Showing papers on "Pore water pressure published in 2020"

Influence of Pore Water (Ice) Content on the Strength and Deformability of Frozen Argillaceous Siltstone

Abstract: Bedrock freezes deeply in high-latitude or high-elevation regions; for instance, permafrost thickness has been estimated to exceed 900 m in Arctic Canada and can even reach 1400 m in Siberian Russia (Sammis and Biegel 2004). Excavation of frozen bedrock therefore presents one of the most difficult challenges faced by engineering constructions in such areas. Understanding the mechanical properties of frozen rocks is the key for safe and efficient excavation. Due to unsettled weather and complex topography, moisture states in natural rocks may vary greatly; this variation can be more severe and complex at subzero temperatures in cold regions (Sass 2005). It is well known that the water content can influence the mechanical responses of rock (Roy et al. 2017). Therefore, investigating the mechanical properties of frozen rocks of various water contents is essential. Rocks of various water contents are basically strengthened after freezing. Static strength of rocks, including the uniaxial compressive strength, tensile strength and pointload strength, are reported to increase at subzero temperatures compared to at room temperature. Several rock types have been tested, including limestone, basalt, granite, sandstone, andesite, marble and welded tuff (Heins and Friz 1967; Mellor 1970; Inada and Yokota 1984; Dwivedi et al. 1998; Kodama et al. 2013). Fracture toughness of rocks (limestone, basalt, granite (Heins and Friz 1967), dolerite, dolomite, quartz–mica schist and agglomerate (Dwivedi et al. 2000) also increases with the decrease of temperature. Deformability of frozen rocks, usually measured by elastic modulus (e.g., Young’s modulus, initial tangent modulus, etc.), is a function of temperature as well. Young’s modulus of limestone, granite (Heins and Friz 1967), and initial tangent moduli of Berea sandstone, Indiana limestone and Barre granite show a significant increase with decreasing temperature (Mellor 1970). Previous research on the mechanical properties of frozen rocks, in most cases tested samples are pre-saturated, has focused on the influence of temperature, while the influence of the initial water content has drawn limited attention, despite the fact that unsaturation is normality. Thus, several fundamental issues remain unfathomed: (1) to what degree is initial water content altering the mechanical behaviors of frozen rocks, (2) what are the microscopic mechanisms of this influence, and (3) what is the essential difference that initial water content introduces to frozen rocks? Trying to answer the above questions, we investigated the influence of the initial water content on the mechanical properties of frozen argillaceous siltstone (at − 20 °C). Both strength (uniaxial compressive strength, tensile strength and point-load strength) and deformability of frozen argillaceous siltstone with six saturation degrees were tested. Moreover, the postfreezing phase composition of pore water (i.e., the relative amount of water, ice and gas) in rocks with nine saturation degrees was measured using the nuclear magnetic resonance (NMR) method. The potential roles of unfrozen * Fan Zi zifan319@stu.xust.edu.cn

Soil Sorptive Potential: Its Determination and Predicting Soil Water Density

Abstract: The soil sorptive potential (SSP) has been recently theorized as the physical source for matric potential and local pore-water pressure. It consists of four distinct physicochemical potenti...

Pressure Controlled Permeability in a Conduit Filled with Fractured Hydrothermal Breccia Reconstructed from Ballistics from Whakaari (White Island), New Zealand

Abstract: Breccia-filled eruption conduits are dynamic systems where pressures frequently exceed critical thresholds, generating earthquakes and transmitting fluids. To assess the dynamics of breccia-filled conduits, we examine lava, ash tuff, and hydrothermal breccia ballistics with varying alteration, veining, fractures, and brecciation ejected during the 27 April 2016 phreatic eruption of Whakaari/White Island. We measure connected porosity, strength, and permeability with and without tensile fractures at a range of confining pressures. Many samples are progressively altered with anhydrite, alunite, and silica polymorphs. The measurements show a large range of connected porosity, permeability, and strength. In contrast, the cracked samples show a consistently high permeability. The cracked altered samples have a permeability more sensitive to confining pressure than the unaltered samples. The permeability of our altered ballistics is lower than surface rocks of equivalent porosity, illustrating that mineral precipitation locally blocked pores and cracks. We surmise that alteration within the conduit breccia allows cracks to form, open and close, in response to pore pressure and confining pressure, providing a mechanism for frequent and variable fluid advection pulses to the surface. This produces temporally and spatially variable geophysical and geochemical observations and has implications for volcano monitoring for any volcano system with significant hydrothermal activity.

Microstructural evolution of expansive clay during drying–wetting cycle

Abstract: This paper presents a comprehensive investigation on the microstructural evolutions of expansive clay during a drying–wetting cycle, including pore size distribution (PSD) via mercury intrusion porosimetry and water distribution via nuclear magnetic resonance (NMR). The soil water characteristic curves at different soil densities and soil shrinkage curve are also obtained, and a threshold suction can be identified to distinguish the adsorptive and capillary regimes of pore water. Combined with the water distribution obtained by the NMR technique, the evolutions of the adsorptive water and capillary water during drying–wetting cycle were addressed. The measured PSD curves of the expansive soils at different suctions showed two distinct peaks, corresponding to micropores and macropores, respectively. Both variations of macropores and micropores are irreversible during the wetting–drying cycle, which partly explain the adsorptive water content decreasing when the suction is small.

Pore Structure Changes Occur During CO 2 Injection into Carbonate Reservoirs.

Abstract: Observations and modeling studies have shown that during CO2 injection into underground carbonate reservoirs, the dissolution of CO2 into formation water forms acidic brine, leading to fluid-rock interactions that can significantly impact the hydraulic properties of the host formation. However, the impacts of these interactions on the pore structure and macroscopic flow properties of host rock are poorly characterized both for the near-wellbore region and deeper into the reservoir. Little attention has been given to the influence of pressure drop from the near-wellbore region to reservoir body on disturbing the ionic equilibrium in the CO2-saturated brine and consequent mineral precipitation. In this paper, we present the results of a novel experimental procedure designed to address these issues in carbonate reservoirs. We injected CO2-saturated brine into a composite core made of two matching grainstone carbonate core plugs with a tight disk placed between them to create a pressure profile of around 250 psi resembling that prevailing in reservoirs during CO2 injection. We investigated the impacts of fluid-rock interactions at pore and continuum scale using medical X-ray CT, nuclear magnetic resonance, and scanning electron microscopy. We found that strong calcite dissolution occurs near to the injection point, which leads to an increase in primary intergranular porosity and permeability of the near injection region, and ultimately to wormhole formation. The strong heterogeneous dissolution of calcite grains leads to the formation of intra-granular micro-pores. At later stages of the dissolution, the internal regions of ooids become accessible to the carbonated brine, leading to the formation of moldic porosity. At distances far from the injection point, we observed minimal or no change in pore structure, pore roughness, pore populations, and rock hydraulic properties. The pressure drop of 250 psi slightly disturbed the chemical equilibrium of the system, which led to minor precipitation of sub-micron sized calcite crystals but due to the large pore throats of the rock, these deposits had no measurable impact on rock permeability. The trial illustrates that the new procedure is valuable for investigating fluid-rock interactions by reproducing the geochemical consequences of relatively steep pore pressure gradients during CO2 injection.

Porosity zoning characteristics of fault floor under fluid-solid coupling

Abstract: During exploitation approaching to faults structure, due to the influence of mining stress, the rock mass inside the fault zone and between the fault and the coal pillar is prone to damage and activate, so it is the key research and protection position. The dynamic response of faults is closely related to the failure process of rocks, and the influence of the change in mining-induced stress will also result in the differential distribution of the mechanical properties of floor, and ultimately affects the failure of floor and the outburst process of the confined water. In order to quantitatively analyze the influence of mining pressure on the porosity change of floor and water pressure distribution in coal seam mining process, the change characteristics of rock material mechanics parameters such as Young’s modulus, porosity, and permeability under the action of mining disturbance and confined water pressure were studied. A similar material simulation study on the water pressure and stress distribution of floor at different depths under different mining distance conditions was carried out. According to the results, the energy dissipation of coal floor rock mass is closely related to the change of porosity. Meanwhile, the changes in the porosity of rock mass under the action of uneven water pressure and the partition and distribution model of pore water pressure of the floor were established. When mining is carried out near the faults, the spatiotemporal position when and where the upper and lower boundaries of the fault zone in the lower water-resisting layer release energy at the same time is critical to the occurrence of water inrush.

Effect of Pressure on Imbibition in Shale Oil Reservoirs with Wettability Considered

Abstract: Recent studies indicate that there is a great potential for enhanced oil recovery in shale oil reservoirs by altering the matrix wetness to induce spontaneous imbibition. The most common method is to add surfactant additives in fracturing fluid during multistage hydraulic fracturing operation. This imbibition process has complex pressure systems involved such as reservoir pore pressure, wellbore hydrostatic pressure, and surface pumping pressure. Without the wells being soaked intentionally, this pressurized state may be sustained for more than a month before flowback. Therefore, it is important to study the effect of pressure on the imbibition-induced oil recovery enhancement and its mechanism. In this study, we conducted forced imbibition tests on core plugs of unconventional sandstone, carbonate, and shale with different wettabilities. The applied pressures were 1000, 2000, 3000, 4000, and 5000 psi, and the results were compared to those of spontaneous imbibition under atmospheric pressure. Experimental results were used further in the numerical simulation study. The results manifested that a more water-wet state is still essential to improve the oil recovery regardless of the soaking pressure. When the rock is oil-wet, higher soaking pressure does not further worsen imbibition because of the minimal negative capillary pressure when oil saturation is high. However, when the rock is water-wet, the soaking pressures can be adverse to the imbibition in shale formations because of a longer pressure transient time which is against the capillary force. Dimensionless pressure (pD) is defined in this study to quantitatively determine the extent of imbibition inhibition during the forced imbibition. This observation indicated that in tight reservoirs, a higher soaking pressure will obtain less oil recovery from imbibition than that of lower soaking pressure cases at a given time.

Investigation on frost heave of saturated–unsaturated soils

Abstract: Frost heave is a process of coupled heat–water–mechanics, which refers to heat transfer, water migration, water–ice phase change, deformation, etc. The mechanism of the frost heave for saturated–unsaturated soils was investigated to establish a frost heave model. As the freezing continues, for saturated soils, because all pores are filled with pore water, the total increased volume due to water migration and water crystallization will separate soil particles and induce frost heave. For unsaturated soils, because of the existence of unsaturated pores, volume expansion generates by water and vapor migration and water–ice transition will firstly fill into the unsaturated pores until a critical state is reached. After that, further increased volume will separate soil particles and induce frost heave. Therefore, effective strain ratio was introduced to establish the relationship between frost heave strain of unsaturated soils and the fields of moisture and temperature. Tests were carried out for silty soil. Then, based on the inverse theory, a simplified criterion was proposed to determine the effective strain ratio, and the effective strain ratio was back-calculated according to the test results. Finally, an innovative universal frost heave model for saturated–unsaturated soils was proposed.

SANISAND-MSf: a sand plasticity model with memory surface and semifluidised state

Abstract: A new constitutive model for sand is formulated by incorporating two new constitutive ingredients into the platform of a reference critical state compatible bounding surface plasticity model with k...

Distribution characteristics of floor pore water pressure based on similarity simulation experiments

Abstract: To analyse the impact of underground pressure on the variation in floor porosity and pore water pressure distribution during the coal seam mining process, this paper uses closed and open water pressure sensors to analyse rock pressure, water pressure zoning and layer distribution characteristics of the coal seam floor under mining effects based on a simulation analysis method using similar rock materials; the results determine that the separation characteristics of the pore water pressure distribution are affected by the rock pressure ‘barrier’ function, which is caused by changes in rock permeability under the coupling effect. To quantitatively analyse the rock pressure barrier’s impact on the floor mechanical structure, this paper proposes the ‘virtual separation layer’ concept and defines the virtual separation layer index to determine the scope of the floor tension fracture zone. Through monitored data of pore water pressure at different depths of the floor during field mining, the variation between the water pressure, porosity, mining distance and floor depth is obtained; similar material simulation results are verified, and a zonal distribution model of pore water pressure in the floor is established, providing a reference to investigate the distribution of pore water pressure in the floor under mining disturbances. The results show that the influence of changes in mining stress will also lead to a differential distribution of the floor mechanical properties and ultimately damage the floor and affect the inrush of the confined water. The research results play an important role in risk classification and prevention of water pressure distribution in coal seam floors.