Decline Curve Analysis (DCA) is an important tool used in the petroleum industry to forecast future production performance. Forecasting future production performance is one of the critical inputs in the economic analysis of Oil and Gas investments. Conventional DCA (Arps' approach) has however been shown to have limited accuracy in forecasting cumulative production for solution – gas – drive reservoirs.
Fetkovich introduced a novel method of combining rate equation and material balance equation for finite systems to obtain rate – time equations for solution – gas – drive reservoirs, using the backpressure exponent (n) in place of the Arps' decline exponent (b). This work extends Fetkovich rate –time equation approach by developing cumulative – time and cumulative – rate models, for the two basic forms of the material balance equation investigated viz: PR is linear with Np (or Gp), PR2 is linear with NP (or Gp).
The models developed were validated with production data from two reservoirs, the first reservoir presented in the original Arps' paper and the second reservoir located in Niger Delta, Nigeria. The models were found to provide more accurate forecast of the cumulative production of a solution – gas – drive reservoir than the Arps' approach. The developed models also yielded results which compared favourably with the actual cumulative production data from the solution – gas – drive reservoir in Niger Delta. For the Niger Delta reservoir, the forecasted oil in place gives 128.844MMstb while the actual (estimated) oil in place was 127MMstb, a difference of 1.69%.