Improving the Prediction of Production Loss in Heterogeneous Tight Gas Reservoirs Using Dynamic Threshold Pressure

Tight gas reservoirs commonly exhibit complex pore throat structures. Conventionally, a constant threshold pressure gradient (TPG) has been used to predict the production loss that arises from overcoming the rock's disinclination to flow water and gas through such complex pore throat structures. In this work, we find that the TPG is not constant during the production lifetime of a reservoir. The TPG varies significantly with both effective stress and water saturation, leading us to rename TPG as dynamic threshold pressure gradient (DTPG). In the first part of this paper, we examine the sensitivity of DTPG to stress and mobile water saturation for cores with different permeabilities, showing that DTPG increases logarithmically with effective stress from 0.17 to 0.5 MPa/m for a change in effective stress from 0.6 to 30.5 MPa. The DTPG also increases exponentially with mobile water saturation (S-m), which is 2.7-6.5 times higher at S-m = 25% compared to the value at irreducible water saturation. The sensitivity of DTPG to both variables shows a decreasing power law trend with increasing rock permeability. These combined effects generally suggest that DTPG is larger than the conventional TPG. In the second part of this paper, we model the effects of using a variable DTPG in place of a constant TPG for the purpose of predicting the production loss associated with the latent pressure barrier in different heterogeneous reservoirs. When the interacting effects of effective stress, water saturation, and permeability are taken into account, we find that the threshold pressure is relatively small in heterogeneous reservoirs with a distribution of increasing permeability in the gas flow direction. The constant TPG approach underestimates the production loss by 34-45% with the greatest difference occurring at low gas pressures encountered at the production well, suggesting that gas production wells should be located in areas with high permeability.