Normal production operations can recover 10-30% of the hydrocarbons originally found in the formation (oil-in-place, or “OIP”) . That leaves a lot of hydrocarbon down there! Can’t we do better?
Sometimes we can, by using enhanced recovery techniques to improve the recovery factor. We talked a little bit about secondary and tertiary recovery techniques earlier. Here, we’ll look at some of the most common techniques in more detail.
According to Darcy’s Law, which governs underground fluid flows, the following actions will increase flow rates to a well:
- Increasing the pressure gradient
- Lowering the viscosity of the oil
- Increasing the permeability of the reservoir rocks
- Decreasing the distance that the fluids must flow
Each of these potential actions is associated with an improved oil recovery method.
Secondary Recovery: Increasing the Pressure Gradient
As primary production declines, the pressure gradient within the reservoir lessens and correspondingly, flow rates decrease. In order to maintain flow, the pressure gradient can be increased artificially.
This is usually accomplished by pumping water into the deposit from a well adjacent to the producing well, a technique called waterflood.
The water begins to flow towards the producing well where pressures have been lowered by extraction. As it passes through the pores of the rocks, it pushes oil and gas towards the well ahead of it. Sometimes water reaches the well through a particularly permeable layer, bypassing much of the oil. As more and more water enters the well, the costs of removing the water from the oil and disposing of it on the surface begin to make production uneconomical. This limits the ultimate ability of waterflooding to increase oil recovery. Nevertheless, the method is used frequently to get more oil out of the ground.
Gases can also be injected to increase the pressure within the formation. When it is not economical to transport and sell the natural gas produced alongside the oil, the gas is often pumped back into the ground in a nearby injection well, just like water in the water flooding technique. This increases oil production rates and stores the gas where it can be retrieved and sold later.
Tertiary Recovery: Lowering the Viscosity of the Oil
By lowering the viscosity of oil, reservoir engineers allow it to flow through the formation more quickly. This can boost an uneconomical production rate enough to make production profitable.
Miscible gas injection is a technique that reduces the viscosity of the oil by dissolving gases such as CO2 in it. These gases act to lower the surface tension of the oil and reduce the resistance to movement along oil-water interfaces.
Another way to lower the viscosity of the oil is to heat it up. A side effect of heating is expansion, which creates pressure and helps drive oil to the well.
Fire flooding is a technique that allows reservoir engineers to cause a controlled underground oil fire, which serves to heat up the surrounding rocks and the oil contained in them. Since combustion requires oxygen, which is not available underground, the rate of burning can be carefully controlled by managing the supply of oxygen to the fire.
Rocks and oil can also be heated using steam flooding. In this case, superheated steam is injected into the ground to heat the oil. This process of in-situ oil recovery is very common in the Canadian oil sands region of Alberta. The problem with this method is that the steam condenses into water and can mix with the oil, requiring separation at the surface.
Both of the above methods require multiple wells – injection of steam and oxygen occurs at wells adjacent to the producing wells in order to force water into the producing wells.
Stimulation Techniques
We covered stimulation in the previous section. Sometimes, well stimulation techniques are included in the same category as the enhanced recovery techniques. Let’s take a quick look at how these methods relate to the four possible changes that increase production rates.
Acidizing: Increasing the Permeability of the Reservoir Rocks
Methods which increase the permeability of the reservoir rocks usually involve the use of chemicals that enlarge pore spaces and the passageways between them. During production, calcium carbonate and other precipitates build up in the pore spaces near the well. Acids are used to dissolve these precipitates and increase the permeability. This is referred to as an acid job in the oil industry.
Fracturing: Decrease the Distance that Fluids Must Flow
Fracturing works by decreasing the distance fluids must flow within a rock of low permeability. High pressure fluids are used to produce a system of fractures, which are held open using a variety of proppant materials. Once gas or oil reaches a fracture, its movement no longer follows Darcy’s Law because it does not need to work its way through interconnected pore spaces. Instead, oil and gas need only to flow through the nearest open fracture. Once the fracture is reached, the journey to the well along the fracture is relatively rapid.
Images: “Enhanced Recovery” by Jim Blecha