Probabilistic forecasting and economic evaluation of pressure-drawdown effect in unconventional oil reservoirs under uncertainty of water blockage severity

Abstract

Applying mild pressure drawdown during early-time oil production from unconventional reservoirs is believed to be beneficial for the ultimate oil recovery because it prevents early fracture conductivity loss and mitigates water blockage. However, previous studies demonstrate mixed results – some studies report mild drawdown as beneficial for the oil recovery, whereas others detrimental.This paper investigates water blockage severity and stress-dependent permeability (SDP) as two of the main reasons for the mixed perceptions. To introduce the uncertainty of water blockage severity, this study builds five distinct reservoir solution models of different levels of water blockage severity to perform probabilistic forecasting. All of the models are validated using the same field production data. Meanwhile, to introduce the uncertainty of SDP, this study used a parametric analysis based on numerous SDP experimental data.The results demonstrate that the mild drawdown tends to result in a higher oil recovery when the reservoir permeability is barely stress-sensitive or when the water blockage is severe. We have also shown that this claim can straighten out the mixed conclusions among some previous papers. However, although the mild drawdown can be beneficial in terms of oil recovery, it can more likely be detrimental in terms of NPV, unless the reservoirs are severely water-blocked. This occurs because such reservoirs tend to generate additional oil recovery soon with a small water production volume.