Experimental study of the quantitative relationship between water uptake and fracture generation in tight reservoirs

Abstract

Water uptake is an essential factor influencing the fate of fluids used during large-scale hydraulic fracturing in tight reservoirs. However, the relationship between water uptake and fracture generation in tight reservoirs is not well understood due to the special properties, such as extra-low porosity, permeability and high salinity. Water uptake, salinity evolution, and triaxial compression experiments were conducted on tight sandstone, shale, and volcanic rock to study the mechanisms of water uptake and its relationship with fracture generation. A new method for evaluating the generation of fractures based on the characteristics of water uptake was proposed. The results show that water uptake due to capillarity and osmosis leads to a decrease in rock compressive strength and Young's modulus but an increase in Poisson's ratio. The dilatancy of the sample changes with the water content for a similar lithology. With the increase of water uptake, the dilatancy of rock will occur earlier and more fractures will be produced. New fractures resulted from the high content of mixed layer of illite and smectite during water uptake lead to a significant increase in the salinity of the fracturing fluid. The difference in water uptake between matrix and fracture-matrix could increase with the larger contact areas and volume of fracture dilation. Owing to the larger contact areas between fractures and the fracturing fluid, the complex fractures lead to a larger final mass difference and more peak time of water uptake. The peak time and final mass difference of water uptake could reflect the generation and morphology of new formed fractures. This study is significant for hydraulic fracturing design and the evaluation of hydraulic fracture complexity in tight reservoirs.