|Introduction: This family uses Decline Curves to forecast oil and gas production, and can accommodate exponential, harmonic or hyperbolic decline.|
The wide use of Decline Curves is based more on pragnatism than a strong theoretical case, because the pure drive mechanisms which the original equations were derived for rarely exist by themselves, and decline curves frequently "just happen to fit the data" rather than come from a strong analytical conviction.
The are main functions are:
These functions, through the ResvsOrDcRate variable, either accept a value for the produced Reserves figure (if the value is positive), or, if the value is negative, a value for the decline rate (known as a or ai). So ResvsOrDcRate=100 indicates reserves of 100 mmb, and ResvsOrDcRate=-0.1 indicates a decline rate of 10% per annum.
- Decline, that calculates either an instantaneous rate or cumulative production at a specific time (number of years) after the start of decline.
- ProdProf which calculates a complete production profile by timeperiod. It can also output the instantaneous rate at the start or end of the period. The function accepts a production build-up too, prior to decline, via the AnnualRates variable.
There are certain other "helper" functions:
- AbandonDate is related to ProdProf, and it calculates the eventual abandonment date, to the nearest day (!). Like ProdProf, it accepts either a decline rate or a reserves figure.
- StartDecline is also related to ProdProf, and it works out when the production reaches the DeclinePoint.
- DeclineRate works out the decline rate given a plateau and abandonment rate.
- DescribeDecline just outputs a text message describing the type of decline.
[What the family does not at present contain are any functions for fitting data to a decline curve. We are looking into exponetial curve-fitting functions - Feb"04]