For those of us who work in and around the oil and gas industry, we never get too far from the geology education we received as children – specifically, the study of the remnants of bygone eras that are trapped in the earth’s crust. Extracting oil and gas from the ground is an art and a science. It requires analysis of our planet’s geophysical characteristics, along with methods and technology to extract valuable materials. 

The outermost of the earth’s four concentric layers is known as the crust. The crust is made up of tectonic plates. The activity at this layer of the earth is dominated by counteracting forces. The tectonic plates move, causing a build-up of pressure. The release of that pressure comes in the form of volcanoes and earthquakes.

This layer also has countless other forces at work. Underground rivers and pools move and contain water trapped beneath the surface of the earth. Beneath all that are reservoirs – subsurface sand, strata, formations, aquifers, caverns or voids. Oil and gas companies invest large amounts of capital, time and employee talent in exploring and producing hydrocarbons and related products in these reservoirs. 

Oil and gas reservoirs also have opposing forces acting on them. Energy that has built up over thousands of years is contained by the geological formation surrounding the reservoir.

The natural energy of an oil or gas reservoir can come from myriad sources. The natural swelling of the hydrocarbons, gravity drainage, or nearby aquifers that communicate with the oil or gas reservoir can supply the necessary energy required to bring the hydrocarbons to the surface. This naturally occurring energy is what makes up the primary production of an oil or gas well, and is what most people think of when they imagine oil coming out of the ground. 

But when the naturally existing energy from a reservoir is depleted, other means of recovery are needed to keep a well producing. 

What Is Enhanced Oil Recovery?

Enhanced Oil Recovery (EOR) is “one or more of a variety of processes that seek to improve recovery of hydrocarbon from a reservoir after the primary production phase.”

While the definition states that it is a process used “after the primary production phase,” when referring to enhanced oil recovery, you are not typically talking about the use of pumpjacks, which by their nature assist in the recovery of oil from a reservoir.

Enhanced oil recovery can be broken down into two categories – secondary recovery and tertiary recovery. 

Secondary Recovery

Secondary recovery is the process of recovering hydrocarbons trapped in the reservoir after the reservoir pressure has been depleted to a predetermined level of a well’s production decline curve. The standard methods of secondary recovery involve increasing the reservoir pressure by either injecting water (waterflooding) or some form of gas mixture (gasflooding).

The process of adding pressure to the well (sometimes called pressure maintenance) typically refers to this phase of the recovery process.

Tertiary Recovery

Tertiary recovery methods are often the third recovery process used to recover the hydrocarbons from a reservoir. Depending on the reservoir models your geologists have built, there could be a case for skipping secondary recovery or using both secondary and tertiary recovery methods simultaneously. 

With a wide variety of tertiary recovery methods, the three that are used most often are thermal, gas injection and chemical flooding.

Thermal recovery can actually be achieved in a number of ways – primarily by injecting fluid hot enough to raise the reservoir temperature by 100° to 200°F by injecting a heated fluid or steam. Thermal recovery can be achieved by in situ combustion or combusting the oil where it is in the reservoir. 

Whatever the method of thermal recovery, the purpose of thermal recovery is to lower the viscosity of the crude oil in the reservoir to a predetermined level where it will flow more easily up the wellbore.

Chemical flooding is used to either reduce the interfacial tension between the reservoir oil and the injected fluid or to improve the sweep efficiency (i.e., make the reservoir oil more viscous).

For more information about Enhanced Oil Recovery (EOR) or any other geological consulting services we offer, reach out and let us know what you’re working on by emailing SPRI at info@sprioilgas.com.