Cause analysis and solutions of water blocking damage in cracked/non-cracked tight sandstone gas reservoirs

Abstract

After hydraulic fracturing treatment, a reduction in permeability caused by the invasion of fracturing fluids is an inevitable problem, which is called water blocking damage. Therefore, it is important to mitigate and eliminate water blocking damage to improve the flow capacities of formation fluids and flowback rates of the fracturing fluid. However, the steady-state core flow method cannot quickly and accurately evaluate the effects of chemical agents in enhancing the fluid flow capacities in tight reservoirs. This paper introduces a time-saving and accurate method, pressure transmission test (PTT), which can quickly and quantitatively evaluate the liquid flow capacities and gas-drive flowback rates of a new nanoemulsion. Furthermore, scanning electron microscopy (SEM) was used to analyze the damage mechanism of different fluids and the adsorption of chemical agents on the rock surface. Parallel core flow experiments were used to evaluate the effects of the nanoemulsion on enhancing flowback rates in heterogeneous tight reservoirs. Experimental results show that the water blocking damage mechanisms differ in matrices and fractures. The main channels for gas channeling are fractures in cracked cores and pores in non-cracked cores. Cracked cores suffer less damage from water blocking than non-cracked cores, but have a lower potential to reduce water saturation. The PTT and SEM results show that the permeability reduction in tight sandstones caused by invasion of external fluids can be list as guar gum fracturing fluid > slickwater > brine. Parallel core flow experiments show that for low-permeability heterogenous sandstone reservoirs with a certain permeability ratio, the nanoemulsion can not only reduce reverse gas channeling degree, but also increase the flowback rate of the fracturing fluid. The nanoemulsion system provides a new solution to mitigate and eliminate water blocking damage caused by fracturing fluids in tight sandstone gas reservoirs.