Decline analysis and reserves estimation based on decline analysis should be fundamentally grounded in a good understanding of what factors control the decline.The same decline techniques cannot, and should not, be applied to all fields and all drive mechanisms. Specifically, arbitrarily using an exponential decline approach (log(qo) versus time assumed to be linear) for water drive, solution gas drive, and gravity drainage systems is neither technically nor empirically correct. Use of exponential decline during periods of increasing volumetric sweep efficiency will generally underestimate ultimate recovery. We caution against the temptation to blithely apply exponential decline analysis; although it may have been the traditional or "easy" decline analysis method, neither the theory nor empirical evidence supports its use in waterfloods. This paper shows the correspondence between physical mechanisms such as volumetric sweep and displacement efficiency with waterflood decline parameters.
Decline analysis is applicable, as a technique, when volumetric sweep is changing slowly and when relative permeability is controlling oil rates. In order to properly use decline techniques to forecast oil rates, the waterflood decline correlation period should have the following criteria:
the watercut should be greater than 50%
the voidage replacement ratio should be close to one
well count should be relatively constant
injection and fluid production rates should be relatively constant
the reservoir pressure should be relatively constant
producing well pressures should be constant
the GOR should be relatively constant
the volume of water injected should be greater than 25% of the hydrocarbon pore volume. Because of the wide range of controlling factors and resultant data scatter in decline plots, we would recommend using a variety of decline techniques.
Conventional decline techniques would include log(qo) versus t, qo versus Qo, and log(qo) versus Qo. As well, the recovery factor versus hydrocarbon pore volume injected, log water-oil ratio (WOR) versus cumulative oil produced (Np), and WOR versus cumulative water injected techniques are recommended to enhance resolution of decline techniques. Conformance plots should supplement decline analysis to indicate influx/efflux and water injection losses in waterflood patterns.
The above techniques are demonstrated in analytical and simulation models. As well, the use of the above plots as diagnostic tools is discussed.
No decline technique or form of the decline equation correctly predicts ultimate recovery (URF) early in the life of the project. RF < 0.5 (URF) (WCUTS < 50%) at this stage means that URF is subject to change. In real field situations, changing volumetric sweep, relative permeability, and fluid injection rates mean early stage conclusions based on decline analysis are not very useful. Simulation and analytical techniques are the only approaches that one may attempt which have a reasonable chance of accurately predicting performance at this stage. However, plots of RF versus HCPVI, log(WOR) versus Np, and log(qo) versus Qo have a better chance of correctly predicting future performance earlier in a waterflood's life than do log qo versus time analysis methods.
