How to simplify opamps in circuits for limit analysis
You don't replace the op-amp with anything. If the circuit provides negative feedback to the op-amp, you use the ideal op-amp rules to solve the circuit.

In this case, if you go to high frequency, replace the capacitor with a short. You'll see an op-amp circuit that you should recognize easily.

If you go to low frequency, replace the capacitor with an open. Now you have no negative feedback, so you're left with the open-loop response of the op-amp. Typically, a second resistor (R2>>R1) is placed in parallel with C1 to avoid this problem.