Core Plug: Petrophysical Analysis Sample

What Is a Core Plug?

Core plug (also called a plug sample or core sample plug) is a small cylindrical specimen drilled from a conventional full-diameter core for quantitative reservoir characterization measurements. Typically 1.0 or 1.5 inches in diameter and 1 to 3 inches in length, the plug is cut perpendicular to the core axis (horizontal plug) or parallel to it (vertical plug) using a diamond-tipped drill bit. Core plugs are the fundamental unit of routine and special core analysis, yielding direct measurements of porosity, permeability, grain density, capillary pressure, and electrical properties that calibrate wireline logs and populate reservoir simulation models.

Key Takeaways

Standard plug diameters are 1.0 inch and 1.5 inches; most routine core analysis laboratories accept both sizes, though 1.5-inch plugs are preferred for heterogeneous rocks because the larger volume reduces sampling bias.
Boyle's law helium porosimetry is the industry-standard method for measuring pore volume; helium expands from a calibrated reference cell into an evacuated plug, and pore volume is calculated from the pressure ratio at equilibrium.
Klinkenberg correction converts gas permeability measured at low differential pressures to equivalent liquid permeability by accounting for gas-slippage effects at pore throats; uncorrected gas permeability can overestimate liquid permeability by 10 to 50% in tight rocks.
Dean-Stark extraction removes residual oil and water from a plug before analysis; the technique uses refluxing toluene or a toluene-acetone azeotrope and typically requires 24 to 72 hours to reach constant weight.
Sampling frequency in routine core analysis typically ranges from one plug per 0.3 meters (1 foot) in homogeneous intervals to one plug per 0.15 meters (0.5 feet) in laminated or heterogeneous sequences.

How Core Plugs Are Drilled and Prepared

A diamond core bit mounted in a benchtop drill press cuts the plug from the parent core while a continuous stream of water or compressed air removes cuttings and prevents heat buildup that could alter pore geometry. The orientation of the plug relative to the core axis determines which permeability component is measured: plugs drilled perpendicular to the core axis (horizontal) capture the horizontal permeability that governs lateral fluid flow in the reservoir, while plugs cut parallel to the axis (vertical) measure vertical permeability relevant to gravity drainage and vertical sweep efficiency. In laminated reservoirs where horizontal and vertical permeability differ by one to three orders of magnitude, both orientations are typically cut at the same depth interval to characterize permeability anisotropy.

After drilling, the plug end-faces are trimmed on a trim saw to produce flat, parallel surfaces that seat correctly in the core holder during flow tests. The plug is then cleaned to remove drilling fluid filtrate, native crude oil, and formation water before measurement. Dean-Stark extraction is the most common cleaning method: the plug is suspended above refluxing solvent in a closed system, condensed solvent drips over the plug surface continuously, and displaced oil and water collect in a graduated receiver. Soxhlet extraction uses a similar reflux principle but immerses the plug in solvent periodically. After cleaning, plugs are dried in a humidity-controlled oven at 60 to 105 degrees Celsius to constant weight, which confirms that all mobile water has been removed without damaging clay minerals that would shrink irreversibly at higher temperatures.

Sampling strategy has a significant effect on the apparent reservoir quality derived from core analysis. In interbedded sand-shale sequences, a plug drilled through a thin shale lamina will show near-zero permeability and inflate the shale volume in the petrophysical model, while a plug positioned entirely within a clean sand lamina will overestimate net-to-gross. Experienced core analysts position plugs to capture representative lithofacies based on core photographs, CT scans, and real-time gamma ray measurements taken at the wellsite before the core is shipped to the laboratory. Whole core analysis, which measures petrophysical properties on the full-diameter core without plugging, avoids sampling bias entirely but is considerably more expensive and time-consuming.

Fast Facts: Core Plug

Standard diameters: 1.0 inch and 1.5 inches (25 mm and 38 mm)
Standard length: 1 to 3 inches (25 to 75 mm)
Porosity method: Boyle's law helium porosimetry (SCAL and RCAL)
Permeability method: Steady-state nitrogen flow; Klinkenberg-corrected to liquid equivalent
Cleaning methods: Dean-Stark extraction, Soxhlet extraction, flow-through solvent flushing
Drying temperature: 60-105 degrees Celsius to constant weight
Sampling frequency: 1 plug per 0.15 to 0.30 meters in routine programs
Primary applications: Porosity-permeability transforms, log calibration, simulation input, capillary pressure curves

Laboratory Tip:

Before shipping cores to the laboratory, request a whole-core CT scan at the wellsite. CT images reveal fractures, vugs, laminations, and drilling-induced damage that are invisible on the core surface, allowing the project team to select plug locations that avoid these features and produce measurements representative of the unaltered matrix. CT scanning also provides a permanent record of core condition before any destructive testing begins.

Core plug is also referred to as:

Plug sample — generic laboratory term used in routine core analysis (RCAL) reports to denote a core plug without specifying orientation.
Core sample — broader term encompassing both full-diameter core sections and plug-sized specimens; often used interchangeably with core plug in field reporting.
Mini-core — informal term for a core plug drilled from cuttings or sidewall core material, typically smaller than 1.0 inch in diameter.
Horizontal plug / vertical plug — orientation-specific designations indicating whether the plug axis is perpendicular (horizontal) or parallel (vertical) to the core axis, corresponding to the horizontal and vertical permeability directions in the reservoir.

Frequently Asked Questions About Core Plugs

Why is helium used instead of nitrogen for porosity measurement?

Helium has a very small molecular diameter (about 0.26 nanometers), allowing it to access pore space in tight carbonates and shales that nitrogen or other gases cannot penetrate within reasonable equilibration times. Helium is also chemically inert, so it does not adsorb onto clay mineral surfaces the way nitrogen or carbon dioxide can, ensuring that the measured pore volume reflects only the connected void space accessible to reservoir fluids rather than surface adsorption sites.

What is the difference between routine core analysis and special core analysis?

Routine core analysis (RCAL) covers the basic petrophysical measurements performed on every plug in a core program: porosity, gas permeability, grain density, and lithology description. Special core analysis (SCAL) is conducted on a subset of selected plugs and includes more complex and time-intensive measurements such as capillary pressure curves (mercury injection or porous plate), relative permeability, wettability (Amott-Harvey index), electrical properties (formation factor, cementation exponent), and nuclear magnetic resonance (NMR) relaxation. RCAL establishes the porosity-permeability framework across the interval; SCAL provides the flow-physics parameters that populate multiphase reservoir simulation models.

How do you correct gas permeability to liquid permeability?

At low confining pressures, gas molecules traveling through narrow pore throats slip along the pore wall rather than flowing as a viscous continuum, producing a measured permeability higher than the true liquid (or Darcy) permeability. The Klinkenberg correction plots gas permeability against the reciprocal of mean flow pressure for several pressure steps; the linear regression extrapolated to infinite pressure (zero reciprocal pressure) gives the Klinkenberg-corrected permeability, which equals the equivalent liquid permeability. This correction is most significant in tight rocks with permeabilities below 1 millidarcy, where the ratio of gas-slip permeability to liquid permeability can exceed 2.0.

Why Core Plugs Matter in Oil and Gas

Core plugs provide the only direct, quantitative measurements of rock properties that can be obtained from the subsurface. Every porosity and permeability value used to calibrate a wireline log interpretation model, every capillary pressure curve used to initialize fluid saturations in a reservoir simulation, and every relative permeability curve used to forecast production performance ultimately traces back to laboratory measurements on core plugs. Without reliable plug data, reservoir engineers must rely entirely on log-derived properties and analog databases, introducing uncertainties that compound through the entire development planning process. The careful selection, preparation, and measurement of core plugs — and the rigorous correction of those measurements for overburden stress, temperature, and fluid type — is therefore one of the most consequential technical activities in field appraisal and development.