Investigation of the effect of foam flow rate on foam-based hydraulic fracturing of shale reservoir rocks with natural fractures: An experimental study

Foam-based hydraulic fracturing has been identified as a good solution for the extraction of natural gas from deep shale reservoirs, due to its unique advantages such as low water consumption and high proppant carrying capacity. However, lack of knowledge of foam-based shale fracturing and complex nature of two-phase foam, make it challenging to use in field conditions, and one unclear area is the effect of the foam flow rate on fracturing and fracture characteristics. Therefore, the aim of this study was to investigate the effect of foam flow rate on breakdown pressure, fracture permeability and generated fracture characteristics. In order to achieve this, a series of hydraulic fracturing experiments was conducted for shale rocks with existing natural fractures, using nitrogen-based foam. Fracture permeability was also evaluated for intact and fractured rock samples. Micro-CT scanning technology was adopted to characterise the geometrical properties of fractures, in order to investigate the effect of foam flow rate within the fracture. Experimental results show that the breakdown pressure increases by 30% while energy release decreases by 78% when the foam flow rate increases from 10 to 70 ml/min. In addition, the results suggest that the permeability of a fractured sample at 10 ml/min injection rate is about 15 times higher than that of a fractured sample at 70 ml/min injection rate. The findings of this study reveal that low foam-flow rates can create a better inter-connected fracture network than higher flow rates.