Porous Plate Technique | Oil Authority

The porous plate technique (also called the porous plate capillary pressure method or porous plate drainage/imbibition apparatus) is a laboratory core analysis method for measuring capillary pressure as a function of fluid saturation in reservoir rock samples — in which a water-saturated core plug is placed on a porous ceramic or metal plate that has a fine pore structure with a known water-entry pressure (threshold pressure), and non-wetting fluid (oil or gas) is applied at controlled pressure to displace water from the core while the plate prevents non-wetting phase entry below its threshold pressure — enabling precise, equilibrium capillary pressure measurements at multiple saturation steps by applying step-wise pressure increments and waiting for fluid redistribution to equilibrate before measuring the saturation.

Key Takeaways

The porous plate technique is considered the most accurate of the three standard capillary pressure measurement methods (porous plate, centrifuge, mercury injection) for determining drainage and imbibition capillary pressure curves at wetting-phase saturations above the irreducible water saturation — it provides true equilibrium measurements because each saturation step is held for hours to days until flow ceases, avoiding the non-equilibrium artifacts that affect high-speed centrifuge and rapid mercury injection methods in low-permeability samples.
The drainage porous plate test (oil displacing water, simulating hydrocarbon migration into a water-wet reservoir) is used to determine: the threshold displacement pressure (entry pressure at which the largest pore throats fill with oil), the irreducible water saturation (Swi — the minimum water saturation achievable at the maximum drainage pressure), and the complete primary drainage capillary pressure curve used to model the initial fluid distribution in a reservoir at static conditions before production begins.
The imbibition porous plate test (water displacing oil, simulating waterflood) measures the spontaneous imbibition curve (at zero capillary pressure, water-wet rock imbibes water without applied pressure until residual oil saturation is reached), the forced imbibition curve (at applied positive capillary pressure), and ultimately the residual oil saturation (Sor) at complete waterflooding — both the spontaneous and forced imbibition data are required to fully characterize the wettability state of the rock and predict waterflood recovery performance.
The porous plate threshold pressure must exceed the maximum capillary pressure applied during the test (to prevent non-wetting phase breakthrough through the plate) but must not be so high that the plate becomes an additional flow resistance that slows equilibration and extends test duration — standard porous plates are rated at threshold pressures of 10 to 150 psi for water-oil systems, with the plate selection depending on the maximum capillary pressure expected for the rock type and the minimum wetting-phase saturation to be measured.
Test duration is the primary practical limitation of the porous plate technique: each saturation step requires hours to days (and sometimes weeks for very tight rocks) for equilibration, making a complete capillary pressure curve with 8 to 15 saturation points require weeks to months of laboratory time — compared to hours for mercury injection or centrifuge methods — limiting porous plate use to higher-permeability samples and situations where the accuracy justification outweighs the time and cost premium.

Fast Facts

API RP 40 (Recommended Practices for Core Analysis) provides the standard procedures for porous plate capillary pressure measurement, specifying sample preparation, porous plate requirements, pressure step sequence, equilibration criteria, and saturation measurement methods. The porous plate technique has been the reference method for capillary pressure measurement in core analysis laboratories since the 1940s, providing the calibration benchmark against which faster methods (centrifuge, mercury injection) are validated for different rock types. Commercial core analysis laboratories offer porous plate capillary pressure as a special core analysis (SCAL) measurement, typically priced at 3 to 10 times the cost of mercury injection capillary pressure due to the longer analyst time and laboratory occupancy. The high accuracy and direct relevance to reservoir conditions (live oil or brine as the test fluids rather than mercury-air) make porous plate the preferred method for calibrating relative permeability end-points and wettability indices used in reservoir simulation.

What Is the Porous Plate Technique?

Capillary pressure is the pressure difference across the interface between two immiscible fluids in a porous medium — it represents the additional pressure needed to force a non-wetting fluid into a pore throat that preferentially contains the wetting fluid. In water-wet reservoir rock, oil (the non-wetting phase) can only enter pores by overcoming the capillary pressure that holds water in those pores. The relationship between capillary pressure and the fraction of pore space filled with each fluid (the capillary pressure curve) is a fundamental rock property that controls: the initial fluid distribution in a reservoir (how much water and oil is in each zone at static conditions), the minimum water saturation achievable by drainage (setting the irreducible water saturation and maximum oil saturation), and the residual oil saturation after waterflood (the minimum oil saturation achievable by imbibition).

The porous plate technique measures this capillary pressure curve directly, at equilibrium, using reservoir-representative fluids — brine and crude oil (or refined oil) rather than the mercury-air proxy used in mercury injection. A saturated core plug is placed on the porous ceramic plate, which acts as a semi-permeable membrane: its fine pores are always water-filled (the plate threshold pressure prevents oil from entering the plate), but water displaced from the core by applied oil pressure can flow through the plate into the collection vessel below. By increasing the pressure step by step and measuring the cumulative water produced at each step, the capillary pressure at each wetting-phase saturation is measured directly.

The resulting capillary pressure curve is directly usable for reservoir engineering calculations without the scaling or conversion required by other methods (mercury injection requires conversion from Hg-air to oil-water or gas-water using fluid property ratios; centrifuge requires solving an integral equation to convert centrifuge speed versus production data to capillary pressure versus saturation). The porous plate curve represents the true equilibrium state of the fluid-rock-capillary-pressure system, making it the most reliable input for reservoir model initialization and wettability quantification.

Porous Plate Procedure and Wettability Assessment

A complete porous plate capillary pressure program for wettability characterization follows the Amott-Harvey protocol or the USBM protocol, which prescribe specific sequences of drainage and imbibition measurements to quantify both the drainage capillary pressure curve and the wettability index of the rock. The standard sequence is: (1) primary drainage from 100% water saturation to irreducible water saturation (Swi), establishing the initial oil-saturated state; (2) spontaneous imbibition at zero capillary pressure until no more water imbibes; (3) forced imbibition using the porous plate at increasing pressure steps to residual oil saturation (Sor); and (4) secondary drainage to re-establish oil saturation for USBM area comparison. This four-step sequence generates the data for both the drainage and imbibition capillary pressure curves and the Amott-Harvey (or USBM) wettability index.

Equilibration time at each saturation step is the critical experimental parameter. Equilibration is reached when fluid production stops at a given applied pressure — practically defined as less than 0.1 mL of fluid produced in 24 hours. For high-permeability sandstones (greater than 100 mD), equilibration takes hours; for tight carbonates (0.1 to 1 mD), equilibration may take days to weeks at each step. The total test duration for a complete Amott sequence in a tight limestone can be 3 to 6 months, requiring stable laboratory conditions and careful sample handling throughout.

Sample conditioning (cleaning and resaturation to restored state) before the porous plate test is critical for obtaining wettability-representative data. Cores cut with water-based drilling fluid are partly altered toward water-wet conditions; cores cut with oil-based mud may have oil-wet filtrate-contaminated zones. The restored state protocol — Dean-Stark or reflux solvent extraction to remove original fluids, drying, brine resaturation, then aging with representative crude oil at reservoir temperature for 4 to 6 weeks — is used to restore the original in-situ wettability as closely as possible before porous plate measurements, ensuring that the capillary pressure curve reflects true reservoir wettability rather than core handling artifacts.

Porous Plate Technique Across International Jurisdictions

Canada (AER / WCSB): WCSB special core analysis programs for SAGD and waterflood design in heavy oil and conventional oil reservoirs include porous plate capillary pressure as the reference measurement for relative permeability end-point calibration and wettability assessment. AER pool scheme submissions for waterflood and EOR approvals typically include capillary pressure data as part of the special core analysis documentation supporting the proposed injection scheme design, with porous plate data preferred over mercury injection for reservoir-representative wettability evaluation. Core analysis laboratories in Calgary (ERCB-accredited labs, Corelab, Reservoir Group) provide porous plate SCAL services for WCSB operators as part of the standard special core analysis program.

United States (API / BSEE): API RP 40 is the governing standard for porous plate core analysis in US operations, providing the method specifications followed by all commercial core analysis laboratories serving US markets. Gulf of Mexico deepwater special core analysis programs for major development projects (Atlantis, Thunder Horse, Tiber) include porous plate capillary pressure as part of the formation evaluation supporting the field development plan, with the Amott-Harvey wettability index from porous plate data providing the wettability input to the reservoir simulation model. Independent reserve evaluators use porous plate capillary pressure data from SCAL programs as part of the technical basis for proved reserve certification under SEC guidelines.

Norway (Sodir / NORSOK): NCS special core analysis programs for major field development plans (PDO submissions to Sodir) include porous plate capillary pressure as the reference wettability measurement for Brent Group sandstone and North Sea chalk reservoirs. The IOR Centre at University of Stavanger has published numerous studies on capillary pressure measurement in NCS core samples, comparing porous plate, centrifuge, and mercury injection methods for specific NCS rock types and providing formation-specific guidance on method selection and data quality. Equinor's core analysis standards specify porous plate capillary pressure for all SCAL programs on NCS development wells where wettability characterization is needed for EOR design.

Middle East (Saudi Aramco): Saudi Aramco's SCAL programs for Arab Formation carbonate reservoir characterization include porous plate capillary pressure as the preferred method for measuring drainage and imbibition curves in the complex pore systems of Arab D and Arab C limestones and dolomites. Aramco's core analysis laboratory in Dhahran conducts porous plate SCAL measurements at reservoir temperature (85°C to 115°C for Arab Formation) using representative Arab Formation crude oil and formation brine to obtain the most representative capillary pressure data for reservoir simulation initialization. The long test times for tight carbonate intervals in Arab Formation SCAL programs (sometimes exceeding 3 months per sample) are accepted given the economic importance of accurate wettability data for Aramco's large-scale waterflood management decisions.