Injectivity
- Also known as
- injectivity index, injection rate
Injectivity is how readily a well and the rock around it accept injected fluid, usually expressed as the injection rate achieved per unit of pressure above reservoir pressure. Falling injectivity means more pressure is needed to inject the same volume.
What is injectivity?
Injectivity describes how easily fluid can be pumped into a well and out into the surrounding rock. It is the injection counterpart of a producing well’s productivity. It matters for any well that injects water or gas: saltwater disposal wells, waterflood and enhanced oil recovery injectors, and wells used for gas storage.
Injectivity index
Injectivity is usually quantified as the injectivity index:
Injectivity index = injection rate ÷ (bottomhole injection pressure − average reservoir pressure)
It is commonly expressed in barrels per day per pounds per square inch. A higher index means the well takes more fluid for the same pressure. Because permits limit the pressure at which many wells may inject, a falling injectivity index directly limits how much water a disposal well can handle.
Engineers track injectivity over time, often with a Hall plot, which plots cumulative pressure against cumulative injected volume. A change in slope signals that the well is plugging or, conversely, that fractures have opened.
What controls injectivity
- Rock properties. Permeability and thickness of the injection zone.
- The near-wellbore region. Damage in the rock close to the well has an outsized effect, because all the fluid must pass through it; see near-wellbore region.
- Fluid properties. Viscosity and the amount of solids, oil and dissolved minerals in the injected water.
- Water compatibility. Mixing injected water with formation water can precipitate mineral scale.
What causes injectivity decline
- Solids plugging, from suspended particles, corrosion products and iron sulfide.
- Oil and organic deposits, including paraffin and oil carried over from poor separation.
- Scale, formed as waters mix or as temperature and pressure change.
- Bacteria, which form slime and produce hydrogen sulfide.
- Emulsions and fluid incompatibilities in the rock.
- Clay swelling or migration triggered by water chemistry.
Together these are forms of formation damage.
Restoring injectivity
Operators first improve the water: better separation, filtration and chemical treatment at the surface. To remove damage already in place, they use acid treatments, solvents, dispersants and surfactants that break up and carry away deposits, and in some cases mechanical cleanouts. Regular monitoring helps schedule treatments before injectivity falls too far.
Injectivity at Locus
Locus’s FloBoost is a biosurfactant injectivity aid that Locus describes as dispersing solids and organic deposits, breaking emulsions and reducing backpressure to restore injection rates in disposal and injection wells.
Frequently asked questions
Injectivity measures how readily a well accepts fluid being pumped in. Productivity measures how readily a well delivers fluid from the reservoir. Both are expressed as a rate per unit of pressure difference.
By removing what is restricting flow near the wellbore, with acid, solvents, dispersants or mechanical cleaning, and by improving the quality of the injected water so the damage does not return.
A Hall plot is a graph used to monitor injection wells. It plots cumulative injection pressure against cumulative injected volume. A steepening slope indicates growing resistance to injection, typically from plugging.
Sources
- injectivity index, SLB Energy Glossary
- Injection well performance and Hall plot analysis, SPE PetroWiki