Enhance has chosen the definition of these terms most relevant to oil and gas operations, CO₂ EOR and storage, and the Alberta regulatory environment as the terms are used in this website.
Glossary of Terms
Active EOR: Period during which the CO₂ EOR project is injecting CO2 and recovering oil until all injection and/or production has ceased and project abandonment has commenced.
Aquifer: A permeable geological formation that is water-saturated. The water may be either saline or non-saline.
Atmosphere: The gaseous envelope surrounding the earth.
CO₂ enhanced oil recovery (CO₂ EOR): The process of injecting CO₂ into an oil reservoir to improve oil recovery. Under the proper conditions of reservoir temperature and pressure and oil properties, the CO₂ acts as a solvent mobilizing oil that cannot be produced by conventional primary and waterflood techniques. This process has been used extensively in the Permian Basin of Texas for over 40 years and is also being used in the world’s largest CO₂ EOR and storage project at Weyburn, Saskatchewan, which began CO₂ injection in 2000.
The CO₂ EOR process involves seven steps, as follows:
- CO₂ is injected into the reservoir via injection wells.
- The CO₂ reduces the oil viscosity (i.e. makes it more like water than honey) and swells the trapped oil so it flows more easily.
- Produced water can be injected in alternating cycles (known as WAG) with CO₂ to help sweep the oil to the producing wells.
- Production wells pump the oil, along with CO₂, produced water and any associated natural gas to the surface.
- The CO₂ and associated natural gas are separated at the production satellite facility so that volumes of each substance produced at individual wells can be measured. The fluids then flow to the central treating facility.
- The oil, gas (which consists of both solution gas from the reservoir and CO₂) and produced water are separated at the central treating facility. The oil is piped to holding tanks where it is metered and sold. CO₂ and solution gas are recycled for reinjection. No CO₂ is released to the atmosphere; this is a closed-loop system.
- The produced water is also re-injected.
Containment: Containment means ensuring that CO₂ does not leak vertically out of an EOR and storage reservoir. Keeping CO₂ in the reservoir protects against biosphere and/or hydrosphere contamination, safety risks, and CO₂ contamination of vertically offset hydrocarbon resources. Containment also ensures CO₂ is being efficiently used for EOR; containment of the CO₂ to the injection zone is critical for the technical and economic success of the EOR project, as it is the miscible action of CO₂ on the oil that improves recovery to offset the added costs of implementing CO₂ EOR. As part of any EOR scheme, regardless of a storage component, the scheme operator would focus significant resources, capital and technology to ensure CO₂ is contained to the injected zone to ensure the best possible oil production response.
Container: A term used to define a geologic horizon in which CO₂ can be stored. In order to ensure long-term, safe storage, a solid container is better than a leaky one.
Enhanced oil recovery (EOR): EOR is an improved hydrocarbon recovery method that usually occurs after all possible oil is pumped out of the ground using traditional methods. Primary and secondary recovery methods have been used for over a hundred years, while EOR—also termed tertiary recovery has been used commercially for over 50 years. Numerous EOR methods can be used, dependent on reservoir properties and the costs of the various options. There are many methods: heating, surfactant flooding, polymer flooding, solvent flooding and CO₂ flooding are among the options.
Miscible: Capable of forming a single fluid when mixed. In terms of CO₂, this fluid is miscible with oil under pressure, deep underground, but is immiscible – will not mix – at surface conditions.
Permeability: A rock property that controls the ease with which fluids can flow through the reservoir. Usually expressed as Darcies (D) or milliDarcies (mD).
Pool: In oil and gas terminology, a pool is a distinct and separate oil and/or gas reservoir. Production from a pool will not produce oil and/or gas from another pool. In Alberta, pools are designated by the Alberta Energy Regulator.
Porosity: The space between grains of rock. Expressed as a percentage of the total volume.
Relative permeability: Relative permeability controls how various fluids flow through the reservoir as relative saturations of the fluids changes. For example, if part of the pore space is occupied by water, the effective (or relative) permeability for other fluids is reduced, since the water restricts the size of pore openings left for the other fluids to move through.
Reservoir: A geological formation containing economically recoverable quantities of oil and/or gas (reserves).
Reservoir simulation: This is a tool used by petroleum engineers and geo-scientists to understand and monitor fluid flow and displacement mechanisms and pathways, in order to optimize economic hydrocarbon recovery. Simulation considers rock and fluid properties, initial saturations of oil, water and gas in the rock and the interactions that occur as conditions change. Once a model is constructed, the first step is to conduct a history match to validate the input parameters and tune them to actual reservoir performance. The validated, history matched model is then used to project future performance. It is also possible to construct a purely predictive model where there is no production history, but this type of simulation has a much higher degree of uncertainty and is not applicable to Clive Leduc given the extensive production history.
A well-developed reservoir simulation can also provide a valuable monitoring tool. As reservoir development continues, variations between actual vs. predicted performance can provide early warning of containment or conformance issues that can then be investigated through additional field monitoring.
Viscosity: The thickness of a fluid that controls how easily it can flow. For example, molasses is more viscous than water.
Although it has been documented that some of the earliest oil wells were drilled in China over 1,650 years ago using percussion spring pole methods, the most relevant description of drilling techniques applicable to Clive is that of rotary drilling.
Rotary drilling uses a rotating drill bit to drill through subsurface rock formations to reach oil reservoirs, just like one would use a cordless drill to drill a hole in a piece of wood. The main difference (apart from the size, cost, power and complexity of the equipment), is that rotary drilling uses hollow pipe “joints” (generally approximately 10m long and screwed together) to allow drill bit extension as it penetrates further into the earth. The hollow pipe also allows circulation of drilling mud, a liquid which can be water- or hydrocarbon- based and which contains specialized additives to control its viscosity and density. Circulation of drilling mud means pumping the mud down through the drill pipe and circulating it back up to surface through the space between the hole and the drill string (the entire assembly of pipe joints and the drill bit). The space between the drill string and the hole is termed the annulus. Drilling mud serves four main purposes:
- It provides cooling to the drill bit (drilling through solid rock generates tremendous frictional heat that would otherwise damage the bit);
- It carries the rock fragments dislodged by the bit (cuttings) back to surface where they are separated from the mud (and often mud logged to determine the geology of the subsurface and assist in determining what formations are being drilled through and the types of fluids encountered). The mud is then pumped back into the hole by mud pumps;
- It provides stability to the hole by exerting hydrostatic pressure (i.e. the pressure exerted by the weight of the column of mud) against the sides of the open hole; and
- It prevents influx of unwanted fluids from formations that are being drilled through by exerting hydrostatic pressure to hold the fluids in their formations.
In some cases, the mud is also used to drive a mud motor, which rotates the drill bit downhole, making it unnecessary to rotate the entire drill string, using energy provided by the mud pumped through the drill string by the mud pumps.
After a section of the well has been drilled, joints of hollow pipe called casing are run into the hole and cemented in place. Casing serves five main purposes:
- To protect potable aquifers from contamination by produced oil, water and gas;
- To stabilize the section of the hole that has just been drilled so that material does not fall, or slough, into the hole as the hole is deepened;
- To provide a means of well control within the well should an unexpected flow of oil, water or gas from other underground formations occur;
- To provide a conduit that allows production to surface or facilitates the injection of fluids into the reservoir; and
- To prevent flow between formations.
There are three main types of casing that are generally used in Alberta wells. Each series of pipes is referred to as a string, which is the basis for the term casing string(s). The three types of casing are as follows:
- Conductor pipe: This casing string is generally set to 10 to 20 metres depth to prevent excess sloughing of poorly cemented surface formations (i.e. the crumbly layers of soil that could easily become unstable and fall into the hole) for the surface hole drilling.
- Surface casing: This casing string provides primary protection of aquifers and well control while drilling the next section of the well. The surface casing is set or run to a depth that is calculated based on the maximum reservoir pressure expected to be encountered while drilling the next section of the well or to a minimum depth of 25m below the depth of the deepest potable water well within 200m of the well, whichever is greater. If the preceding calculation shows the surface casing could be set shallower than the BGWP, the owner of the well may elect to set the surface casing deeper in order to isolate the BGWP (the typical procedure). Alternately, the owner must ensure the BGWP is completely protected when cementing the next casing string.
- Production casing: This casing string is run into or slightly below the formation targeted by the well to allow the production or injection of fluids. In very deep or complex wells, intermediate casing strings may be run to a shallower depth than the final production string to improve the stability of the hole and to provide well control.
Casing strings are cemented in place after being run to a programmed depth. While there are many different types of oil field cement and additives depending on the specific application, using cement prevents fluid from moving on the outside of each casing string by providing a strong bond between the string and the rock formation. Once the casing has been run into the hole, cement is circulated down inside the casing and displaced with fluid, pushing it upwards outside the casing until the space (or annulus) between the outside of the casing string and the rock formations is fully covered by cement. Once the cement sets, it provides a seal along the length of the casing string to prevent unwanted fluid movement along the well.
Oil field cements have different compositions and physical properties than cements used in typical construction projects and are formulated for quick curing time and high strength to provide hydraulic isolation.