Preserved Core | Oil Authority

A preserved core (also called a native-state core or preservation core) is a whole-core or sidewall core sample that has been cut from the subsurface formation and handled in a manner specifically designed to retain the original in-situ fluid distribution, wettability, and pore fluid chemistry of the reservoir rock — typically by sealing the core immediately at the wellsite in aluminum foil, plastic wrap, or wax to prevent evaporation and oxidation, storing the sealed core at or below reservoir temperature, and maintaining the original connate water and crude oil saturation in the pore space so that laboratory measurements on the preserved sample reflect the true reservoir conditions rather than the altered state created by conventional core handling that allows fluids to evaporate, oxidize, or redistribute during transport and storage.

Key Takeaways

Conventional core handling (allowing core to air-dry at surface conditions after retrieval) irreversibly alters the wettability of reservoir rock — as light hydrocarbons evaporate and crude oil oxidizes, the polar compounds (resins, asphaltenes) in the crude oil adsorb more strongly onto the rock surfaces, artificially increasing the oil-wetness of pore surfaces beyond the original in-situ condition; laboratory measurements of relative permeability and capillary pressure on conventionally handled cores therefore reflect an artificially oil-wet state rather than the mixed-wettability or water-wet conditions that may have existed in the reservoir, leading to incorrect predictions of oil recovery efficiency from waterflooding and other secondary recovery methods.
The critical measurements that require preserved core rather than conventional cleaned-and-dried core include: Amott-Harvey wettability index (comparing spontaneous imbibition of oil versus water), USBM wettability index (comparing capillary displacement curves), unsteady-state relative permeability (sensitive to initial wettability and fluid distribution), formation water resistivity and salinity from extracted connate water (used for Archie water saturation calibration), and initial oil and water saturation determination by retort or Dean-Stark distillation without intermediate evaporative losses.
Native-state core preservation begins at the wellsite immediately after core retrieval — the typical preservation procedure wraps the trimmed core plug or whole-core section in water-impermeable aluminum foil, overwraps in plastic, seals all ends with wax, and places the preserved core in a refrigerated cooler at 4°C to 10°C for transport to the core analysis laboratory; laboratory storage continues at refrigerated temperature until analysis, with total preservation time from retrieval to analysis typically less than 6 months to prevent long-term oxidation through the seal; some operators use liquid nitrogen freezing for deep-ocean cores or for time-sensitive preservation of very volatile crude systems.
Restored-state core (also called fresh-state core, distinguished from true native-state) is a compromise procedure used when preservation is impractical — a conventionally cleaned core plug is saturated with representative formation brine and crude oil at reservoir temperature and pressure conditions, then aged at these conditions for several weeks to allow the polar compounds in the crude to re-adsorb onto the rock surfaces and restore the wettability state toward the original in-situ condition; while not as accurate as true native-state preservation, restored-state cores are more representative than conventionally cleaned cores and are widely used when preserved native-state core is unavailable for the planned measurements.
Preserved core programs are significantly more expensive than conventional coring programs because of the specialized handling requirements at the wellsite (trained preservation crew, refrigerated transport), higher laboratory analytical costs (special core analysis (SCAL) on preserved plugs costs 5 to 10 times more per sample than routine core analysis on conventional plugs), and the extended timeline for laboratory analysis (months versus weeks for routine analysis); the cost is justified for reservoir characterization programs where relative permeability and wettability data will be used to calibrate reservoir simulation models that drive multibillion-dollar secondary or tertiary recovery investment decisions.

Fast Facts

The importance of core preservation for wettability-sensitive measurements was recognized in pioneering laboratory work by Taber, Archie, and Buckley in the 1940s and 1950s, but widespread adoption of preservation protocols in industry practice did not occur until the 1970s and 1980s when the economic importance of waterflood recovery efficiency predictions became clear to major operators. API Recommended Practice 40 (Recommended Practices for Core Analysis, 2nd edition 1998) devotes an entire chapter to core preservation procedures, defining native state, restored state, and cleaned state conditions and specifying preservation handling requirements for each. The North Sea operator consortium Core Analysis Forum (SCAL Group) published influential preservation protocols in the 1980s that became industry standards for North Sea SCAL programs and were subsequently adopted globally.

What Is a Preserved Core?

Core samples retrieved from a wellbore go through significant stress changes from their original in-situ state — the pressure drops from thousands of psi to atmospheric, the temperature drops from reservoir temperature to surface ambient, and the fluids in the pore space begin to redistribute and change chemical state the moment the core leaves the formation. For many routine measurements — porosity, permeability, grain density, mineralogy, mechanical strength — this change of state does not affect the result significantly, and cores can be handled conventionally (allowed to reach ambient conditions) before analysis. But for measurements that depend directly on the in-situ fluid distribution and rock surface chemistry — particularly wettability and the capillary-controlled relative permeability to oil and water — the conventional handling process destroys the very information the measurement is designed to capture.

The fundamental issue is wettability alteration. Reservoir rock surfaces in hydrocarbon-bearing formations are coated with crude oil polar compounds (asphaltenes, resins, naphthenic acids) that establish the wettability state — the tendency of the rock surfaces to prefer contact with oil rather than water or vice versa. This wettability directly controls the capillary pressure curves and relative permeability characteristics that determine how efficiently water injection can displace oil from the pore space. When a core is cut, the light volatile hydrocarbons evaporate rapidly (within hours), and the remaining crude oil oxidizes (over days to weeks), causing changes in the polar compound distribution on the rock surfaces that shift the wettability from the original state toward a more oil-wet condition. Measurements made on this altered surface reflect an artifact of the handling process, not the true reservoir wettability.

Preserved core programs are the solution — by sealing the core immediately at the wellsite to prevent evaporation and oxidation, the original fluid distribution and surface chemistry are locked in place, allowing the wettability measurements made weeks or months later in the laboratory to reflect the true in-situ conditions. The investment in preserved core handling is repaid many times over when it prevents the expensive miscalculation of waterflood efficiency that would result from using altered-state core data to calibrate the reservoir simulation model driving a multi-billion-dollar secondary recovery program.

Preserved Core Handling and Laboratory Procedures

The preservation protocol begins at the wellsite before the core is opened — the core barrel is laid horizontally after retrieval and the exterior is inspected for damage before opening, but the plugging and sampling procedure must be completed rapidly to minimize air exposure time. Each core section for preservation is cut to the required length (typically 50 to 100 mm for plug-size preserved samples, or 1 to 2 meter sections for whole-core preservation), wrapped immediately in two layers of water-impermeable aluminum foil with the shiny side facing the core, overwrapped in two layers of polyethylene film, and all joints sealed with petroleum wax or epoxy putty. The sealed packages are labeled with depth, orientation, and handling instructions, and placed in refrigerated storage (4°C) for transport.

At the laboratory, preserved cores are maintained refrigerated until analysis. The wettability test procedure (Amott-Harvey method per API RP 40) involves measuring the spontaneous imbibition and forced displacement of oil and water in the preserved plug to calculate the wettability index without prior cleaning — the entire test is conducted on the preserved core without exposing it to any cleaning solvents that would remove the native crude oil coating from the surfaces. Special core analysis (SCAL) relative permeability tests on preserved plugs use unsteady-state flooding with native connate water and representative crude oil to measure relative permeability curves that reflect the original wettability conditions, providing the most accurate relative permeability data available for reservoir simulation history-matching and production forecast calibration.

Handling deviations from the preservation protocol — particularly any period of air exposure, elevated temperature above 25°C during transport, or mechanical damage to the seal that allows partial evaporation — create an intermediate state between true native preservation and full air-exposure alteration that may be difficult to detect and may bias subsequent wettability measurements in unpredictable directions. Preservation quality is assessed at the laboratory by measuring the initial saturation of the preserved plug (before any analysis) and comparing it to the expected initial saturation from formation evaluation data; a significant deviation (more than ±10 saturation units) indicates that the preservation may have been compromised.

Preserved Core Applications Across International Jurisdictions

Canada (AER / WCSB): WCSB heavy oil and oil sands reservoirs — characterized by very high crude oil viscosities and complex wettability states between the Athabasca bitumen and the clays and sands of the McMurray Formation — require preserved core for SCAL programs that characterize the thermal recovery efficiency of SAGD and cyclic steam stimulation operations. Cenovus, CNRL, and MEG Energy conduct preserved core programs on exploration and appraisal wells in the Cold Lake and Peace River heavy oil areas to measure wettability, capillary pressure, and end-point saturations that define the steam-oil ratio and recovery efficiency for thermal recovery scheme design. AER requires that development plans for in-situ oil sands projects include SCAL data from preserved or restored-state core to support the production forecast commitments in the commercial scheme application.

United States (API / BSEE): Gulf of Mexico deepwater operators (Chevron, Shell, BP) conduct preserved core programs on Paleogene turbidite exploration wells to characterize the wettability and capillary pressure properties of the deepwater sand reservoirs, which have unusual wettability characteristics due to the low-API-gravity waxy crude oils that characterize some Paleogene accumulations. API RP 40 preservation protocols are specified in BSEE-compliant core handling programs for all OCS exploration core programs, with chain-of-custody documentation requirements ensuring that the preserved core handling quality can be verified by regulatory review. Permian Basin unconventional Wolfcamp and Bone Spring core programs increasingly include preserved samples for wettability evaluation because the mixed-wettability pore systems of these tight oil formations control the relative permeability characteristics relevant to gas injection EOR design.

Norway (Sodir / NORSOK): NCS operators are legally required to submit a percentage of all core samples to Sodir's Norwegian Core Repository at the Norwegian Petroleum Directorate in Stavanger, where preserved and conventional cores from NCS exploration wells are archived and made available for future research and comparison studies. NORSOK G-001 (Marine Site Investigations) and Sodir's core handling guidelines specify preservation procedures for NCS cores, with particular attention to the preservation of Ekofisk chalk and Brent Group sandstone cores for SCAL programs that define the multiphase flow properties essential for NCS field development planning. Equinor's SCAL laboratory at the Trondheim Technology Centre has published extensively on preserved chalk core relative permeability and wettability, establishing the North Sea chalk wettability characteristics that underpin the highly efficient Ekofisk waterflood design.