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