Drainage Area of Oil/Gas Well?

Drainage area is primarily controlled by the permeability of the reservoir rock. In theory, one well can drain a complete reservoir but it might take a very long time.

In most reservoirs that have water present, the oil is displaced by the water as the well is produced. It has little to do with the drainage area of a well but it might greatly slow down the drainage of the oil from the reservoir. In the case of water drive reservoirs, the water table actually rises as the oil is removed.

Drainage Area Definition

One way of establishing the drainage area is by going back to the seismic data and comparing the strata in which the oil is being produced from to the surrounding strata.

This is made a lot easier now with the latest technology in that they can show a three dimensional image of the seismic data and therefore the extent of the oil bearing reservoir.

A more expensive way is to drill step out wells to establish the edge of the reservoir. These wells can later be used as driving wells for water or gas injection to drive the oil to the producing wells.

I'm sure it varies due to such factors as how heavy (thick) the crude oil is and how deep the inlet portion of the well is compared to the upper most extent of the oil. In other words, the more head, the wider the area that will drain into the well. The geology and type of rock probably also has a lot to do with it.

As far as actually determining the true area that is draining into a particular well, you could probably measure the water table in surrounding water wells. Water is often found in conjunction with gas or oil, so removing oil and water is going to lower the water table.