Coreflooding: Definition, Core Analysis Laboratory Testing, and Reservoir Characterisation

What Is Coreflooding?

Coreflooding is a laboratory test in which fluids are injected through a core plug or core composite at controlled temperature, pressure, and flow rate to measure reservoir rock properties under simulated reservoir conditions. The core sample — typically a 1.5-inch diameter plug cut perpendicular or parallel to the bedding plane from a whole core — is mounted in a hassler sleeve inside a pressure vessel, confined at reservoir net stress, and flooded with reservoir-representative brine, oil, gas, or EOR chemicals. Coreflooding measures absolute permeability, relative permeability, residual oil saturation, fluid compatibility, formation damage propensity, and EOR recovery efficiency — the essential laboratory data that characterises how a reservoir rock will behave in the field and that cannot be derived from log data alone.

Types of Coreflood Tests

The most common coreflood test types serve different objectives. Absolute permeability measurement floods the core with a single fluid (brine or gas) at multiple flow rates and applies Darcy's Law (k = qμL / AΔP) to determine permeability. Unsteady-state (USS) displacement floods the core with one fluid while monitoring production of the displaced fluid — the JBN (Johnson-Bossler-Naumann) method back-calculates kr at multiple saturations from the production history. USS is faster than steady-state (SS) but requires careful interpretation to avoid viscous instability artefacts at unfavourable mobility ratios. Steady-state (SS) corefloods co-inject oil and water at fixed fractional flow ratios until each saturation equilibrates — more accurate than USS, particularly at low and high saturations, but takes 5–10× longer.

EOR corefloods — the highest-value application — inject EOR slugs (polymer, surfactant, alkaline-surfactant-polymer, CO2, solvent, low-salinity water) following a waterflooded core to quantify incremental oil recovery and measure chemical adsorption. The test validates that the EOR method is compatible with the specific rock mineralogy and formation brine, that adsorption losses are within economic limits, and that the incremental oil mobilised (typically as tertiary recovery above Sor from waterflood) justifies the chemical cost. A typical ASP coreflood runs 3–6 months including waterflooding to Sor, then chemical slug injection, then water chase — total cost $30,000–80,000 per test, trivial against a $500 million EOR field implementation decision.

Coreflooding Synonyms and Related Terminology

Coreflooding is also referred to as:

• Core flood / core flow test — operational terms used in laboratory context
• Displacement test — describes the fluid displacement mechanism being studied
• SCAL (Special Core Analysis) — the category of core measurements that includes coreflooding, as distinguished from routine core analysis (porosity, permeability, grain density)
• EOR coreflood — specifically when the test evaluates an enhanced oil recovery chemical or process

Related terms: Relative Permeability, Formation Damage, Polymer Flooding, Wettability

Frequently Asked Questions About Coreflooding

Why do coreflood results sometimes not match field performance?

Coreflood-to-field scale-up failures have several common causes. Core plug scale (2.5 cm × 5 cm) cannot capture reservoir-scale heterogeneity — a permeability streak that controls field waterflood performance may not intersect any single plug. Wettability of cleaned and restored-state cores may differ from in-situ wetting state. Flooding rate in the lab may differ from field reservoir Darcy velocity, affecting capillary number and residual saturation. Temperature and pressure effects may be incompletely reproduced. Composite cores from multiple plugs introduce end-effects at plug interfaces that distort saturation distribution. The most reliable approach combines coreflood data with field-scale geological heterogeneity models in compositional simulation — using coreflood kr and Sor at the pore scale and propagating these through the geological model to predict field behaviour.

What is the difference between SCAL and routine core analysis?

Routine core analysis (RCA) measures basic reservoir properties on a large number of plugs: porosity, air permeability, grain density, and sometimes formation factor. These measurements are fast (1–2 days per plug), inexpensive ($50–100/plug), and run on cleaned, extracted plugs without reservoir fluid. Special core analysis (SCAL) includes advanced measurements — relative permeability corefloods, capillary pressure curves, wettability, formation factor at reservoir stress, NMR T2 distribution, acoustic velocity, and EOR tests — that require native or restored-state cores, reservoir-condition pressure and temperature, and complex experimental setups. SCAL tests run for weeks to months each and cost $5,000–100,000 per test. A typical field evaluation programme runs RCA on 200–500 plugs and SCAL on 20–50 representative plugs from the full formation column.

Can coreflooding test CO2 EOR performance?

Yes — CO2 corefloods are routinely used to test miscibility conditions, minimum miscibility pressure (MMP), and incremental oil recovery from CO2 injection above and below MMP. The MMP is determined by slim-tube tests (an alternative to conventional corefloods) that displace oil through a long, small-diameter tube at varying pressures — the pressure at which oil recovery exceeds 80% at 1.2 pore volumes injected is the MMP. Above MMP, CO2 mixes with oil in all proportions (first-contact or multi-contact miscible) and can theoretically displace 100% of contacted oil. CO2 corefloods then validate realistic recovery including gravitational override, viscous fingering, and early breakthrough effects that slim-tube tests do not capture. MMP data from corefloods directly sets the minimum reservoir pressure for which CO2 EOR is economic — a critical design parameter for carbon capture and utilisation (CCUS) field projects.

Why Coreflooding Matters in Oil and Gas

Coreflooding provides the only direct measurement of how reservoir rock will respond to injected fluids — whether injection water, EOR chemicals, or stimulation acids — under realistic reservoir conditions. Every waterflood design, every EOR project, and every completion fluid selection programme depends on coreflood data to validate compatibility, quantify recovery potential, and de-risk the significant capital commitments involved. A well-executed SCAL programme on 30–50 representative core plugs provides the reservoir engineering team with the definitive relative permeability, residual saturation, and fluid interaction data that underpins decades of field development decisions. The $500,000–1,000,000 cost of a comprehensive SCAL programme is typically recovered within months from improved field performance decisions.