Productivity index and field behavior: a case study

Abstract

This study is an investigation of different factors' phics. influence on the productivity index and its behavior. The objectives of this research are (1) to develop an overview of how different factors influence the productivity index', and (2) to explain the irrational behavior of the productivity index in a case study presented. The problem has its origin in a field in north Africa, where irrational behavior of the productivity index (PI) has made it difficult to forecast the field performance. By irrational behavior we meant that the PI ants the opposite of what is expected. Normally we think PI will increase when the production oil rate of the field increases, at the same pressure drawdown. Or for the same well, PI should be constant over time. In some wells in this particular field we can see the P1 increase as production oil rate decreases and vice versa. Numerical simulation was used to simulate the influence different factors had on the productivity index, and to match wellness PI's with calculated PI's from field data in the case study. An overview of which factors can cause the P1 to go in unexpected directions is presented. Finally the theory obtained about the PI behavior is linked to the case study, and the E6incon-ect'' behavior of the PI is explained. It was shown that transient flow and two-phase flow are the two main reasons for the productivity index to decrease as production oil rate increases. It was also shown that dual porosity, non-Darcy flow, permeability changes, formation compressibility, and skin affect the length of the transient flow period and the magnitude of the difference between transient PI and pseudo steady state (PSS) PI. The behavior of the PI in the field case presented is explained by the transient flow effect and bad test data.