Coupled flow and heat transfer of power-law Nanofluids on a non-isothermal rough rotary disk subject to magnetic field

In this paper, we study coupled flow and heat transfer of power-law nanofluids on a non-isothermal rough rotating disk subject to a magnetic field. The problem is formulated in terms of specified curvilinear orthogonal coordinate system. An improved BVP4C algorithm is proposed and numerical solutions are obtained. The influence of volume fraction, types and shapes of nanoparticles, magnetic field and power-law index on the flow and heat transfer behavior are discussed.<br/>Results show that the power-law exponents (PLE), nanoparticle volume fraction (NVF) and magnetic field inclination angle (MFIA) are almost no effects on velocities in wave surface direction, but have small or significant effects on azimuth direction. NVF have remarkable influence on local Nusselt number (LNN) and friction coefficients (FC) in radial and azimuth directions (AD). LNN increases with NVF while FC in AD decrease. The types of nanoparticles, magnetic field strength and inclination have small effects on LNN, but they have remarkable effects on the friction coefficients with positively correlated while the inclination is negatively correlated with heat transfer rate. The size of the nanoparticle shape factor is positively correlated with LNN.

Iteratively Coupled Flow and Geomechanics in Fractured Poroelastic Reservoirs: A Phase Field Fracture Model

Fluid-solid coupling in fractured reservoirs plays a critical role for optimizing and managing in energy and geophysical engineering. Computational difficulties associated with sharp fracture models motivate phase field fracture modeling. However, for geomechanical problems, the fully coupled hydromechanical modeling with the phase field framework is still under development. In this work, we propose a fluid-solid fully coupled model, in which discrete fractures are regularized by the phase field. Specifically, this model takes into account the complex coupled interaction of Darcy-Biot-type fluid flow in poroelastic media, Reynolds lubrication governing flow inside fractures, mass exchange between fractures and matrix, and the subsequent geomechanical response of the solid. An iterative coupling method is developed to solve this multifield problem efficiently. We present numerical studies that demonstrate the performance of our model.

Discrete Fracture Model for Hydro-Mechanical Coupling in Fractured Reservoirs

The process of coupled flow and mechanics occurs in various environmental and energy applications, including conventional and unconventional fractured reservoirs. This work establishes a new formulation for modeling hydro-mechanical coupling in fractured reservoirs. The discrete-fracture model (DFM), in which the porous matrix and fractures are represented explicitly in the form of unstructured grid, has been widely used to describe fluid flow in fractured formations. In this work, we extend the DFM approach for modeling coupled flow-mechanics process, in which flow problems are solved using the multipoint flux approximation (MPFA) method, and mechanics problems are solved using the multipoint stress approximation (MPSA) method. The coupled flow-mechanics problems share the same computational grid to avoid projection issues and allow for convenient exchange between them. We model the fracture mechanical behavior as a two-surface contact problem. The resulting coupled system of nonlinear equations is solved in a fully-implicit manner. The accuracy and generality of the numerical implementation are accessed using cases with analytical solutions, which shows an excellent match. We then apply the methodology to more complex cases to demonstrate its general applicability. We also investigate the geomechanical influence on fracture permeability change using 2D rock fractures. This work introduces a novel formulation for modeling the coupled flow-mechanics process in fractured reservoirs, and can be readily implemented in reservoir characterization workflow.

Effects of passive storage on modelling hydrological function and isotope dynamics in a karst flow system in southwest China

Representing passive storage in coupled flow-isotope models can facilitate simulation of mixing and retardation effects on tracer transport in many natural systems, such as catchments or rivers. However, the effectiveness of incorporating passive storages in models of complex karst flow systems remains poorly understood. In this study, we developed a coupled flow-isotope model that conceptually represents both “fast” and “slow” flow processes in heterogeneous aquifers to represent hydrological connections between hillslopes and low-lying depression units in cockpit karst landscapes. As this model originally included a varying number of passive storages at different positions of the flow system (e.g. fast/slow flow reservoirs combined with different hillslope/depression units), the model structure and relevant parameters were optimized using a multi-objective optimization algorithm. This was used to match detailed observational data of hydrological processes and isotope concentration in the Chenqi catchment in southwest China. Results show that the optimal structure for a coupled flow-isotope model incorporated only two passive storages in fast flow and slow flow paths of the hillslope unit. Using fewer or greater numbers of passive stores in the model could lead to under- or over-mixing of isotope signatures. This optimized model structure could effectively improve simulation accuracies for outlet discharge and isotope signatures, with > 0.65 of the modified Kling-Gupta efficiency. Additionally, the optimal tracer-aided model yields reasonable parameter values and estimations of hydrological components (e.g. more than 80 % of fast flow in the total discharge). Furthermore, results imply that the solute transport is primarily controlled by advection and hydrodynamic dispersion in steep hillslope unit, which is a remarkable phenomenon in the karst flow system. The study resulted in new insights, more realistic catchment conceptualizations and improved model formulation.

New Insights on Characteristics of the Near-Wellbore Fractured Zone from Simulated High-Resolution Distributed Strain Sensing Data

Summary Rayleigh frequency-shift-based distributed strain sensing (RFS-based DSS) is a fiber-optic-based diagnostic technique, which can measure the strain change along the fiber. The spatial resolution of RFS-based DSS can be as low as 0.2 m, and the measuring sensitivity is less than 1 μɛ. Jin et al. (2021) presented a set of DSS data from the Hydraulic Fracture Test Site 2 project to demonstrate its potential to characterize near-wellbore fracture properties and to evaluate perforation efficiency during production and shut-in periods. Extensional strain changes are observed at locations around perforations during a shut-in period. At each perforation cluster, the observed responses of strain changes are significantly different. However, the driving mechanisms for the various observations are not clear, which hinders accurate interpretations of DSS data for near-wellbore fracture characterization. In this study, we applied a coupled flow and geomechanics model to simulate the observed DSS signals under various fractured reservoir conditions. The objective is to improve understanding of the DSS measurements and characterize near-wellbore fracture geometry. We used our in-house coupled flow and geomechanics simulator, which is developed by a combined finite-volume and finite-element method, to simulate strain responses within and near a fracture during shut-in and reopen periods. Local grid refinement was adopted around fractures and the wellbore, so that the simulated strain data can accurately represent the DSS measurements. The plane-strain condition is assumed. Numerical models with various fracture geometries and properties were constructed with representative parameters and in-situ conditions of the Permian Basin. The simulated well was shut-in for 4 days after producing 240 days, and reopened again for 1 day, following the actual field operation as shown in Jin et al. (2021). The characters of the strain changes along the fiber were analyzed and related to near-wellbore fracture properties. A novel diagnostic plot of relative strain change vs. wellbore pressure was presented to infer near-wellbore fracture characteristics. The impacts of permeability and size of the near-wellbore-stimulated region, fracture length, and near-perforation damage zone on strain responses were investigated through sensitivity analysis. The strain responses simulated by our model capture the observed signatures of field DSS measurements. During the shut-in period, clear positive strain changes are observed around the perforation locations, forming a “hump” signature. The shape of the “hump” region and peak value of each “hump” are dependent on the size and permeability of the near-wellbore fractured zone. Once the well is reopened, the strain changes decrease as the pressure drops. However, in one cycle of shut-in and reopen, the strain-pressure diagnostic plot shows path dependency. The discrepancy between the shut-in and reopen periods is highly influenced by the properties of near-wellbore fractured zones. The differences in the strain-pressure diagnostic plots can help to identify the conductive fractures. This study provides better understandings of the DSS measurements and their relations to the near-wellbore fracture properties, which is of practical importance for near-wellbore fracture characterization and completion/stimulation optimization.

Study on the Rock Fracture during Fluid Injection Using a Coupled Flow-Stress-Damage (FSD) Model: Insight into the Stress Shadow Effect

In order to investigate the influence of pore pressure on hydraulic fracturing behavior in the local and whole model, the coupled flow-stress-damage (FSD) analysis system RFPA-Flow was used to study the influence of rock heterogeneity, natural stress ratio, double-hole spacing, and water pressure gradient on the stress shadow effect. The numerical results show that the tensile crack induced by pore water pressure is significantly affected by the pore water pressure and water pressure gradient. The larger the pore pressure gradient is, the more asymmetrical the crack development pattern and the smaller the instability pressure of the model. In addition, the shape of hydraulic fracture becomes much more irregular with the increase in rock heterogeneity. The number and shape of tip microcracks under the influence of local water pressure are closely related to the homogeneity of rock. Moreover, when the natural stress difference is large, the hydraulic fracture propagates parallel to the maximum principal stress; when the stress field is close and the spacing between two holes is less than 5 times the diameter, the propagation direction of hydraulic fractures between holes is perpendicular to the maximum principal stress. It is found that no hydraulic fractures occur between the two holes when the distance between holes is greater than 5 times the diameter.

Workshop: Best-practice for laboratory testing low-permeable materials

The testing of low-permeable materials is challenging. Yet, for the disposal of radioactive waste, it is essential, too. This workshop is aimed at gathering ambitious scientists to discuss and to collaborate on their experiences in the laboratory testing of low-permeable materials. The focus here is on the methods: What method is best for what kind of low-permeable host rock (salt/clay) and for what kind of technical barrier material (bentonite/crushed salt)? How can measurement errors be correctly determined? What are the crucial “bottlenecks” in the device setups? How can high porous but low permeable samples best be pre-saturated? How can coupled flow and cumbersome gas traps in the tests be dealt with? What is the best-practice analysis of permeability from pressure decay recordings? Is there a hope of defining a standardized procedure for low-permeability testing? These points will be reflected in the light of radioactive waste disposal and in the need to find a best-practice solution when it comes to eventual evaluation and comparison of potential underground disposal sites.