AP-608 Neo Automated Gas Permeameter and Porosimeter

Coretest · technical description

Automated Gas Permeameter–Porosimeter · CTS-AP608N-TD-01 · Rev. A · July 2026

1. Purpose

The AP-608 Neo measures gas permeability and open porosity on cylindrical plugs at a set confining pressure. Permeameter and porosimeter share one pneumatic manifold and one program, so both properties come off a single loading of the core holder.

A run returns open porosity and pore volume, absolute gas permeability, Klinkenberg-corrected permeability, the slip factor, the inertial coefficient and grain volume. Confining pressure is applied in steps, up to twenty per run, so the stress dependence of porosity and permeability is acquired in the same experiment instead of as separate work.

The instrument is built for routine core analysis throughput: reservoir characterisation, incoming quality control of plug preparation, screening of tight and conventional rock, stress-sensitivity studies. There is no ionising radiation source in the instrument, so no licence, shielding or radiation safety programme is required.

2. Method

2.1 Permeability: unsteady-state pressure falloff

The plug is sealed in the core holder by an elastomer sleeve under confining pressure, so gas flows only along the axis of the sample. A reservoir of known volume above the sample is charged with the measurement gas to the working pore pressure. The valve opens, gas discharges through the sample to atmosphere, and reservoir pressure falls.

The instrument records the whole falloff curve and reduces it with the full-transient unsteady-state solution after Jones (API RP 40). The entire curve enters the calculation rather than a single operating point, so one transient yields the Klinkenberg permeability k∞, the slip factor b and the Forchheimer inertial coefficient β together. No flowmeters are required, and neither is a separate multi-point sequence at different mean pressures. The reservoir is the flowmeter: instantaneous flow into the sample follows from the reservoir volume and the rate of pressure decay, so nothing in the instrument requires per-gas calibration or carries a zero drift.

Several reference reservoirs are each calibrated individually. The program selects the reservoir to suit the sample: on tight rock a small volume gives time resolution, on highly permeable rock a large volume keeps the transient from collapsing into a few seconds.

2.2 Porosity and pore volume

A known volume of gas at a known pressure expands from a calibrated reservoir into the chamber holding the sample, which remains under confining pressure. The sleeve seals the curved surface, so the gas occupies only the connected pore space. Pore volume follows from the pressures before and after the expansion (Boyle's law, isothermal), and open porosity from the pore volume and the bulk volume of the sample. Sample dimensions are entered into the program directly from the digital caliper.

2.3 Grain volume

Grain volume is measured in a separate chamber by the same gas expansion, with calibrated billets and inserts sized to the sample; the chamber is part of the base configuration. It is a second, independent route to porosity: disagreement with the pore-volume result points straight at an incompletely extracted or incompletely dried sample. Chamber volumes are not computed from geometry. They are calibrated by expansion against certified steel billets, and porosity is exactly as sound as that calibration chain. Grain volume is taken at ambient pressure, pore volume in the core holder under confining stress; the instrument records which route a porosity value came from.

2.4 Gases

The working gases are helium, nitrogen and air; all three are inert and do not adsorb on reservoir rock. Adsorbing gases are unsuitable for permeability measurement in principle, since adsorption adds a storage term the flow models do not contain. Air is measured, not back-calculated from a helium result. Separately, the program computes air permeability at a mean pressure set by the operator, to match the basis a laboratory keeps its historical database on.

3. Construction

The instrument is a self-contained pneumatic and electronic assembly. Main units:

• confining system, which generates and holds hydrostatic pressure on the sleeve; an automatic pump runs the stepped programme;
• pore-pressure circuit;
• calibrated reference reservoirs with automatic selection;
• manifold with pneumatic valve block, routing gas between the reservoirs, the sample chamber and the grain-volume chamber;
• core holder with end-face loading, pressure-sealed core insertion, and removal without tools and without draining the confining fluid;
• grain-volume chamber with billets and inserts;
• pressure and temperature transducers, calibrated per channel including the decay channel;
• control electronics and PC.

Wetted parts are stainless steel and Hastelloy; the sleeve is Viton. At up to 70 kg the instrument sits on a laboratory bench, with no lifting equipment and no foundation. Heating, a support stand and a thermal insulation jacket are supplied separately.

4. Automation and software

A measurement is set up once and then runs on its own: leak check, confining steps, reservoir selection, transient acquisition, stabilisation check, computation, report. No operator is needed between steps, which removes the main source of scatter in stress-dependence work.

The leak check runs before every measurement and shows where the leak is, not merely that there is one. In an unsteady-state method this is not an auxiliary function: a leak indistinguishable from flow through the sample biases tight-rock results low, and a temperature drift over the decay presents itself as a pressure change. Leak tightness and thermal stability of the manifold are the measurement.

Equilibrium is judged against a criterion of 0.2 % per minute rather than against a timer. Helium warms on expansion, and reading pressure off the clock before the curve has flattened builds an error into the Boyle's law computation. The raw transient is stored together with the result, so a measurement can be re-reduced later without repeating the experiment. Reports are produced as tables and plots, including porosity and permeability against effective stress and the Klinkenberg plot. The program runs under Windows and is protected against unintended and unauthorised modification.

5. Technical characteristics

Figures apply to the current production version of the instrument.

5.1 Measurement methods

5.2 Measuring ranges and accuracy

5.3 Operating envelope

5.4 Samples

5.5 Computed parameters

5.6 System and automation

5.7 Construction and handling

5.8 Utilities

6. Scope of supply

Reference reservoir volumes are nominal; actual values from individual calibration are recorded in each instrument's calibration data. Core holders and check plugs for other diameters, and a compressor, are supplied on request.

7. Installation and service

The instrument is a benchtop unit and requires no foundation. It needs a stable bench, a single-phase supply, clean dry compressed air at 87–116 psi (6–8 bar) and a measurement gas supply regulated to 203–362 psi (14–25 bar). Installation and commissioning are carried out on site by a manufacturer's engineer and closed with a signed commissioning certificate. Operator and maintenance training is delivered on site, with a training report and an attendance record. A full spare parts list and the operating documentation set are handed over with the instrument.