shear dilation
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Energies ◽  
2021 ◽  
Vol 14 (6) ◽  
pp. 1667
Author(s):  
Shuai Yang ◽  
Yan Jin ◽  
Yunhu Lu ◽  
Yanru Zhang ◽  
Beibei Chen

PetroChina’s Xinjiang oilfield has a large quantity of tight oil reserves and hydraulic fracturing technology has been widely used to achieve commercial production. Some parts of this tight glutenite formation are laumontite-rich and the actual productivity of the hydraulically fractured wells is less than expected. To figure out the ways that laumontite affects tight glutenite well productivity, comprehensive experimental and numerical simulation studies have been conducted to investigate the rock mechanical properties, fluid flow behaviors and the major controlling factor of productivity. Laboratory results indicate that the tight glutenite formation with higher laumontite content has higher initial porosity, permeability but lower yield strength and more severe stress sensitivity in both permeability and fracture conductivity. For laumontite-rich glutenite rocks, there are commonly three types of rock deformation during the loading process: elastic compression, shear dilation and shear enhanced compaction. Both elastic compression and shear enhanced compaction will cause the reduction on rock porosity and permeability. A fully coupled finite element model (FEM) considering stress-induced permeability evolution was introduced to simulate the production process. Permeability evolution models of three different deformation stages were presented, respectively. Simulation results showed that our model is in good agreements with the well testing data. The simulated oil production characteristics for permeability evolution coupled and uncoupled models were discussed. Results showed the strong stress-induced permeability reduction is the major factor that laumontite causing the low and quickly declining oil rates. Initial permeability has a positive effect on productivity and stress-induced fracture conductivity reduction has slight influence on productivity. The results of this paper indicate that the stress-induced permeability evolution in the oil production process must be considered to accurately evaluating reservoirs in the studied area.


2021 ◽  
Author(s):  
shun wang ◽  
Wei Wu ◽  
Dichuan Zhang ◽  
Jong-Ryeol Kim

This paper presents a new rate-dependent hypoplastic constitutive model for overconsolidated clays. The model is developed based on a basic hypoplastic model proposed recently for sand. New density and stiffness factors are introduced to account for history dependence. The Matsuoka-Nakai failure surface is incorporated for the limit stress criterion. With six constitutive parameters, the model is capable of predicting the hardening/softening, shear dilation/contraction, and asymptotic state for overconsolidated clays. Comparison between numerical predictions and experimental results shows this model can properly describe the main features of both reconstituted and undisturbed clays with different overconsolidation ratios.


2021 ◽  
Author(s):  
Katarzyna Warburton ◽  
Duncan Hewitt ◽  
Jerome Neufeld

<p>The dynamics of soft-bedded glacial sliding over saturated till are poorly constrained and difficult to realistically capture in large scale models. While experiments characterise till as a plastic material with a pressure dependent yield stress, large scale models rely on a viscous or power-law description of the subglacial environment to efficiently constrain the basal sliding rate of the ice. Further, the subglacial water pressure may fluctuate on timescales from annual to daily, leading to transient adjustment of the till.</p><p>We construct a continuum two-phase model of coupled fluid and solid flows, using Darcy flow for the fluid phase and a recently described saturated granular model for the solid. After verifying our model against the steady-state experiments, we force the model with a fluctuating effective pressure at the ice-till interface and infer the resulting relationships between basal traction, porosity, rate of deformation, and till flux. Shear dilation introduces internal pressure variations, leading to hysteretic behaviour in low-permeability materials, resulting in a time-dependent effective sliding law.</p>


Geofluids ◽  
2020 ◽  
Vol 2020 ◽  
pp. 1-11
Author(s):  
Zhiqiang Zhou ◽  
Yu Zhao ◽  
Chaolin Wang

In this paper, a new approach has been developed for predicting the hydraulic and mechanical relationship of individual fractures subjected to normal stress and compression-shear stress. Considering that the closure process of rough fracture subjected to normal stress can be divided into two phases (linear behavior and nonlinear behavior), a relationship between normal stress and fracture aperture is derived through the minimum potential energy principle. Then, a formulation for calculating fracture permeability during shearing and compression processes is developed. Furthermore, a formulation for determining fracture aperture during the crack growth process is obtained, which is further implanted into the permeability model to predict the hydraulic behavior of fractured rock during fracture propagation. This new model not only considers the normal deformation of the fracture but also, and more importantly, integrates the effect of fracture propagation and shear dilation. Theoretical studies demonstrate that fracture permeability increases nonlinearly during fracture propagation. At last, experimental results and analytic results are compared to demonstrate the usefulness of the proposed models, and satisfactory agreements are obtained.


2020 ◽  
Vol 13 (19) ◽  
Author(s):  
Pan Ding ◽  
Yayuan Hu ◽  
Wanhuan Zhou ◽  
Xingwang Liu ◽  
Riqing Xu

Author(s):  
Mostafa E. Mobasher ◽  
Juan G. Londono ◽  
Pawel B. Woelke

Abstract We present VistaDam, a physics-based ductile fracture material model that is tailored to predict failure in thin metal sheets. VistaDam is based on a three invariant plasticity model in which metal fracture is dependent on the combined evolution of the triaxial stresses as well as the third invariant of deviatoric stress. Thus, VistaDam can predict damage due to combined volumetric void growth and shear dilation; which provides VistaDam with a superior capability to describe and predict fracture in a wide range of loading ranges. VistaDam relies on three independent material parameters that can be calibrated from experimental data at different triaxiality. Calibration is achieved through the automated calibration tool VistaCal. The calibrated VistaDam material card can be readily used in explicit FEM packages such as Abaqus and LS-DYNA. In addition, the calibrated VistaDam model can be used as a virtual testing platform that can generate data required by data-driven models such as GISSMO and Johnson-Cook. This process is currently automated within VistaCal’s graphical user interface. VistaDam and VistaCal have been developed for Navy applications and have been deployed successfully to predict pressurized pipes and vessel deformation and fracture under extreme loading conditions.


Solid Earth ◽  
2020 ◽  
Vol 11 (2) ◽  
pp. 627-655 ◽  
Author(s):  
Linus Villiger ◽  
Valentin Samuel Gischig ◽  
Joseph Doetsch ◽  
Hannes Krietsch ◽  
Nathan Oliver Dutler ◽  
...  

Abstract. We performed a series of 12 hydraulic stimulation experiments in a 20m×20m×20m foliated, crystalline rock volume intersected by two distinct fault sets at the Grimsel Test Site, Switzerland. The goal of these experiments was to improve our understanding of stimulation processes associated with high-pressure fluid injection used for reservoir creation in enhanced or engineered geothermal systems. In the first six experiments, pre-existing fractures were stimulated to induce shear dilation and enhance permeability. Two types of shear zones were targeted for these hydroshearing experiments: (i) ductile ones with intense foliation and (ii) brittle–ductile ones associated with a fractured zone. The second series of six stimulations were performed in borehole intervals without natural fractures to initiate and propagate hydraulic fractures that connect the wellbore to the existing fracture network. The same injection protocol was used for all experiments within each stimulation series so that the differences observed will give insights into the effect of geology on the seismo-hydromechanical response rather than differences due to the injection protocols. Deformations and fluid pressure were monitored using a dense sensor network in boreholes surrounding the injection locations. Seismicity was recorded with sensitive in situ acoustic emission sensors both in boreholes and at the tunnel walls. We observed high variability in the seismic response in terms of seismogenic indices, b values, and spatial and temporal evolution during both hydroshearing and hydrofracturing experiments, which we attribute to local geological heterogeneities. Seismicity was most pronounced for injections into the highly conductive brittle–ductile shear zones, while the injectivity increase on these structures was only marginal. No significant differences between the seismic response of hydroshearing and hydrofracturing was identified, possibly because the hydrofractures interact with the same pre-existing fracture network that is reactivated during the hydroshearing experiments. Fault slip during the hydroshearing experiments was predominantly aseismic. The results of our hydraulic stimulations indicate that stimulation of short borehole intervals with limited fluid volumes (i.e., the concept of zonal insulation) may be an effective approach to limit induced seismic hazard if highly seismogenic structures can be avoided.


2020 ◽  
Vol 152 (10) ◽  
pp. 104708
Author(s):  
Rong-Guang Xu ◽  
Yuan Xiang ◽  
Stefanos Papanikolaou ◽  
Yongsheng Leng

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