Probing the mechanism of external fluids invasion of nanopores in fractured tight sandstone reservoir

The oilfield development process is accompanied by reservoir damage, which affects the reservoir permeability and reduces production. In this study, the mechanism of external-fluid invasion was analyzed to maintain oil-well productivity. The characteristics of the damage caused by external fluids to tight sandstone reservoirs were examined via scanning electron microscopy and high-pressure mercury-invasion experiments. Then, seepage models for fluids in the nanopores of tight sandstone reservoirs were established to investigate the effects of the natural fracture parameters and fluid properties on the seepage characteristics and damage mechanism of fractured tight sandstone reservoirs. The results indicated that the damage rates of water-based drilling fluid, oil-based drilling fluid, guar-gum fracturing fluid, and slick-water fracturing fluid for the natural core of tight sandstone are 53.20%, 33.87%, 23.64%, and 19.10%, respectively. The proportion of nanopores in fractured tight sandstone reservoirs ranges from 44.62% to 62.45%. The invasion of external fluids leads to a reduction in the permeability as well as the number of microfractures and their lengths. The changes in permeability barely affect the seepage characteristics of the fluid in the nanopores. The reductions in the lengths and number of microfractures change the connectivity of the entire flow space, affecting the seepage characteristics. The effect of the lengths of the microfractures is more significant than that of the number of microfractures. Additionally, the invasion speed of the external fluids has the most significant effect on the seepage characteristics. A higher viscosity of the external fluids corresponds to less damage to the nanopores. Drilling fluid causes less damage to nanopores than fracturing fluid does. The recovery of fractured tight oil reservoirs with nanopores can be enhanced by reducing the degree of external-fluid invasion.