Sea,I made a survey of the various production parameters and used the results in the prodcasting software.