I have had some experience with both gas injection and water flooding. I think it depends more on where the play is and what has been best known for working in that geological picture.
Some key points about gas injection:
Gas reinjection is presently the most-commonly used approach to enhanced recovery. In addition to the beneficial effect of the pressure, this method sometimes aids recovery by reducing the viscosity of the crude oil as the gas mixes with it.
Gases used include CO2, natural gas or nitrogen. Air cannot be used to repressurize the reservoir because the oil will quickly catch on fire.
Oil displacement by carbon dioxide injection relies on the phase behaviour of the mixtures of that gas and the crude, which are strongly dependent on reservoir temperature, pressure and crude oil composition. These mechanisms range from oil swelling and viscosity reduction for injection of immiscible fluids (at low pressures) to completely miscible displacement in high-pressure applications. In these applications, more than half and up to two-thirds of the injected CO2 returns with the produced oil and is usually re-injected into the reservoir to minimize operating costs. The remainder is trapped in the oil reservoir by various means.
Some key points about water flooding:
Flooding an oil field with extraneous water has been a widely accepted method for increasing a reservoir's recovery since the 1950's, but to the uninitiated it may seem odd. After all, water production is a bad thing; it increases lifting costs, puts more strain on equipment, and may even prevent flowing wells from flowing. Plus, the produced water must be dealt with in an environmentally sound way, which also adds to the operating costs.
So why add water? For two reasons: First, injecting anything into a reservoir will increase the reservoir pressure, and second, water and oil don't mix. This second reason may again seem odd, but because they don't mix water, under higher pressure, will displace the oil it contacts.
So what does this mean? First we need to understand that most oil reservoirs are solution gas drive reservoirs*. This means as the oil is produced the reservoir's pressure is reduced and the gas that was held in solution begins to breakout and expand, thus "driving" the oil towards the producing wells. This is a familiar process we see when opening a bottle of soda (Mentos added for emphasis).
The problem with a solution gas drive reservoir is when the gas breaks out of solution it is free to flow to the producing well and be produced, and once the gas is produced the reservoir's energy is lost. Typically a solution gas drive reservoir will only recover 5-20% of the reservoir's original volume of oil leaving a large portion behind.
By injecting water in a controlled manner, the loss of reservoir pressure can be controlled and reversed. Water is injected into dedicated injection wells strategically located throughout the reservoir, and the water itself can be used to displace the remaining oil towards the producing wells. If properly designed and operated, a water flood can double the reservoir's oil recovery.
Even with double the recovery (10-40%), we are leaving large volumes of oil behind in these solution gas drive reservoirs, and with the ever-growing oil-thirsty economies around the world we need to do better. This is where enhanced oil recovery (EOR) techniques come to play, but that discussion is for another day.
This all sounds great and water flooding has been used successfully for decades, however, it is important to take care to design and operate the flood appropriately, otherwise all the bad things we mentioned at the beginning may be all you get. There are may factors to consider when designing a successful water flood including:
reservoir permeability (both absolute and relative)
beginning and ending fluid saturations (oil, water and gas)
reservoir heterogeneity
oil gravity and viscosity
water source and compatibility
formation clay content
depth and lifting costs
But if done right, a well run water flood will significantly improve oil recovery and produce attractive returns for many years.
