injection-well testing, Oil & Gas Glossary

What Is Injection Well Testing?

Injection well testing analyses the pressure response of wells in which fluid is being injected into a reservoir — most commonly in water injection for pressure maintenance or enhanced recovery — using falloff tests that measure pressure decline after injection ceases to determine permeability, skin factor, reservoir boundaries, and injectivity, applying the same pressure transient analysis theory as conventional buildup tests but with sign-reversed pressure change and a pressure-dependent mobility contrast at the water-oil bank.

Key Takeaways

The falloff test — shutting in an injection well and measuring pressure decline versus time — is the standard injection well test, analogous to the buildup test for producing wells.
In waterflood injection wells, the pressure transient may probe two regions with different mobilities: the near-wellbore water bank and the more distant oil bank, creating a characteristic composite system response on the Horner or log-log diagnostic plot.
Injectivity tests measure the relationship between injection rate and bottomhole pressure to establish the maximum injection rate that does not exceed the formation fracture pressure, preventing unintended hydraulic fracturing of the injection interval.
Step-rate tests inject at incrementally increasing rates to directly identify the formation parting pressure — the rate and pressure at which the formation matrix fractures — which must not be exceeded in injection operations.
Injection well pressure data can often be monitored at surface rather than downhole in reservoirs with sufficient pressure to maintain a full fluid column, simplifying data acquisition compared to producing well testing.

How Injection Well Testing Works

The theory of injection well testing mirrors pressure buildup analysis for producing wells. When injection ceases, the excess pressure created at the wellbore by injection begins to dissipate radially into the reservoir. The rate of pressure decline is controlled by reservoir permeability, fluid mobility, wellbore storage, and skin — the same parameters that control pressure buildup after production. The falloff test data is analysed using the same semi-log straight-line (Horner) and log-log diagnostic plot (derivative) methods applied to buildup tests, with the Horner time ratio adapted for the injection period.

The mobility contrast between injected water and in-situ oil or gas creates a complication absent in single-phase testing. Near the wellbore, the invaded region contains water with its mobility (kw/muw); beyond the water front, undisturbed formation contains oil or gas with a different mobility (ko/muo or kg/mug). A pressure transient propagating through this composite system encounters two different diffusivity environments and produces a response that deviates from single-zone behaviour. Composite reservoir analysis, which models the inner and outer zones separately, is required to accurately determine reservoir parameters from falloff tests in active waterflood injectors.

Injection Well Testing Across International Jurisdictions

In Canada, injection well testing is required as part of scheme approvals for enhanced recovery projects under AER Directive 065 (Reservoir Pressure Management and Waterflood Scheme Applications). Injection-zone permeability and skin factor determined from falloff tests are key inputs to the material balance and reservoir simulation models that AER reviews when approving secondary recovery schemes for Cardium, Viking, and Pembina Nisku oil pools in Alberta. The AER requires operators to conduct a minimum injectivity test programme before initiating waterflood on any new injection well, with results submitted as part of the scheme approval documentation.

In the United States, EPA Underground Injection Control (UIC) Class II well regulations require permitted injection wells to demonstrate injectivity within permitted pressure limits; falloff tests and step-rate tests are the standard tools for demonstrating compliance with formation parting pressure constraints before and after any change in injection operations. BSEE offshore injection well permits require demonstration of injectivity parameters and fracture pressure for OCS waterflood projects. Gulf of Mexico deepwater waterflood projects, such as Perdido Norte and Cascade-Chinook, have employed sophisticated falloff test programmes with permanent downhole gauges to characterise the highly heterogeneous Pliocene and Miocene turbidite reservoir connectivity. In Norway, Sodir requires operators to submit reservoir performance data including injection well test results as part of annual Field Development Plan updates; Equinor's waterflood performance at Johan Sverdrup is monitored using permanent downhole gauge data that enables continuous injection well pressure analysis without interrupting the injection programme. NORSOK D-010 well integrity requirements apply to injection well completion design and the pressure management procedures during testing. In Australia, NOPSEMA-regulated carbon capture and storage (CCS) injection projects in the Carnarvon Basin use step-rate tests to characterise the injectivity and fracture pressure of the storage formation, with regular falloff tests to monitor formation response as injection proceeds. In the Middle East, Saudi Aramco's maximum flood effort waterflood management at Ghawar requires systematic injection well testing programmes to track injectivity changes through the producing life of the field, with permeability and skin data used to optimise injection pattern and targeting for the next five-year field development cycle.

Fast Facts

The largest waterflood in the world — Saudi Aramco's seawater injection programme at Ghawar field — injects approximately 8 million barrels of seawater per day through hundreds of injection wells to maintain reservoir pressure in the Arab Formation carbonate reservoir. The pressure management of this injection programme relies on systematic injection well testing, including falloff tests and interference tests between injector-producer pairs, to characterise the reservoir permeability distribution and optimise injection pattern targeting across the 280 km by 30 km field area.

Step-Rate and Injectivity Tests

Before fracture-pressure constraints can be managed, the formation parting pressure must be known. Step-rate tests inject at incrementally increasing rates (each rate maintained until pressure stabilises) while recording bottomhole pressure. The injection pressure versus rate plot shows a linear trend at sub-fracture rates — reflecting matrix injectivity — then a break to a steeper or flatter slope at the rate where matrix fracturing begins. The formation parting pressure identified from this break defines the maximum allowable injection pressure for matrix (non-fractured) injection and guides the injection rate envelope for waterflood optimisation and disposal well operations where fracturing must be avoided to prevent unintended migration of injected fluid out of the target zone.

Tip: When interpreting a falloff test in a waterflood injector that has been operating for many months, account for the effect of the water bank radius on the derivative response. A composite system with an inner water bank will show a first radial flow regime reflecting water bank permeability, a transition zone, and a second radial flow regime reflecting total reservoir mobility. If the first semilog straight line is used to calculate permeability without recognising the composite character, the result will reflect only the water bank properties — potentially underestimating or overestimating the far-field reservoir permeability by 30 to 50% depending on the relative mobilities. Log-log derivative plots with composite system type curves are essential for identifying and correctly interpreting the multi-zone response.

Injection well testing is also known as:

Falloff test — the specific test type most commonly used for injection wells; analogous to the pressure buildup test for producers; used when the discussion focuses on the test procedure rather than the well type
Injectivity test — the assessment of the relationship between injection rate and wellbore pressure; used specifically when measuring injectivity index and fracture pressure limits
Step-rate test — the specific procedure for determining formation parting pressure; a subcategory of injection well testing used before and during waterflood and disposal well operations

Frequently Asked Questions

What is a falloff test in injection well testing?

A falloff test is conducted by shutting in an injection well after a period of steady injection and recording the bottomhole pressure decline versus time. The pressure declining back toward the original reservoir pressure follows a semi-log straight line on the Horner plot, from which reservoir permeability-thickness product and skin factor are calculated using the same equations as pressure buildup analysis. The test provides the reservoir characterisation data needed to monitor injectivity, detect wellbore damage or stimulation effects, and verify that injection is penetrating the intended zone at the expected rate.

How does injection well testing differ from production well testing?

Injection well testing uses pressure decline (falloff) where production well testing uses pressure buildup, but the mathematical analysis is otherwise symmetric. The main practical difference is that injection wells in active waterflood programmes can often be monitored at surface pressure rather than requiring a downhole gauge, because full wellbore fluid columns maintain a predictable hydrostatic gradient. The main theoretical difference is that injection wells in waterfloods create a composite reservoir system with two different fluid mobilities, requiring composite analysis to accurately extract far-field reservoir properties from the falloff test response.

Why Injection Well Testing Matters in Oil and Gas

Waterflood and pressure maintenance by injection are the primary mechanisms for recovering oil and gas beyond primary depletion — accounting for the majority of global incremental production above primary recovery factors. The efficiency of any injection programme depends on understanding how well injectivity is distributed between wells and zones, whether injected water is channelling through high-permeability paths, and whether injection pressure is approaching the fracture limit that would cause loss of containment. Injection well testing through falloff and step-rate tests provides the reservoir characterisation data that answers these questions at the individual well level and supports the field-scale pattern analysis and simulation model updates that guide waterflood optimisation decisions governing billions of dollars of incremental oil production annually.