Why does reservoir pressure decline as oil is produced?

In petroleum engineering, reservoir pressure declines primarily because the extraction of fluids—oil, gas, and water—removes the material that occupies the reservoir's pore space, thereby reducing the internal energy of the system.12

As production proceeds under primary conditions, gas that was previously in a miscible state within the oil begins to escape as bubbles. This process is analogous to opening a carbonated beverage; as the solution gas is released, the overall pressure within the reservoir drops. Because the initial formation pressure is responsible for driving crude oil into the wellbore and toward the surface, this inevitable decline eventually leads to the end of natural primary recovery.12

When the natural pressure is no longer sufficient to sustain commercial flow, engineers may implement secondary recovery methods, such as the injection of gas or water, to reenergize the reservoir and maintain or increase the pressure required for continued production.1