Hydrostatic stress

About: Hydrostatic stress is a research topic. Over the lifetime, 1568 publications have been published within this topic receiving 37773 citations.

Stress-Dependent Flow, Mechanical, and Acoustic Properties for an Unconventional Oil Reservoir Rock

Abstract: Summary Accurate estimation of rock properties has significant impact on reservoir simulation, geophysical interpretation, hydraulic fracture design, and geomechanical analysis. B ecause reservoirs are buried at depth, measurement of these properties should consider a s tress environment that is as close to in-situ as possible. Stress-dependence of permeability, pore volume compressibility, elastic moduli, and acoustic anisotropy has been widely studied in conventional reservoirs. H owever, due to challenges in sample preparation and laboratory setup, most measurements of unconventional rock are still limited to hydrostatic stress conditions. This paper investigates a nano-Darcy unconventional oil reservoir rock under triaxial stress conditions. Rock permeability and pore volume compressibility, strength and Young’s modulus, acoustic velocities, six stiffness coefficients, and Thomsen anisotropy parameters are measured under different levels of loading and confining stresses. The relationship of these properties to the differential stress between loading and confining stresses is identified and compared with conventional hydrostatic tests. The differential stress level reaches as high as 20,000 psi for rock mechanics tests and 10,000 psi for flow tests. S teady pressure and pulse decay methods have been applied to measure permeability with gas or oil fluid flows. Despite high rock strength (greater than 10,000 psi), the rock permeability and acoustic anisotropy strongly depend on the differential stress level. Following a typical slow permeability reduction with increasing stress, a distinguished stage of accelerated permeability reduction is observed when the differential stress exceeds 3,160 psi. Interestingly the initial pore volume compressibility of the strong rock is measured as high as 50×1/Mpsi. After slight decreases until the differential stress reaches a certain level, the compressibility reverses the trend and increases significantly. Both rock strength and Young’s modulus vary greatly with the angles to bedding planes. The stress-strain curves under different confining stresses challenge the traditional modulus-based brittleness. Thomsen anisotropy parameters reduce as much as 80% with the differential stress after observing an initial increase of anisotropy when the stress is low. Among the six stiffness coefficients, the two shear components along vertical and horizontal directions (C12 and C13) vary much more than the others. T his highlights the necessity of applying differential stress, rather than hydrostatic stress, in the laboratory. It also indicates that the studied nano-Darcy rock may be much more compressible and stress-dependent than its strength suggests.

A fiber-reinforced thermoelastic medium with an internal heat source due to hydrostatic initial stress and gravity for the three-phase-lag model

Abstract: Purpose The purpose of this paper is to investigate the effect of a hydrostatic initial stress and the gravity field on a fiber-reinforced thermoelastic medium with an internal heat source that is moving with a constant speed. Design/methodology/approach A general model of the equations of the formulation in the context of the three-phase-lag model and Green-Naghdi theory without energy dissipation. Findings The exact expressions for the displacement components, force stresses, and the thermal temperature for the thermal shock problem obtained by using normal mode analysis. Originality/value A comparison made between the results of the two models for different values of a hydrostatic initial stress as well as an internal heat source. Comparisons also made with the results of the two models in the absence and presence of the gravity field as well as the reinforcement.

Evaluation of constraint methodologies applied to a shallow-flaw cruciform bend specimen tested under biaxial loading conditions

Abstract: A technology to determine shallow-flaw fracture toughness of reactor pressure vessel (RPV) steels is being developed for application to the safety assessment of RPVs containing postulated shallow surface flaws Matrices of cruciform beam tests were developed to investigate and quantify the effects of temperature, biaxial loading, and specimen size on fracture initiation toughness of two-dimensional (constant depth), shallow surface flaws The cruciform beam specimens were developed at Oak Ridge National Laboratory (ORNL) to introduce a prototypic, far-field out-of-plane biaxial stress component in the test section that approximates the nonlinear stresses resulting from pressurized-thermal-shock or pressure-temperature loading of an RPV Tests were conducted under biaxial load ratios ranging from uniaxial to equibiaxial These tests demonstrated that biaxial loading can have a pronounced effect on shallow-flaw fracture toughness in the lower transition temperature region for RPV materials The cruciform fracture toughness data were used to evaluate fracture methodologies for predicting the observed effects of biaxial loading on shallow-flaw fracture toughness Initial emphasis was placed on assessment of stress-based methodologies namely, the J-Q formulation, the Dodds-Anderson toughness scaling model, and the Weibull approach Applications of these methodologies based on the hydrostatic stress fracture criterion indicated an effect of loading-biaxiality on fracture toughness, the conventional maximum principal stress criterion indicated no effect

Computer simulation of martensitic transformations in constrained, two-dimensional crystals under external stress

Abstract: This article reports a computer simulation study of the microstructures produced by martensitic transformations. In the present work, the transformation strain is dyadic, and the transformation is athermal and irreversible. The transformation occurs in a two-dimensional crystal that is constrained in a matrix that has no net transformation strain and may be subject to external stress. The crystal is divided into elementary cells. The transformation is simulated by computing the elastic strain energy in the linear elastic approximation and transforming the most-favored cell in each step to generate the minimum-energy transformation path. The simulation generates the microstructure at each step of the transformation and plots a temperature-transformation (TT) curve by computing the chemical driving force required to maintain the transformation and assuming that it is proportional to the undercooling. The results show that the matrix constraint causes complex, multivariant microstructures and separatesMsandMf. Multiple variants partly relax the shear part of transformation strain but interfere so that the transformation is difficult to maintain. The dilational part of the transformation strain produces interesting microstructures, such as “butterfly martensite,” in partially transformed crystals. It also increases ΔM since it produces a hydrostatic stress that cannot be compensated by mixing variants. The applied stress can be divided into hydrostatic and deviatoric components. The hydrostatic component changesMswithout altering the microstructure or ΔM. The deviatoric stress changes the relative energies of the variants and produces a microstructure that is rich in the favored variant. It also increases ΔM, since single-variant transformations must be sustained against an accumulating, uncompensated shear. The thermal resistance (ΔM) increases with the magnitude of the deviatoric stress until a high-stress limit is reached and only one variant appears. The microstructure is most complex at intermediate stress, where both variants develop in a complex internal stress field. Cyclic stress dramatically increases the extent of transformation at given maximum load. The martensite that has already formed becomes a source of intense internal stress when the stress is reversed, promoting further transformation.