Abstract
Shale and fractured cores often exhibit dual-continuum medium characteristics in pulse decay testing. Dual-continuum medium models can be composed of different flow paths, interporosity flow patterns, and matrix shapes. Various dual-continuum medium models have been used by researchers to analyze the results of pulse decay tests. But the differences in their performance for pulse decay tests have not been comprehensively investigated. The characteristics of the dual-permeability model and the dual-porosity model, the slab matrix, and the spherical matrix in pulse decay testing are compared by numerical modeling in this study. The pressure and pressure derivative curves for different vessel volumes, storativity ratios, interporosity flow coefficients, and matrix-fracture permeability ratios were compared and analyzed. The study found that these models have only a small difference in the interporosity flow stage, and the difference in the matrix shape is not important, and the matrix shape cannot be identified by pulse decay tests. When the permeability of the low permeability medium is less than 1% of the permeability of the high permeability medium, the difference between the dual-permeability model and the dual-porosity model can be ignored. The dual-permeability model approaches the pseudo-steady-state model as the interporosity flow coefficient and vessel volume increase. Compared with the dual-porosity model, the dual-permeability model has a shorter horizontal section of the pressure derivative in the interporosity flow stage. Finally, the conclusions were verified against a case study. This study advances the ability of pulse decay tests to investigate the properties of unconventional reservoir cores.
Highlights
In the past two decades, the development of unconventional oil and gas, such as tight sandstone oil and gas, shale gas, and coalbed methane, has received great attention and made has achieved major breakthroughs
Han et al [29] discussed in detail the pulse decay test of the pseudo-steady-state dual-porosity model and transient dual-permeability model with a slab matrix, and the corresponding models and numerical solution methods are given in detail
When the interporosity flow coefficient is small (λ = 0.01), the pressure and pressure derivatives of the two models coincide in the early and equilibrium stages, and they only have a slight difference in the interporosity flow (1 < tD < 103 ) stage
Summary
In the past two decades, the development of unconventional oil and gas, such as tight sandstone oil and gas, shale gas, and coalbed methane, has received great attention and made has achieved major breakthroughs. Dejam et al [4] obtained the analytical solution of the pressure transient of radial non-Darcy flow by the generalized Boltzmann transformation method Another method is to perform laboratory core permeability testing. Numerical modeling studies have shown that when the pressure pulse is large, the gas compressibility [14,15], slippage effect, and stress sensitivity [16] will affect the test results At this time, the analytical methods cannot be applied, and the numerical modeling for historical fitting becomes the basic means. Han et al [29] systematically simulated pulse decay tests for a dual-porosity model and proposed a pressure derivative method to analyze the data, which can diagnose whether the cores conform to dual-porosity models through early time pressure data and interporosity flow models by transition stage data. The characteristics of different models in the pulse decay test are clarified and verified by examples
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