Permeability evolution of shale under anisotropic true triaxial stress conditions

doi:10.1016/j.coal.2016.08.017

International Journal of Coal Geology

Volume 165, 1 August 2016, Pages 142-148

https://doi.org/10.1016/j.coal.2016.08.017 Get rights and content

Highlights

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Permeability evolution of shale under anisotropic true triaxial stress conditions
•
A permeability model with considerations of the effects of principal stresses
•
Shale permeability under true triaxial stress is matched accurately by the model.

Abstract

Although permeability data for shale have been reported previously and permeability models have been proposed, studies on permeability for shale under anisotropic true triaxial stress conditions are lacking. In this work, the permeability of shale under true triaxial stress conditions was measured using a newly developed true triaxial geophysical apparatus. The permeability always decreased with increasing principal stress. The permeability variations induced by varying each principal stress were distinct. The greatest permeability reductions were observed with increasing stress normal to the bedding planes. Based on a consideration of shale anisotropy, a permeability model was established to be applicable to shale under anisotropic true triaxial stress conditions (TTP model). This model was verified by matching the permeability data in our experiments. It was found that a more accurate match could be obtained using the TTP model, which showed advantages in reflecting the permeability changes related to increments in each principal stress.

Introduction

Shale is usually considered to be a naturally fractured, dual-porosity material with its fractures providing the main pathway for fluid flow. Fluid transport in fractured shale is generally described using Darcy's law. Permeability is an intrinsic component of Darcy's law when quantifying the fluid flow in reservoir rock (Yang and Aplin, 2007, Karacan et al., 2011, Pan and Connell, 2012).

The permeability of reservoir rocks under conventional triaxial stress conditions have been studied. Liu et al. (2015) investigated the permeability evolution of a coal sample over time under conventional triaxial stress conditions and proposed a novel permeability model. Cai et al. (2014) explored the contribution of the interactions between stress and damage to the evolution of permeability through X-ray computed tomography images and acoustic emission profiling, together with the concurrent measurement of the P-wave velocity. The mining-enhanced permeability of coal has been tested under conventional triaxial stress conditions (Yin et al., 2015). Some researchers studied the shale permeability of different samples. Soeder (1988) measured the gas permeability of shale samples from the Appalachian basin. Based on the assumption of a sheet-type fracture structure, a fracture aperture based permeability model was proposed. Ohmyoung et al. (2001) measured the permeability as a function of the effective pressure for bedding-parallel flow in illite-rich shale from the Wilcox formation. In their study, as the effective pressure was increased from 3 to 12 MPa, the permeability decreased from 300 × 10^− 21 m² to 3 × 10^− 21 m². In addition, it was shown that the permeability was dependent on the flow direction relative to the bedding (Ohmyoung et al., 2004). Ross and Bustin (2008) and Bustin et al. (2008) tested shale from the Western Canada sedimentary basin producing a measured permeability of less than 0.04 × 10^− 15 m² for the entire sample suite, also showing an exponential decrease in permeability with increasing effective stress. The investigation by Dong et al. (2010) showed that the permeability of shale is highly sensitive to stress. Several empirical correlations have been proposed to describe the experimental data (Shi and Durucan, 2004, Shi and Durucan, 2010, Chen et al., 2015).

The permeability of shale is strongly anisotropic with the presence of bedding (Ohmyoung et al., 2004, Ghanizadeh et al., 2014). Some researchers used cubic rock samples to measure the anisotropy of the permeability. King (2002) developed a polyaxial stress loading system, which is designed to measure the fluid permeability and elastic properties for 51 mm-side cubic rock specimens. The pore pressure can be varied in the range 0–3 MPa. The sides of a cubic rock sample were sealed by magnesium plates, while the edge of the sample was chamfered and sealed by silicone rubber. Massarotto et al., 2001, Massarotto et al., 2003 developed a true triaxial rig to measure the anisotropic permeability of cubic coal samples. The apparatus can apply three individual and mutually-orthogonal stresses to a specimen. Pan et al. (2015) designed a membrane to hold cubic rock samples. The membrane and cubic sample assembly forms a standard-sized cylindrical core sample, allowing it to be installed in any triaxial rig for gas permeability measurement without modifying the rig. It was used to measure the anisotropy of shale permeability. An advanced true triaxial testing system equipped with seismic and resistivity sensors was developed combined with the option of three-dimensional (3-D) permeability to evaluate the transport properties and fracture delineation of rock (Lombos et al., 2013, Nasseri et al., 2014, Goodfellow et al., 2015).

Despite shale having strong anisotropy in fracture distributions, aperture, tortuosity and connectivity, the response of anisotropic fractures in shale to each principal stress under true triaxial stress conditions is not well represented in previously published data and models. Differences in the morphology and deformation modulus of fractures along the three principal stress directions cause anisotropy in fractures. This work presents measurements of shale permeability under true triaxial stress conditions using the newly developed multi-functional true triaxial geophysical (TTG) apparatus. We then use these permeability data to verify our permeability model, which was designed to be applicable to shale under anisotropic true triaxial stress conditions (TTP model).

Section snippets

Study area and characteristics of shale

The samples used in testing were taken from outcrops of the Silurian Longmaxi formation in Shizhu, Sichuan Basin, southwest China, which are extensions of reservoirs in the Pengshui shale gas plays (Longmaxi Shale). The study area is located in the high and steep structural zones in the Eastern Sichuan Basin (Fig. 1(a)). Because of the influence of north-northeast tectonic motion, the study area contains a series of broad and gentle synclines and anticlines trending in a northeasterly

Apparatus used

The experiments were conducted using the newly developed TTG apparatus. This apparatus is capable of conducting mechanical and permeability experiments on samples under true triaxial stress conditions. The apparatus allows the continuous monitoring of stresses, strains, flow rates, fluid pressures, and acoustic emission signals in three dimensions. The apparatus is shown in Fig. 2.

The design of the internally sealed fluid flow system of the apparatus caters for the control and measurement of

Permeability evolutions of shale under anisotropic true triaxial stress conditions

Fig. 4 presents the variations of permeability as a function of the changing principal stresses. Fig. 4 reveals that there was always a decrease in permeability with increasing principal stress. Under condition 1, the permeability decreased from 1.49 × 10^− 17 m² at σ₁ = 10 MPa to 5.57 × 10^− 18 m² at σ₁ = 60 MPa; from 5.57 × 10^− 18 m² at σ₂ = 10 MPa to 5.00 × 10^− 19 m² at σ₂ = 60 MPa; and from 5.00 × 10^− 19 m² at σ₃ = 10 MPa to 3.13 × 10^− 19 m² at σ₃ = 60 MPa (Fig. 4(a)). Similar permeability evolutions were observed under condition 2

Conclusions

In this work, the permeability of shale under true triaxial stress conditions was measured using a newly developed TTG apparatus. The results showed that the permeability always decreased with increasing principal stress. The permeability variations induced by varying each principal stress were distinct. The greatest permeability reductions were observed during the increase in the stress normal to the bedding planes. Conventional triaxial stress experiments ignore the effects of σ₂ on shale

Acknowledgments and data

This study was financially supported by National Natural Science Foundation of China (51434003, 51374256) and Chongqing Research Program of Application Foundation and Advanced Technology (CSTC2015JCYJBX0076).

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Permeability evolution of shale under anisotropic true triaxial stress conditions

Highlights

Abstract

Introduction

Section snippets

Study area and characteristics of shale

Apparatus used

Permeability evolutions of shale under anisotropic true triaxial stress conditions

Conclusions

Acknowledgments and data

Int. J. Coal Geol.

Fuel

Int. J. Rock Mech. Min. Sci.

Int. J. Coal Geol.

Int. J. Coal Geol.

Int. J. Rock Mech. Min. Sci.

Fuel

Int. J. Rock Mech. Min. Sci.

Int. J. Coal Geol.

Int. J. Coal Geol.

J. Nat. Gas Sci. Eng.

Int. J. Rock Mech. Min. Sci.

Impact of Shale Properties on Pore Structure and Storage Characteristics

Volumetric strain associated with methane desorption and its impact on coalbed gas production from deep coal seams

AAPG Bull.

Interpretation of Fracture Spacing and Intensity