It is not enough to recover every last hydrocarbon from a geological reservoir. The recovery method has to make economic sense for the business. Secondary recovery methods are used once the primary production pressure has declined to a level below that which is required to bring a desired volume of hydrocarbons to the surface.
While your oil or gas well may still be producing, the recovery rate may not be enough to meet your production requirements or the minimum requirements to maintain your lease on the property.
While there are a number of secondary and tertiary recovery methods you can use to increase the production of a single well or a field with multiple wells, few give the economic benefits of waterflooding.
What Is Waterflooding?
Waterflooding is a secondary recovery method in which water is injected into a reservoir in an effort to push the trapped hydrocarbons toward a producing area of the existing field.
The combination of adding water and moving the hydrocarbons to another part of the existing reservoir effectively increases the reservoir pressure and therefore the recoverable hydrocarbons in the field.
The waterflooding process was discovered by accident in the Pithole City area of Pennsylvania in 1880. Little is known about how the process was developed because it was an illegal practice in parts of the United States until 1921. Today, the methods used for waterflooding or water-injection wells follow a pattern injection program in the geology of the well, combined with surface pressure injections that help operators to significantly increase the production of a field in an economical way.
Important Factors in Waterflooding
According to the Society of Petroleum Engineers Petroleum Engineering Handbook, there are nine key factors for determining the suitability of a reservoir for waterflooding:
- Reservoir geometry
- Reservoir depth
- Continuity of reservoir rock properties
- Magnitude and distribution of fluid saturations
- Fluid properties and permeability
- Optimal time
While this might seem somewhat obvious, the geometry of the reservoir plays a large part in whether or not a waterflooding operation will be successful.
The presence of natural water, unfavorable structural features such as faults or stratigraphic features such as shale-outs could negatively affect the success of a waterflood operation.
Lithology – the general physical characteristics of the rock formations in and around the reservoir – is an important factor in the success of a waterflood operation. A field with little permeability will cause water injection to have little effect on the recoverability of the hydrocarbons in the reservoir.
If a reservoir is too deep for economical redrilling, and if old wells have to be used as injection and producing wells, operators can expect lower recoveries than in new wells. Shallow-depth fields require caution since the maximum pressure is limited by the depth of the reservoir.
Porosity determines the amount of oil that is present for any given percentage of oil saturation. Because the fluid content of reservoir rock can vary widely, it is important to collect reliable porosity data.
If you have access to the data, isoporosity maps can be used to give operators a much better picture of the porosity of the reservoir geology.
To a large extent, the permeability of the reservoir rock controls the rate of water injection that can be sustained in an injection well for a specific pressure. It is essential to have relatively uniform permeability for a successful waterflood.
Continuity of Reservoir Rock Properties
Fluids in a reservoir generally flow in the direction of bedding planes, so horizontal continuity is of primary interest. If a reservoir is split into layers by partings of shale or dense rock, a study should indicate whether individual layers have a tendency to shale out in relatively short lateral distances, or whether sand development is uniform.
Magnitude and Distribution of Fluid Saturations
In examining a reservoir for waterflooding, a high oil saturation would be more suitable than a low saturation. Usually, the higher the oil saturation at the beginning of flood operations, the higher the recovery efficiency.
Fluid Properties and Permeability
The physical properties of reservoir fluids substantially impact whether waterflooding is advisable for a reservoir – especially the viscosity of the oil. Oil viscosity affects the mobility ratio. Other factors affecting the mobility ratio include the permeability of the reservoir rock to the fluids and the viscosity of the water.
The ideal time to waterflood a reservoir depends on the operator’s primary objective, which may have to do with the desired amount of hydrocarbons and/or the profitability of the project. The most common way to determine the optimal time to begin flooding is to compute the anticipated oil recovery, production rate, monetary investment and income for a few different time frames and comparing them.
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