Showing papers on "Pore water pressure published in 2021"

Repetitive Mining Stress and Pore Pressure Effects on Permeability and Pore Pressure Sensitivity of Bituminous Coal

Abstract: Coalbed methane is a lucrative energy source, but its development relies heavily on the fracturing of coal seam and underground extraction during mining. This study of mining stress paths, pore pressure, and permeability of a coal body was aimed to overcome the constraints of low coal permeability under the stress applied to a coal seam group. Variation in the applied stress changes the effective stress in coal and its permeability (which also depends on pore pressure). The effects of different mining intensities, repetitive cyclic loading and unloading, and pore pressure on gas permeability in a coal mine are revealed and discussed. The permeability evolution curve can be described by quadratic polynomial equations and split into three (decreasing, stable, and increasing) stages. When the stress environment with low axial deviatoric stress changes to that with high axial deviatoric stress and when the stability stage becomes longer, the goodness of quadratic polynomial data fitting is reduced. Cyclic loading and unloading with different axial stress paths have different effects on coal permeability. Increased axial and confining pressures decrease the permeability and pore pressure sensitivity of permeability. Because the obtained permeability-pore pressure curves contain a stable stage, this sensitivity is more pronounce in the low and high pore pressure stages.

Seismic performance assessment of unsaturated soil slope in different groundwater levels

Abstract: Soil slopes located in more rainfall region have been damaged significantly in the previous earthquakes due to the earthquake-induced excess pore water pressure (EPWP), which is among primary factors causing slope failure. For the purpose of evaluating seismic behaviors of an unsaturated soil slope at various groundwater levels, we established a simple approach for calculating earthquake-induced EPWP, which is importable to the numerical simulation software through the custom interface. Based on this, we investigate the seismic performance of the unsaturated soil slope. It is observed that the seismic performance of the slope has much difference at various groundwater levels; the slope deformation at a high groundwater level increases greatly while the groundwater reduced the vibration of the slope. Also, it was found that the slope shows different failure processes with the groundwater influence: the failure of slope with high groundwater is mainly the flow slide and tensile crack around the slope toe while the slope presents the collapse and slip failure without the groundwater influence. Moreover, by strict similarity law formula derivation, the shaking table test of two slope models, one without groundwater and one with groundwater, was performed, and the test results show that our calculation results are accurate and reasonable, and our establishment calculation method of EPWP is practical and convenient.

State of the Art Review of Emerging and Biogeotechnical Methods for Liquefaction Mitigation in Sands

Abstract: Earthquake-induced liquefaction causes soil to exhibit fluidlike behavior due to a sudden increase in pore water pressure and a concurrent decrease in effective stress. The liquefaction ca...

Effect of Depletion and Fluid Injection in the Mesozoic and Paleozoic Sandstone Reservoirs of the October Oil Field, Central Gulf of Suez Basin: Implications on Drilling, Production and Reservoir Stability

Abstract: This work attempted to understand the behavior of the Upper Cretaceous Nezzazat and Lower Cretaceous–Carboniferous Nubia sandstone reservoirs in response to production-induced depletion and fluid injection for enhanced hydrocarbon recoveries from the October oil field, Gulf of Suez, Egypt. Pore pressure (PP), vertical stress (Sv) and minimum horizontal stress (Shmin) magnitudes were modeled based on well logs, drilling data and subsurface measurements. The latest measurements indicated 11.7–12.7 MPa pressure drop (ΔPP) in the Nezzazat reservoirs, while the Nubia sandstone reservoir was depleted by 19–21 MPa. Revised PP and Shmin gradients offer a narrow mud weight window of 9–10.7 PPG (pore pressure gradient) if the entire Lower Miocene–Carboniferous section was planned to be drilled with a single casing in the infill/injector wells. A more conservative approach will be to drill the depleted reservoirs with 5.5–9.3 PPG mud window and case separately, although that may incur an additional cost. Based on the PP–Shmin poro-elastic coupling, stable stress path values of 0.61 and 0.65 are interpreted in the Upper and Lower reservoirs, indicating depletion-induced normal faulting is unlikely to occur at the present rate of depletion. The reservoir stability threshold during pressurization was assessed for fluid injection optimization to sustain production and curtail the bypassed oil. The maximum allowable pressure build-up during injection was estimated using various possible pore pressure–stress coupling scenarios at their maximum depletion state. Based on the PP–Shmin coupling approach, maximum pressure increments of 23 and 27 MPa can be permitted in the depleted Nezzazat and Nubia sandstone reservoirs during injection, without exceeding the lower limit of caprock Shmin, as applicable for both the reservoirs. This will ensure the geomechanical stability of the reservoirs as well as the caprock integrity. This geomechanical study provides crucial comprehensions regarding the optimization of drilling, production, and fluid injection by reducing the risk of reservoir instabilities and formation integrity.

Experimental Investigation of Mechanical Properties and Failure Behavior of Fluid-Saturated Hot Dry Rocks

Abstract: Thousands of cubic meters of fluid are continuously injected for a long term to create complex fracture patterns in hydraulic fracturing of hot dry rocks. However, the physics and mechanics behind the interaction of fluid–rock are not fully understood at present. To reveal the related damage mechanisms of saturated rock samples such as damage initiation and evolution at various alternative stress levels, a series of in-house laboratory tests were performed on a TAW-series triaxial rock mechanics testing system, combined with ultrasound measurement and acoustic emission (AE) monitoring. After saturation with nano-emulsion and distilled water, ultrasound velocity of longitudinal wave was increased by 40%. Saturation weakens these mechanical parameters such as the crack damage stress ratio, fracture toughness and cohesive strength under different stress conditions. Fluid-saturated rock sample has higher AE hit rate than dry rock sample. Meanwhile, many step-like jumps appeared on the curve of cumulative AE events. Failure envelop, b-value and frequency spectrum were analyzed out to compare the mechanical difference between fluid-saturated and dry rock samples. The experimental results demonstrate that the saturation increased the pore pressure in rocks and further promoted crack propagation in hydraulic fracturing. Moreover, nano-emulsion liquid is more advantageous than distilled water for enhancing fracture complexity. This investigation provides for better understanding of the mechanisms of complex fracture formation in deep geothermal reservoirs.

Prediction of shallow landslides in pyroclastic-covered slopes by coupled modeling of unsaturated and saturated groundwater flow

Abstract: Slopes covered with unsaturated shallow pyroclastic deposits, lying upon fractured limestone bedrock, are widespread in the mountains around Naples (southern Italy). Rainfall infiltration, reducing soil suction, eventually triggers shallow landslides. While drastic reduction of suction is unanimously recognized as the triggering mechanism, there is still debate about the hydrological processes controlling slope drainage and causing the establishment of landslide predisposing conditions. Field observations at the slope of Cervinara suggested that temporary storage of water in a perched aquifer, in the upper part of the fractured bedrock, may affect the leakage through the soil-bedrock interface. Hence, a physically based model, coupling flows in the unsaturated soil cover and in the perched aquifer, has been applied to three large rainfall events which occurred in December 1999 (when a landslide was triggered), January 2009, and November 2012. The results highlight that the different responses of soil and aquifer to precipitations, related not only to rainfall event characteristics (i.e., duration and mean intensity) but also to the initial conditions of the slope, determined by antecedent precipitations, can play a prominent role in the triggering of landslides. In fact, further simulations with synthetic rainfall events and different initial conditions provide a possible interpretation of the triggering of the landslide of December 1999, indicating that a soil profile with dry conditions at the base and a low level in the perched aquifer, typical of late autumn, can impede the drainage of infiltrating water through the soil-bedrock interface, thus favoring the build-up of pore pressure within the soil cover.

Experimental and Theoretical Study on Frost Deformation and Damage of Red Sandstones with Different Water Contents

Abstract: Volumetric expansion of water by 9% in saturated pores and cracks causes substantial frost deformation in rock masses. Frost deformation is an important index reflecting the frost resistance of rocks; however, water saturation has a great influence on the frost deformation characteristics. In this research, the frost strains and acoustic emission activities of red sandstone with different water saturations are monitored under freeze–thaw conditions. The experimental results show that both the peak and the residual frost heaving strains greatly increase for sandstone beyond 85% water saturation. However, there is no significant frost heaving strain that occurs in low-saturation red sandstone (less than 85% water saturation). The acoustic emission activities show the same change trend and further confirm the existence of this critical saturation. In addition, the pore size distribution also has a great influence on the frost heaving strain and freeze–thaw damage. All the liquid pore water in this red sandstone is frozen at − 20 °C because the pores are larger than the critical freezing radius (2.58 nm at − 20 °C) according to the measured pore size distribution. Based on the pore micromechanics and Gibbs–Thomson equation, a developed frost heaving model is proposed considering the effects of water saturation and the pore size distribution. The proposed model can be used to predict the frost heaving strain at any freezing temperature for unsaturated red sandstone. This study thus provides the frost deformation characteristics of red sandstone and contributes to a better understanding of the freeze–thaw damage mechanism of unsaturated sandstone.

Initiation mechanism and quantitative mass movement analysis of the 2019 Shuicheng catastrophic landslide

Abstract: The 2019 Shuicheng landslide was a typical landslide in that failure occurred in the slope deposits and there was a close correlation with the antecedent rainfall. The initiation mechanism was analysed using field investigations, laboratory tests and numerical simulations and the characteristics of mass movement were quantitatively assessed. The numerical results indicated that the infiltration of rainfall decreased the shear resistance of the slope and increased the pore water pressure and soil weight, resulting in a large deformation of the slope and a gradual deterioration in its stability. After saturation, as the pore water pressure coefficient (ru) approached 0.3, the overall safety factor decreased to 1 and failure eventually occurred. Quantitative mass assessment showed that the unique terrain aggravated the erosion and entrainment effects and that the inclusion of loose material and water promoted the movement of the debris avalanche by changing it into a fluidized state, resulting in a much larger landslide volume with a long runout distance. As a result, the volume of the failed mass, which was calculated by an unmanned aerial vehicle and terrestrial laser scanning modelling techniques to be c. 4.7 × 105 m3 in the source area, almost quadrupled to >2.0 × 106 m3.

Study on the seepage characteristics of deep buried tunnels under variable high-pressure water heads

Abstract: Well-connected seepage channels are the key components of tunnel water inrush. To study the trends of the pore water pressure in the rock mass in a seepage channel, the physical experiment and corresponding numerical model were established. First, in the seepage experiment under excavation disturbance, the propagation rate of the high-pressure water source and the growth rate of the pore water pressure in the rock mass were analyzed by considering the different water pressures. Second, under the same model size, parameters, and working conditions, the finite element method was used to simulate the seepage process of water in the rock mass. From the microlevel, the evolution process of the pore water pressure isosurface was obtained. The results showed that the pressure water head is positively correlated with the pore water pressure in the rock mass, which displays multiple approximate relationships. Moreover, when the water pressure is high, excavation disturbance can significantly increase the pore water pressure in the rock mass. The disturbance caused by the first excavation is larger than the subsequent excavations. In addition, the research results were compared with previous results. Reasonable agreements that would provide a reference for the conditions of high water-pressure disasters in deep buried tunnel construction were obtained.

New Mechanical Model of Slotting–Directional Hydraulic Fracturing and Experimental Study for Coalbed Methane Development

Abstract: When hydraulic fractures do not expand in the direction required by a project, it is difficult to enhance coal seam permeability effectively. Slotting–directional hydraulic fracturing (SDHF) has been proposed as a possible alternative, but there is not enough theoretical mechanism to guide the construction. Based on preliminary study of the directional mechanism of single slotted hole, we established a new slotting–directional hydraulic fracturing (NSDHF) mechanical model by using the complex function theory of elasticity, and the influence of stress interference between adjacent slotted holes and non-uniform pore water pressure was considered. We carried out true triaxial double slotted holes SDHF experiments and used large-scale nondestructive computer tomography to scan the fractured samples to ensure accurate measurement of directional distance. The measured directional distance was used to verify the NSDHF model; the maximum deviation was 5.1%. Taking the experimental data in this paper as example, the stress interference between adjacent slotted holes decreased the fracture directional distance by 20.3%, and the non-uniform pore water pressure increased the fracture directional distance by 47.6%. NSDHF mechanical model realized the quantitative description of the influence of non-uniform pore water pressure on directional distance. The contribution of non-uniform pore water pressure to directional distance accounted for more than 25% of the total directional distance; the effect of non-uniform pore water pressure on fracture direction distance was almost twice the stress interference between adjacent slotted holes. The verified NSDHF model was used to study further the influence of horizontal stress difference, azimuth of slotted hole, slotting size and fluid injection pressure on directional distance with different slotting spacing. The work discussed in this paper will contribute to promoting and apply SDHF on a large scale in low-permeability coal mines.

Modelling Rock Fracture Induced By Hydraulic Pulses

Abstract: Soft cyclic hydraulic fracturing has become an effective technology used in subsurface energy extraction which utilises cyclic hydraulic flow pressure to fracture rock. This new technique induces fatigue of rock to reduce the breakdown pressure and potentially the associated risk of seismicity. To control the fracturing process and achieve desirable fracture networks for enhanced permeability, the rock response under cyclic hydraulic stimulation needs to be understood. However, the mechanism for cyclic stimulation-induced fatigue of rock is rather unclear and to date there is no implementation of fatigue degradation in modelling the rock response under hydraulic cyclic loading. This makes accurate prediction of rock fracture under cyclic hydraulic pressure impossible. This paper develops a numerical method to model rock fracture induced by hydraulic pulses with consideration of rock fatigue. The fatigue degradation is based on S–N curves (S for cyclic stress and N for cycles to failure) and implemented into the constitutive relationship for fracture of rock using in-house FORTRAN scripts and ABAQUS solver. The cohesive crack model is used to simulate discrete crack propagation in the rock which is coupled with hydraulic flow and pore pressure capability. The developed numerical model is validated via experimental results of pulsating hydraulic fracturing of the rock. The effects of flow rate and frequency of cyclic injection on borehole pressure development are investigated. A new loading strategy for pulsating hydraulic fracturing is proposed. It has been found that hydraulic pulses can reduce the breakdown pressure of rock by 10–18% upon 10–4000 cycles. Using the new loading strategy, a slow and steady rock fracture process is obtained while the failure pressure is reduced.

Investigation of pore pressure, in-situ stress state and borehole stability in the West and South Al-Khilala hydrocarbon fields, Nile Delta, Egypt

Abstract: This work interprets the pore pressure distribution and in-situ stress magnitudes in the ~ 10,600 feet thick Neogene succession of the West and South Al-Khilala gas fields, onshore Nile Delta. The Messinian incised valley-fill (IVF) fluvio-marine sediments host the principal gas-bearing zones having a 0.47 PSI/feet pore pressure gradient, as seen from the downhole measurements. Early Pliocene is marked by a dramatic regional sea-level rise which deposited thick marine shales unconformably above the Abu Madi IVF reservoir facies. A high sedimentation rate during Pliocene retained excess porosity and translated to a mild overpressure of around 0.55 PSI/feet in the Kafr El Sheikh Formation due to compaction disequilibrium. Vertical (Sv), minimum (Sh), and maximum (SH) horizontal stress gradients are estimated as 0.97–0.98; 0.64–0.76, and 0.86–1 PSI/feet, respectively. Interpreted stress magnitudes decipher a normal to strike-slip stress state in the studied fields. B-Quality wellbore breakouts with ~ 600 feet cumulative length infer NNE-SSW SH orientation, almost parallel to the motion direction of the African plate. Wellbore stability analysis indicated a minimum mud weight of 11.45 PPG needs to be maintained against the Pliocene shales which exhibited major wellbore instability issues in the exploratory drilling campaign. Based on the interpreted pore pressure, Sh, and collapse pressure gradients, a safe drilling mud window is proposed to ensure borehole stability.