Experimental Quantification of the Evolution of the Static Mechanical Properties of Tight Sedimentary Rocks during Increasing-amplitude Load and Unload Cycling

Understanding the linearly and nonlinearly elastic behaviors of tight reservoir rocks is crucial for numerous geophysical and geomechanical applications in hydrocarbon exploration and production, geological repositories for greenhouse gases, and geothermal energy exploitation. We perform a suite of triaxial load and unload cycling tests with increasing stress amplitudes on three tight sedimentary rocks to explore the evolution of their static mechanical properties (Young’s modulus and Poisson’s ratio). We intend to depict the transition from linear to nonlinear elasticity by combining static measurements with dynamic measurements. The experimental results suggest that static mechanical properties increase upon load stress cycling but decrease upon unload stress cycling. Upon the increasing-amplitude unload cycling, static mechanical properties gradually decrease from values approaching dynamic properties to values closer to static properties upon load cycling. By quadratically fitting the static mechanical properties as functions of the strain amplitude in the process of unload cycling, we define a characteristic strain amplitude of about 5 × 10−5 to distinguish the linearly elasticity-dominated and nonlinearly elasticity-dominated behaviors for three tight rocks. Such transitional behavior in tight sedimentary rocks can be microscopically explained by the gradual activation of friction-controlled sliding from the beginning of the cyclic stress unload. These observations provide direct experimental evidence of the transition from linear to nonlinear elasticity for tight sedimentary rocks during the laboratory static measurements, which will facilitate understanding of the dynamic-static parameter correlation and the modeling of rock deformations in geoscience or geoengineering applications.

Flow-Induced Vibration Screening of a Thermoplastic Composite Pipe Water Injection Jumper

An active subsea field in the Gulf of Mexico has adopted a thermoplastic composite pipe (TCP) water injection jumper for its waterflood upgrade. The jumper assembly is composed of a TCP span attached to steel piping on either end. The TCP spool is lightweight and flexible relative to the traditional steel-only M-shaped subsea jumpers. As such, the flow-induced vibration (FIV) threat from internal fluid flow must be assessed for the intended service. A three-tiered approach is used to assess the level of FIV threat expected in this TCP subsea jumper application. A high-level screening based on widely used industry guidelines indicates a susceptibility to FIV fatigue failure for the steel piping in the TCP jumper assembly. A comprehensive screening based on structural finite element analysis and computational fluid dynamics shows that the vibration levels and stress cycling due to FIV will be acceptable for the intended water injection application and a 30-year design life, when adopting a factor of safety of 10 for subsea service. We evaluate the effect of doubling the length of the steel piping on either end of the TCP span, as a means to increase the overall span of the TCP jumper assembly. Lastly, we draw a comparison between a traditional all-steel M-shaped jumper and the TCP jumper in terms of FIV fatigue life, for the same operating conditions and the same total suspended span.

Impact of One-Way Streets and Contraflow on Low-Stress Bicycle Network Connectivity

One-way restrictions on local streets, which tend to have low traffic stress, can create a significant barrier to low-stress cycling. Contraflow, a treatment that undoes one-way restrictions on bike travel, has the potential to improve low-stress connectivity. Although contraflow is applied routinely in the Netherlands and Belgium, it has been sparingly applied in the United States. We propose refined measures of connectivity and accessibility that account for one-way restrictions by requiring a low-stress round trip path between origins and destinations. Different methods of associating origin–destination demand from polygons with a street network were analyzed. These methods are particularly important where there are one-way restrictions and irregular street networks because of the assumptions they entail in relation to first- and last-segment travel. In a case study of Greater Boston, we found that with the current bike network, low-stress connectivity between homes and jobs would increase from 1.2% to 8.7% if one-way restrictions on local streets were eliminated. We also found that even with a dense mesh of low-stress main bike routes, connectivity would still be 16% lower without contraflow on local streets than with. These results suggest that creating a network of main bike routes is not always enough; it is also important to provide contraflow on local streets. The Boston study also found that providing contraflow on selected links representing only 3% of local one-way street mileage delivered 40% of the connectivity impact of universal contraflow. Based on this finding, a method is proposed for prioritizing streets for contraflow conversion.

Mechanical Response of Castlegate Sandstone under Hydrostatic Cyclic Loading

The stress history of rocks in the subsurface affects their mechanical and petrophysical properties. Rocks can often experience repeated cycles of loading and unloading due to fluid pressure fluctuations, which will lead to different mechanical behavior from static conditions. This is of importance for several geophysical and industrial applications, for example, wastewater injection and reservoir storage wells, which generate repeated stress perturbations. Laboratory experiments were conducted with Castlegate sandstone to observe the effects of different cyclic pressure loading conditions on a common reservoir analogue. Each sample was hydrostatically loaded in a triaxial cell to a low effective confining pressure, and either pore pressure or confining pressure was cycled at different rates over the course of a few weeks. Fluid permeability was measured during initial loading and periodically between stress cycles. Samples that undergo cyclic loading experience significantly more inelastic (nonrecoverable) strain compared to samples tested without cyclic hydrostatic loading. Permeability decreases rapidly for all tests during the first few days of testing, but the decrease and variability of permeability after this depend upon the loading conditions of each test. Cycling conditions do affect the mechanical behavior; the elastic moduli decrease with the increasing loading rate and stress cycling. The degree of volumetric strain induced by stress cycles is the major control on permeability change in the sandstones, with less compaction leading to more variation from measurement to measurement. The data indicate that cyclic loading degrades permeability and porosity more than static conditions over a similar period, but the petrophysical properties are dictated more by the hydrostatic loading rate rather than the total length of time stress cycling is imposed.

EXPERIMENTAL AND NUMERICAL FATIGUE ANALYSIS OF BRASS SHAFT SPECIMEN UNDER CYCLIC BENDING MOMENTS

Fatigue is a form of failure that occurs in structures subjected to dynamic and fluctuating stresses, where failure can occur at a stress level significantly lower than the tensile or yield strength of a static load under these circumstances. The term “fatigue” is used because, after a long period of repetitive stress or stress cycling, this form of failure typically occurs. Fatigue is important because it is the single largest cause of metal failure, estimated to account for about 90% of all metal failures; polymers and ceramics (except glasses) are also prone to this form of failure. This research is studying the failure analysis, fatigue life and endurance limit of brass metal experimental and numerical under cyclic bending moments

Flow-Induced Vibration Screening of a Thermoplastic Composite Pipe Water Injection Jumper

An active subsea field in the Gulf of Mexico has adopted a thermoplastic composite pipe (TCP) water injection jumper for its waterflood upgrade. The TCP spool is lightweight and flexible — relative to the traditional steel-only spool segments used in subsea jumpers. As such, the flow-induced vibration (FIV) threat from internal fluid flow must be assessed for the intended service. A three-tiered approach is used to assess the level of FIV threat expected in this TCP subsea jumper application. A high-level screening based on widely used industry guidelines indicates a high susceptibility to FIV fatigue failure for the steel product in the jumper, with no applicability to the TCP material. A comprehensive screening based on structural finite element analysis and computational fluid dynamics shows that the vibration levels and stress cycling due to FIV will be acceptable for the intended water injection application and a 30-year design life, when adopting a factor of safety of 10 for subsea service.