Ultimate recovery data are frequently modeled as being lognormal without consideration of other possible distributions. A typical approach is to use lognormal probability paper and a mean value (Swanson's mean) is calculate as well as chance factors. Engineers have identified a competing probability model for lifetime data called the Weibull distribution. In this paper, we consider the Weibull distribution as a potential alternative probability model for ultimate recovery data. Optimal methodologies of estimating the mean ultimate recovery for both the lognormal and the Weibull distribution are presented. Statisticians refer to this optimal approach as maximum likelihood methodology. Further, probability plots and hypothesis testing methods are presented that help reveal whether one should use the Weibull model or the lognormal model for available ultimate recovery data. Finally, a simulation approach is proposed to predict the average ultimate recovery value for "n" additional wells. An example with real data concludes the paper.
