Comparison of Chemical-Induced Fracturing by Na₂S₂O₈, NaClO, and H₂O₂ in Marcellus Shale

Abstract

Shale−oxidant reactions can enhance the permeability of organic-rich shale reservoirs through chemical-induced fracturing. Choosing the best oxidant for this purpose is explored in this study. To compare the effects of chemical-induced fracturing with sodium persulfate (Na 2 S 2 O 8 ), sodium hypochlorite (NaClO), and hydrogen peroxide (H 2 O 2 ) on organic-rich shale, Marcellus shale samples were used to conduct spontaneous imbibition experiments in three oxidants' aqueous solutions under no confining pressure. The mass loss of samples after oxidant imbibition, the pH change of the oxidants' aqueous solutions, the bubble density, the morphology of chemical-induced cracks, and the mineral composition alteration before and after Na 2 S 2 O 8 treatment were analyzed and discussed to evaluate the effects of three oxidants on chemical-induced cracking. Results show that the samples treated by Na 2 S 2 O 8 had the highest mass loss, while the samples treated by H 2 O 2 had the lowest mass loss. pH change results and bubble density results indicate that the rate of the chemical reactions between Marcellus shale and Na 2 S 2 O 8 or H 2 O 2 aqueous solutions during the first 24 h was the highest compared to that during the rest of the period. After 24 h, the rate of the chemical reactions decreased and then became flat until the end of the imbibition tests. Pervasive multiscale cracks formed in response to acidic and oxidative dissolutions in Na 2 S 2 O 8 imbibition tests. Although induced cracks were initiated on the surfaces of the samples treated by NaClO aqueous solutions, Fe(OH) 3 precipitates plugged the pores and prevented the cracks from developing into a favorable dimension. XRD results show that the contents of calcite, pyrite, and illite decreased after Na 2 S 2 O 8 treatment, where calcite had the largest decrease. Gypsum crystallization occurred during Na 2 S 2 O 8 imbibition. Na 2 S 2 O 8 was evaluated as the optimal oxidant among three for the initiation and propagation of chemical-induced cracks. Three concentrations (6, 12, and 15 wt %) of Na 2 S 2 O 8 solutions effectively caused chemical-induced cracks in samples, and the difference in concentrations had no significant difference in the fracturing effect. The synergy of chemical reactions between oxidant shale samples and the generated swelling stress due to crystallization contributes to crack development.

Study on Formation Damage of Fracturing and Its Prevention through an Environmentally Friendly Method of Oxidative Stimulation in Shale Gas Reservoirs

“…Organic matter and pyrite are easily oxidized by different acidic or alkaline oxidants such as KMnO 4 , H 2 O 2 , and NaClO. The dissolution capacity of different oxidants in shale was in the order Na 2 S 2 O 8 > NaClO > H 2 O 2 . Strong oxidants such as (NH 4 ) 2 S 2 O 8 and (Na 2 S 2 O 8 ) are widely used in the decomposition of gelling agents in shale hydraulic fracturing, but the main mechanism of interaction between persulfate and shale is calcium sulfate crystals replacing the metasomatism of calcite and dolomite particles .…”

“…The core plug sample (Table 2) used in the nuclear magnetic resonance (NMR) test was drilled from the unweathering outcrop sample, and some rock fragments were obtained during the plug preparations. 18 Strong oxidants such as (NH 4 ) 2 S 2 O 8 and (Na 2 S 2 O 8 ) are widely used in the decomposition of gelling agents in shale hydraulic fracturing, but the main mechanism of interaction between persulfate and shale is calcium sulfate crystals replacing the metasomatism of calcite and dolomite particles. 19 The stimulation of (NH 4 ) 2 S 2 O 8 will produce significant carbonate precipitation and abundant gypsum precipitation, with a limited oxidation effect with organic matter.…”

“…Cheng et al confirmed that oxidative stimulation plays a positive role in improving the permeability of shale matrix, protecting and improving the conductivity of unsupported fractures under effective stress, delaying the rapid decline of gas well production, and improving shale gas recovery . Liang et al also found that the selective dissolution of the fracture surface by chemicals can increase the width of unsupported fracture and roughness of the fracture surface, so as to maintain high fracture conductivity after fracturing for a long time …”

Experimental and Numerical Studies on Oxidation–Acidification Interactions that Dominate Shale Stimulation with Persulfate

“…Therefore, chemical reactions are inevitably enhanced after persulfate addition, making the shale–persulfate interaction a critical geochemical process during hydraulic fracturing. There is also research studying potential alterations of the matrix by oxidation of persulfate and evaluating the effects of oxidative dissolution on shale conductivity. − Influencing factors such as an oxidant type, concentration, and shale composition have been compared. , Persulfate is preferred because more fractures result from acidification and crystallization of gypsum compared with thermal decomposition by hydrogen peroxide and secondary precipitation with alkaline sodium hypochlorite. ,, …”

Laboratory investigation on slip transition of rock fractures

“…It has been demonstrated that the fracture frictional stability and frictional strength are correlated with rock properties, and the rock microstructure and components can largely affect the fracture slip behaviors ranging from regular earthquakes to slow slip and creep Collettini et al, 2011;Fang, et al, 2018). Abundant laboratory experiments and numerical simulations are performed to study the impact of chemical alternation on the physic-mechanical properties of various rocks, such as the effect of chemical solutions on the frictional properties of a quartz-rich sandstone (Feucht and Logan, 1990) and rock fracturing (Karfakis and Akram, 1993;Liang and Sheng, 2020), chemically assisted crack and comminution in frictional deformation (Dunning, et al, 1994), evolution of crack permeability in fluid-rock interaction (Sausse et al, 2001), chemical induced micro-fracturing evolution under real-time computerized tomography experiments (Feng et al, 2004), the compaction creep of sands over time in the chemical environment (Brzesowsky et al, 2014), chemically and mechanically mediated influences on the mechanical and transport characteristics of fractured rock using a fracture aperture model (Min et al, 2009). Despite these progresses, the impact of physic-mechanical properties of rock on the fracture slip stability (e.g., the bifurcation near the stability transition) remains poorly understood.…”