A method and cell for freezing formation layer sample tests

Abstract: A method for determining temperature dependant stress-strain characteristics of a formation layer sample (3) comprises the following steps: a) simulating characteristic underground conditions for said sample (3), b) creating an axial central hole (6) in said sample (3) which functions as a coolant conduit, c) arranging said sample in a triaxial cell (1) in which said sample (3) is loaded to predetermined test pressures, d) cooling said sample (3) during a freezing process outwardly from said central hole (6) by circulating a coolant (8) through the central hole (6) while exerting said test pressures, and e) monitoring during said freezing process temperature dependant sample parameters with which said stress-strain characteristics are determined.

Description: The present invention relates to a method and cell for determining temperature dependant stress-strain characteristics of a formation layer sample during a freezing process. The present invention particularly relates to a method and cell wherein a freezing process in a formation layer is simulated such as when a cavern therein is filled with a liquified gas. More specifically caverns in soil and rock layers are intended for storage of liquified gases such as propane, ethylene and liquified natural gas, LNG, with respective boiling points of -42 °C, -104 °C and -162 °C. As known from A. Boulanger et al. "Pilot project shows liquified gas is stored successfully at low temperature in clay", Oil & Gas Journal, vol. 81, No. 10, March 1983, in this field of application pilot tests have been carried out in Belgian formation layers, so-called Boom-clay layers. In said tests a pilot cavity was created at 23 m beneath the earth surface. Said cavity had a diameter of 3 m and a length of 30 m. The lowering of the temperature was accomplished by spraying liquid nitrogen into said cavity. During the entire 10 week test period the behaviour of the cavity and the surrounding area was monitored by more than three hundred measurement points. Among the different types of sensors thermocouples, vibrating wire extensometers, clinometers and bore-hole extensometers were used. After said 10 week period the cavity was found to be perfectly sealed and surrounded by approximately 6 m of frozen clay which was not fractured in any way. Moreover, a laboratory test programme on samples from the above formation layer revealed some "in situ" layer characteristics i.e. the thermal diffusivity and the compressive strength at various temperatures. The said characteristics were compared with and were said to be in good agreement with the above pilot test results. However, it is very expensive to carry out such pilot tests in different formation layers, particularly when investigating rock formation layers. Moreover, soil and/or rock properties are in terms of material science not well known, since they are influenced by numerous parameters such as the ratio of mineral:water:void content, the grain size distribution of the mineral content, the minerals and composition of the mixture, the stress boundary conditions, the permeability or drainage conditions, the strain rate dependance and strain history and the effective particle stress level. As can be read from P. Sage, "Instruments and Tests in Soil Mechanics", Instrument Practice, vol. 25, No. 11, November 1971, specifically in soil mechanics this has led to the development of triaxial soil testing techniques, in which a generally cylindrical sample is exposed to representative boundary conditions with respect to horizontal and vertical stresses and drainage conditions (Due to the fact that samples are retained from bore holes, or in soft soil using a continuous core sampler, the diameters are in the range of 60 to 100 mm). Under ideal conditions the tests are then performed on undisturbed samples of the soil under conditions that represent characteristic underground conditions for the loading of which the effect must be studied. For samples taken at larger depths or from overconsolidated clay layers the test procedure, prior to the test itself, requires readaption to the representative conditions since these cannot be maintained throughout the handling of a sample. This process is referred to as consolidation, and can be explained as reachieving the conditions as in situ with respect to the magnitude and ratio of vertical (overburden) and horizontal (confining) pressure and the original corresponding porewater pressure and content. In Sovjet Inventions Illustrated, week 8702, January 14, 1987, accession No. 87-013705/02, Derwent Publications, London, a soil test device for determination of water permeability and filtration capacity of a soil sample is shown. In order to start from well defined conditions said soil sample is frozen thereby locking the water within the sample. Subsequent thawing the sample by means of a pressurized water stream allows the above determination. In no way cooling the sample from an inner conduit to the outer sample parts can be seen from or is suggested in this document. From Sovjet Inventions Illustrated, week 8539, November 8, 1985, accession No. 85-241845/39, Derwent Publications, London, a ground soil stability measuring device is known. In particular a pressure control system is shown for controlling vertical and horizontal pressures upon a soil sample. Via an outlet conduit in the upper part of the device filtration flow is measured. No further temperature characteristics of such a sample are addressed. In such above triaxial tests extended temperature behaviour of formation layer samples during a freezing process is unknown up to now. In particular it is of importance to test samples under drained or undrained conditions. In the first case porewater can flow in and out of the sample during the test, while the volume flow is measured. In the second case the sample is sealed after consolidation and the porewater flow is bounded within the sample during the test. In this case the porewater pressure can be measured. Thus, it is an object of the invention to improve such tests in which soil and/or rock parameters can be measured during a freezing process thereby reaching deep cooled circumstances, so-called cryogenic circumstances. It is a further object of the invention to measure soil and/or rock parameters which are highly representative for the above-mentioned characteristic underground conditions. It is another object of the invention to obtain reproducible results from which storage criteria can be derived allowing a reliable underground storage of liquified gases such as LNG. The invention therefore provides a method for determining temperature dependant stress-strain characteristics of a formation layer sample said method comprising the following steps: a) simulating characteristic underground conditions for said sample, b) creating an axial central hole in said sample which functions as a coolant conduit, c) arranging said sample in a triaxial cell in which said sample is loaded to predetermined test pressures, d) cooling said sample during a freezing process outwardly from said central hole by circulating a coolant through the central hole while exerting said test pressures, said coolant being supplied into and discharged from said hole through conduit means, and e) monitoring during said freezing process temperature dependant sample parameters with which said stress-strain characteristics are determined. Alternatively in the above method step b) is effected before step a). The invention further provides a method wherein said test pressures comprise simulating in situ condition pressures. In a further embodiment of the above method said axial central hole is created continuously throughout said sample. Advantageously for said sample horizontal and vertical deformation, the transport and volume of unbound water, and the pressure and volume of unbound water in an undrained sample are measured, and cracking investigation is performed during such a freezing process. The mentioned parameters can be measured separately or in combination as desired. Furthermore, a controlled coolant supplying rate is provided. The invention further provides a triaxial cell for determining temperature dependant stress-strain characteristics of a formation layer sample arranged in said cell, said cell comprising: means for loading said sample to predetermined test pressures, conduit means for supplying, circulating and discharging a coolant for cooling said sample outwardly from a central hole during a freezing process, means for measuring during said freezing process temperature dependant sample parameters with which said stress-strain characteristics are determined, and means for thermally isolating said sample during said freezing process. The cell in accordance with the invention further comprises a bellow means supplied with a pressurizing fluid for vertically loading said sample to said predetermined test pressures, laser optical means for measuring horizontal and vertical deformations of said sample and transport means connected with said sample for measuring the transport and volume of unbound water. The invention will now be described by way of example in more detail with reference to the accompanying drawings, wherein Fig. 1 is a plot which schematically shows the behaviour of the proportional change of volume for a clay sample and a rock sample when lowering the temperature, Fig. 2 shows schematically an embodiment of a triaxial cell in accordance with the invention, Fig. 3A and 3B are graphs, giving curves as measured for a clay sample in accordance with the invention, and Fig. 4A and 4B are graphs, giving curves as measured for a rock sample in accordance with the invention. Referring to fig. 1 a plot schematically gives the behaviour of the proportial change of volume, or volumetric change, for a clay sample I and a rock sample II as function of the temperature. The clay curve I first shows a contraction due to the presence of mineral compounds in the clay sample whereafter an expansion occurs caused by the presence of water within such a clay sample. When the temperature is lowered substantially again a contraction mechanism will dominate the sample behaviour. It will be clear that such a minimum as shown in the clay curve I will be an important criterion for the utility of such formation layers in case liquified gases have to be stored therein. Possible cracking phenomena will be revealed thereby. On the contrary the rock curve II only shows a contraction because hardly any free water will be present in such rock samples. Referring to fig. 2 an embodiment of a triaxial cell 1 in accordance with the present invention has been shown. Particularly deep cooling of such a formation layer sample in a cryogenic test is required when LNG storage has to be simulated. For that purpose the cell 1 has vacuum isolated walls 2. A formation layer sample 3, having for example a cylinder form, is arranged within said cell 1. The sample 3 is placed within a load frame 4 to achieve and/or to maintain predetermined test pressures. More in detail one has to distinguish separately the vertical load conditions, or overburden pressures, and the horizontal conditions, or confining pressures. In such tests various levels and combinations of overburden and confining pressures are exerted onto such a sample including in situ condition pressures which correspond with characteristic underground conditions. For clay samples in general the overburden pressures are about a factor two greater than the confining pressures. On the contrary for rock samples vertical and horizontal pressures are more or less equal. To achieve said pressure conditions a sample 3 is arranged in an overpressurized environment, for example by means of an overpressurizing fluid such as silicon oil. When deep cooled conditions are desired helium gas, as shown in fig. 2, is used advantageously. Then for a clay sample, the load frame 4 can comprise a bellow means 7 as a load frame, supplied with additional helium gas in order to obtain the required vertical pressure. Generally a sealing against penetration of pressurized fluid will be necessary for both clay and rock samples. Such sealing can be achieved for example by means of a fluid tight sleeve covering the whole free sample surface, i.e. the side and top surfaces of the sample 3 as shown in fig. 2. In order to determine temperature dependant stress-strain characteristics of such a formation layer sample 3 during a freezing process monitoring of parameters has to be done. Relative thereto one has to measure, separately or in combination, vertical and horizontal deformation, transport and volume of unbound water, pressure and mass of unbound water within undrained samples and temperature gradients within such samples. Also crack inspection has to be performed. Implementing the above measurements can be effected on various ways. Conventionally vertical and horizontal deformations are measured respectively by means of displacement transducers and gap sensors. However, in deep cooling circumstances the said measuring equipment may give inaccurate and unreliable data. Therefore an other type of measuring has to be used, for example the optical type. Also linear voltage displacement transducers (LVDT) may be used. As can be seen in fig. 2 looking glasses 5 are provided in the wall of the triaxial cell 1 to perform the said optical measurements. When simulating said underground conditions a pore water flow can occur. More specifically the gradient of the pore water flow presents vital information about the sample freezing conditions. Thereby the transport and volume of unbound water will be measured. Therefore a transport means, such as a drain pipe, (which is not shown in fig. 2 for reason of clarity) is arranged within the sample 3 as a conduit to transport the unbound water from and into the sample. Advantageously the transport means is connected with a water reservoir. On the contrary an undrained sample is used when the pressure of unbound water within said sample has to be measured. Therefore a pressure detector (not shown in fig. 2 for reason of clarity) has to be connected with said sample. In addition to measuring of transport volume and pressure of unbound water as mentioned above also the volume of the unbound water within said sample can be monitored with aid of advanced technology like nuclear magnetic resonance (NMR). Thereby two NMR techniques, i.e. the continuous wave techniques and the pulsed NMR technique, can complement each other. Furthermore, temperature measuring means (not shown for reason of clarity) are arranged within the sample 3. Advantageously a plurality of temperature sensors, for example a number of four sensors, is inserted into the sample from the bottom, the sensors being positioned at different distances from the central axis of the sample. Then the temperature gradient from the inner region to the outside region can be monitored, thereby simulating in situ monitoring when storing liquefied gases in a cavern beneath the earth surface. To achieve the required cryogenic temperatures in such samples a coolant is introduced into the sample 3 through a hole 6 created in the sample. Advantageously said hole 6 is created along the central axis of the sample 3. Furthermore said hole is bored not entirely throughout said sample, thus simulating a cavernlike hole. A conduit means 8 communicates with the inner region of the sample, supplying and discharging the coolant into and from said region. It has been proven advantageous to supply said coolant in a controlled way in which different coolant supply rates can be obtained. Alternatively the central hole 6 is created continuously throughout the sample. Then a radially symmetric temperature loading of the sample is achieved. Advantageously the coolant used in simulating cavern test procedures is nitrogen vapour or liquid nitrogen, the latter having a boiling temperature of -196 °C. Therewith a freezing process from ambient temperature to the boiling temperature of -162 °C of liquefied natural gas can be simulated suitably. In a complementary way after the above cooling a controlled warming-up can be accomplished. Both temperature courses will give specific parameter informations. The temperature measurements can be performed by standard type temperature sensors. For example Pt-100 sensors will be used during such freezing periods. Last mentioned, but not least of interest, crack investigation has to be performed. It appeares advantageous to use acoustic emission measurements to reveal possible cracks. Just like in the seismic field cracks will reflect (and also diffract) acoustical energy. Accurate processing of data so obtained will be accomplished in automated procedures performed by computer devices. Also crack inspection by means of video imaging accomplished with aid of an endoscope device can be employed. It will be clear to those skilled in the art that measurements of the above type will provide a lot of information about the stress-strain characteristics of such formation layers. Thereby elastic and plastic properties like Young's modulus, Poisson ratio and yield strength can be calculated. Also permeability and porosity can be determined. To further elucidate the above method and corresponding test procedures two examples will be shown. EXAMPLE 1 In example 1 test results relative to an artificial clay sample are disclosed. Some properties as determined for said clay sample are shown in Table 1. density (dry) 1755 ± 10 kg/m³ water content 18.9% (weight % relative to dry sample) grain size distribution 49.5% < 2 µm; 69% < 16 µm; (weight %) 85.0% < 63 µm plastic limit 18.5% liquid limit 38.1% plasticity index 19.6% permeability 1.0x10⁻¹⁰ m/s at 5 bar porosity 33.3% mineral content (weight %) quartz 50%, kaolinite 25%, illite 20%, chlorite 5% The sample was adopted (consolidated) to the mechanical boundary conditions, isotropic pressure of 5 bar for 17 hours, a period after which the gradients in volume change and porewater flow were negligible, and inserted into the cell for a drained triaxial test at constant stress level. The cooling rate chosen was linear at 20 °C per hour, measured at the temperature sensor nearest to the central cooling hole and the minimum temperature at this sensor was set to -50 °C. The results of the deformation measurements as recorded in an investigation run are presented in two graphs, shown in fig. 3A and fig. 3B. Fig. 3A shows the change in height as function of time the most integral measurement since it is averaged over the total height of the sample. Conventionally the contraction region is situated above the horizontal axis (see also fig. 1). Therefore the change of heigth (and also diameter) is indicated as -Δh(and -Δd). Furthermore it is noticed that because of the linear cooling process as stated above the shown time dependant courses are similar to temperature dependant courses such as explained relative to fig. 1. It can be distinguished that the axial component contributing to the volumetric strain first shows a decrease and after 1.5 hours, corresponding to an average sample temperature of ± -5 °C, reaches its original size and then shows an increase. The second deformation measurement is the change of diameter measured at two levels of the sample, generally ± 1/3 and ± 2/3 of the total height with three gap sensors each. The resulting value represents the diameter as shown in fig. 3B shows very similar characteristics as the change in height. The diameters decrease in the first stage of the loading and then increase. The initial diameter however is reached slightly earlier than the initial height. The difference in the amplitude of change in diameter for the two levels can be assigned to a temperature gradient in axial direction of the sample. Deriving the volumetric strain from the product of the average change in the cross section derived from diameter changes and the change in height related to the original volume would thus result in a curve qualitatively following curve I as presented in fig. 1. EXAMPLE 2 In example 2 test results relative to a dry granite rock sample are disclosed. The boundary conditions and geometrical dimensions were kept the same as for the clay samples, and with exception of the consolidation phase which does not apply for granite (only the elastic deformation due to the loading must be considered), the same test and measuring procedure was followed. The effected cooling was also a linear process. However, a temperature discontinuity (not shown in a separate graph for reason of simplicity) in the final condition occurred. A preliminary explanation of this is that local micro cracking provided short cut cooling channels between the temperature sensors. The deformation measurements presented in figures 4A and 4B can be described very briefly, since they require less interpretation than clay tests. The granite rock shows a steady contraction as reaction to the cooling. The derived volumetric strain thus would confirm the predictions.

Claims: A method for determining temperature dependant stress-strain characteristics of a formation layer sample, said method comprising the following steps: a) simulating characteristic underground conditions for said sample, b) creating an axial central hole (6) in said sample (3) which functions as a coolant conduit, c) arranging said sample (3) in a triaxial cell (1) in which said sample (3) is loaded to predetermined test pressures, d) cooling said sample (3) during a freezing process outwardly from said central hole (6) by circulating a coolant through the central hole while exerting said test pressures, said coolant being supplied into and discharged from said hole (6) through conduit means (8), and e) monitoring during said freezing process temperature dependant sample parameters with which said stress-strain characteristics are determined. The method as claimed in claim 1, wherein step b) is effected before step a). The method as claimed in claim 1 or 2, wherein said test pressures are in situ condition pressures. The method as claimed in claim 2 or 3, wherein said axial central hole (6) is created continuously throughout said sample (3). The method as claimed in any one of the claims 1-4, wherein the coolant is liquid nitrogen. The method as claimed in any one of the claims 1-5, wherein horizontal and vertical deformation of said sample (3) are measured. The method as claimed in any one of the claims 1-6, wherein the transport and volume of unbound water are measured. The method as claimed in any one of the claims 1-6, wherein the pressure of unbound water contained in an undrained sample is measured. The method as claimed in any one of the claims 1-8, wherein the volume of unbound water within said sample is measured by using a nuclear magnetic resonance (NMR) technique. The method as claimed in claim 9, wherein said NMR-technique is a pulsed NMR-technique. The method as claimed in claim 9, wherein said NMR-technique is a continuous wave technique. The method as claimed in any one of the claims 1-9, wherein a crack inspection is performed during said freezing process. The method as claimed in claim 12, wherein said crack inspection is accomplished using an acoustical investigation technique. The method as claimed in claim 12, wherein said crack inspection is accomplished using an endoscope technique. The method as claimed in any one of the claims 1-14, wherein a controlled coolant supplying rate is provided. A triaxial cell (1) for determining temperature dependant stress-strain characteristics of a formation layer sample (3) arranged in said cell (1), said cell (1) comprising: means (4,7) for loading said sample (3) to predetermined test pressures, conduit means (8) for supplying, circulating and discharging a coolant for cooling said sample (3) outwardly from a central hole (6) during a freezing process, means for measuring during said freezing process temperature dependant sample (3) parameters with which said stress-strain characteristics are determined, and means (2) for thermally isolating said sample (3) during said freezing process. The cell as claimed in claim 16 further comprising a bellow means supplied with a pressurizing fluid for vertically loading said sample (3) to said predetermined test pressures. The cell as claimed in claim 16, further comprising laser optical means for measuring horizontal and vertical deformations of said sample (3). The cell as claimed in claim 16, further comprising transport means connected with said sample (3) for measuring the transport volume of unbound water. Use of the method as claimed in any one of the claims 1-15 on samples (3) taken from formation layers.

Inventors: HORVAT ENDRE, UNSWORTH JOHN FRANCIS, ZORN NILS FRIEDRICH, HORVAT, ENDRE, UNSWORTH, JOHN FRANCIS, ZORN, NILS FRIEDRICH
Applicants: SHELL INT RESEARCH, SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1989
Main classification: G01N3
IPC codes: G01N3, G01N1, G01N33