Impact of Heterogeneity on the Transient Gas Flow Process in Tight Rock

Abstract

There exits a great challenge to evaluate the flow properties of tight porous media even at the core scale. A pulse-decay experiment is routinely used to measure the petrophysical properties of tight cores including permeability and porosity. In this study, 5 sets of pulse-decay experiments are performed on a tight heterogeneous core by flowing nitrogen in the forward and backward directions under different pressures under pore pressures approximately from 100 psi to 300 psi. Permeability values from history matching are from about 300 nD to 600 nD which shows a good linear relationship with the inverse of pore pressure. A preferential flow path is found even when the microcrack is absent. The preferential path causes different porosity values using differential initial upstream and downstream pressure. In addition, the porosity values calculated based on the forward and backward flow directions are also different, and the values are about 1.0% and 2.3%, respectively, which is the primary novelty of this study. The core heterogeneity effect significantly affects the very early stage of pressure responses in both the upstream and downstream but the permeability values are very close in the late-stage experiment. We proposed that that there are two reasons for the preferential flow path: the Joule–Thomson effect for non-ideal gas and the core heterogeneity effect. Based on the finding of this study, we suggest that very early pressure response in a pulse-decay experiment should be closely examined to identify the preferential flow path, and failure to identify the preferential flow path leads to significant porosity and permeability underestimation.

Simulation Analysis of the Characteristics of Layered Cores during Pulse Decay Tests

“…As the two ends of the core can be swapped, there are two arrangements for testing: forward sequence and reverse sequence (Figure 3). The experiment of Jia et al [22,23] has shown that the arrangement of cores can affect the results of pulse decay tests. At present, there is a lack of systematic research on this.…”

“…Jia et al [22] simulated pulse decay tests on rock cores containing fractures and Vugs and found that the pulse decay curves exhibited a dual-porosity behavior. The experiments they conducted showed that core heterogeneity has a significant impact on the early response of pulse decay testing, and the porosities obtained from the forward and backward flow tests are different, but little effect on the permeability was indicated from the later-stage data [23]. Arnoaimi et al [24,25] carried out pulse decay experiments and numerical simulations on Eagle Ford and Haynesville shale samples containing microcracks.…”

A pulse-decay method for low permeability analyses of granular rock media

“…For pulse-decay methods, the early-time solution has the advantage of capturing the anisotropic information contained in reservoir rocks (Jia et al, 2019;Kamath, 1992). However, it suffers from the shortcoming of uncertainty in data for initial several seconds, which as a result is not recommended for data processing (Jia et al, 2019;Kamath, 1992). This is due to: (1) the Joule-Thompson effect, which causes a decrease in gas temperature from the expansion;…”

A pulse-decay method for low (matrix) permeability analyses of granular rock media

“…For pulse-decay methods, the early-time solution has the advantage of capturing the anisotropic information contained in reservoir rocks (Jia et al, 2019;Kamath, 1992). However, it suffers from the shortcoming of uncertainty in data for the initial several seconds, which is why it is not recommended for data processing (Brace et al, 1968;Cui et al, 2009).…”