In Situ Sequestration of a Hydraulic Fracturing Fluid in Longmaxi Shale Gas Formation in the Sichuan Basin

Abstract

Due to high salinity and complex components including varied inorganic salts, organic agents, released heavy metals, radioactive elements and clastic particles, environmental and economical treatments of a huge amount of flowback wastewater from shale gas wells have attracted both public concerns and industrial interest. Unlike the existing methods such as deep well reinjection and recycling, this paper explores the feasibility of directly reserving an injected hydraulic fracturing (HF) fluid in a shale formation utilizing its strong capillary imbibition and permanent sequestration capacity. Fluid imbibition-flowback and nuclear magnetic resonance experiments are conducted to check the microscopic reliability: once being imbibed into shale, only less than 20% of the total fluid is driven out even under a pressure gradient of 22.1–62.6 MPa/m. Macroscopic flowback data from Longmaxi shale gas wells in the Sichuan Basin further demonstrates that by adding 10–30 days to current shut-in operations the flowback rates of a HF fluid decrease approximately 5–15%, meaning a reduction of 2000–6000 m3 wastewater per single well without extra technical procedures or additives. Moreover, fractured sample displacement experiments, simulations, and field data statistics confirm that extending shut-in time causes an overall improvement in well productivity and flowback fluid quality.

Hydraulic fracturing flowback chemical composition diversity as a factor determining possibilities of its management

“…In the Sichuan basin, the gas exploitation is conducted in four regions of the Longmaxi shale formation: Fuling, Changning, Weiyuan, and Pengshui. The thickness of the formation varies between 100 and 120 m; however, the layers rich in gas are located at the bottom of the formation, and their thickness is around 30 m. The top of the formation is located between 2000 (in the Pengshui part) and 4500 m (Yang 2019). Organic matter content is above 2% and thermal maturity above 2.2% Ro (Changbo and Pieńkowski 2015).…”

“…The scope of physicochemical analysis of flowback fluid reported in the literature is very diverse (He et al 2013;Hayes 2012;Olsson et al 2013;Yang 2019;Konieczyńska et al 2011;Konieczyńska et al 2015;Owen 2017;Owen and Bustin 2017;Liang et al 2018;Busby 2018;EA 2011;Zolfaghari et al 2015). Total dissolved solids concentration (TDS), pH, concentration of sodium, potassium, calcium, barium, strontium, iron, chloride, bromide, and sulfate are the most frequently reported in the literature.…”

“…After 6 months, the flowback volume is in the range from 20 to 50% (Kondash et al 2017 ). In Canada, the amount of flowback fluid that returns after the fracturing process is in the range from 10 to 30% of injected volume (Bustin et al 2018 ), whereas in the Sichuan Basin in China, the amount is in the range from 10 to 60% (Yang et al 2019 ). In the case of Europe, hydraulic fracturing was performed only for exploration purposes; thus, produced water was not collected.…”

“…Chinese studies concerned flowback fluids from boreholes in the Longmaxi shale formations of the Sichuan Basin (Yang et al 2019 ) and from the Tarim Basin (Liang et al 2018 )(Fig. 1 ).…”

“…Concentration of other constituents varied as follows: chloride from 13,110 to 36,470 mg/l, bicarbonate from 310 to 1263 mg/l, bromide from 180 to 470 mg/l, sulfate from 41 to 172 mg/l, sodium from 8296 to 19,830 mg/l, calcium from 564 to 3870 mg/l, potassium from 157 to 768 mg/l, barium from 138 to 412 mg/l, magnesium from 105 to 336 mg/l, and strontium from 67 to 320 mg/l. Concentrations of ammonia, aluminum, and iron were at trace level, or those species were not detected in samples of investigated flowback fluids (Yang et al 2019 ). In the Tarim Basin, samples of flowback fluid were collected from one borehole, and only fluid density, pH, and concentration of chloride were tested.…”