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Edited by: Hang Lin, Central South University, China

Reviewed by: Fuqiang Yang, Fuzhou University, China; Jia Lin, University of Wollongong, Australia

This article was submitted to Earth and Planetary Materials, a section of the journal Frontiers in Earth Science

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

In order to study the effects of stress and water pressure on the permeability of fractured sandstone, an ultra-deep gas layer of fractured tight sandstone in WengFu Mine in GuiZhou province, was selected for experimental design and research, and the effects of confining pressure, internal pressure, effective stress, and water pressure on gas permeability were studied. The results show that the effective stress and slippage effect compete with each other when the confining pressure is kept fixed and the internal pressure is increased in the fractured tight sandstone reservoir. At the beginning of the experiment, the gas slippage effect is strong while the rock stress sensitivity is weak. The gas permeability increasingly decreases with the increase of internal pressure. The effective stress is stronger than slippage effect, and the gas permeability increases with the increase of internal pressure when crossing the low point of permeability. The greater the confining pressure is, the greater the internal pressure needed to reach the low permeability point will be. With the increase of axial stress, horizontal contact hydraulic hole increasingly closed. Horizontal hydraulic holes are more sensitive to axial stress than vertical ones.

Tight sandstone gas is considered unconventional gas in general conditions, but it can also be considered conventional gas for extracting when the burial depth is shallow and the mining conditions are good (Gao et al.,

As a kind of important unconventional reservoirs, fractured tight sandstone gas reservoirs have attracted increasing attention due to their considerable productivity and high economic benefits (Li et al.,

Since the 20th century, domestic and foreign scholars have made rich achievements in studying the relationship between the slippage effect of unconventional gas and the effective stress. A great number of methods have been used to study rock permeability, including field measurement, triaxial seepage experiment, and numerical simulation. It is usually reliable to measure the rock permeability on the scene. However, because of the complexity of the geological environment, repeated mining interference and the expensive testing required, these methods are not widely used. Instead, most rock samples are tested for permeability in the laboratory. In the past few years, the permeability evolution of rock in stress-strain process has been widely studied. Zhao et al. studied the rheological fracture behavior of rock fractures through a series of calculations and analyses of crack rheological fractures under different water pressure, and proposed the equivalent boggs model of rock fracture rheological fracture, laying a foundation for revealing the rheological characteristics of fractured rock under the combined action of water pressure and stress (Zhao et al.,

The relationship between volume change and permeability is very close. Ord (

Firstly, the volume change (or volume strain) of a compressed rock is a continual process with continual deformation phases, which are divided into regions of volume reduction (compression) or volume increase (expansion). In a word, the process of volume change starts with fracture closure and elastic compression, followed by the collapse of pore structure and the increase of microcrack density. The point of turnover between the compaction and expansion stages is often referred as the expansion boundary. Due to the disturbance of the original stress distribution, the expansion boundary of rock depends on its mechanical and hydraulic characteristics. Geological environment, temperature and time are also important factors for expansionary development. With the increase of steam and gas pressure, the expansion rate tends to increase with the increase of temperature, which will increase the tensile stress at the crack tip and facilitate the crack opening. The time-varying effect is characterized by a significant static quiescent period, resulting in a sudden energy release and pressure drop (Li et al.,

During fault slip events, mechanical expansion leads to the creation of pores, which vary widely in fault zones, depending on rock type and rheology, stress state, displacement and so on. Many scholars have carried out research on rock permeability. Oda et al. (

For this reason, this paper adopts the method that from low pressure to high pressure environment to conduct experimental research on non-steady-state method of high-pressure helium gas permeability measurement for the first time, and further discusses the slippage effect and effective stress of fractured tight sandstone reservoir permeability comprehensive control effect, which provides a new understanding for the study of mechanism of fractured tight sandstone gas reservoir permeability change. At the same time, it also has a certain practical guiding significance for predicting and improving gas reservoir of the gas production.

The samples were derived from the ultra-deep fractured tight sandstone gas layer which depths 4,525 m in the Wongfu Mine area of Guizhou province, and the samples were cylindrical samples with a length of 100 mm and a diameter of 50 mm. There were 10 groups, which were washed with oil and dried at 150°C. The experimental sample is shown in

Sandstone sample.

Sample parameters and number.

1# | 100.52 | 50.00 | 6# | 100.86 | 50.35 |

2# | 100.96 | 49.89 | 7# | 100.48 | 50.75 |

3# | 100.90 | 50.13 | 8# | 100.66 | 50.23 |

4# | 100.33 | 50.01 | 9# | 100.12 | 50.78 |

5# | 100.01 | 50.45 | 10# | 100.63 | 50.33 |

Permeability is an inherent property of porous media materials, and it is a mark to measure the difficulty of sandstone gas flow. Measuring sandstone permeability has important guiding significance for the development of tight sandstone gas. Permeability is evaluated based on laminar flow state and hydraulic head. The flow rate at a point is defined as the volume of fluid(q) passing through the unit area per unit time (A) and is proportional to the pore pressure gradient at that point. The effect of pressure gradients in samples is known, and it is difficult to measure the gradients under triaxial test conditions, especially to express this change with an equation. Therefore, the permeability of hydraulic coefficient should be taken into cautiously consideration. This paper uses the Darcy formula to measure permeability using the steady state method, that is:

q is the flow through the rock at a pressure difference of Δp, cm^{3}/s; A is the cross-sectional area of the rock, cm^{2}; l is the contact length between the rocks, cm; μ is the viscosity of the fluid passing through the rock, mPa·s; Δp is the pressure difference before and after the fluid passes through the rock, atm; K is the proportionality coefficient, which is called the absolute permeability of the porous medium.

The sandstone permeability test experimental simulation model is shown in

Simulation process.

For

Comparing Equations (1) with (3), the following equation can be obtained:

Considering

This experiment mainly uses a nitrogen bottle, a pressure gauge, an intermediate container, a core holder, and a flow meter to measure the gas permeability of a sandstone sample. The experimental flow chart is shown in

K is the gas permeability, μm^{2}; Q_{0} is the gas flow at the core outlet end, cm^{3}/s; L is the core length, cm; A is the cross-sectional area of the core, cm^{2}; P_{0} is the absolute atmospheric pressure, MPa; P_{1} is the absolute core inlet end Pressure, MPa; P_{2} is the absolute pressure at the outlet end of the core, MPa; μ is the viscosity of the gas at the experimental temperature and atmospheric pressure, mPa·s.

Flow chart of permeability measurement experiment.

The response surface method uses a series of deterministic experiments to approximate the implicit limit state function with a polynomial function, which can greatly reduce the number of experiments, improve the efficiency of the experiment, and explore the relationship between multiple input variables and dependent variables. In this paper, Design-Expert 8.0.6 software is used to compare and analyze the change law of gas permeability under the interaction of water pressure, effective stress, confining pressure, and internal pressure.

As is shown in the

Surface of permeability (10^{−3} μm^{2}) vs. effective pressure and water pressure.

It can be seen from

Surface of permeability (10^{−3} μm^{2}) vs. effective pressure and confining pressure.

Surface of permeability (10^{−3} μm^{2}) vs. water pressure and confining pressure.

Surface of permeability (10^{−3}μm^{2}) vs. effective pressure and internal pressure.

It can be seen from

Surface of permeability (10^{−3} μm^{2}) vs. confining pressure and internal pressure.

In this study, a coupling model was used to research the effect of water pressure on gas permeability. The model provides information on the effects of pressure and water pressure changes on permeability of fractured sandstone (

Surface of permeability (10^{−3} μm^{2}) vs. water pressure and internal pressure.

An ultra-deep fractured tight sandstone gas layer from Wengfu mine in Guizhou province was selected for experimental design and study, and the effects of confining pressure, internal pressure, effective stress, and water pressure on gas permeability were investigated. The results show that in the fractured tight sandstone reservoir, keeping the confining pressure constant and constantly increasing the internal pressure, the effective stress and slippage effect compete with each other. At the beginning of the experiment, the gas slippage effect was strong while the rock stress sensitivity was relatively weak, and the gas permeability gradually decreased with the increase of internal pressure. When crossing the low point of permeability, the effective stress is stronger than slippage effect, and the gas permeability increases with the increase of internal pressure. The greater the confining pressure is, the greater the internal pressure needed to reach the low permeability point will be. With axial stress increasing, horizontal contact hydraulic hole increasingly closed. Horizontal hydraulic holes are more sensitive to axial stress than vertical ones. The damage which caused by the effective stress changing from large to small to the pore structure is irreversible, and the loss of gas permeability is large. However, the effective stress changes from small to large, and the destruction of pore structure is progressive, the loss of gas permeability is relatively small.

The datasets generated for this study are available on request to the corresponding author.

BY designed the tests and wrote the paper. JX performed the tests and processed the data. SS provided funds and experimental apparatus. HLiu performed the lab tests and optimized the experimental scheme. YL collected samples. HLi was responsible for contacting the journal editor.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

The authors gratefully acknowledge the support from the National Natural Science Foundation of China (51704110) and the Key Scientific Research Project of Hunan Education Department (18A183). The reviewers are gratefully acknowledged for their valuable comments on the manuscript.