Laboratory True Triaxial Hydraulic Fracturing of Granite Under Six Fluid Injection Schemes and Grain-Scale Fracture Observations

Abstract

Laboratory hydraulic fracturing tests on cubic granite specimens with a side length of 100 mm were performed under true triaxial stress conditions combined with acoustic emission monitoring. Six different injection schemes were applied to investigate the influence of the injection scheme on hydraulic performance and induced seismicity during hydraulic fracturing. Three of these schemes are injection rate controlled: constant rate continuous injection (CCI), stepwise rate continuous injection (SCI), and cyclic progressive injection (CPI); the other three are pressurization rate controlled: stepwise pressurization (SP), stepwise pulse pressurization (SPP) and cyclic pulse pressurization (CPP). The test results show that the SPP scheme achieves the highest increase in injectivity among the six schemes. The CPI scheme generates the lowest induced seismicity while the improvement in injectivity is the least pronounced. The CPP scheme allows increasing injectivity and decreasing induced seismicity, and is suggested as a promising alternative injection scheme for field applications. Thin section microscopic observations of fractured specimens show that intragranular fractures splitting microcline, orthoclase and quartz grains dominate the hydraulic fractures independent of the injection scheme. The SPP scheme creates the largest fracture length, which explains the highest injectivity value among all schemes. Tests with relatively low magnitude of maximum AE amplitude correspond to short fracture length and small portions of intragranular fractures in microcline grains. Quartz grains are more fractured than microcline and orthoclase grains, and quartz chips (natural proppants) are frequently observed adjacent to hydraulic fractures. The laboratory test results show the potential for hydraulic fracture growth control in field applications by advanced fluid injection schemes, i.e. cyclic pulse pressurization of granitic rock mass.

Microscale mechanical modeling of deformable geomaterials with dynamic contacts based on the numerical manifold method

“…The microscale structure of geomaterials contains minerals, pores, and fractures of complex shapes that evolve as a result of coupled fluid, heat, mechanics, and chemical reactions. In order to understand such dynamic multiphysics problems, in the past decade, new technologies have been developed for visualization and characterization of the multiphysics processes of geomaterials at the pore scale, microscale, and even smaller scales [16,20,31,33,38].…”

Special Issue “Observations of Coupled Processes in Fractured Geological Media at Various Space and Time”

“…The Special Issue covers recent developments in the understanding of permeability evolution of fractured rock during hydro-shearing (Kakurina et al 2020;Shen et al 2020), fracturing mechanisms of fractured rock (Zhuang et al 2020;Pan et al 2020;Li et al 2020;Zhou et al 2020), effect of high and low temperature on various rock properties (Yang et al 2020;Weng et al 2020) and characterization of transport and mechanical characteristics of rock under different conditions (Suzuki et al 2020;Zhang et al 2020). Kakurina et al (2020) investigate the orientation of injection-induced fault reactivation in two in situ experiments, one performed in carbonate rock at the Rustrel Low Noise Underground Laboratory (LSBB URL), France, and another in shale rock at the Mont Terri Rock laboratory, Switzerland.…”

“…The study shows that the characterization of fracture permeability increase during shear is not straightforward and extensive research is necessary in terms of the role of normal stress, roughness and formation of fault gouge. Zhuang et al (2020) examine the injectivity enhancement and acoustic emissions when creating hydraulic fractures in cubic granite samples under six fluid injection schemes in true triaxial experiments. The stepwise pulse pressurization scheme creates the largest hydraulic fracture, providing the highest permeability increase.…”

Experimental Investigation of Hydraulic Fracturing and Stress Sensitivity of Fracture Permeability Under Changing Polyaxial Stress Conditions

“…In an attempt to reduce seismicity during hydraulic fracturing and improve efficiency, fatigue hydraulic fracturing through cyclic injection schemes has been proposed. Hofmann et al (2018), Patel et al (2017), Zhuang et al (2020Zhuang et al ( , 2019 introduced experimental and field work on cyclic injection schemes which reduce the breakdown pressure and reduce the maximum amplitude of associated acoustic emission (AE) events released by fracture generation. The final major area of research concerning fracture propagation has been the investigation of fracture propagation with respect to anisotropic mechanical properties and pre-existing interfaces such as natural fracture networks and bedding planes.…”

“…The majority of hydraulic fracture propagation investigations focus on the propagation mechanisms, using these methods to better understand the fracture geometries and networks produced, as well as the likely fracture extent. AEs are also used to show the accumulation of damage during cyclic injection prior to fracture propagation (Diaz et al, 2020; Zhuang et al, 2020). In some cases following breakdown when fluid injection is sustained, AEs have been recorded at stable fluid pressures, for example, Hu et al (2020).…”