Responses of Laterally Loaded Single Piles Subjected to Various Loading Rates in a Poroelastic Soil

Groundwater table has an important role in soil–structure interaction problems. However, analysis of laterally loaded single piles has often been conducted by solely considering the mechanics of the soil skeleton or decoupling the interactive mechanics of the soil skeleton and the fluid flux; in other words, most analyses were performed without taking into consideration the coupling effect between the soil skeleton and the fluid flux. To improve our understanding of the hydromechanical coupling effect on laterally loaded single piles, a series of finite element study on laterally loaded single piles in saturated porous media was conducted. The effect of pile cap geometries, cap widths, cap embedment depths, and pile lengths, on the response of laterally loaded single piles was also studied. The loading condition of the pile was found to have a significant effect on the generation of excess pore-water pressure. The lateral displacement and bending moment computed at the maximum excess pore water pressure, which in turn, is equivalent to an undrained analysis, produced the minimum responses among all the other loading conditions. The effect of pile cap geometries was found to be much less significant than anticipated.

A sensitivity analysis of a single extraction well from deep geothermal aquifers in the Cheshire Basin, UK

Deep hot sedimentary aquifers (HSAs) are targeted for geothermal exploitation in the Cheshire Basin, UK. In this study, a single extraction well targeting the Collyhurst Sandstone Formation was modelled on MATLAB coupling heat and fluid flux. The Collyhurst Sandstone Formation in the Crewe area of the Cheshire Basin is expected to be found at a depth of 2.8 to 3.5 km, and was chosen as an area for geothermal exploration due to the high demand for energy.Model results suggest that low-enthalpy, deep geothermal systems with thick HSAs are affected by both geological and engineering parameters. The results of this study highlight that the thermal gradient, hydraulic conductivity, production rate, length and position of the well screen are the key parameters capable of affecting the success and viability of any single well scheme. Poor planning during exploration and development can hinder the productivity of any single well scheme and these parameters must be considered to fully understand the risk. Engineering parameters, such as the length of the well screen, can be used during well planning to mitigate geological risks in the aquifer, whilst the results presented can also be used as a guide for energy potential under varying conditions.

A Model for Stokes Flow in Domains with Permeable Boundaries

We derive a new computational model for the simulation of viscous incompressible flows bounded by a thin, flexible, porous membrane. Our approach is grid-free and models the boundary forces with regularized Stokeslets. The flow across the porous membranes is modeled with regularized source doublets based on the notion that the flux velocity across the boundary can be viewed as the flow induced by a fluid source/sink pair with the sink on the high-pressure side of the boundary and magnitude proportional to the pressure difference across the membrane. Several validation examples are presented that illustrate how to calibrate the parameters in the model. We present an example consisting of flow in a closed domain that loses volume due to the fluid flux across the permeable boundary. We also present applications of the method to flow inside a channel of fixed geometry where sections of the boundary are permeable. The final example is a biological application of flow in a capillary with porous walls and a protein concentration advected and diffused in the fluid. In this case, the protein concentration modifies the pressure in the flow, producing dynamic changes to the flux across the walls. For this example, the proposed method is combined with finite differences for the concentration field.

The Human Meniscus Behaves as a Functionally Graded Fractional Porous Medium under Confined Compression Conditions

In this study, we observe that the poromechanical parameters in human meniscus vary spatially throughout the tissue. The response is anisotropic and the porosity is functionally graded. To draw these conclusions, we measured the anisotropic permeability and the “aggregate modulus” of the tissue, i.e., the stiffness of the material at equilibrium, after the interstitial fluid has ceased flowing. We estimated those parameters within the central portion of the meniscus in three directions (i.e., vertical, radial and circumferential) by fitting an enhanced model on stress relation confined compression tests. We noticed that a classical biphasic model was not sufficient to reproduce the observed experimental behaviour. We propose a poroelastic model based on the assumption that the fluid flow inside the human meniscus is described by a fractional porous medium equation analogous to Darcy’s law, which involves fractional operators. The fluid flux is then time-dependent for a constant applied pressure gradient (in contrast with the classical Darcy’s law, which describes a time independent fluid flux relation). We show that a fractional poroelastic model is well-suited to describe the flow within the meniscus and to identify the associated parameters (i.e., the order of the time derivative and the permeability). The results indicate that mean values of λβ,β in the central body are λβ=5.5443×10−10m4Ns1−β, β=0.0434, while, in the posterior and anterior regions, are λβ=2.851×10−10m4Ns1−β, β=0.0326 and λβ=1.2636×10−10m4Ns1−β, β=0.0232, respectively. Furthermore, numerical simulations show that the fluid flux diffusion is facilitated in the central part of the meniscus and hindered in the posterior and anterior regions.

Well Testing Based Method to Identify the Complex Fracture Geometry and Changing Drainage Radius Using an Efficient Boundary Element Model

Fracture geometries and drainage radius are important parameters for developing a reasonable development plan of a single fractured well. In some unconventional gas reservoir, some scholars observed the phenomenon of single well controlled reserves increasing through the material balance curve, and put forward the idea of district supply. In addition, owing to fracture hits, the fracture geometries of fractured wells are sometimes more complex. Thus, those complex factors bring challenges for parameter estimations. In order to study the variation of the drainage radius and complex fracture geometries in the single model, a well testing based model for a finite-conductivity fractured vertical well in radial composite reservoirs with dynamic supply and fracture networks is established. Based on "successive steady state method", the point source function, pressure superposition principle and boundary element method are used to solve the reservoir model, and the methods of discrete fracture and pressure superposition are used to solve the fracture model. By introducing the rate normalized pseudo-pressure and material balance time, the variable fluid flux is equivalent to the constant fluid flux. Combined with the inversion idea of well test, the drainage radius value and fracture geometries are solved by fitting the log-log curves of pressure response, and case studies are performed. The results show that the drainage radius increases with the increase of production time and finally tends to a certain value, and it has a good exponential relationship with time. Also, the fracture geometries of the typical well are multiple-radial fracture networks. Through the study of dynamic drainage radius, the controlled reserves of single well in unconventional gas reservoir can be better determined, and it can also provide theoretical basis for fracture evaluation, productivity prediction and enhanced recovery study of the same type of unconventional gas reservoir.

Clogging and un-clogging of the subduction plumbing system may generate tremor-like patterns

<p>Many subduction zones host intermittent, low-frequency, low-magnitude seismic activity emitted from the vicinity of the plates' interface. For instance, in Guerrero, Mexico, deep (30--50 km) low-frequency earthquakes (LFEs) occur in bursts, and migrate in cascades along the subduction interface. Those patterns are often attributed to episodic pulses of fluid pressure and slow slip that travel within the fault zone. However, the dynamic behavior of the permeable system in which fluid-pressure circulates remains a blindspot in most models of tremor generation, even as geological observations report pervasive imprint of strong, localized fluid pressure and permeability variations in its source region.</p><p>In order to analyze the role of such processes in generating tremor, we design a simple model of how fluid pressure and permeability can interact within the subduction interface, and generate realistic, tremor-like patterns. It is based on seismic source triggering and interaction in a permeable channel. The latter contains a number of low-permeability plugs acting as elementary fault-valves. In a mechanism akin to erosive burst documented in porous media, valve permeability abruptly opens and closes in response to the local fluid pressure. The brutal pressure transient and/or mechanical fracturing associated with valve opening acts as the seismic source of an LFE-like event. The strong fluid pressure transient that it triggers allows valves to interact constructively: as a valve breaks open, neighbor valves are more likely to break. This interaction therefore leads to cascades and migrations of synthetic seismicity along the model fault channel, that can synchronize into larger bursts of activity that migrate more slowly along the channel. In our model, valve activity draws patterns of that closely resemble tremor patterns in Guerrero and other subduction zones.</p><p>The input metamorphic fluid flux at the base of the channel exerts a key control on the occurence of and distribution of synthetic tremor in space and time. A weak input flux will not allow valves to open, conversely a strong flux will not allow them to close. In both cases, no activity will occur. However when the value of the fluid flux is intermediate, permanent regimes of sustained activity arise. Depending on its value, activity can be strongly time-clustered, quasi-periodic or random but constant in time.</p><p>Our model is based on a simple yet powerful and realistic description of the permeability and its dynamics in fault zones. It allows for new interpretations of low-frequency seismicity in terms of effective flux and fault-zone permeability, both for long-term regimes and finer scale, transient dynamics. Eventually, it could lead to deep enhancements of our understanding of fault-zone hydraulic processes and how they are coupled with fault-slip.</p>

Fluid Flux in Fractured Rock of the Alpine Fault Hanging-Wall Determined from Temperature Logs in the DFDP-2B Borehole, New Zealand

©2018. American Geophysical Union. All Rights Reserved. Sixteen temperature logs were acquired during breaks in drilling of the 893m-deep DFDP-2B borehole, which is in the Alpine Fault hanging-wall. The logs record various states of temperature recovery after thermal disturbances induced by mud circulation. The long-wavelength temperature signal in each log was estimated using a sixth-order polynomial, and residual (reduced) temperature logs were analyzed by fitting discrete template wavelets defined by depth, amplitude, and width parameters. Almost two hundred wavelets are correlated between multiple logs. Anomalies generally have amplitudes <1°C, and downhole widths <20m. The largest amplitudes are found in the first day after mud circulation stops, but many anomalies persist with similar amplitude for up to 15 days. Our models show that thermal and hydraulic diffusive processes are dominant during the first few days of re-equilibration after mud circulation stops, and fluid advection of heat in the surrounding rock produces temperature anomalies that may persist for several weeks. Models indicate that the fluid flux normal to the borehole within fractured zones is of order 10−7 to 10−6 m s−1, which is 2–3 orders of magnitude higher than the regional flux. Our approach could be applied more widely to boreholes, as it uses the thermal re-equilibration phase to derive useful information about the surrounding rock mass and its fluid flow regime.

Fluid Flux in Fractured Rock of the Alpine Fault Hanging-Wall Determined from Temperature Logs in the DFDP-2B Borehole, New Zealand

©2018. American Geophysical Union. All Rights Reserved. Sixteen temperature logs were acquired during breaks in drilling of the 893m-deep DFDP-2B borehole, which is in the Alpine Fault hanging-wall. The logs record various states of temperature recovery after thermal disturbances induced by mud circulation. The long-wavelength temperature signal in each log was estimated using a sixth-order polynomial, and residual (reduced) temperature logs were analyzed by fitting discrete template wavelets defined by depth, amplitude, and width parameters. Almost two hundred wavelets are correlated between multiple logs. Anomalies generally have amplitudes <1°C, and downhole widths <20m. The largest amplitudes are found in the first day after mud circulation stops, but many anomalies persist with similar amplitude for up to 15 days. Our models show that thermal and hydraulic diffusive processes are dominant during the first few days of re-equilibration after mud circulation stops, and fluid advection of heat in the surrounding rock produces temperature anomalies that may persist for several weeks. Models indicate that the fluid flux normal to the borehole within fractured zones is of order 10−7 to 10−6 m s−1, which is 2–3 orders of magnitude higher than the regional flux. Our approach could be applied more widely to boreholes, as it uses the thermal re-equilibration phase to derive useful information about the surrounding rock mass and its fluid flow regime.