Quantifying Controls on Threshold Pressure during CO₂ Injection in Tight Gas Reservoir Rocks

Abstract

The simultaneous flow of gas and water is controlled by a threshold pressure gradient (TPG) effect during CO 2 injection of tight gas reservoirs. The TPG effect is dynamic because it varies with both the effective stress and the water saturation. The sensitivity of TPG to effective stress and mobile water is affected by the porethroat microstructure. In this paper, we report the results of dynamic TPG tests on six cores with similar permeability. The influence of the pore-throat microstructure on the sensitivity of the TPG to stress and to mobile water was also quantitatively studied using a fractal method, and the distribution of the threshold pressure and corresponding gas production loss were calculated during CO 2 injection in tight gas reservoirs. The test results show that TPG decreases logarithmically with the increase of the pore fluid pressure during CO 2 injection, a change of 0.1−50 MPa in the pore fluid pressure corresponding to a 1.8−3.5 times increase of the TPG variation. The TPG increases exponentially by 3.5−6.7 times from irreducible water saturation to a mobile water saturation of 30%. The fractal dimension (D) of the heterogeneity of the rock pore-throat microstructure has a linear relationship with both the stress sensitivity coefficient (λ) and mobile water sensitivity coefficient (η), with the larger values of λ and η being associated with more heterogeneous pore-throat microstructures. The reservoir threshold pressure showed a significant nonlinear distribution in near-well reservoirs at low bottom-hole flow pressures of the production well during CO 2 injection. The calculated gas well production loss as a result of a dynamic threshold pressure is 7−16% higher than that of the fixed threshold pressure, and the difference is larger at low pressures.

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

“…This parameter is defined as the DTPG mobile water sensitivity coefficient. 22 The parameter η determines how quickly the curve increases, and the coefficient b is the DTPG value at S wi .…”

“…The dynamic nature of the TPG makes it difficult to predict the threshold pressure distribution in reservoirs at different reservoir pressures. 22 The determination of the optimal well spacing lacks a basis for prediction. 23 In addition, the prediction of gas well production is complicated by the DTPG effect.…”

“…Consequently, the combination of low permeabilities and high mobile water saturations results in tight reservoirs with large values of TPG. 31,32 The sensitivity of TPG to mobile water saturation has been tested in our previous studies, 22 showing that mobile water has a greater effect on TPG than irreducible water. 11 Moreover, the TPG values increase with effective stress, 33 which is attributed to increased capillary resistance due to a decrease in pore throat size.…”

“…Measurements of the DTPG of tight sandstone cores with different permeabilities have been carried out by using the improved bubble method. 22 The variation in the characteristics of DTPG at different effective stress values and saturations of mobile water has also been studied. The impact of rock permeability on effective stress sensitivity coefficient and mobile water sensitivity coefficient of DTPG has been analyzed quantitatively.…”

“…The threshold pressure values were measured by an improved bubble method. 22 First, a small displacement differential pressure is applied to the core by injecting CH 4 . A thin pipette containing air bubbles in dyed water is connected to the outlet end of the core.…”