With the high global demand for oil, attention turns to more unconventional reservoirs as a means to improve oil recovery rates. There are billions of barrels of oil in North America’s shale reservoirs, but the industry’s average recovery rate is less than 10% as opposed to an average rate of 30% for conventional reservoirs. Industry experts are scrambling to improve oil recovery rates from unconventional reservoirs as a result.
The Problem with Unconventional Reservoirs
Initial oil production rates from shale formations often decline quickly because oil can’t flow easily through the low-permeability rock. In the early weeks of first oil, productions teams usually must resort to placing shale wells on artificial lift methods like electric submersible pumps or sucker rod. Even so, since the reservoir fluids can’t move easily to the wellbore from the formation, production rates will decline.
In an attempt to keep the production economically viable, production teams will try to counter declining production with drilling. Doing so is very expensive in good times, even more so when oil prices plummet.
EOR to Increase Oil Recovery Rates
Enhanced oil recovery (EOR) methods are often used to help low oil recovery rates. Production teams can use EOR to inject fluids into conventional reservoirs to substitute in depleted natural resources that push oil through a given formation to the wellbore and then the surface.
With many unconventional reservoirs being very sizeable, EOR methods can boost recovery rates by a small number of percentage points. The end result is several additional billion barrels of recovered oil and a potential solution for some oil recovery challenges.
But not all EOR methods can be used in such cases. Where waterflooding and gas injection don’t get the job done in low-permeability situations, inject fluids are sent on low-resistance paths that exist in induced and natural fracture-matrix interfaces. But even traditional EOR techniques must be adapted for these unique shale formations.
Effective EOR techniques on unconventional reservoirs are based on fluid injection. These methods depend on counter-current flow and imbibition as opposed to traditional secondary recovery methods.
The Benefits of Cyclic Injection
Since fluids can’t be injected under rock matrixes to force oil out to separate production wells as is done in conventional EOR projects, cyclic gas injection is being used by production teams. They inject the gas into the production well which is shut for a period of time so the pressurized gas can seep into the reservoir from the fractures. During the period where the gas is sealed within, the gas ends up dissolving into the oil, causing it to swell and lose viscosity.
The oil with its lowered viscosity flows through the matrix once the well is returned to production to the fractures and then the surface. The process of injection, sealing, and flow is repeated in what’s known as a huff-n-puff method and extra oil is recovered making it a potential answer to oil recovery challenges.
Additional EOR Methods for Unconventional Reservoirs
In addition to cyclic inject, researchers are developing methods involving the injection of surfactants, causing water to penetrate the shale rock matrix and relocate the oil through a process known as spontaneous imbibition. With shale being an oil-wet rock, it naturally absorbs oil as opposed to water.
The surfactants used have the ability to change the rock’s affinity to water-wet from oil-wet so the rock is more apt to absorb water instead of oil. Like cyclic injection using gas or C02, an injection well is used and sealed once the surfactants have been injected into the reservoir to give it time to saturate the matrix before reopening both well and production well.
Preparing for the Future
With the development of EOR techniques to utilize shale reservoirs and raise low oil recovery rates, the US has gone from a country most said would be dependent on imports for as much as 70% of their crude supplies by 2015 to one that actually only imports beneath a third of its oil needs.
The continuing development of EOR processes for shale resources may soon make a big impact on the unconventional oil industry in a way very much like the combined forces of hydraulic fracturing and horizontal drilling did only a few years ago.