Differences in Performance of Models for Heterogeneous Cores during Pulse Decay Tests

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.

Keywords

• pulse decay test
• dual-porosity model
• dual-permeability model
• pseudo-state interporosity flow
• transient interporosity flow