Thermal reactivation of microfractures and its potential impact on hydraulic fractures efficiency

Core studies have proven the presence of abundant cemented micro-fractures in many tight formations. Furthermore, these studies have revealed the opening of some of these microfractures on the wall surface of main hydraulic fractures. Additionally, early-production well-testing analysis in some of these cases provides estimates for hydraulically induced fracture surface area which is much larger than fracture dimensions estimated in fracturing design or provided by the location of microseismic events. Existence of open small-size fractures could be a possible cause behind this discrepancy. In this paper, we show in what extent thermal stresses induced by temperature difference between fracturing fluid and formation fluid could provide the driving force to open a portion of these small cemented natural fractures laying on the surface of hydraulic fractures. Moreover, through combination of stress analysis and empirical fracture distribution models obtained from outcrops, we calculate the increase of total reservoir/fractures contact surface under the condition of microfractures activation. Our thermoelasticity analysis reveals the effect of the pump rate and temperature of the fracturing fluid on the number of activated microfractures. The results show that the volume of the micro-fractures varies depending on the length of the microfracture, rock properties and time. At the end, through an example, we show that activation of only a small portion of these microfractures can increase the total formation contact surface considerably and consequently initial production. Reservoir pressure changes due to production might partially close or re-open these micro-cracks during production. Hence, the role of these microfractures is mainly restricted to the early life of the reservoirs.