Secondary fractures and their potential impacts on hydraulic fractures efficiency

Abstract

Outcrop studies have revealed the presence of abundant cemented fractures in many low permeable formations. Recovered cores have also revealed the opening of some of these smaller size fractures on the wall surface of main hydraulic fractures. Furthermore, early-production well-testing analysis in some of these cases provide estimates for hydraulically induced fracture surface areas which are much larger than the fracture dimensions estimated in fracturing design. Re-opening of these small-size fractures could be a possible reason for this discrepancy. In this paper, we show how and to what extent tensile stresses induced by temperature difference between fracturing fluid and formation fluid or plastic unloading of the formation rock could provide a large enough driving force to open a portion of these small cemented natural fractures laying on the surface of hydraulic fractures. Our thermo-elastoplasticity analysis reveals the effect of net pressure, stratigraphy and also temperature of the fracturing fluid on the number of activated microfractures. Accordingly, potential distributions of activated micro-fractures are estimated. At the end, through an example, we show that the activation of only a small portion of cemented microfractures can increase the total formation contact surface considerably, and consequently increase the initial production by many folds.