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High pressure-high temperature laboratory apparatus for the measurement of deep sea sediment physical properties
0029-~8018/83$3.00 + .00 Pergamon Press Ltd.
Ocean Engng, Vol. 10, No 6, pp.481--487, 1983. Printed in Great Britain.
TECHNICAL NOTE
HIGH PRESSURE-HIGH TEMPERATURE LABORATORY APPARATUS FOR THE MEASUREMENT OF DEEP SEA SEDIMENT PHYSICAL PROPERTIES RooEs MORNS* and ARMANDJ. SILVA Department of Ocean Engineering, University of Rhode Island, Kingston, R102881, U.S.A. Almtrect--Laboratory equipment has been built which will measure the permeability and thermal conductivity of deep-sea sediments at their in-sire conditions of hydrostatic pressure, temperature, and void ratio. The apparatus has the capability of uniaxially consolidating a sediment sample to simulate compaction within the sediment colunm, while exposing the specimen to hydrostatic pressures ranging from atmospheric to 62 MPa and to temperatures from 22 to 220"C. The equipment includes a hypodermic needle mounted vertically through the base of the pressure vessel from which thermal conductivity is determined by the needle probe method. The system also features a combination of dead-weight testers which produces a small hydraulic gradient across the sample and permits the measurement of sediment permeability at large hydrostatic pressures. The physical property data generated from this apparatus will be important in understanding the mechanisms of heat transfer through the ocean floor and in analysing the coupled flow of heat and pore fluid in the vicinity of a heat source, such as a radioactive waste canister, buried in the seabed.
INTRODUCTION LABORATORYstudies have shown that tests designed to measure sediment permeability directly should be performed under hydrostatic pressures large enough to assure complete pore fluid saturation (Nickerson, 1978; Crowe and Silva, in press). This back-pressure, usually 0.5-1.0 MPa, is necessary to dissolve gas bubbles which tend to impede water flow through the voids. There has been speculation that much larger hydrostatic pressures (beyond 30 MPa) might affect sediment permeability by producing physico-chemical changes in the sediment skeleton (Lang, 1967). In investigations concerning hydrothermal activity and convective heat transfer through the ocean floor, discrepancies have been found between values of fluid velocity inferred from non-linear thermal profiles and other values derived from Darcy's Law and laboratory permeability results (Langseth, 1980). The existence of a pressure-induced alteration of the sediment skeleton has been proposed as a possible explanation for the conflicting data (Abbott et al., 1981). Ratcliffe (1960) introduced a pressure correction term for sediment thermal conductivity based on the behavior of water alone, independent of porosity. He suggested adding a 1% correction per 1000 fathoms (1830 m) of water depth to the measured sediment conductivity at atmospheric pressure. The accuracy of this generalized correction factor has not been tested against actual laboratory results. Studies to determine the effect of temperature on thermal conductivity of sediments have only been performed for a narrow range of temperatures. Ratcliffe (1960), and MacDonald and Simmons (1972) found approximately a 6% increase in this property from 4 to 25°C. *Presently at the Department of Earth and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139. 481
482
ROGER MOroN and ARMAND J, SILVA
Preliminary work designed to determine the dependence of permeability on temperature has also been performed over a relatively narrow range. Silva et al. (1981) found the increase in permeability of an illitic clay at 80°C to be due entirely to a decrease in the viscosity of the permeating medium. The change in permeability could be accounted for by applying a viscosity correction within this temperature range. The variation in the permeability and the thermal conductivity of sediments at much higher temperatures has not been determined. This information is vital to the Subseabed Disposal Program, where the thermal response and fluid transport in the vicinity of waste canisters implanted in the sediment column must be analysed (Anderson et al., 1976). Under this set of conditions, sediments will be exposed to substantial heat fluxes at large hydrostatic pressures. The apparatus described permits systematic laboratory studies to be performed on various sediment types. The data generated from these test programs will be used to address the above problems. A separate paper reporting the results of tests on biogenic and red clay sediments is being published (Morin and Silva, in press). CONSTRUCTION AND CAPABILITIES The equipment is designed to measure sediment permeability and thermal conductivity at hydrostatic pressures ranging from atmospheric (0.1 MPa) to 62 MPa and at temperatures from 22 to 220°C. Permeability is measured directly by producing a differential pressure across a specimen and measuring the corresponding volume of fluid which permeates through the sample. The basic concept has been employed previously with a modified back-pressure consolidometer for measurement of permeability at pressures up to 2.0 MPa and temperatures up to 800C (Silva et al., 1981). Because of the larger hydrostatic pressures and higher temperatures demanded by the latest investigations, a novel method of measuring volumetric flow has been devised. This new apparatus is very different from conventional geotechnical laboratory equipment. Stainless steel bellows, equipped with linear variable displacement transformers (LVDT), provide the interfaces between hydraulic oil on the applied pressure side and the seawater pore fluid (Fig. 1). As seawater permeates through the sediment sample, it flows out of one bellows and into the other, with the resulting bellows movement being monitored using the LVDT. This linear displacement is calibrated against actual volume change and fluid flow is recorded as a function of time. The capacity of each bellows is 72 cm 3, with a resolution of 0.01 cm 3. The pressure gradient produced across a sediment sample during a test must be essentially constant in order to properly determine the permeability. This is accomplished with a unique pressure system composed of two dead-weight testers (DWT) and a hydraulic pump. Each DWT consists of a piston, precision ground and honed to a tolerance of +0.008 ram, on top of which is a pedestal and a series of weights. Oil pressure is initially generated by the pump and pressure regulation is achieved in the same manner as a conventional dead-weight calibration device. A prescribed weight exerts a downward force on a piston, pressure from the pump is increased until the piston rises and floats in the oil reservoir, and the resulting hydrostatic pressure is proportional to the given weight divided by the piston area. Friction is reduced by designed oil leakage around the piston and further minimized with an electric motor which continuously spins the piston and weight assembly. The floating, spinning pedestals dampen pressure
Technical Note
S
4
8
483
F
'
I. 2. 3, 4. 5. 6. 7.
Diagram of apparatus High pressure triaxlal cell 8. LoQdlng frame with weights Heating band 9. Dead weight tester Sea war.or 10. Rl~ircutating hydraulic pump Confining ring I I. Oil. reservoir Thernml conductivity probe 12. VoLume change device with bellows Sediment sompto 13. High pressure valve Porous filter 14. High pressure roughing pump FIG. 1
fluctuations originating from the pump as they gently rise and fall corresponding to slight changes in hydraulic pressure, thereby helping to maintain the desired constant pressure conditions within the test chamber. The hydraulic pump itself is a Racine piston-type pump specifically designed to minimize pressure fluctuations. It has seven outlet ports, with one port going to each of the two DWT's and the remaining five ports by-passing these devices and returning directly to the oil reservoir. The relatively large volume of by-pass oil reduces hydraulic pressure variations at the two outlet ports connected to the DWT's. Each dead-weight tester acts independently as a pressure regulator and sensitive relief valve. The result is a differential pressure across the sediment sample whose variation can be controlled to within __.3.5 KPa at absolute pressures to 62 MPa. This translates to a permeability test accuracy of about ± 10% when conducted at a gradient of 40. A heat exchanger also is incorporated into the oil line to cool the warm, pressurized hydraulic fluid and prevent damage to the pump. Permeability tests can thus be run at substantial absolute pressure over several hours. A stainless steel hypodermic needle, 0.15 cm in diameter, is mounted vertically, through the base of the pressure cell and the sediment sample is pushed down axially over it (Fig. 1). The needle contains a thermistor and a nickel-chromium resistor wire. A
484
ROGER Moron and ARMAND J. SILVA
new feature of this probe is that it is filled with a high temperature epoxy resin which allows tests to be performed at temperatures up to 220°C. Thermal conductiv;tv is measured by the needle probe technique (Jaeger, 1958; Von Herzen and Maxwell, 1959). In this method, which has an accuracy of approximately +3%, the temperature response of the thermistor to a specified heat input from the resistance wire is monitored as a function of time and directly related to the sediment thermal conductivity. The pressure vessel, with base plate and piston, is made of Inconel 600 in order to resist the corrosive nature of high temperature seawater, and is designed to accommodate a sediment sample 5 cm in dia. by 10 cm long. The sample holder, a thick-walled tube lined with teflon, laterally confines the sample during testing. The piston, which incorporates a double O-ring seal design, enters the vessel through the top cap and permits one-dimensional loading and consolidation of the specimen. A load cell and an LVDT are attached to record axial load and displacement, respectively. This process simulates compaction through the sediment column, and the thermophysical properties of the sample can thus be measured as a function of water content. However, because of the friction created by the seals, only an approximate value of stress transmitted to the sample can be determined. Three resistance heater bands, each with a 1500-W capacity, surround the vessel, heating the cell and its contents. Asbestos insulation is provided to reduce heat loss, and heat input is regulated with a proportional power controller. The sediment temperature is monitored from the thermistor which is inside the thermal conductivity needle. During the initial heat-up phase of a test, the bellows rise in response to the thermally expanding seawater within the pressure cell and a constant hydrostatic pressure is, therefore, maintained while the vessel temperature increases. The magnitude of this pressure assures that no seawater phase change occurs. Once the system has reached a steady state condition, sediment temperatures are maintained to within +_½°C. Standard viscosity corrections are applied to permeability test results performed at elevated temperatures. A photograph of the completed apparatus is included as Fig. 2. Acknowledgements--We would like to thank the personnel at Structural Behavior Engineering Laboratories. Inc. of Phoenix, Arizona for their valuable assistance in the design and fabrication of this equipment. Support was provided by the National Science Foundation, Grant No. OCE 79119426, and the Department of Energy in cooperation with Sandia Laboratories, Contract Nos 13-2561 and 13-9927.
REFERENCES AaaoTr, D., MENKE, W., H o a ^ ~ , M. and ANDERSON, R. 1981. Evidence for excess pore pressures in southwest Indian Ocean sediment. J. geophys. Res. 86, 1813-1827. ANDF_~,SON, D.R., HOLUSTER, C.D. and TALBEIt~, D.M. eds. 1976. Report to the Radioactive Waste
Management Committee on the First International Workshop on Seabed Disposal of High-Level Wastes. Woods Hole, Mass., Feb. 16--20, 1976, SAND 76--00224. Sandia Laboratories, Albuquerque, NM. CaowE, J. and SILVA, A.J. In press. Permeability measurements of Equatorial Pacific carbonate oozes using a direct measurement, backed-pressured technique. J. geophys. Res. JAEOEIt, .I.C., 1958. The measurement of thermal conductivity and diffusivity with cylindrical probes. Trans. Am. geophys. Un. 39, 708-710. LANG, W.J. 1967. The influence of pressure on the electrical resistivity of clay-water systems. Proc. of the Fifteenth Conference on Clays and Clay Minerals, pp. 455-468. LANC,SEI'n, M.G. 1980. Towards a submarine hydrology. Nature Lond. 286, 554-555. MACDONALD, K. and SIMMONS, G. 1972. Temperature coefficient of the thermal conductivities of ocean sediments. Deep Sea Res. 19, 669-671.
Y
FI¢. 2. Unit at left contains two volume change devices (a) behind the two dead-weight testers (b), a 5 H P motor and a hydraulic pump. Pressure cell (c) is shown inside loading frame (d) and electronicinstrumentation controls are at right.
Technical Note
487
MoroN, R. and SILVA,A.J. In press. The effects of high pressure-high temperature on some physical properties of ocean sediments. J. geophys. Res. NICKEaSON, C.R. 1978. Consolidation and permeability characteristics of deep sea sediments: North Central Pacific Ocean. M. S. Thesis, Worcester Polytechnic Institute. RATCUFFE, E.H. 1960. The thermal conductivities of ocean sediments. J. geophys. Res. 6S, 1535-1541. SILVA, A.J., HErHERMAN,J.R. and CALN^N, D.I. 1981. Low gradient permeability testing of fine-grained marine sediments. In Permeability and Ground Water Contaminant Transport ASTM STP 746, pp. 121-136. Edited by ZIMM1ET.F. and PdGOS C.O. Am. Soc. of Testing and Materials. VON HERZEN R.P. and MAXWELLA.E., 1959. The measurment of thermal conductivity of deep sea sediments by a needle-probe method. J. geophys. Res. 64, 1557-1563.
Ocean Engng, Vol. 10, No 6, pp.481--487, 1983. Printed in Great Britain.
TECHNICAL NOTE
HIGH PRESSURE-HIGH TEMPERATURE LABORATORY APPARATUS FOR THE MEASUREMENT OF DEEP SEA SEDIMENT PHYSICAL PROPERTIES RooEs MORNS* and ARMANDJ. SILVA Department of Ocean Engineering, University of Rhode Island, Kingston, R102881, U.S.A. Almtrect--Laboratory equipment has been built which will measure the permeability and thermal conductivity of deep-sea sediments at their in-sire conditions of hydrostatic pressure, temperature, and void ratio. The apparatus has the capability of uniaxially consolidating a sediment sample to simulate compaction within the sediment colunm, while exposing the specimen to hydrostatic pressures ranging from atmospheric to 62 MPa and to temperatures from 22 to 220"C. The equipment includes a hypodermic needle mounted vertically through the base of the pressure vessel from which thermal conductivity is determined by the needle probe method. The system also features a combination of dead-weight testers which produces a small hydraulic gradient across the sample and permits the measurement of sediment permeability at large hydrostatic pressures. The physical property data generated from this apparatus will be important in understanding the mechanisms of heat transfer through the ocean floor and in analysing the coupled flow of heat and pore fluid in the vicinity of a heat source, such as a radioactive waste canister, buried in the seabed.
INTRODUCTION LABORATORYstudies have shown that tests designed to measure sediment permeability directly should be performed under hydrostatic pressures large enough to assure complete pore fluid saturation (Nickerson, 1978; Crowe and Silva, in press). This back-pressure, usually 0.5-1.0 MPa, is necessary to dissolve gas bubbles which tend to impede water flow through the voids. There has been speculation that much larger hydrostatic pressures (beyond 30 MPa) might affect sediment permeability by producing physico-chemical changes in the sediment skeleton (Lang, 1967). In investigations concerning hydrothermal activity and convective heat transfer through the ocean floor, discrepancies have been found between values of fluid velocity inferred from non-linear thermal profiles and other values derived from Darcy's Law and laboratory permeability results (Langseth, 1980). The existence of a pressure-induced alteration of the sediment skeleton has been proposed as a possible explanation for the conflicting data (Abbott et al., 1981). Ratcliffe (1960) introduced a pressure correction term for sediment thermal conductivity based on the behavior of water alone, independent of porosity. He suggested adding a 1% correction per 1000 fathoms (1830 m) of water depth to the measured sediment conductivity at atmospheric pressure. The accuracy of this generalized correction factor has not been tested against actual laboratory results. Studies to determine the effect of temperature on thermal conductivity of sediments have only been performed for a narrow range of temperatures. Ratcliffe (1960), and MacDonald and Simmons (1972) found approximately a 6% increase in this property from 4 to 25°C. *Presently at the Department of Earth and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139. 481
482
ROGER MOroN and ARMAND J, SILVA
Preliminary work designed to determine the dependence of permeability on temperature has also been performed over a relatively narrow range. Silva et al. (1981) found the increase in permeability of an illitic clay at 80°C to be due entirely to a decrease in the viscosity of the permeating medium. The change in permeability could be accounted for by applying a viscosity correction within this temperature range. The variation in the permeability and the thermal conductivity of sediments at much higher temperatures has not been determined. This information is vital to the Subseabed Disposal Program, where the thermal response and fluid transport in the vicinity of waste canisters implanted in the sediment column must be analysed (Anderson et al., 1976). Under this set of conditions, sediments will be exposed to substantial heat fluxes at large hydrostatic pressures. The apparatus described permits systematic laboratory studies to be performed on various sediment types. The data generated from these test programs will be used to address the above problems. A separate paper reporting the results of tests on biogenic and red clay sediments is being published (Morin and Silva, in press). CONSTRUCTION AND CAPABILITIES The equipment is designed to measure sediment permeability and thermal conductivity at hydrostatic pressures ranging from atmospheric (0.1 MPa) to 62 MPa and at temperatures from 22 to 220°C. Permeability is measured directly by producing a differential pressure across a specimen and measuring the corresponding volume of fluid which permeates through the sample. The basic concept has been employed previously with a modified back-pressure consolidometer for measurement of permeability at pressures up to 2.0 MPa and temperatures up to 800C (Silva et al., 1981). Because of the larger hydrostatic pressures and higher temperatures demanded by the latest investigations, a novel method of measuring volumetric flow has been devised. This new apparatus is very different from conventional geotechnical laboratory equipment. Stainless steel bellows, equipped with linear variable displacement transformers (LVDT), provide the interfaces between hydraulic oil on the applied pressure side and the seawater pore fluid (Fig. 1). As seawater permeates through the sediment sample, it flows out of one bellows and into the other, with the resulting bellows movement being monitored using the LVDT. This linear displacement is calibrated against actual volume change and fluid flow is recorded as a function of time. The capacity of each bellows is 72 cm 3, with a resolution of 0.01 cm 3. The pressure gradient produced across a sediment sample during a test must be essentially constant in order to properly determine the permeability. This is accomplished with a unique pressure system composed of two dead-weight testers (DWT) and a hydraulic pump. Each DWT consists of a piston, precision ground and honed to a tolerance of +0.008 ram, on top of which is a pedestal and a series of weights. Oil pressure is initially generated by the pump and pressure regulation is achieved in the same manner as a conventional dead-weight calibration device. A prescribed weight exerts a downward force on a piston, pressure from the pump is increased until the piston rises and floats in the oil reservoir, and the resulting hydrostatic pressure is proportional to the given weight divided by the piston area. Friction is reduced by designed oil leakage around the piston and further minimized with an electric motor which continuously spins the piston and weight assembly. The floating, spinning pedestals dampen pressure
Technical Note
S
4
8
483
F
'
I. 2. 3, 4. 5. 6. 7.
Diagram of apparatus High pressure triaxlal cell 8. LoQdlng frame with weights Heating band 9. Dead weight tester Sea war.or 10. Rl~ircutating hydraulic pump Confining ring I I. Oil. reservoir Thernml conductivity probe 12. VoLume change device with bellows Sediment sompto 13. High pressure valve Porous filter 14. High pressure roughing pump FIG. 1
fluctuations originating from the pump as they gently rise and fall corresponding to slight changes in hydraulic pressure, thereby helping to maintain the desired constant pressure conditions within the test chamber. The hydraulic pump itself is a Racine piston-type pump specifically designed to minimize pressure fluctuations. It has seven outlet ports, with one port going to each of the two DWT's and the remaining five ports by-passing these devices and returning directly to the oil reservoir. The relatively large volume of by-pass oil reduces hydraulic pressure variations at the two outlet ports connected to the DWT's. Each dead-weight tester acts independently as a pressure regulator and sensitive relief valve. The result is a differential pressure across the sediment sample whose variation can be controlled to within __.3.5 KPa at absolute pressures to 62 MPa. This translates to a permeability test accuracy of about ± 10% when conducted at a gradient of 40. A heat exchanger also is incorporated into the oil line to cool the warm, pressurized hydraulic fluid and prevent damage to the pump. Permeability tests can thus be run at substantial absolute pressure over several hours. A stainless steel hypodermic needle, 0.15 cm in diameter, is mounted vertically, through the base of the pressure cell and the sediment sample is pushed down axially over it (Fig. 1). The needle contains a thermistor and a nickel-chromium resistor wire. A
484
ROGER Moron and ARMAND J. SILVA
new feature of this probe is that it is filled with a high temperature epoxy resin which allows tests to be performed at temperatures up to 220°C. Thermal conductiv;tv is measured by the needle probe technique (Jaeger, 1958; Von Herzen and Maxwell, 1959). In this method, which has an accuracy of approximately +3%, the temperature response of the thermistor to a specified heat input from the resistance wire is monitored as a function of time and directly related to the sediment thermal conductivity. The pressure vessel, with base plate and piston, is made of Inconel 600 in order to resist the corrosive nature of high temperature seawater, and is designed to accommodate a sediment sample 5 cm in dia. by 10 cm long. The sample holder, a thick-walled tube lined with teflon, laterally confines the sample during testing. The piston, which incorporates a double O-ring seal design, enters the vessel through the top cap and permits one-dimensional loading and consolidation of the specimen. A load cell and an LVDT are attached to record axial load and displacement, respectively. This process simulates compaction through the sediment column, and the thermophysical properties of the sample can thus be measured as a function of water content. However, because of the friction created by the seals, only an approximate value of stress transmitted to the sample can be determined. Three resistance heater bands, each with a 1500-W capacity, surround the vessel, heating the cell and its contents. Asbestos insulation is provided to reduce heat loss, and heat input is regulated with a proportional power controller. The sediment temperature is monitored from the thermistor which is inside the thermal conductivity needle. During the initial heat-up phase of a test, the bellows rise in response to the thermally expanding seawater within the pressure cell and a constant hydrostatic pressure is, therefore, maintained while the vessel temperature increases. The magnitude of this pressure assures that no seawater phase change occurs. Once the system has reached a steady state condition, sediment temperatures are maintained to within +_½°C. Standard viscosity corrections are applied to permeability test results performed at elevated temperatures. A photograph of the completed apparatus is included as Fig. 2. Acknowledgements--We would like to thank the personnel at Structural Behavior Engineering Laboratories. Inc. of Phoenix, Arizona for their valuable assistance in the design and fabrication of this equipment. Support was provided by the National Science Foundation, Grant No. OCE 79119426, and the Department of Energy in cooperation with Sandia Laboratories, Contract Nos 13-2561 and 13-9927.
REFERENCES AaaoTr, D., MENKE, W., H o a ^ ~ , M. and ANDERSON, R. 1981. Evidence for excess pore pressures in southwest Indian Ocean sediment. J. geophys. Res. 86, 1813-1827. ANDF_~,SON, D.R., HOLUSTER, C.D. and TALBEIt~, D.M. eds. 1976. Report to the Radioactive Waste
Management Committee on the First International Workshop on Seabed Disposal of High-Level Wastes. Woods Hole, Mass., Feb. 16--20, 1976, SAND 76--00224. Sandia Laboratories, Albuquerque, NM. CaowE, J. and SILVA, A.J. In press. Permeability measurements of Equatorial Pacific carbonate oozes using a direct measurement, backed-pressured technique. J. geophys. Res. JAEOEIt, .I.C., 1958. The measurement of thermal conductivity and diffusivity with cylindrical probes. Trans. Am. geophys. Un. 39, 708-710. LANG, W.J. 1967. The influence of pressure on the electrical resistivity of clay-water systems. Proc. of the Fifteenth Conference on Clays and Clay Minerals, pp. 455-468. LANC,SEI'n, M.G. 1980. Towards a submarine hydrology. Nature Lond. 286, 554-555. MACDONALD, K. and SIMMONS, G. 1972. Temperature coefficient of the thermal conductivities of ocean sediments. Deep Sea Res. 19, 669-671.
Y
FI¢. 2. Unit at left contains two volume change devices (a) behind the two dead-weight testers (b), a 5 H P motor and a hydraulic pump. Pressure cell (c) is shown inside loading frame (d) and electronicinstrumentation controls are at right.
Technical Note
487
MoroN, R. and SILVA,A.J. In press. The effects of high pressure-high temperature on some physical properties of ocean sediments. J. geophys. Res. NICKEaSON, C.R. 1978. Consolidation and permeability characteristics of deep sea sediments: North Central Pacific Ocean. M. S. Thesis, Worcester Polytechnic Institute. RATCUFFE, E.H. 1960. The thermal conductivities of ocean sediments. J. geophys. Res. 6S, 1535-1541. SILVA, A.J., HErHERMAN,J.R. and CALN^N, D.I. 1981. Low gradient permeability testing of fine-grained marine sediments. In Permeability and Ground Water Contaminant Transport ASTM STP 746, pp. 121-136. Edited by ZIMM1ET.F. and PdGOS C.O. Am. Soc. of Testing and Materials. VON HERZEN R.P. and MAXWELLA.E., 1959. The measurment of thermal conductivity of deep sea sediments by a needle-probe method. J. geophys. Res. 64, 1557-1563.