Options for the AutoLab Series

Strains are measured with LVDTs and/or strain gauges.  LVDTs are commonly used on samples jacketed with an elastomer such as Viton tubing.  The standard configuration for LVDTs is three devices: two measure the axial shortening and one monitors the radial displacement at the midpoint of the sample.

Strain gauges are used on low porosity rocks jacketed with metal foil.  The gauges are epoxied directly on a metal jacket, such as copper, that has previously been pressure compressed onto the rock surface.  Five strain gauge channels are standard to enable measurement of anisotropic elastic coefficients (VTI).

Both techniques are rated to 175^oC at pressures to 200 MPa. Typical results obtained during a confined compression to failure test using LVDTs are shown.   A compressibility test on a sample of Barre granite using strain gauges is also presented.

One compressional and two orthogonally polarized shear waves are acquired with NER’s PS² velocity transducers.  The PS² transducers:

• withstand dynamic, brittle rock failure,
• generate high signal quality waveforms for all loading protocols,
• operate at temperatures to 190^oC and pressures to 200 MPa

Each transducer consists of a piezoelectric crystal stack with a resonant frequency of 0.50 or 1.0 MHz mounted to a titanium substrate. For time-of-flight measurements, a source generator provides a recurrent signal to a transmitting transducer located at one end of the sample. At the far end of the sample, a receiving transducer records the arriving signal.  The signal is then passed through an amplifier, displayed directly on an oscilloscope, and uploaded to a computer for further processing in our AutoLab v6 software.

Traditional approaches to velocity anisotropy characterization require separate tests on three oriented samples (a sample parallel to bedding, a sample normal to bedding and a sample at 45^o to the bedding). For samples with bedding parallel to the core axis, the five independent VTI coefficients can be measured in a single sample in one test.  The sample assembly shown below is setup to measure both axial and radial velocity.  Each transducer pair measures one P and two orthogonally polarized S waveforms.  The radial transducers are either epoxied to a copper jacket or permanently mounted in a re-useable Viton jacket.  The maximum operating temperature for the radial ultrasonic transducers is 150^oC.

Our PS² transducers have been modified to support velocity captures at temperatures to 350^oC, at pressures to 300 MPa.  By incorporating waveguides into the transducer assembly, the piezoelectric elements can be positioned in the cooler portions of the vessel (below the Curie temperature of the transducers) while the sample resides in the center of the high temperature furnace.

One compressional and one torsional shear wave are propagated through the sample positioned between the waveguides integral to each transducer.  Due to the length of the transducer assembly, torsional waves are the measurement of choice for shear.  Torsion is a fundamental mode for shear and produces excellent signal quality in the high temperature environment.

Compressional waves are dispersive and because of the long travel path yield a longitudinal wave and not a P-wave measurement.  The longitudinal velocity, c, is a directly related to Young’s modulus, E, and density,  by

c= √(E/ρ)

Examples of longitudinal and torsional timeseries collected on a sample of Berea sandstone are shown below.

NER has developed equipment that can perform various transient methods on low permeability rocks (5 nanodarcies to 50 microdarcies) including NER’s proprietary complex transient method.  Included in the equipment overview below is a brief description of the challenges with these methods and how NER has provided tools for our users to proactively assess the errors in the measurements as they are made.

When the storage parameter, S, becomes significant, the transient methods become challenging.  While analytical solutions to the re-equilibration process are available in a number of forms, in this general case there is no closed-form expression to compute permeability, k, from the equilibration process. It is also important to understand that the equilibration process is a strong function of the ratio k/S, and thus in many cases it is difficult to accurately determine k and S individually. The strong dependence on k/S creates a situation where an error in the determination of one parameter (i.e. specific storage) will lead to an error in the determination of the other (i.e. permeability). AutoLab processing is designed to assist the user in quantifying the uncertainties which are a function of the sample dimensions and the choice of transient employed.

Utilizing a special downstream set up with a small dead volume and integral pressure transducer, the standard configuration is designed for low permeability materials of 5 nanodarcies to 50 microdarcies. This option uses NER’s complex transient method for permeability analysis, allowing use of customizable pressure transients as well as more traditional sinusoidal oscillation and pulse decay. Time series data for a Barre granite is shown below.  The permeability was 75 nD and the storage was 6.58 x 10^-7 m^-1

Resistivity is measured as a function of frequency, stress, and temperature using both two- and true four-electrode techniques. NER’s ZMeter impedance analysis is used to perform true four-electrode measurements at frequencies between 0.02 Hz and 100 kHz. Data collected on a brine-saturated sample of Indiana limestone are shown.

For rocks that are permanently deformed during the initial pressurization, simultaneous measurement of velocity and electrical resistivity offers solution to the difficulty. If both measurements are required on a large number of samples, the number of pressurization cycles to obtain the data is cut in half. Both the resistivity and velocity options are required to support this transducer. A transducer for 1.00 inch diameter samples is shown, as well as data collected on brine saturated Indiana limestone at a confining pressure of 10 MPa and a pore pressure of 4.3 MPa.

The permanent deformation of brittle rocks at elevated stress is the result of the initiation and propagation of microcracks. These events often give rise to acoustic emissions (AE).  With an array of acoustic detectors on the surface of the sample, the AE events can be detected and located.  AE events occur not only as the external stress changes, but can also be generated by fluid flow, rock-fluid chemical interaction, phase changes, etc.  This is a useful technique to track in real time the damage in the rock.

NER does not independently produce AE detection systems, rather we recommend the Vallen AMSY-6AE System.  A typical setup consists of eight (8) AE amplifiers, an independent computer, and eight (8) piezoelectric sensors manufactured by NER.  The AE sensors are embedded into a rubber sample jacket in a documented, fixed position.  During a confined compression experiment, the development of microcracks emit stress waves.  If the energy is sufficiently large, the piezoelectric transducers are excited, the Vallen system recognizes them as an AE event, and stores the data in an independent computer.   After a specified dead-band, the system is rearmed and ready to collect another AE.  Hundreds of AE events are collected during a typical experiment.

After the experiment, the database is examined. A large number will have insufficient data or poor signal quality and are discarded.  The remaining candidate events are then analyzed by picking the first arrival on each trace (relative to the transducer that first detected the event).  Then, by knowing the location of the transducers in the array, the relative travel times, and the velocity of the rock, the location of the damage, AE event, can be located.  It is the same process as locating an earthquake detected within a seismic network.  The AE events are then displayed on a 3D representation of the sample.

AE events collected on a sample of Berea tested in confined compression to failure were processed and displayed on the sample (BLUE dots).  Six AE transducers were positioned on the sample (BLACK dots).  The AE events concentrated on a diffuse plane cutting  diagonally through the sample.  This plane coincides with the orientation of the fracture plane  observed on the sample when it was removed from the vessel.

For triaxial loading in the large bore AutoLab systems, flat jacks are used to generate a triaxial state of stress on a rock sample.  The flat jacks are mounted on the base plug of the vessel    A jacketed sample is positioned between the flat jacks.  The confining pressure and a small axial stress are exerted on the sample. To develop a state of triaxial stress, the flat jacks are pressurized to a pressure greater than the confining pressure causing the loading plates to advance and exert a force normal to the axis of the sample.  The reaction force for the principal stress is developed at the vessel bore.  The sample is now in a state of triaxial stress; for most tests this is the intermediate principal stress.

The samples have either a square or circular cross section can be used depending on the application.   The test specimens can be instrumented to measure velocity, acoustic emissions (AE), displacement, strain, and permeability.  A common application is the study of hydrofracture on cylindrical samples; hydrofractures are initiated by varying the radial principal stress and the pore pressure injected into the sample.  To control the position of the hydrofracture, a focused stress on the surface of a cylindrical sample is sufficient.  The faces of the flat jacks are machined to match the radius of the sample. The width of the contact area is adjusted depending on the elastic and strength properties of the rock.  A schematic and photo of a cylindrical test set up are shown below.

system or as a stand-alone fluid mixing and delivery system for general use with other equipment. The module is configured as a stand-alone mobile cart that can be easily moved to various testing apparatuses and docked for use during active flooding of CO₂ saturated brines or supercritical CO₂. Typical pressure and temperature limitations are 70 MPa and 120°C.

The computer controlled system includes a dual piston electromechanical pump that provides pressure control and a measure of volume of CO₂ injected. The heated reaction chamber contains an ultrasonic waveguide with internal reflector to provide a real-time measurement of the acoustic velocity in the brine. Detection of the reflection off the CO₂/brine interface provides a measure of CO₂ and brine volumes in the reaction vessel. Outlets top and bottom provide delivery of humidified sc CO₂ or CO₂ (aq) saturated brine to the test system. Optional line heaters are available for applications that require temperature control between the cart and apparatus.