What could cause flow reversals in a reservoir which might lead to hysteresis?

• A. Rising reservoir temperature
• B. Lowering oil viscosity
• C. Increasing permeability
• D. Changing a flood pattern, changing injection rates, or shutting in a well

Answer: (D)

Flow reversals come from changing the boundary conditions that drive fluid movement in the reservoir. When you alter how the flood is applied, adjust injection rates, or shut in a well, you change the pressure field and the direction of the displacing fluid. That can cause the downstream flow to reverse or for fluids to redistribute back toward regions that were previously swept. This shift in flow direction means the system can follow a different saturation and capillary-pressure path than before, which is the essence of hysteresis in multiphase flow.

Rising temperature, lower oil viscosity, or increased permeability modify mobility or ease of flow, but they don't by themselves reverse the overall flow direction or create the history-dependent path changes that lead to hysteresis.