Wells in unconventional, ultra-low permeability reservoirs have significantly longer duration of transient flow regimes than wells in conventional reservoirs due to different reservoir characteristics and production methodologies. This calls for new methodologies developed specifically for transient flow analysis, such as the pioneering work by Duong (SPE 137748).
This paper presents a critique of the Duong method and proposes a modified and improved version of the method which enhances the accuracy of forecasting production in ultra-low permeability reservoirs.
Our study examined the derivation of the Duong model and highlighted the lack of taking fracture damage into account. This causes inconsistent expressions of flow rate and cumulative production as functions of time. Further, most wells produce with changing pressure drawdowns, and this also creates an inconsistency between the flow and cumulative production equations that Duong assumed in deriving his decline model.
We therefore propose a modified and improved version of the Duong method using basic transient flow equations that account for the presence of fracture skin and changing pressure drawdown. We used simulated production histories to carry out a comparative study of the predictions from the original and modified Duong methods to highlight the effect of fracture skin and varying pressure drawdown on the production forecast results.
Results from our analysis show that Duong method experiences problems in accuracy when early time pressure drawdowns increase and, independently, when fracture damage is present. Our modified method removes the inaccuracies due to fracture damage and can remove inaccuracies due to changing pressure drawdown when pressures are known. For unknown pressure histories, our method can at least minimize the impact of changing pressure drawdown.
This study shows that the Duong method is most accurate under constant pressure drawdown and no fracture damage. With changing bottomhole pressure, and presence of fracture damage, production forecasts using the original Duong model can be quite inaccurate. Our modified Duong method, which uses pressure-normalized rates and accounts for the presence of fracture damage, leads to more accurate production forecasts. The parameters calculated using the modified Duong method also provide meaningful insight into reservoir and fracture properties, whereas the empirical parameters calculated using the original Duong model fail to do so.
