The Utrecht HPT lab

The HPT or High Pressure and Temperature Laboratory at Utrecht University is one of the largest and best-known university laboratories for rock and fault mechanics research at elevated pressures and temperatures in Europe. It is run by the Experimental Rock Deformation Group, within the Department of Earth Sciences, led by Prof. C.J. Spiers and colleagues Prof. J.H.P. de Bresser, Dr. C.J. Peach, Dr. A.R. Niemeijer and Dr. S.J.T. Hangx. From around January 2019, the HPT Lab will be located at a new site on the UU campus, within the newly built Earth Simulation Laboratory (ESL) complex.

The Experimental Rock Deformation Group, also known simply as the HPT Lab, specializes in research on the mechanical behaviour and transport properties of Earth materials at conditions ranging from the upper crust to upper mantle, with a focus on the effects of fluid-rock interaction. The HPT laboratory is equipped with a wide variety of facilities for addressing these topics and is supported by a team of three dedicated technicians.

Equipment available ranges from apparatus for deformation and fault friction/rheology studies at pressures and temperatures up to 600 MPa and 1200 °C, to equipment for permeametry and impedance measurement, as well as high temperature furnaces and apparatus for thermal, microstructural and IR analysis. Transnational access to the HPT lab facilities is available in the framework of EPOS TCS Multi-scale laboratories.

The main facilities are summarized below.

Deformation apparatus for axisymmetric compression and direct shear (friction) experiments

Triaxial compression apparatus with dilatometry and permeametry plus direct shear option (Heard apparatus)

This apparatus consists of an externally heated, conventional triaxial compressive testing machine, with silicone oil as the confining medium, linked to logging and several pressure control systems. Key features of the apparatus are a main pressure vessel, containing the sample, a linked auxiliary pressure vessel, and a load transmitting yoke/piston assembly driven by a motor/gearbox/ball-screw system. The set-up is equipped with a pore fluid pressure control system plus sample dilatometry and permeametry systems (gas, transient step and liquid-steady state flow-through capabilities). Confining and pore fluid pressure are controlled using ISCO 65D servo-controlled pumps.

The Heard triaxial apparatus is suited for axisymmetric compression testing of cylindrical samples at confining pressures up to 100 MPa, and temperatures up to 400 °C, either at constant stress (creep tests) or constant strain rate (10-4-10-7 s-1). In addition, it is possible to perform stress stepping and relaxation tests, as well as strain rate stepping tests. This machine has been used to study subsurface CO2 mineralisation and fault rock permeability. The Heard apparatus can be used for direct shear friction experiments as well, making use of a two-component direct shear assembly. The direct shear assembly consists of two L-shaped forcing pistons, sandwiching a gouge layer of dimensions 47x35 mm and 1-2.5 mm thickness in between. Both constant strain rate and strain rate stepping tests can be carried out in the direct shear configuration (same conditions as for axisymmetric compression testing), with the purpose of measuring the frictional resistance and the time- and velocity-dependence of friction.

Triaxial compression apparatus with pore fluid pressure control, pore volumometry and direct shear option (Shuttle vessel)

This internally heated, 100 MPa confining pressure vessel is mounted on a standard Instron 1362 loading frame with a servo-controlled positioning system. This machine can be used to deform cylindrical samples at a constant piston velocity, ranging from 10-7 to 10-2 m/s. The displacement is measured outside the vessel using a (Instron standard) linear variable differential transformer (LVDT), with 2 μm resolution. The apparatus is equipped with two (internally and externally located) load cells, each of 100 kN capacity. The confinement on jacketed samples is done by silicone oil, with the pressure being controlled by a servo pump, which also serves as dilatometer to carefully monitor the compaction/dilation of the sample under study. It is also facilitated with an internal heating furnace to locally heat up the sample, up to a maximum temperature of 150 °C. Recently, a high-resolution stress-relaxation method was developed for simulating rheology at in-situ conditions (strain rates down to 10-9 s-1). Like the Heard apparatus, the Shuttle vessel can also be used for direct shear friction experiments, by installing a sample assembly consisting of two L-shaped forcing pistons, sandwiching a gouge layer of dimensions 47x35 mm and 1-2.5 mm thickness in between.

High precision triaxial compression apparatus with pore fluid plus full axial and radial servo control (NAM Machine)

Externally heated, 120 MPa oil pressure medium, servo controlled triaxial deformation apparatus with 200kN axial load capability. Using an internal axial force gauge with low pressure sensitivity, this machine features programmable pore fluid and confining fluid pressure feedback control of ISCO 65D syringe pumps to allow high precision, constant radial strain deformation, to failure, of jacketed 50 mm long cylindrical x 25 mm diameter, rock samples at temperatures up to 150 °C. Unique design with drive mechanics within the pressure vessel to eliminate control hysteresis from seal friction.  Controlled loading is possible via a programmable motor drive subsystem with an automatic gear changer. The overall control system developed by UU using LabView links all features of deformation apparatus. The machine and system development is funded by NAM.

Gas medium, high precision servo-controlled triaxial testing machine (gas apparatus, 600 MPa, 1200 °C)

This internally heated machine can generate pressures of up to 600 MPa and temperatures of up to 1200 °C. Additional differential loading may be applied via an Instron loading frame, to deform materials under lower crustal to upper mantle conditions. Typical applications involve studies of rock rheology and deformation mechanisms at high pressure and temperatures. Recently, the gas apparatus has been used to constrain the strain-dependent rheological behaviour of the lithosphere.

Rotary shear deformation/friction facilities

High PT hydrothermal rotary shear apparatus

This machine is located in an Instron 1362 testing machine for normal stress control. The ring shear machine can be used for shear experiments with a temperature up to about 700 °C, normal stresses up to 300 MPa, shear velocities in the range from 3 nm/s up to 0.3 mm/s, a pore fluid pressure up to 200 MPa and ultrahigh shear strains beyond 100. Currently, this machine is used for the research projects described under fault mechanics.

Room T low-medium velocity rotary shear apparatus with AE capabilities

The apparatus is housed inside a 100 kN capacity Instron 8862 testing machine equipped with a servo-controlled electromechanical actuator that may be operated either in position control or in load control mode. A Parker MH205 motor provides rotary motion to the base plate via a 1:160 harmonic drive gearbox, allowing a maximum rotation rate of 12.5 rpm. Using the motor’s onboard servo-controller, it is possible to control either the rotation rate (and thus shear displacement) or the torque (and thus the shear stress). The base plate is equipped with two angular potentiometers that measure its rotation and is capable of housing samples or pistons of up to 100 mm in diameter. The exact possible range of normal stress and sliding velocities depends on the sample size but are in general between 0.5-40 MPa and 0.01 to 50 mm/s. Some piston sets allow the use of a pressurized pore fluid. A dedicated piston set is equipped with 8 holes on both sides that house AE sensors which are capable of registering acoustic emissions generated during laboratory earthquakes (“stick-slips”). The apparatus has no heating capabilities.

Uniaxial compaction (oedometer) and sorption/dilatometry facilities

Uniaxial (oedometer) compaction cell with pore fluid pressure control (Monel vessel)

This cell is located in an Instron 8862 loading frame. The compaction cell is equipped with an internal load cell (max. axial load 100 kN) and an acoustic emission sensor. Fluids can be introduced into the sample using an ISCO 65D syringe pump (vessel can accommodate fluid pressures of up to 137.9 MPa). Temperature (range up to 100 °C, if the internal load cell is used) is controlled using an external furnace. This cell is primarily used to investigate the effect of fluid chemistry on the compaction of various earth materials, including coals, shales, various evaporite minerals, sands and sandstones.

Uniaxial (oedometer) compaction cell with flow-through capabilities (Remanit vessel)

An Instron 8562 servo-controlled loading frame, furnished with a 100 kN load cell, is coupled with a Remanit compaction vessel in which the temperature (up to 150 °C) and fluid pressure (up to 20 MPa) are precisely controlled. This oedometer-type testing system is further equipped with ISCO 65D syringe pumps and instrumentation to facilitate concurrent mechanical testing and reactive flow-through permeametry. The setup is mainly used to investigate the mechanical response of earth materials undergoing any physical-chemical reactions with reservoir fluids under in-situ hydro-thermo-mechanical conditions. Examples include measuring the swelling stress developed in shale and coal due to CO2/CH4 adsorption, which is of great importance for CO2 sequestration and enhanced shale gas and coal bed methane recovery, and studying potential force of crystallization development in mineral reactions such as metal oxide hydration and peridotite carbonation.

100 MPa sorption dilatometer (eddy current dilatometer)

To measure dimensional changes of mm-sized, cylindrical samples, we have a purpose-built dilatometer consisting of a high-pressure, eddy current sensor, housed in a Remanit stainless steel pressure vessel (inner diameter of 20 mm). Fluid pressures of up to 100 MPa can be introduced to the sample. The assembly allows for measurement of axial expansion or contraction of the sample upon exposure to fluid. The entire set-up can be submerged in a thermobath, to achieve a constant experimental temperature.

3D Dilatometer

This setup consists of three high resolution (~25 nm) eddy current sensors, which are assembled in orthogonal directions, sealed in a glass chamber. With this setup one can measure sorption/desorption induced swelling/shrinkage of tiny cubic samples (<15 mm) exposed to low-pressure gas (e.g. water vapour) environments.

Permeametry and conductivity apparatus

Transient step gas permeameter

Permeabilities of cores (diameters of 25mm and 35 mm) and powders can be measured in the lab using the pressure transient pulse decay method. This apparatus is suitable for number of different gasses (Argon, Helium, CO2 and methane) and can be used over a wide range of confining pressures. The transient step gas permeameter can also be used in conjunction with the Heard apparatus, to measure the permeability of samples deformed by triaxial compressive loading.

Contact resistivity cell (Solartron 1206A, Solartron 1296A dielectric interface)

This cell can be used at (approximately) room temperature and under atmospheric pressure conditions to measure the electric resistivity of a pressure dissolving, annular contact between a glass lens and a salt crystal. At the same time, the growing contact can be directly observed through the glass lens. This cell has been used for example in the investigation of the structural and diffusional properties of grain boundaries under stress.

Pressurised conductivity cell

This cell is submerged in an oil bath and the temperature may be controlled to approximately 0.1 °C. Impedance spectroscopy with this cell can be done by use of a Solartron 1260A frequency response analyzer and 1296 dielectric interface. This cell is used for impedance spectroscopy on loaded powders with pore fluid and has recently been used on coal samples.

Auxiliary facilities

• Vertical Tube furnace with controlled atmosphere

• Nabertherm Superkanthal 1600 °C furnace

• Impedance Spectroscopy Solartron 1260

• Lab presses suitable for pressing material under loads up to 400 kN

• Du Pont 1090 DTA/TGA Thermal Analyser

• Malvern Instruments Mastersizer S long bed laser diffraction particle size analyser

• JASCO 620 infrared/optical microscope with FTIR microanalysis system

• Leica DMRX light optical polarization microscope with image analysis QWIN Pro