Elsevier

Engineering Fracture Mechanics

Volume 194, 1 May 2018, Pages 117-135
Engineering Fracture Mechanics

Investigation of effects of fracturing fluid on hydraulic fracturing and fracture permeability of reservoir rocks: An experimental study using water and foam fracturing

https://doi.org/10.1016/j.engfracmech.2018.03.009Get rights and content

Highlights

  • Enhanced gas production from deep tight geological formations.

  • Foam based fracturing fluids has better fracturability compared to water based fluid.

  • High pressure triaxial experiments were conducted.

  • The results indicate that foam fracturing can induce a complex twisted fracture with greater surface area.

Abstract

Hydraulic fracturing is a promising technique to enhance gas production from deep geological formations by enhancing reservoir rock permeability. However, permeability enhancement may vary with reservoir conditions and fracturing fluid. The main objective of this study is to investigate the effect of fracturing fluid on the fracture pattern and permeability of fractured rock. Fracturing experiments were conducted on siltstone specimens with water and foam (N2 + water) as the fracturing fluids, using a high-pressure triaxial apparatus. CT scanning and 3-D scanning technology were used to examine the fracture pattern and the fracture surface of the fractured specimens. The fractured specimens were then used to conduct permeability experiments for five different confining pressures (5 – 25 MPa) and six different injection pressures (1 – 6 MPa) with N2, and fracture permeability values were compared, based on the fracturing fluid type as well as the intact rock permeability. The results indicate that foam fracturing can induce a complex twisted fracture with greater surface area and the fracture is throughout the specimen. Moreover, foam fracturing emits more energy than water fracturing during the fracturing process, which is favourable for secondary fracture development. According to the measured permeability values, the fracture permeability of foam fractured specimens is about 5 × 105 times greater than that of intact and water-fractured specimens. With increasing confining pressure, N2 permeability decreases, regardless of the specimen (intact or fractured), and interestingly permeability decreases with increasing injection pressure. These observations are related to the effective stress principle and the Klinkenberg effect.

Introduction

Natural gas extraction from deep geological formations has been identified as an important solution to the world energy crisis. Hydraulic fracturing is one of the most popular methods to fracture deep geological rock formations to enhance their permeability. Therefore, hydraulic fracturing has been identified as a promising technique to enhance reservoir permeability in almost impermeable rocks such as shale and siltstone [7], [8], [12], [39], [3]. The key elements in hydraulic fracturing are the fracturing fluid and the proppants [26]. The fracturing fluid is the medium used to generate the pressure in the reservoir rock via a wellbore. Conventionally vertical wells were used and research has shown that the use of horizontal wells is more economical and efficient than vertical wells [32], [24]. Therefore, after vertical well drilling, the use of horizontal wells within the impermeable rock layer is the current practice. Proppants are small particles about 1 to 5 mm in diameter which are used to prop open generated fractures after releasing the fracturing fluid pressure. The selection of the fracturing fluid depends on the associated costs as well as the type of reservoir. Water, gas (CO2, N2), foams (liquid + gas), energized liquids, and acids are some common fracturing fluids, each of which has different advantages and disadvantages [35]. Moreover, the fracturing fluid type is a key variable in reservoir breakdown pressure and reservoir permeability after fracturing. Therefore, to ensure maximum gas production from an optimum fracture network, a clear understanding of the influence of fracturing fluid on the hydraulic fracturing and fracture permeability of reservoir rock is required.

Rock testing for hydraulic fracturing characterisation in engineering is typically carried out for almost impermeable rocks using water as the fracturing fluid. The direct use of these hydraulic fracturing properties (breakdown pressure, energy release, etc.) for other fracturing fluids would constitute a significant error in the well stimulation process. As an example, foam-based fracturing fluids have been used in the oil and gas industry, with the use of various types of foams. However, the behaviour of foam-based fracturing is completely different from the conventional water fracturing. Gu and Mohanty [13], have been studied the effect of foam quality on field scale fracturing. However, the enhancement in permeability was not directly quantified. In addition, the two-phase nature of foam and high compressibility are identified as major issues in field-scale foam fracturing [35]. Furthermore, rock testing for fracture permeability in petroleum engineering is typically carried out for mechanically-fractured rock specimens. Direct use of these permeability values to estimate gas production from hydraulic fracturing would constitute a significant error in well production. Fracture permeability must be calculated for hydraulically-fractured specimens to account for gas production through hydraulic fracturing well stimulation.

The main objectives of this study are to investigate the effect of fracturing fluid on hydraulic fracturing characteristics, including breakdown pressure and energy release in siltstone and to evaluate the fracture permeability of hydraulically-fractured siltstone specimens. Hydraulic fracturing experiments were conducted to study the effect of fracturing fluid on breakdown pressure and energy release, using water and foam (water + N2) as the fracturing fluids. Permeability experiments were then conducted to study the effect of injection pressure and depth of reservoir, using N2 for hydraulically fractured specimens using water and foams as the fracturing fluid. Since the use of foam-based fluids for hydraulic fracturing is a recent technique, this study provides a comprehensive comparison of foam fracturing and water fracturing and the corresponding fracture permeability in siltstone reservoirs.

Section snippets

Previous work

Hydraulic fracturing is a technique which is used to enhance reservoir permeability in almost impermeable reservoirs. In this technique, fracturing fluids are injected under high pressure into the wellbore. In the gas extraction industry, water is a popular fracturing fluid and fracturing with water has therefore been widely studied [22], [16]. Matsunaga et al. [22] explored the use of water and oil as fracturing fluids on granite, marble and andesite specimens, and found that hydraulic

Specimen preparation

The siltstone specimens in this study were collected from an outcrop in the Eidsvold siltstone formation in Queensland, Australia, which was formed in the Triassic, Jurassic and Cretaceous periods. The location of the formation is shown in Fig. 1. According to scanning electron microscope (SEM) observations, the siltstone is nearly homogeneous with a 0.01 – 0.05 mm grain size distribution. Energy dispersive X-ray (EDX) analysis shows a presence of high amounts of Si, O, Al and Ti within the

Effect of fracturing fluid on breakdown pressure and AEs

All the fracturing experiments were conducted by injecting fluids at a constant flowrate of 10 ml/min. Foam and water were used as the fracturing fluids and tests were duplicated to confirm the results. Table 4 shows the breakdown pressures and fracture orientations for the duplicated specimens. Although the experiments were conducted under the same stress conditions, temperature and fluid injection rate, the water- and foam-fractured specimens fractured at different pressures due to the

Conclusion

Hydraulic fracturing is a promising technique to enhance reservoir permeability in deep geological formations which may enhance gas production. The selection of the most suitable fracturing fluid is a governing fact for the effectiveness of the fracturing process, which can alter the fracture pattern, the energy release and the reservoir permeability. Therefore, in this study, water and foam fracturing experiments were conducted on siltstone specimens to investigate the effect of fracturing

References (40)

  • M.S.A. Perera et al.

    The effects of sub-critical and super-critical carbon dioxide adsorption-induced coal matrix swelling on the permeability of naturally fractured black coal

    Energy

    (2011)
  • A. Qajar et al.

    Modeling fracture propagation and cleanup for dry nanoparticle-stabilized-foam fracturing fluids

    J Petrol Sci Eng

    (2016)
  • A. Abou-Sayed et al.

    In situ stress determination by hydrofracturing: a fracture mechanics approach

    J Geophys Res Solid Earth

    (1978)
  • Y. Chen et al.

    Observations of fractures induced by hydraulic fracturing in anisotropic granite

    Rock Mech Rock Eng

    (2015)
  • Y. Cho et al.

    Pressure-dependent natural-fracture permeability in shale and its effect on shale-gas well production

    SPE Reservoir Eval Eng

    (2013)
  • A. Farough et al.

    Evolution of fracture permeability of ultramafic rocks undergoing serpentinization at hydrothermal conditions: an experimental study

    Geochem Geophys Geosyst

    (2016)
  • Y.A. Fialko et al.

    Numerical simulation of high-pressure rock tensile fracture experiments: evidence of an increase in fracture energy with pressure?

    J Geophys Res Solid Earth

    (1997)
  • K.B. Gregory et al.

    Water management challenges associated with the production of shale gas by hydraulic fracturing

    Elements

    (2011)
  • GUO F, MORGENSTERN N, SCOTT J. Interpretation of hydraulic fracturing breakdown pressure. International journal of rock...
  • HAIMSON B. Hydraulic fracturing in porous and nonporous rock and its potential for determining in-situ stresses at...
  • Cited by (0)

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