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Volume 31 Issue 5
Oct.  2020
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Yanjun Liu, Teng Ma, Juan Chen, Ziqi Peng. Compaction Simulator: A Novel Device for Pressure Experiments of Subsurface Sediments. Journal of Earth Science, 2020, 31(5): 1045-1050. doi: 10.1007/s12583-020-1334-6
Citation: Yanjun Liu, Teng Ma, Juan Chen, Ziqi Peng. Compaction Simulator: A Novel Device for Pressure Experiments of Subsurface Sediments. Journal of Earth Science, 2020, 31(5): 1045-1050. doi: 10.1007/s12583-020-1334-6

Compaction Simulator: A Novel Device for Pressure Experiments of Subsurface Sediments

doi: 10.1007/s12583-020-1334-6
More Information
  • Increasing overburden pressure is a key factor that alters the chemical and physical properties of soils and sediments. However, limited information is presently available on how aquifer compression impacts water quality. We introduced a novel compaction device, which is composited of four parts, including pressure simulator reactor system (PSRS), gas-liquid separator (GLS), automatic collector (AC) and composite control system (CCS). We conducted experiments at various pressures to test the functionality and outcomes of the device. In general, this device can be used to examine changes in water chemistry associated with aquifer compression resulting from compaction (overburden pressure) or groundwater overdraft.
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Compaction Simulator: A Novel Device for Pressure Experiments of Subsurface Sediments

doi: 10.1007/s12583-020-1334-6

Abstract: Increasing overburden pressure is a key factor that alters the chemical and physical properties of soils and sediments. However, limited information is presently available on how aquifer compression impacts water quality. We introduced a novel compaction device, which is composited of four parts, including pressure simulator reactor system (PSRS), gas-liquid separator (GLS), automatic collector (AC) and composite control system (CCS). We conducted experiments at various pressures to test the functionality and outcomes of the device. In general, this device can be used to examine changes in water chemistry associated with aquifer compression resulting from compaction (overburden pressure) or groundwater overdraft.

Yanjun Liu, Teng Ma, Juan Chen, Ziqi Peng. Compaction Simulator: A Novel Device for Pressure Experiments of Subsurface Sediments. Journal of Earth Science, 2020, 31(5): 1045-1050. doi: 10.1007/s12583-020-1334-6
Citation: Yanjun Liu, Teng Ma, Juan Chen, Ziqi Peng. Compaction Simulator: A Novel Device for Pressure Experiments of Subsurface Sediments. Journal of Earth Science, 2020, 31(5): 1045-1050. doi: 10.1007/s12583-020-1334-6
  • Recognition of compaction on water chemistry remains largely unknown. Unlocking this mystery will provide critical information on groundwater contamination, pollutant migration, mineralization and other related phenomena (Xiao et al., 2016; Lloret et al., 2003; Collin et al., 2002; Eugene and Shinn, 1983). Soil compaction is an especially important environmental problem now which can be caused by agricultural machinery, but also some natural conditions without human or animal involvement (Beylich et al., 2010; Batey, 2009). And the sediments can be also compacted with burial depth by the increasing overburden pressure (Mondol et al., 2007; Audet, 1995). Amount of previous researches suggested that gravitational compaction was able to control the permeability and connection of gases (such as air, CO2 of respiration) and water in soils and sediments via the reduction of porosity, the pore fluid pressure and mineral structures (Dasgupta and Mukherjee, 2020; Liu et al., 2017; Ruser et al., 2006). In this regard, compaction is a key factor for altering the chemical and physical properties of soils and sediments.

    Physical devices used to study compaction can be mainly divided into uniaxial, biaxial and triaxial categories according to the types of compressive stress. Early in 1986, Sandbaekken et al. (1986) introduced a high-stress uniaxial oedometer with the feature of a vertical effective stress, with three different heights, which could be placed with or without a chamber for back-pressuring. It was widely modified for investigating the compaction behavior and rock properties of various sands concerning the textural properties and mineralogical composition (Koochak Zadeh et al., 2016; Mondol et al., 2008, 2007), evaluating the vertical swelling pressure parameter (Langroudi and Yasrobi, 2013) and other numerous related researches. The first biaxial oedometer was mentioned by Bishop and Donald in 1961. Then there were several different designs that were presented (Alabdullah et al., 2009; Sivakumar et al., 2006; Ng et al., 2002; Yin, 2002; Toyota et al., 2001; Cui and Delage, 1996). Later, a biaxial device was designed to measure the axial stress and strain to account for hypoplastic constitutive impacts (Pincus et al., 1993). The triaxial compactor usually was applied for the super high-pressure strain (CRS) compression testing of soils and sediments (Muna and McCartneyb, 2015). Neveux et al. (2014) also proposed a triaxial device that was able to be used to simulate deeply buried reservoirs and record the results of stresses, fluid pressure, and strains in-situ.

    Previous studies, and the properties of the compaction devices, have focused on the physical properties of soils and sediments, including pore structure, densities and porosity, and a few studies also examine chemical reactions under super-high pressure (usually up to 50 MPa) (Nooraiepour et al., 2017; Fawad et al., 2011). Considerable information found that the porosity and water content, which had great variations in the initial period of compaction with a low pressure, were key factors for controlling the microbe and chemical reactions of soils and sediments (Rubol et al., 2013; Saha and Hossain, 2011; Zeglin et al., 2011). There is, however, lack of a specialized in-situ instruments for the experiments on the initial compaction of soils and sediments and especially resulting impacts on water chemistry. The research presented a novel compaction instrument with an auto collector and an analysis system which include analytical instruments; capable of measuring the chemistry of gases and liquids squeezed from sediment samples under the pressure. It was developed to explore the chemistry of samples under the different pressures. This device was subject to a series of pressure tests to demonstrate its reliability, robustness and safety. According to the overburden pressure (Weller, 1959), we estimated the pressure at 20 m thick layers to be 0.6 MPa. Here we use pressure tests ramping to 0.6 MPa at different rates to illustrate impact on water and gas chemistry.

  • The entire system is composited of four major parts characterized with their own special functions, including pressure simulator reactor system (PSRS), gas-liquid separator (GLS), automatic collector (AC) and composite control system (CCS). They are connected with each other (as shown in Fig. 1) to realize the online monitor for the compaction experiments under a long time operation. PSRS, the core component of this device, is composed of a sampling chamber, pressure unit, heating cover and in-line sensor. The function of GLS is to separate the gases and the liquids flowing out of the PSRS. AC is able to split the separated gas and liquid, allowing them to then be measured in the connecting analytical instruments. The control and data processing is performed by CCS.

    Figure 1.  Schematic diagram of the pressure testing device showing the four primary components.

    Figure 2 presents the assembly diagram of two sets of the PSRS. According to the various purposes of the experiments, two sets of reaction chambers are made from two different materials. The first chamber (Fig. 2a) is made from Hastelloy Alloy C-276 (HC Chamber), a kind of special alloy material which is able to resist high pressure, high temperature and corrosion (Leonard, 1969). The chamber are covered by heating jackets (designed as hollow cylinders of 98.00 cm (38.58 inch) high and 8.00 cm (3.15 inch) diameter). The heating jacket provides the temperature which is supported by a circulating temperature controller and determined by the characters of the media materials. Here, we choose the phenylmethyl silicone oil as the heating fluid which allows for a temperature range from -50 to 200 ℃. A precise actuating motor combined with a long screw as the power system is used to drive a piston moving that increases the overburden pressure. The range of the pressure is from 0 to 15 000 kPa (0-15 MPa), and can be set with different rates of increase in pressure (i.e., 100 kPa per day, 100 kPa per hour, 100 kPa per min). With a height of 20 cm sampling cells, this chamber is able to study the changes of pore water and sediments at various depths. The sampling cells are vertically (in uniform arrangement) to facilitate fluid flow and sediments, and they are mounted to the chamber. Furthermore, the filters are equipped in outputs and their materials can be changed according to various types of samples. In order to add additional sources into the chambers during the pressurization process, a pressure injection unit (as shown in Fig. 2a) is used. During this process, the injection pressure of additional sources should not be less than the pressure applied in the chamber. Therefore, water is pumped by a pressure pump into a piston column to provide pressure for the injection solutions. When the additional sources are gases, we can connect a gas valve to an air supply directly. And this part also can be used to collect the liquid samples from the top. For monitoring and recording the displacement of piston, a displacement sensor is placed on the top of the shaft attached to the piston. There are two temperature sensors monitoring the center and the edge of samples and three pressure sensors monitoring the top and the bottom of solid and liquid samples.

    Figure 2.  Two configuration of PSRS. (a) A chamber made of HC-267, a pressure injection unit and circulating temperature controller; (b) a chamber made of PEEK, a pressure injection unit and circulating temperature controller.

    The other chamber (as shown in Fig. 2b) made of Polyether ether ketone (PEEK) can be examined by the computed tomography (CT scan) without the pressure relief (O'Reilly et al., 2015; Rae et al., 2007). Different from the HC Chamber, the main function of this chamber is able to have CT scan, so the materials of its accessories must be non-metallic. In this chamber, we use a high quality actuating motor combined with a long screw and another piston chamber to provide the overburden pressure. The water in the piston chamber can be forced into the PEEK Chamber to give pressure on the samples. We can close the valve which is connected the piston chamber and the PEEK Chamber, and bring the whole PEEK Chamber with keep the overburden pressure to have the CT scan. Based on the some characters of the pressure methods on the two chambers, there are some differences between two sets of the pressure injection units. Its pressure unit can only add an extra channel to the piston chamber when the pressure of additional sources is same as its pressure on the samples (Fig. 1b), but the same pressure unit as shown in Fig. 1a is needed when the experiments need a greater pressure for additional sources. As for the sampling cells, the PEEK Chamber only has two sampling ports at bottom for collecting liquid and solid respectively. In addition, the sensors in this chamber is arranged to the HC Chamber.

    The liquid squeezed from the samples is always a mixer with some gases. The design of gas-liquid system is based on gravity flow as illustrated in Fig. 3. It is also a simple piston system which can adjust the volume and pressure, according to water flow from the PSRS. The piston can be fastened by two pins on the top of the GLS, and is similar to a "sifter" full of small holes. The top Teflon film allows for gas but not liquid transmission. In contrast, an alternate film attached at the bottom allows for fluid flow and collection. Two tubes are set in the piston. The longer one is used for supplying mixed gas-liquid from the PSRS, and the shorter one for retrieving the gas with connecting an extracting pump. In addition, micro-air pump connects with the shorter tube for collecting the gas and sending them into the gas chromatography (GC). A solenoid valve connects with the bottom tube to control.

    Figure 3.  Configuration of gas liquid system (GLS). The longer tube is connected to PSRS. When mixed gas-liquid from PSRS flows into the GLS, the solenoid valve is closed until micro-air pump extracts the gas out. The solenoid valve and the micro-gas pump can be set to open and close intermittently based on the experiments demands. The liquid will flow into the next component of the system.

    As the key part of connecting the various analysis instruments, the schematic diagram of automatic collector is displayed in Fig. 4. Unlike a standard fraction collector, this AC have two mechanical arms for injection needles (designed for the outputs of PSRS or GLS) and sampling needles (designed for the inputs of various analysis instruments) separately. In the AC, two mechanical arms able to moving in three directions (up and down, front and back, right and left) are set to make "collecting" and "taking" actions. "Collecting" makes the liquid flow into the sample bottles. The second arm can carry out the "taking" actions for the analysis instruments.

    Figure 4.  Schematic view of the Automatic Collector. This sampling needle is used to connect the analyze devices. The injection needle is used to collect the liquid from different outputs.

    Composite control system (CCS) is the "brain" of this device and allows automated control and monitoring. It allows setting of temperature, pressure, temperature change rate, pressure change rate, displacement rate and other parameters, as well as data acquisition and processing. It is able to execute the data files (in excel format) and some line-graphics automatically. AC is also controlled by it and easily to be enforced the operations based on the experimental requirements.

  • To test the device, clayey sediments were collected from a depth of 2 m below a rice paddy in Jianghan Plain. The water table of the rice paddy was below 1.1 m, and the collected samples were soaked in the groundwater with an average initial water content around 40.1%. Before filled the chamber with the samples, they were air dried for several days, then ground and sieved through a 200-mesh screen to ensure homogeneity. Sediments were added to the columns to a height of roughly 2 cm and then thoroughly compacted by a PMMA rod. This progress was repeated until the chamber was filled with the sediment-packed columns. After adding sediment, the plunger was placed at the surface of samples. Deionized water was then pumped into the chamber at a rate of 0.2 mL∙min-1 from the bottom sampling cell. The water content of the samples was determined by two methods. One was by calculating the mass of influent solutions and the total mass, the other was by mass differences of the samples before and after drying in an oven at 105 ℃.

    Our experiments represented three pressure modes: constant rate, acceleration rate and deceleration rate. We also included five different pressure rates: 0.02 MPa per 12 h, 0.04 MPa per 12 h, 0.06 MPa per 12 h, 0.04 MPa per 24 h, 0.04 MPa per 36 h, from 0.04 MPa per 12 h to 0.02 MPa per 12 h and from 0.04 MPa per 12 h to 0.06 MPa per 12 h. After preparing the samples as mentioned above, we input the final pressure and the pressure rate in the CCS. Then the flow rate, calculated by the liquid volumes from the PSRS at a certain time, could be adjusted for realizing the online monitoring. After the tests above, we also carried out unilateral and bilateral drainage experiments to present an additional example of this device. Here, we collected liquid samples from the bottom sampling cells (unilateral drainage) and both the bottom and top (bilateral drainage), and measured their masses.

  • For testing the pressure accuracy, we carried out 14 pressure tests and calculated the difference between add-value and test-value. It was able to reach ±1.01‰. And we used the mixed N2-H2O samples to go through GLS for verifying its accuracy. The result showed 89.61% N2 can be collected from the GLS. The temperature accuracy was less than ±1% only on a higher moisture samples within the range 18.0 and 37.5 ℃.

  • Table 1 showed the initial moisture and porosity data of the samples for the pressure tests. The maximum initial moisture of samples in five pressure modes is 39.76%, the minimum is 38.92%. And the initial porosity of that is around 62%. There were five tests carried out to test the pressure function. The errors between results tested by the pressure sensors and added values were within ±0.001 MPa. Figure 5 illustrates the relationship between added pressure values and tested pressure values, and the slight errors might occur in a long experiment. Comparing the displacements of the five modes, the instruments could perform the changeable pressures during the experiments well. Additionally, we tested the functions of top and bottom sampling cells (Table 2). We chose 0.02 MPa per 12 h, 0.04 MPa per 12 h and 0.06 MPa per 12 h as the pressure rates. The fluids were effectively displaced from the top and bottom sampling cells. Interestingly, the water masses of unilateral groups were lower than the bilateral. More importantly, there were great differences between the chemical results of the top and the bottom samples.

    Pressure mode Initial moisture (%) Initial porosity (%)
    0.04 MPa/12 h 39.61 62.13
    0.04 MPa/24 h 39.28 61.75
    0.04 MPa/36 h 39.76 62.51
    0.04-0.02 MPa/12 h 39.61 62.13
    0.04-0.06 MPa/12 h 38.92 62.51

    Table 1.  Initial moisture and porosity data of samples on various pressure modes

    Figure 5.  Relationship between test pressure values and added pressure values.

    0.02 MPa/12 h 0.04 MPa/12 h 0.06 MPa/12 h
    Unilateral Bilateral Unilateral Bilateral Unilateral Bilateral
    Water mass (g) 1 941.81 2 208.42 2 100.66 2 483.91 2 073.9 2 565.33

    Table 2.  Water mass results on three pressure modes

  • We tested the pressure device at 0.6 MPa, but it is capable of extending to 15 MPa. The device is also able to perform experiments at varying temperature and vertical flow rates, with or without the pressure. The pressure injection coupling unit can be used as an input or sampling cell. And the functions on vertical flows supply the possibility for finding out the differences of the complicate reactions with kinds of external sources (e.g., solutions or gases). In addition, the PEEK chamber can help to explore the change of pore size and pore structure of various solid samples (e.g., sand, clay, silt) or liquid samples under different pressures. Furthermore, with the auto sampling collector and computer controlling system, high-throughput sampling can easily be accomplished by tight experimental control. In general, this device can be used for studying and solving some key issues on diverse areas including compaction, land subsidence, and hydraulic engineering.

  • The authors would like to thank the National Natural Science Foundation of China (Nos. 41630318, 41521001) and the Wuhan Institute of Industrial Technology of Geological Resources & Environment for their financial supports. We appreciate some great advices provided by Prof. Guoli Zhu from Huazhong University of Science and Technology and Prof. Scott Fendorf from Stanford University. The final publication is available at Springer via https://doi.org/10.1007/s12583-020-1334-6.

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