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A Study of Variation in the Initiation Pressure and Fracture Distribution Patterns of Raw Coal in SC-CO2 Fracturing Under the True Tri-axial System

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Abstract

Supercritical CO2 (SC-CO2) fracturing is a water-less fracturing technology that has attracted increasing attention in the coalbed methane mining industry. However, the mechanisms of fracture initiation and propagation have not been studied in detail. In this study, SC-CO2 and hydraulic fracturing of coal was conducted using a true tri-axial testing system. The change in injection pressure and in fracture morphology, both before and after fracturing, was analyzed and results presented. In comparison to hydraulic fracturing, using SC-CO2 as a fracturing fluid reduces the pressure required to initiate fractures by approximately 32–41%. This reduction is due to the increased percolation and pore pressure effects arising from the use of SC-CO2. The low viscosity and high diffusivity of SC-CO2 in conjunction with the highly developed pore/fracture of coal result in the formation of a complex fracture network. The fractal dimension is positively correlated with the injection rate and temperature during SC-CO2 fracturing, indicating that using SC-CO2 as fracture fluid under high injection flows and temperature can achieve increased Coalbed methane production.

Highlights

  • SC-CO2 and hydraulic fracturing experiments were conducted on coal utilizing a true tri-axial testing system.

  • The variation law in the initiation pressure of coal in SC-CO2 and hydraulic were obtained.

  • 3D reconstruction were performed to study the internal fracture distribution patterns of the specimens.

  • Based on the unique physicochemical properties of SC-CO2 and its interaction with coal, the formation mechanism of fracture network was revealed.

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Abbreviations

\(\sigma_{{\text{H}}} ,\sigma_{{\text{h}}}\) :

The maximum and minimum horizontal principal stresses (MPa)

\(\sigma_{{\text{v}}}\) :

The vertical principal stress (MPa)

\(\sigma_{{\text{t}}}\) :

The tensile strength of coal (MPa)

\(\sigma_{{\text{n}}}\) :

The normal stress acting on the natural fracture (MPa)

\(P_{{\text{w}}}\) :

The water pressure in the borehole (MPa)

\(P_{{\text{s}}}\) :

The SC-CO2 pressure in the borehole (MPa)

\(P_{{\text{g}}}\) :

The initial pore pressure of coal (MPa)

\(P_{{\text{i}}}\) :

The fluid pressure at the intersection (MPa)

\(P_{{{\text{net}}}}\) :

The net pressure during construction in wellbore (MPa)

\(\Delta P_{{{\text{nf}}}}\) :

The net pressure (MPa), that is, effective pressure that directly acts on the natural fracture which causes it to expand

\(P_{1}\) :

The injection pressure (MPa)

\(P_{2}\) :

The outlet pressure (MPa)

\(k\) :

The permeability of coal (mD)

\(k_{{\text{s}}}\) :

The permeability of coal to SC-CO2 (mD)

\(k_{{\text{w}}}\) :

The permeability of coal to water (mD)

\(q\) :

The gas flow rate (m3/s)

\(\mu\) :

The dynamic viscosity of gas (Pa s)

\(\mu_{{\text{w}}}\) :

The dynamic viscosity of water (Pa s)

\(\mu_{{\text{s}}}\) :

The dynamic viscosity of SC-CO2 (Pa s)

\(L\) :

The length of specimen (m)

\(\kappa\) :

The Boltzmann’s constant (J/K)

\(T\) :

The temperature (K)

\(A\) :

The specimen cross section area (m2)

\(v\) :

The diffusion speed of fluid in coal (m/s)

\(v_{{\text{s}}}\) :

The diffusion speed of SC-CO2 in coal (m/s)

\(v_{{\text{w}}}\) :

The diffusion speed of water in coal (m/s)

\(R\) :

The diameter of the borehole (m)

\(r\) :

The distance away from the fracturing borehole axis (m)

\(r_{{\text{a}}}\) :

The pore radius, m

\(\Delta h\) :

The hydraulic gradient

\(w\) :

The equivalent hydraulic aperture (m)

\(\theta\) :

The angle between \(\sigma_{{\text{r}}}\) and \(\sigma_{{\text{x}}}\)(°)

\(\nu\) :

The dimensionless Poisson’s ratio of the coal

\(\varepsilon\) :

The dimensionless Biot’s porous elastic coefficient

\(\phi\) :

The porosity of coal (%)

\(N(\delta_{m} )\) :

The number of effective grids that cover the fracture

\(\delta_{m}\) :

The side length of the mth covering grid

\(m\) :

The mth covering

\(D_{{\text{F}}}\) :

The fracture fractal dimension

\(c\) :

The dimensionless constant

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Acknowledgements

This study was financially supported by the National Natural Science Foundation of China (51774055, 51804050), the Natural Science Foundation of Chongqing, China (cstc2019jcyj-bshX0041), the Postdoctoral Science Foundation Project Funded by State Key Laboratory of Coal Mine Disaster Dynamics and Control (2011DA105287-BH201908).

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Authors and Affiliations

  1. State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University, Chongqing, 400044, China

    Yiyu Lu, Jingwei Zheng, Zhaolong Ge, Zhe Zhou, Haoming Wang & Liang Zhang

  2. School of Resources and Safety Engineering, Chongqing University, Chongqing, 400044, China

    Yiyu Lu, Jingwei Zheng, Zhaolong Ge, Zhe Zhou, Haoming Wang & Liang Zhang

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  1. Yiyu Lu
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  2. Jingwei Zheng
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  3. Zhaolong Ge
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  4. Zhe Zhou
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  5. Haoming Wang
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  6. Liang Zhang
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Contributions

YYL and JWZ conceived and designed the experiments; JWZ, ZZ and HMW performed the experiments; JWZ and ZLG analyzed the data; JWZ and ZLG contributed materials and analysis tools; YYL, JWZ and ZLG wrote the paper.

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Correspondence to Zhaolong Ge.

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Lu, Y., Zheng, J., Ge, Z. et al. A Study of Variation in the Initiation Pressure and Fracture Distribution Patterns of Raw Coal in SC-CO2 Fracturing Under the True Tri-axial System. Rock Mech Rock Eng 55, 3425–3438 (2022). https://doi.org/10.1007/s00603-022-02800-3

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