High-precision stress determination in photoelasticity

Stress separation is usually achieved by solving differential equations of equilibrium after parameter determination from isochromatics and isoclinics.The numerical error resulting from the stress determination is a main concern as it is always a function of parameters in discretization.To improve the accuracy of stress calculation,a novel meshless barycentric rational interpolation collocation method(BRICM)is proposed.The derivatives of the shear stress on the calculation path are determined by using the differential matrix which converts the differential form of the equations of equilibrium into a series of algebraic equations.The advantage of the proposed method is that the auxiliary lines,grids,and error accumulation which are commonly used in traditional shear difference methods(SDMs)are not required.Simulation and experimental results indicate that the proposed meshless method is able to provide high computational accuracy in the full-field stress determination.

A novel binary effective medium model to describe the prepeak stressstrain relationship of combined bodies of rock-like material and rock

Combined bodies of rock-like material and rock are widely encountered in geotechnical engineering,such as tunnels and mines.The existing theoretical models describing the stress-strain relationship of a combined body lack a binary feature.Based on effective medium theory,this paper presents the governing equation of the“elastic modulus”for combined and single bodies under triaxial compressive tests.A binary effective medium model is then established.Based on the compressive experiment of concretegranite combined bodies,the feasibility of determining the stress threshold based on crack axial strain is discussed,and the model is verified.The model is further extended to coal-rock combined bodies of more diverse types,and the variation laws of the compressive mechanical parameters are then discussed.The results show that the fitting accuracy of the model with the experimental curves of the concretegranite combined bodies and various types of coal-rock combined bodies are over 95%.The crack axial strain method can replace the crack volumetric strain method,which clarifies the physical meanings of the model parameters.The variation laws of matrix parameters and crack parameters are discussed in depth and are expected to be more widely used in geotechnical engineering.

Field experience and numerical investigations of minifrac tests with flowback in low-permeability formations

In this study,flowback-assisted minifrac tests were conducted in low-permeability shale and salt formations to measure the in situ stress.An injection/flowback testing protocol was implemented in each test to achieve accuracy and efficiency.Accurate and efficient injection/flowback testing is very important,given the impermeable nature of these formations and the need to complete each test as quickly as possible.Each flowback cycle yields a distinct and repeatable fracture closure signature,simplifying the interpretation of the fracture closure pressure.The objective of this paper is to share our field experience and to present a numerical analysis of the flowback test pressure responses,fracture closure behaviors,and fracture closure diagnostic methods.Examples from open-hole and casedhole minifrac tests are used to demonstrate site operation procedures.Then,two numerical models are presented for simulating the fracture closure behavior during a flowback test.Field evidence is provided to demonstrate that the fracture closure pressures from the flowback tests are identical to those from tests without flowback.The fracture closure diagnostic methods for flowback tests are discussed,and it is found that the G-function diagnostic method yields a distinct fracture closure signal during the flowback tests.This study is intended to provide additional insights regarding flowback tests by sharing our successes,experience,and knowledge,thereby benefiting the industry.