ABSTRACT
To evaluate the permeability changes and the underline mechanism during hydraulic fracturing, well shut-ins and flow back process, we perform a series experimental work by using tight core samples. Firstly, we build a physical model to simulate the whole hydraulic fracturing process by injecting fluids from different ends of core samples. According to the injection pressure and inject fluids flow rate, we calculate permeability according Darcy’s law. What’s more, we use nuclear magnetic resonance (NMR) method to detect aqueous phase migration to explain this phenomenon. Core flooding results showed that under no water invasion condition, flow-back pressure rise linearly at the beginning and reaches a plateau of 3.1 MPa. While under water invasion condition, flow-back pressure curves present a pressure peak and drop area then followed by a plateau. Then after different length of well shut-ins, the original pressure peak and plateau decreased which indicating rock permeability regained. Results demonstrated that permeability reduced 32.43% after water invasion while regained 11.0%, 24.7% and 32.9% after 2h, 4h and 7h’s shut-ins. NMR results showed that aqueous phase redistribute and migrate to smaller pores spaces and deeper distance during well shut-ins. Via aqueous phase migration, near fracture water blocks can be weaken or removed and this is the reason for rock permeability enhancement. Study of permeability enhancement and aqueous phase migration can provide a reference to explain hydrocarbon production increase after hydraulic fracturing shut-ins.
