“Comparison of Numerical vs Analytical Models for EUR Calculation
and Optimization in Unconventional Reservoirs”
Analytical models available in Rate-Transient-Analysis (RTA) packages are widely used as fast tools for history matching and forecast in unconventional resources. In addition, recently, there has been an increasing interest in numerical simulation of unconventional reservoirs. In this study, we use both methods to history match fractured unconventional wells, followed by forecast calculations. This study aims to reveal large differences in Estimated Ultimate Recovery (EUR), predicted by analytical models and numerical simulation in unconventional reservoirs.
First, we consider a single-phase shale oil reservoir as a base case for this study. The base case also satisfies other assumptions inherent in analytical models such as homogenous reservoir properties and fully-penetrating planar fractures with constant half-length and conductivity. We then impose different real-world deviations from RTA assumptions and investigate reliability of EUR predictions made by both approaches.
Example results show that, in the presence of real-world deviations from RTA assumptions, analytical models can still match the historical production data; however, key reservoir and fracture parameters need to be modified drastically to compensate lack of sufficient physics in analytical models. For the cases presented in this study, analytical models under-predicts EURs by 10-20% although history match of two-year production looks good. For all cases, we also apply an efficient simulation workflow for probabilistic forecasting of brown fields. This workflow provides multiple history-matched models that are constrained by historical production data. The probabilistic forecast provides P90 (conservative), P50 (most likely), and P10 (optimistic) values for EUR. In all examples, range of P90 to P10 values includes the reference EUR and the P50 values are within 7% error of the reference EUR.