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

Abstract

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.

Productivity Analysis of Fractured Horizontal Wells in Tight Gas Reservoirs Using a Gas–Water Two-Phase Flow Model with Consideration of a Threshold Pressure Gradient

“…As for the influence of the TPG on the productivity of tight gas reservoirs, these studies mainly utilize vertical well models to calculate gas productivity, − and the productivity model of fractured horizontal wells with the TPG, including gas–water two-phase flow effects, is rarely reported. For instance, Tian et al obtained the TPG correlation by considering the influences of permeability, connate water, movable water, and vertical well models that accounted for the TPG, slip, and diffusion effects.…”

“…Zafar et al 25 introduced two new TPG correlations based on the pore pressure and permeability and the pore pressure and water saturation. Wang et al 26 showed that the TPG varies significantly with both effective stress and water saturation, and a constant TPG approach underestimates production loss. Dong et al 27 established a dynamic pseudothreshold pressure gradient (PTPG) and showed that the vertical well performance considering dynamic PTPG was smaller than that of Darcy flow but larger than that of conventional non-Darcy flow.…”

Zoning Productivity Calculation Method of Fractured Horizontal Wells in High-Water-Cut Tight Sandstone Gas Reservoirs under Complex Seepage Conditions

“…Wang (2022)'s experimental results indicated that under water conditions, seepage in tight gas reservoirs exhibits nonlinear characteristics and forms the TPG. The TPG for the gas phase has a close power-law relationship with water saturation [32]. For the water-producing gas wells in water-bearing-inclined gas reservoirs considering stress sensitivity, Fu (2022) has developed a new gas well production equation that accurately determines the relationship between gas well production and stress sensitivity and water production [33].…”