High Effective Stress Research Articles

3D CFD-DEM modeling of sand production and reservoir compaction in gas hydrate-bearing sediments with gravel packing well completion

Sand production is one of the bottlenecks restricting the safe, efficient, and controllable production of hydrates. Enhancing the understanding of mesoscopic sand production responses is essential for sand production risk management. Yet, existing mesoscopic sand production models inadequately capture the effects of hydrate cementation, resulting in an incomplete assessment of the mechanical impacts of hydrates on sand production. Herein, we developed a new three-dimensional model for sand production in gas hydrate-bearing sediments (GHBSs) with gravel packing well completion, utilizing the coupled computational fluid dynamics and discrete element method (CFD-DEM). The model considers the coupled interactions of mechanical weakening and permeability variation in GHBSs caused by hydrate cementation reduction. Simulations are analyzed to clarify the responses of sand production and reservoir compaction under the coupled mechanical, hydraulic, and sand control completion in GHBSs during depressurization. The high fluid flow rate induced by a high production pressure differential can promote sand production and reservoir compaction. Additionally, the high effective stress and high hydrate dissociation rate induced by a high production pressure differential are beneficial for initial sand production, but they can also prematurely lead to gravel packing layer obstruction, inhibiting the final sand production. This also results in a dual impact on compaction deformation, enhancing it through compaction while decelerating it by inhibiting sand production. This work provides a viable simulation idea and preliminary insights into the mechanism of sand production from GHBSs.

The Coefficient of Earth Pressure at Rest K0 of Sands up to Very High Stresses

The mechanical behaviour of soils subjected to any stress path in which deviatoric stresses are present is heavily characterised by non-linearity, irreversibility and is strongly dependent on the initial state of stress. The latter, for the majority of geotechnical applications, is normally determined by the at-rest earth pressure coefficient K0, even though this state is valid, strictly speaking, for axisymmetric conditions and for zero-lateral deformations only. Many expressions are available in the literature for the determination of this coefficient for cohesive and granular materials both for normal consolidated and over-consolidated conditions. These relations are available for low to medium stress levels. Results of an extensive experimental investigation on two sands of different mineralogy up to very high stress (120 MPa) are reported in the paper. For reach very high vertical stresses, a special oedometer has been realised. In the loading phase (normal consolidated sands), the coefficient K0n depends on the stress level. It passes from values of about 0.8 to values of about 0.45 in the range of effective vertical stress σ′v = 0.5–4 MPa. Subsequently, K0n is about constant and varies between 0.45 to 0.55 up to very high vertical effective stresses (120 MPa). For the sands employed in the tests, Jaki’s relation did not lead to reliable results at relatively low pressures, while at high pressures, the same relationship seems to lead to reliable predictions if it refers to the constant volume angle of shear strength. For the over-consolidated sands, K0C strongly depends on the OCR, and for very high values of OCR, K0C could be greater than Rankine’s passive coefficient of earth pressure, Kp. This result is due to the very locked structure of the sands caused by the grain crushing, with intergranular contact of sutured and sigmoidal, concavo-convex and inter-penetrating type, that confer to the sand a sort of apparent cohesion and make it similar to weak sandstone.

Unstable Sliding of Plagioclase Gouge and Deformation Mechanisms Under Hydrothermal Conditions With Effective Normal Stresses of 100–300 MPa

AbstractPlagioclase feldspar is a major mineral in mafic crustal rocks. To better understand the deformation mechanism of plagioclase feldspar during frictional faulting, we conducted shearing experiments on simulated plagioclase gouge in a wide range of effective normal stress of 100–300 MPa, pore‐water pressure of 30–100 MPa, and temperatures ranging from 100°C to 600°C. The coefficient of friction is found to range from 0.65 to 0.74 across the entire temperature range, showing no significant thermal weakening process. Except for a case at 200°C with an effective normal stress of 300 MPa, the frictional sliding is velocity weakening over the whole temperature range, showing a steady‐state rate dependence (a−b) ranging from −0.5 × 10−3 to −8.6 × 10−3. This property facilitates nucleation of unstable slips in frictional faulting. Above 200°C, the direct rate effect parameter (a) and the evolution effect parameter (b) of friction increase with temperature up to a threshold of 400°C or 500°C, depending on the effective normal stress. This thermal enhancement suggests thermally activated creep at contact junctions governed by intergranular pressure solution, as evidenced by microstructural signatures indicating the prevalence of very fine precipitates formed at the surfaces of gouge particles as a result of pressure solution. In frictional sliding of plagioclase, a low effective normal stress of 100 MPa corresponds to a higher degree of velocity weakening and tends to facilitate seismic slip rather than slow slips, whereas the high effective normal stress of 300 MPa corresponds to a minor velocity weakening which may cause slow‐slip events in faults of limited size.

Padé acoustoporoelasticity finite-difference simulation of elastic wave propagation in prestressed porous media

Abstract Acoustoelasticity has significant implications in seismic exploration, enabling the inference of geological structures and the properties of oil and gas reservoirs through rock physical acoustoelastic parameters, as well as elucidating the propagation of elastic waves in subsurface rocks. In traditional acoustoporoelastic numerical simulations, third-order elastic constants are derived from the Taylor expansion of the strain energy function. However, under conditions of high effective stress, the Taylor approximation can lead to divergent elastic wave velocities. To address this limitation, we propose employing the Padé approximation for the expansion of the strain energy function, as it provides superior accuracy compared to the Taylor approximation. By calibrating the Padécoefficients using experimental data for various rock types, the resultant acoustoelastic constants exhibit a reasonable theoretical limit on elastic wave velocities as effective stress increases. This method allows for a more precise characterization of velocity variations with stress, especially under high-pressure conditions. The acoustoporoelasticity numerical simulation is mainly limited to the 2D cases, so we consider confining prestressed mode in the 3D cases.

Subduction Zone Geometry Modulates the Megathrust Earthquake Cycle: Magnitude, Recurrence, and Variability

AbstractMegathrust geometric properties exhibit some of the strongest correlations with maximum earthquake magnitude in global surveys of large subduction zone earthquakes, but the mechanisms through which fault geometry influences subduction earthquake cycle dynamics remain unresolved. Here, we develop 39 models of sequences of earthquakes and aseismic slip (SEAS) on variably‐dipping planar and variably‐curved nonplanar megathrusts using the volumetric, high‐order accurate code tandem to account for fault curvature. We vary the dip, downdip curvature and width of the seismogenic zone to examine how slab geometry mechanically influences megathrust seismic cycles, including the size, variability, and interevent timing of earthquakes. Dip and curvature control characteristic slip styles primarily through their influence on seismogenic zone width: wider seismogenic zones allow shallowly‐dipping megathrusts to host larger earthquakes than steeply‐dipping ones. Under elevated pore pressure and less strongly velocity‐weakening friction, all modeled fault geometries host uniform periodic ruptures. In contrast, shallowly‐dipping and sharply‐curved megathrusts host multi‐period supercycles of slow‐to‐fast, small‐to‐large slip events under higher effective stresses and more strongly velocity‐weakening friction. We discuss how subduction zones' maximum earthquake magnitudes may be primarily controlled by the dip and dimensions of the seismogenic zone, while second‐order effects from structurally‐derived mechanical heterogeneity modulate the recurrence frequency and timing of these events. Our results suggest that enhanced co‐ and interseismic strength and stress variability along the megathrust, such as induced near areas of high or heterogeneous fault curvature, limits how frequently large ruptures occur and may explain curved faults' tendency to host more frequent, smaller earthquakes than flat faults.

Numerical analysis of stress and distortion in bulk deposited structures of Inconel 625 alloy: Influence of deposition strategies

This study explores the numerical analysis of effective stress and distortion (dimensional variations) in bulk deposited structures of Inconel 625 alloy with seven different deposition strategies using wire arc additive manufacturing (WAAM) process. Due to the challenges in detecting in-situ stress changes during bulk deposition, numerical analysis is employed to approximate stress distribution. The goal is to investigate stress and distortion (dimensional variations) in the build direction (BD), longitudinal and transverse directions, and understand their impact on anisotropy and asymmetry. The findings reveal distinct patterns in effective stress, with initial decrease in bottom, followed by a gradual increase at the middle and rapid increase thereafter, regardless of deposition strategies. Parallel and contour deposition strategies exhibit lower effective stress compared to spiral strategies. Distortion (dimensional variations) along the BD increases with height, and parallel strategies result in higher distortion. The parallel strategies show increasing distortion from one edge to the other, while spiral and contour strategies lead to high distortion at the center. The parallel and contour deposition strategies exhibit higher compressive stress at the bottom and middle compared to the spiral strategies. Anisotropy analysis indicates higher stress anisotropy in parallel and contour patterns, but lower distortion anisotropy compared to spiral patterns. Empirical equations are developed to understand the relationship between stress, distortion (dimensional variations) and hardness. The study suggests that higher effective stress can adversely affect material yielding behavior. This study undertakes a comparison between FEA and analytical model which reveals a close alignment in the residual stress distributions in the build direction.

Seepage process understanding at long-existing landslide dams through numerical analysis and hydrological measurements

Landslide dams consist of unconsolidated heterogeneous material and lack engineering measures to drain water and control pore water pressure. They may be porous and seepage through them could potentially lead to piping failure. In this research, the internal processes within a long-existing landslide dam are assessed under transient seepage force. The implemented approach includes a 3D finite element numerical simulation executing fully coupled flow-deformation and consolidation methods based on hydraulic data measurements and geotechnical laboratory tests. The nonlinear constitutive model ‘Hardening Soil’ is applied to accurately calculate the stress-induced pore water pressure, effective stress, deformation, and flow. Further, the possibility of slope failure due to seepage force is investigated through the strength reduction method. The results highlight the dependency of the seepage flow on the corresponding variation of the relative permeability and saturation in the soil mediums under different rates of seepage force. Small rates of seepage force, however, impose deformation at the dam's crown. High effective stress is obtained at negative small rates of seepage force where the long duration of fluctuation is modeled. In the drawdown simulation, there is a reverse relation between effective stress and the rate of the seepage force. Through the modeling process and based on the measured data, two seepage paths are detected within the landslide dam, while their activation depends on the lake level. The modeling approach and the required data analysis are suggested for utilization in further studies regarding the seepage process understanding at the long-existing landslide dams and their hazard assessments in addition to the common geomorphological approaches.

Strength and creep characteristics of methane hydrate-bearing clayey silts of the South China Sea

Natural gas hydrates are abundantly detected in the clayey-silty deposits of the South China Sea (SCS). The long-term hydrate extraction process may trigger creep deformation and failure of the clayey-silty reservoir. Hence, it is essential to evaluate the creep properties of the clayey-silty hydrate reservoir for safe production. Current research mainly focused on rapid failure, while neglecting time-dependent creep deformation of the clayey-silty hydrate reservoir. In this study, a number of triaxial shear and creep experiments were carried out on methane hydrate-bearing clayey silts (HBCSs) in the SCS in order to investigate their strength and creep deformation characteristics. The results indicated that: (1) the increase of effective stress enhances the shear strength and deformation, and the presence of hydrates enhances the shear strength while reducing the deformation; (2) the creep strain and strain rate increase as the creep stress ratio and effective stress rise; (3) the accelerated creep stage only occurs at higher effective stress in HBCSs and not in hydrate-free clayey silts (HFCSs), and the creep strain and strain rate of the HBCSs are lower than those of the HFCSs. The experimental data will contribute to the construction and verification of creep constitutive models for HBCSs in the SCS.

Uniqueness of the Normal Consolidation Line for Gold Tailings

Abstract Depending on the stress state, mine tailings are generally accepted to be susceptible to static liquefaction. A common method to assess the in situ stress state of tailings in relation to static liquefaction susceptibility involves the use of the state parameter. Because most tailings materials are normally consolidated (NC), this type of assessment requires knowledge about the normal consolidation line (NCL). It has been shown experimentally that the uniqueness of the NCL is vastly different for fine-grained and coarse-grained soils, with clays usually exhibiting a unique NCL and clean sands exhibiting an infinite number of parallel NCLs. Gold tailings, a sandy silt, fall between clays and clean sands, and there are limited experimental data regarding their compression behavior over a range of initial void ratios. This lack of data results in inconsistent interpretation of the uniqueness of the NCL for gold tailings in the industry. This can influence the results of designs and safety evaluations of tailings dams. In this study, a number of oedometer tests were conducted on gold tailings sourced from an active tailings dam in South Africa. Several specimens were prepared at various initial densities and were consolidated in small increments to a high effective stress. The oedometer tests were supplemented with triaxial compression tests, from which a unique critical state line was identified. Across the oedometer and triaxial tests, it was found that the behavior of the NC and overconsolidated samples was consistent with that typically observed for fine-grained soils. Therefore, for practical purposes, it appears that the gold tailings tested can be viewed in a framework with a unique NCL. No significant influence of particle crushing was noted.

Effect of High Effective Stress on the Permeability of Shale Rock Undergoing Shear Deformation Fracturing: A Comprehensive Study

Effect of High Effective Stress on the Permeability of Shale Rock Undergoing Shear Deformation Fracturing: A Comprehensive Study

Link between the Nankai underthrust turbidites and shallow slow earthquakes

Trench sediments such as pelagic clay or terrigenous turbidites have long been invoked to explain the seismogenic behavior of the megathrust fault (i.e., décollement). Recent numerous studies suggest that slow earthquakes may be associated with huge megathrust earthquake; however, controls on the slow earthquake occurrence remain poorly understood. We investigate seismic reflection data along the Nankai Trough subduction zone to understand the correlations between the spatial distribution of the broad turbidites and along-strike variations in shallow slow earthquakes and slip-deficit rates. This report presents a unique map of regional distribution of the three discrete Miocene turbidites that underthrust apparently along the décollement beneath the Nankai accretionary prism. A comparison of distributions of the Nankai underthrust turbidites, shallow slow earthquakes, and slip-deficit rates enables us to infer that the underthrust turbidites may cause primarily low pore-fluid overpressures and high effective vertical stresses across the décollement, leading to potentially inhibiting the slow earthquake occurrence. Our findings provide a new insight into potential role of the underthrust turbidites for shallow slow earthquakes at subduction zone.

An experimental study on the effects of acidization on coal permeability: Implications for the enhancement of coalbed methane production

Acidization can enhance gas/oil production by increasing permeability. The coal permeability at one effective stress (ES) before/after acidization was compared in many studies, but the ES changes greatly during coalbed methane (CBM) production. Moreover, the coal permeability enhancement after acidization varies from −30% to 20,000%, and it is unclear which parameters contributed to those variances. In this study, the pore structure/mineral composition/permeability at different ES of coal before/after acidization are determined. The results show that the key parameter affecting the acidization performance is the total carbonate mineral content, which is almost zero after acidization. The permeability at atmospheric pressure (k0) increases after acidization, with an average increase of 780.97%, but a high increase in the compression coefficient (cf) leads to a lower permeability than that before acidization at high ES in some samples, indicating a negative effect of acidization. This study finds that coals with a porosity of 5–9%, ash yield of 8–18%, carbonates of 10–40%, clays of 30–70%, and more filled-in-fracture minerals are more suitable for acidization. This study also establishes empirical equations for predicting permeability after acidization, with average predicted k0/cf errors of 17.94%/7.19%, providing foundations for preliminarily evaluating the potential of CBM production enhancement by acidization.

The mechanism of inward and outward expansion of multiscale dynamic permeability of coalbed methane at different temperatures

AbstractThe multiscale micro‐nano pores in coal can result in the ultra‐low permeability of coal, which restricts the efficiency of gas extraction. It is difficult for the conventional seepage‐enhancement measures to affect the nanoscale pores within the coal matrix. Thermal stimulation can reach deep into the micro‐nano pores within coal matrix to improve the permeability. Therefore, it is important to study the diffusivity and permeability of the multiscale micro‐nano pores at different temperatures. In this study, the experiments of diffusion‐seepage measured by the methods of GRI (Gas Research Institution) and steady‐state were conducted using a cylindrical coal at different temperatures and pressures. The experimental results show that the apparent diffusion coefficient of cylindrical coal is not constant but variable dynamically; and the classical diffusion model fails to describe the full‐time process of gas flow accurately. On this basis, a model of multiscale dynamic apparent diffusion‐seepage that can accurately describe the full‐time flow process was proposed. As is observed, the apparent permeability attenuates dynamically with time without stress loading, and the initial apparent permeability and the attenuation coefficient increase monotonically with the rise of temperature. Under the stress constraint, the steady‐state permeability increases after a decrease as the temperature rises, displaying a “U‐shaped” pattern. Without stress constraint, the increasing temperature causes the exterior multiscale pores to expand outward by different degrees so as to increase permeability, while the interior micro‐nano pores show three inward and outward expansion mechanisms. Under stress constraint, at low temperature and high effective stress, the increasing temperature causes pores to expand inward and the permeability decreases accordingly. When temperature continues to increase, coal expands outward because the effective stress is counteracted by the thermal stress, leading to an increase in permeability. This study is of significance for thermal gas extraction engineering.

Compressibility, permeability and microstructure of fine-grained soils containing diatom microfossils

Fine-grained soils containing diatom microfossils of biological origin are found worldwide. However, although these soils are acknowledged to have unique physical and mechanical properties, the exact role of diatoms in determining their compression and hydraulic behaviours remains unclear, especially when high effective stress is involved, and the underlying micromechanism is yet to be revealed. This study investigated the compression and permeability of diatomaceous soils prepared by mixing pure diatom and kaolin powders through one-dimensional constant rate of strain compression tests with a maximum vertical stress of 10 MPa. The microstructure of the soils studied was observed by atomic force microscopy, scanning electron microscopy and mercury intrusion porosimetry, and the microstructural evolution during compression was traced. The results indicate that the special hollow structure of diatom particles with internal pores contributes significantly to the high void ratio, compressibility and permeability of diatomaceous soils, and increasing the diatom content improves the pore distribution non-uniformity of the diatom–kaolin mixtures. In addition, the diatom particles have high brittleness and breakage potential, leading to microstructural rearrangement during compression, especially pore-structure adjustment. Lower compressive stress deforms the inter- and intra-aggregate pores, but higher vertical stress damages the diatoms’ hollow structure, which changes the compression and hydraulic parameters in a different manner from the case of conventional clay. This paper enriches the knowledge concerning the multilevel behaviour of fine-grained soils containing diatom microfossils and provides a fundamental dataset.

Experimental and Numerical Assessment of the Influence of Bottomhole Pressure Drawdown on Terrigenous Reservoir Permeability and Well Productivity

During oil and gas fields development, a decrease in reservoir and bottomhole pressure has often a detrimental effect on reservoir properties, especially permeability. This study presents the results of laboratory tests conducted to determine the response of terrigenous reservoir core-sample permeability to changes in the effective stresses and a decrease in the reservoir pressure. The considered samples were exposed for a long time to a constant high effective stress for a more reliable assessment of the viscoplastic deformations. According to these experiments, the decrease of the core samples permeability may reach 21% with a decrease in pressure by 9.5 MPa from the initial reservoir conditions. Numerical simulations have been also conducted. These have been based on the finite element modeling of the near-wellbore zone of the terrigenous reservoir using poroelasticity relations. The simulation results show a limited decrease in reservoir permeability in the near-wellbore zone (by 17%, which can lead to a decrease in the well productivity by 13%).

Numerical Simulation of the Wormhole Propagation in Fractured Carbonate Rocks during Acidization Using a Thermal-Hydrologic-Mechanics-Chemical Coupled Model

Acidizing is a widely adopted approach for stimulating carbonate reservoirs. The two-scale continuum (TSC) model is the most widely used model for simulating the reactive process in a carbonate reservoir during acidizing. In realistic cases, there are overburden pressure and pore pressure at present. When the injected acid reacts with the rock, the dissolution of the rock and the consumption of the acid in the pore will break the mechanical balance of the rock. Many experimental studies show that cores after acidizing have lower strength. However, it is still not clear how the deformation of rocks by the change of ground stress influences the acidizing dynamics. For fractured carbonate reservoirs, fractures play a leading role in the flow of injected acid, which preferentially flows into the fractures and dissolves the fracture walls. The effect of the combined action of rock mechanical balance broken and fracture wall dissolution on the formation of wormholes in fractured carbonate reservoirs remains to be studied. To address the above-mentioned issues, a thermal-hydrologic-mechanical-chemical coupled model is presented based on the TSC model for studying the wormhole propagation in fractured carbonate reservoirs under practical conditions. Linear and radial flow cases are simulated to investigate the influences of fracture distribution, reaction temperature, and effective stress on acidizing dynamics. The simulation results show that more wormhole branches are formed by acidizing if the fractures are perpendicular to the flow direction of acid. Temperature is a key parameter affecting the acidification dissolution patterns, so the influence of temperature cannot be ignored during the acidification design. As the effective stress of the formation increases, the diameter of the wormhole gradually decreases, and the branching decreases. More acid is needed for the same stimulation result under higher effective stress.

Dynamic evolution of shale permeability under coupled temperature and effective stress conditions

The permeability of shale in unconventional subsurface reservoirs under high-temperature and high effective stress conditions is of great significance for evaluating the extraction efficiency of shale gas and determining reservoir development programs. In this study, a series of permeability experiments using nitrogen gas were conducted on a sample from the lower Longmaxi Formation over a range of temperature (25–100 °C), effective stress (22.5–52.5 MPa), and pore pressure (0.5–8 MPa) conditions. The dynamic relationships between permeability and temperature, effective stress, and pore pressure are analyzed in detail. The results show that the apparent permeability decreases approximately linearly with increasing pore pressure, temperature, and effective stress. The absolute permeability also decreases with increasing temperature or effective stress. Upon increasing the reservoir depth by 1000 m, the permeability decreases by an average of approximately 25%. Approximately two-thirds of this decline is caused by changes in the effective stress and approximately one-third by changes in temperature. The Klinkenberg parameter is also found to be positively correlated with temperature and effective stress. The results herein provide important guidance for evaluating permeability changes and the extraction efficiency for deep shale gas development.

Weakening behavior of the shallow megasplay fault in the Nankai subduction zone

The Nankai Trough megasplay fault hosts diverse modes of fault slip, ranging from slow slip events to megathrust earthquakes, and is responsible for related phenomena such as tsunamis and submarine landslides. All types of slip events require some kind of frictional weakening process in order to nucleate and propagate. We tested fluid-saturated, powdered megasplay fault samples in a direct shear apparatus under effective normal stresses of 2–18 MPa to investigate their friction velocity- and slip-dependence. The experiments show that for short distances (1 mm) after a velocity step, there is an evolution from velocity weakening at low effective normal stress to velocity strengthening at high effective normal stresses. Over a longer distance (5 mm), large velocity weakening is observed over all tested effective normal stresses. In all experiments, slip weakening behavior occurred with relatively large weakening rates at low effective normal stresses and smaller weakening rates at higher effective normal stresses. Critical stiffnesses for slip instability were calculated for both the velocity and slip dependence of friction to determine their relative importance. At shallow depths, velocity weakening would be the main cause of frictional instability for both small and large slip perturbations, whereas at greater depth instability requires either slip weakening over small slip distances, or velocity weakening induced by larger slip. Regardless of the underlying mechanism, the observed slip instability at lower effective stresses increases the likelihood for fault slip events to travel to the seafloor which may cause submarine landslides and tsunamis.Graphical

Gas permeability variation during methane hydrate dissociation by depressurization in marine sediments

Accurate acquisition of permeability change in hydrate-bearing sediments (HBSs) is the key to predicting reservoirs' multiphase fluids distribution and production capacity. Hydrate saturation and effective stress present significant effects on reservoir permeability. Thus, the influence of hydrate saturation and effective stress on gas permeability change of remolded core during decompression was studied. The results showed that effective stress compression and hydrate decomposition determine the flow channel structure within hydrate sediments. Under unconsolidated and low effective stress(σ' = 1.2 MPa) conditions, the gas permeability increased gradually as the decomposition of hydrate, and the magnitude of permeability variation before and after decomposition increased with increasing hydrate saturation, indicating that the effect of hydrate saturation on permeability dynamic change dominated at low effective stresses. When σ′ was increased to 3.2 MPa and 5.2 MPa, the results were contrary, indicating that the effective stress variation on gas permeability dynamic change dominated at higher effective stresses. In addition, it was found that compression led to higher heat transfer efficiency in high-saturation samples. When hydrate saturation was increased from 23.37% to 48.7%, the effect of increasing effective stress on gas production changed from inhibition to facilitation.

The influence of hydrogen on the low cycle fatigue behavior of strain-hardened 316L stainless steel

The present study focuses on the low cycle fatigue (LCF) behavior of strain-hardened 316L stainless steel under plastic strain control, aiming to understand the influence of internal hydrogen on fatigue performance and cyclic deformation behavior for energy-related technologies. The strain-hardened 316L specimens tested at plastic strain amplitudes between 0.1% and 0.7% with and without hydrogen displayed continuous cyclic softening. Internal hydrogen increased the cyclic strength of this steel, with a greater difference in strength at low plastic strain amplitudes. A Bauschinger analysis revealed that effective stresses represented the major contribution to the flow stress for all material conditions and amplitudes. At low plastic strain amplitudes, effective stresses were responsible for differences in cyclic strength between hydrogen precharged and non-charged specimens. In contrast, back stresses became more significant at high amplitudes, and more similar deformation structures were observed. Although internal hydrogen reduced the total LCF lifetime at all amplitudes, this degradation was more significant in the low amplitude regime. This result is attributed to high effective stresses that led to the onset of multiple slip at lower cumulative plastic strains in the hydrogen precharged condition compared to the non-charged condition at low amplitudes. In addition, the evolution of the apparent elastic response of the material suggests that the primary crack(s) nucleated at a smaller percentage of the total lifetime and propagated in fewer number of cycles in the hydrogen precharged than in the non-charged condition. Fracture surface and gage section observations revealed a transgranular crack path and planar slip traces in both material conditions, with internal hydrogen promoting multiple slip at lower values of cumulative plastic strain than in the non-charged condition.