Analytical modelling of wettability alteration-induced micro-fractures during hydraulic fracturing in tight oil reservoirs

摘要

Hydraulic fracturing technique is of vital importance to increase fuel energy, in particular to unlock the unconventional resources (e.g., tight oil, shale oil and shale gas reservoirs). However, low recovery of hydraulic fracturing fluid has been in the centre of attention from both technical and environmental perspectives in the last decade. To gain a deeper understanding of controlling factor(s) over low recovery of hydraulic fracturing fluids, we hypothesized that hydraulic fracturing fluid (usually low salinity water) increases hydrophilicity of reservoir rocks, thus stress intensity factor, which in return facilitates in-situ micro-fractures extension. To test this hypothesis, we developed a physical model with consideration of capillary pressure which is associated with wettability. Moreover, we calculated stress intensity factor using our previous contact angle results with presence of low salinity and high salinity water [Xie et al., The low salinity effect at high temperatures, Fuel, 200 (2017) 419-426]. Furthermore, we examined the effect of tip width of in-situ micro-fractures, distance from main hydraulic fractures on micro-fractures extension at different wetting systems.Our results demonstrate that low salinity water indeed increases stress intensity factor as compared to high salinity water due to the wettability alteration. Our results also show that the length of micro-fracture extension increases up to 15 times than the original micro-fracture length due to the wettability alteration when the tip width of in-situ micro-fracture reaches 10 nm. This likely explains the extension of micro-fractures due to water uptake by shale. Moreover, micro-fracture extension is more pronounced for micro-fractures located nearby a wellbore, suggesting that a huge amount of hydraulic fluids likely remains at the vicinity of the wellbore. Knowing wettability thus stress intensity factor, the disappearance of hydraulic fracturing fluids and EOR potential in tight sandstone and shale reservoirs can therefore be quantified.