Abstract
Fracability of unconventional gas reservoirs is an important parameter that governs the effectiveness of subsequent gas extraction. Since reservoirs are saturated with various pore fluids, it is essential to evaluate the alteration of fracability of varyingly saturated rocks. In this study, varyingly saturated (dry, water, and brine with 10%, 20% and 30% NaCl by weight) siltstone samples were subjected to uniaxial compressive loading to evaluate their fracability variation. Acoustic emission (AE) and ARAMIS photogrammetry analyses were incorporated to interpret the crack propagation. SEM analysis was carried out to visualize the micro-structural alterations. Results show that siltstone strength and brittleness index (BI) are reduced by 31.7% and 46.7% after water saturation, due to water-induced softening effect. High NaCl concentrations do not reduce the siltstone strength or brittleness significantly but may contribute to a slight re-gain of both values (about 3–4%). This may be due to NaCl crystallization in rock pore spaces, as confirmed by SEM analysis. AE analysis infers that dry siltstone exhibits a gradual fracture propagation, whereas water and brine saturated specimens exhibit a hindered fracturing ability. ARAMIS analysis illustrates that high NaCl concentrations causes rock mass failure to be converted to shear failure from splitting failure, which is in favour of fracability.
Highlights
With growing concerns about rising energy demands, the petroleum industry is turning to exploring new energy resources such as extraction of natural gas from geological reservoirs
The tested siltstone has a significant amount of kaolinite, and this may cause significant kaolinite deposition in the rock mass pore space upon interaction with NaCl. This clay mineral deposition in the rock mass pore space may increase the cohesion of the rock, which in turn offer more opportunities for stable fracture propagation [50]. This indicates that brine has some ability to reduce the sudden failure effect caused by water, which is favourable for the hydraulic fracturing process
The fracability of unconventional gas reservoirs is a key parameter that controls the effectiveness of rock stimulation processes such as hydraulic fracturing
Summary
With growing concerns about rising energy demands, the petroleum industry is turning to exploring new energy resources such as extraction of natural gas from geological reservoirs. Due to the ultra-low permeability, the stored gas is tightly trapped inside the reservoirs and cannot be extracted with conventional gas production techniques, such as reservoir pore pressure depletion through pore water removal [3] This indicates the necessity of using appropriate reservoir permeability enhancement techniques to produce an economically-viable amounts of gas from unconventional reservoirs. New fractures are created, when the injecting fluid pressure exceeds the least principal stress acting on the formation [7] These newly-created fractures connect with the natural fractures in the formation to form a fracture network connected to the wellbore [8]. The rock mass tends to heal the fractures during the fracturing process due to the required high energy, which is not favourable in terms of the fracking process This indicates the importance of identifying the brittleness characteristics of unconventional reservoirs before proceeding with hydraulic fracturing. It should be noted that these brittleness indices have mainly been derived empirically, are applicable only under certain defined conditions
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