Abstract

The pulse decay test is the main method employed to determine permeability for tight rocks, and is widely used. The testing gas can be strongly adsorbed on the pore surface of unconventional reservoir cores, such as shale and coal rock. However, gas adsorption has not been well considered in analysis pulse decay tests. In this study, the conventional flow model of adsorbed gas in porous media was modified by considering the volume of the adsorbed phase. Then, pulse decay tests of equilibrium sorption, unsteady state and pseudo-steady-state non-equilibrium sorption models, were analyzed by simulations. For equilibrium sorption, it is found that the Cui-correction method is excessive when the adsorbed phase volume is considered. This method is good at very low pressure, and is worse than the non-correction method at high pressure. When the testing pressure and Langmuir volume are large and the vessel volumes are small, a non-negligible error exists when using the Cui-correction method. If the vessel volumes are very large, gas adsorption can be ignored. For non-equilibrium sorption, the pulse decay characteristics of unsteady state and pseudo-steady-state non-equilibrium sorption models are similar to those of unsteady state and pseudo-steady-state dual-porosity models, respectively. When the upstream and downstream pressures become equal, they continue to decay until all of the pressures reach equilibrium. The Langmuir volume and pressure, the testing pressure and the porosity, affect the pseudo-storativity ratio and the pseudo-interporosity flow coefficient. Their impacts on non-equilibrium sorption models are similar to those of the storativity ratio and the interporosity flow coefficient in dual-porosity models. Like dual-porosity models, the pseudo-pressure derivative can be used to identify equilibrium and non-equilibrium sorption models at the early stage, and also the unsteady state and pseudo-steady-state non-equilibrium sorption models at the late stage. To identify models using the pseudo-pressure derivative at the early stage, the suitable vessel volumes should be chosen according to the core adsorption property, porosity and the testing pressure. Finally, experimental data are analyzed using the method proposed in this study, and the results are sufficient.

Highlights

  • The pulse decay test is the most used method for determining the permeability of low permeability rocks, and was proposed by Brace et al in 1968 [1]

  • If the testing pressure is low, the non-correction total compressibility is less than the equivalent total compressibility, which is contrary at high pressure (Figures 11 and 12)

  • For the two non-equilibrium sorption models, the pseudo-pressure derivative decreases with the Langmuir volume, which is the opposite observed for the pseudo-pressure (Figure 19)

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Summary

Introduction

The pulse decay test is the most used method for determining the permeability of low permeability rocks, and was proposed by Brace et al in 1968 [1]. Modifications are made to the primary testing method in order to improve the accuracy and the flexibility, and to decrease the testing time [6,7] All of these analysis methods are based upon the homogeneous model. Pulse decay method widely used in testing shale, coal rockoil/gas and tight sandstone pulse decay method is widely used in testing shale, coal rock and tight sandstone cores [8,9,10,11,12,13,14,15]. A few experiments of gas that the unipore diffusion model is not good enough for representing cores flow characteristics [25,26,27,28]. New correction is involving volume, and volume, an identifying method for non-equilibrium sorption models proposed adsorbed involvingphase adsorbed phase and an identifying method for non-equilibrium is suggested

Mathematical
Equilibrium Sorption Model
Non-Equilibrium Sorption Model
Sensitivity Analysis
12. The influence of of V
45% (Figures
Findings
Conclusions
Full Text
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