Oxidative dissolution kinetics of organic-rich shale by hydrogen peroxide (H₂O₂) and its positive effects on improving fracture conductivity

Highlights

• Oxidative dissolution mechanism of organic matter in shale samples is further revealed from the molecular structure.

• Reaction kinetic and activation energy equations of organic-rich shale oxidation by peroxide hydrogen are established.

• The mechanism of oxidative dissolution improving the fracture conductivity is revealed.

• It is the first attempt at illustrating the potential of oxidation improving shale unpropped fracture conductivity..

Abstract

Maintaining the fracture network created by hydraulic fracturing remains challenging because unpropped fractures close during production, resulting in shale gas production decline. Chemical dissolution is being implemented to maintain unpropped fracture conductivity, and the effect of oxidative dissolution on organic-rich shale may have the potential to maintain these fractures. In this paper, the change of organic matter (OM) in crushed shale samples exposed to hydrogen peroxide (H₂O₂) or deionized water were characterized. Reaction kinetics experiments were run using sliced shale samples and H₂O₂ with a mass concentration of 2–10% at temperature of 40–80 °C. Stress sensitivity of fractured shale plugs after oxidation for 72 h was evaluated. Results show H₂O₂ can remove 56.87% of solid OM and 55.34% of extractable OM in the crushed sample. Total organic carbon in H₂O₂ filtrate after the reaction is 91.36 mg/L,while that of deionized water is 30.88 mg/L. A significant decrease of C–O according to the C1s and O1s spectra of the crushed samples after oxidation means that some of oxygen contained functional groups oxidized and cleaved. However, reaction rate of oxidative dissolution occurs slowly and significantly depends on the concentration and temperature. The relationship between reaction rate and concentration can be described as a power function, and the relationship between reaction rate and temperature can be described as an exponential function. Activation energy of oxidative dissolution ranges from 3.51 to 12.10 kJ/mol when H₂O₂ mass concentration ranges from 2% to 10%. Oxidative dissolution can increase fracture width judging by estimation based on the mass loss. Stress sensitivity coefficient of the plugs treated with deionized water are 0.52 and 0.59, while the plugs treated with H₂O₂ are 0.48 and 0.52. It indicates that oxidative fluids can play a role in maintaining unpropped fracture conductivity enhancing shale gas recovery.

Introduction

China has the world's largest shale gas reserves estimated at ~134 trillion m³ (EIA, 2013). There are 13 prioritized provinces for shale gas exploitation to satisfy growing energy demands (Qiu, 2011). Shale gas reservoirs are characteristically ultra-low porosity and permeability formations (Loucks et al., 2009), and their successful exploitation has relied on a combination of horizontal drilling, multi-stage hydraulic fracturing completions, innovative fracturing, and other stimulation methods. (Warpinski et al., 2009). Due to complex geological conditions and current hydraulic fracturing techniques, the demand for fracturing fluid, which contains over 90% freshwater, ranges from 10–24 × 10³ m³ per well (Wang et al., 2013a, b). Unfortunately, a large volume of the fracturing fluid (over 70%) is still retained in the subsurface after flowback operation by the positive pressure differential and spontaneous imbibition during hydraulic fracturing due to a subcritical water saturation and high capillary pressure in the shale (Makhanov et al., 2014; Ghanbari and Dehghanpour, 2015).

Interestingly, an abnormal phenomenon has occurred in some shale gas wells from the Horn River basin in Canada where low (or high) flowback efficiency corresponds to a high (or low) early gas production (Ghanbari and Dehghanpour, 2016). Some previous publications insist that this may be caused by a beneficial effect of fracturing fluid retention during shut-in on gas transport, especially such as microfracture creation and propagation improving the gas transport capacity due to water-rock interaction (Roychaudhuri et al., 2011; Dehghanpour et al., 2013; Sun et al., 2015; Xue et al., 2018). Clay hydration in shale inducing hydration forces is regarded as a primary mechanism of microfracture generation (Kang et al., 2017; Qian et al., 2017), and anisotropic stress conditions compared with isotropic stress conditions can play a more positive role in inducing microfractures with the help of hydration (Liu and Sheng, 2019). However, both in the Eagle Ford Shale and the Mancos Shale from the United States, a high confining pressure usually results in a decrease in fracture width and even eventually closure, and the closure of the fractures is more severe with increased content of swelling clays (Zhang and Sheng, 2017). The generation of water-induced microfractures based on the mechanical mechanism under the formation conditions has therefore always been a controversial topic. The fracture network created by hydraulic fracture consists of propped fractures and unpropped fractures, and the reduction of unpropped fracture aperture with production time typically results in a decline in gas production (Deng et al., 2013; Wu and Sharma, 2017). Therefore, the phenomenon occurring may be caused by the large volume of fracturing fluids retained contributing to maintaining unpropped fractures open therefore promoting high early gas production (Ehlig-Economides and Economides, 2011). This indicates that methods need to be developed to maintain open apertures under formation conditions during the production to enhance shale gas recovery.

Recent studies have focused on chemical-mechanical coupled action stimulation on shale reservoirs. For calcite-rich shale samples, diluted acid has a significant effect on improving matrix permeability, but causes a reduction in the effective permeability for unpropped fractures due to the decrease of fracture roughness and fracture width at formation conditions (Pournik and Tripathi, 2014; Teklu et al., 2019). Of course, some shale microfractures' conductivity can be maintained or improved under closure stress because of non-uniform etching of the “fracture faces” (Wu and Sharma, 2017), or results from a change in shale's porosity due to carbonates dissolution or development of cracks (Harris, 2017). Unfortunately, the dilute acid imbibed into the clay-rich or calcite-poor shale may also impair matrix permeability (Teklu et al., 2017&2019). Organic-rich shale commonly is rich in clay but has a low calcite content, thus dilute acid may not work well on improving porosity and permeability in these systems. You et al. (2017) proposed that an oxidative burst in organic-rich shale reservoirs by oxidative fluids can have a significant effect in enhancing shale gas recovery. Oxidative dissolution of organic-rich shale components like organic matter (OM), pyrite and carbonate minerals, such as calcite and dolomite, can generate considerable dissolution pores and induced microfractures (Li et al., 2016; Chen et al., 2017; Xu et al., 2018; Mahoney et al., 2019). This also influences water imbibition behaviors due to a change in the imbibition path, correlating to an enhancement of shale porosity and permeability (You et al., 2018). Furthermore, oxidative dissolution of ductile and elastomeric OM may play an essential role in minimizing proppant embedment and improving fracture face conductivity (permeability) in maintaining long-term well productivity (Hull et al., 2019). However, previous studies did not take the influence of temperature and concentration of oxidative fluids on the oxidative dissolution into account, and the reaction kinetics between the organic-rich shale and oxidative fluid, which can characterize the process resulting in induced-pores and microfractures, have not been entirely determined. Moreover, the impact of oxidative dissolution on the conductivity of unpropped fracture of organic-rich shale is not clear.

In this study, the Longmaxi (LMX) organic-rich shale from the Sichuan Basin was used to determine changes in the reaction rate, fracture width, and stress sensitivity of fracture permeability using hydrogen peroxide (H₂O₂). The mass concentration of H₂O₂ and experimental temperatures were varied to determine reaction rates at various conditions. These results are then applied to characterize the chemical reaction kinetic behaviors of shale oxidation and to reveal the effect on improving unpropped fracture conductivity in organic-rich shale using H₂O₂. Finally, we also discussed the implication of organic-rich shale oxidation for developing higher performance fracturing fluid, optimizing the conductivity of unpropped fracture network created by hydraulic fracturing, and enhancing shale gas recovery.