Investigation of buoyant plumes in a quasi-2D domain : characterizing the influence of local capillary trapping and heterogeneity on sequestered CO₂ – : a bench scale experiment

textLeakage of stored bulk phase CO₂ is one risk for sequestration in deep saline aquifers. As the less dense CO₂ migrates upward within a storage formation or in layers above the formation, the security of its storage depends upon the trapping mechanisms that counteract the migration. The trapping mechanism motivating this research is local capillary trapping (LCT), which occurs during buoyancy-driven migration of bulk phase CO₂ within a saline aquifer with spatially heterogeneous petrophysical properties. When a CO₂ plume rising by buoyancy encounters a region where capillary entry pressure is locally larger than average, CO₂ accumulates beneath the region. One benefit of LCT, applied specifically to CO₂ sequestration and storage, is that saturation of stored CO₂ phase is larger than the saturation for other permanent trapping mechanisms. Another potential benefit is security: CO₂ that occupies local capillary traps remains there, even if the overlying formation that provides primary containment were to be compromised and allow leakage. Most work on LCT has involved numerical simulation (Saadatpoor 2010, Ganesh 2012); the research work presented here is a step toward understanding local capillary trapping at the bench scale. An apparatus and set of fluids are described which allow examining the extent of local capillary trapping, i.e. buoyant nonwetting phase immobilization beneath small-scale capillary barriers, which can be expected in typical heterogeneous storage formation. The bench scale environment analogous to CO₂ and brine in a saline aquifer is created in a quasi-two dimensional experimental apparatus with dimension of 63 cm by 63 cm by 5 cm, which allows for observation of plume migration with physically representative properties but at experimentally convenient ambient conditions. A surrogate fluid pair is developed to mimic the density, viscosity and interfacial tension relationship found at pressure and temperature typical of storage aquifers. Porous media heterogeneity, pressure boundary conditions, migration modes of uprising nonwetting phase, and presence of fracture/breach in the capillary barrier are studied in series of experiments for their influences on LCT. A variety of heterogeneous porous media made of a range of sizes of loosely packed silica beads are used to validate and test the persistence of local capillary trapping mechanism. By adjusting the boundary conditions (fluid levels in reservoirs attached to top and to bottom ports of the apparatus), the capillary pressure gradient across the domain was manipulated. Experiments were conducted with and without the presence of fracture/potential leakage pathway in the capillary seal. The trapped buoyant phase remained secure beneath the local capillary barriers, as long as the effective capillary pressure exerted by the trapped phase (proportional to column height of the phase) is smaller than the capillary entry pressure of the barrier. The local capillary trapping mechanism remained persistent even under forced imbibition, in which a significantly higher hydraulic potential gradient, and therefore a larger gradient in capillary pressure, was applied to the system. The column height of buoyant fluid that remained beneath the local capillary barrier was smaller by a factor corresponding to the increase in capillary pressure gradient. Mimicking a breach of the caprock by opening valves at the top of the apparatus allowed buoyant mobile phase held beneath the valves to escape, but buoyant phase held in local traps at saturations above residual, and therefore potentially mobile, was undisturbed. This work provides systematic validation of a novel concept, namely the long-term security of CO₂ that fills local (small-scale) capillary traps in heterogeneous storage formations. Results from this work reveal the first ever unequivocal experimental evidence on persistence of local capillary trapping mechanism. Attempts to quantify the nonwetting phase saturation and extent of LCT persistence serve as the initial steps to potentially reduce the risks associated with long-term storage security.Petroleum and Geosystems Engineerin

Development of a novel Fast-Warm stamping (FWS) technology for manufacturing high-strength steel components

Hot and warm stamping are preferable sheet metal forming technologies used in manufacturing high-strength parts with the twofold objectives of reducing fuel consumption and improving automotive crashworthiness. Great efforts have been made to improve the production rate in these processes and it is difficult to further improve productivity. Therefore, the development of new forming technologies may be an alternative solution to form high-strength steels into complex shapes whilst reducing the cycle time. The present work aims to develop a novel lightweight forming technology, namely fast-warm stamping (FWS) technique, to manufacture high-strength steel components with the desired properties. The concept of this process is to utilise ultra-fast heating of a steel blank to an appropriate temperature, whilst minimising the major negative changes to microstructure which are detrimental to the post-form strength. Mechanical properties such as ductility and post-form strength (PFS) of the MS-W900Y1180T (MS1180) steel were examined via uniaxial tensile tests at various temperatures (25–500°C) and strain rates (0.01–5/s). Special attention has been afforded to the effect of heating rate on thermo-mechanical properties and microstructure of the MS1180 steel with different heating rates. The results suggest that the ductility and post-form hardness of the MS1180 steel were simultaneously improved by 25.7% and 5%, with an increase in heating rate from 1 to 150°C/s. The increased hardness is attributed to the finer precipitated carbides and lower recovery at fast heating rate conditions, which was validated by microstructural observations. The validation of the FWS technology was conducted by forming U-shaped components through a dedicated pilot production line caller Uni-form. The fast-warm stamped components exhibited over 92% mechanical strength of the original as-received material consisting of 1140MPa post-form strength and 370HV hardness. The overall manufacturing cycle time in the FWS process was within 10 seconds. Springback of the formed parts under FWS conditions IV was successfully characterized at various temperatures and forming speeds. Close agreements were achieved between the experimental and simulated results for temperature, thickness distribution and springback prediction of the formed parts which validated the accuracy of the developed finite element (FE) model. FWS technology is a promising solution to manufacture components with desirable mechanical properties and dimensional accuracy. In this work, a feasibility study of the FWS technology was extended from martensitic steels to 60Si2Mn spring steel by producing commercialized disc springs. A separate forming tool set with a replaceable forming surface was developed to reduce manufacturing cost. Experimental results showed that a disc spring was successfully formed using the proposed forming process with the required dimensional precision, post-form strength and surface roughness. This forming technique has shown to enable a tremendous reduction of overall cycle time from 30 minutes to less than 20 seconds and subsequent productivity improvement for a mass-production setting.Open Acces

Penerapan Metode CPM dan PERT Pada Proyek Konstruksi Gereja Kemah Tabernake PIK 2 Jakarta Utara

PT. TCI is a company operating in the construction sector which handles the construction of several projects, one of which is the Tabernacle Church project. In carrying out the project work, several factors/problems arose, resulting in an extension of the project foundation construction stage and increasing the project period and costs. The use of the CPM and PERT methods for implementing the foundation construction of the GKT project using the planned and actual schedules has been able to optimize the work, namely 123 days using the planned schedule and 150 days using the actual schedule. These results have found the critical path, so that by applying the CPM and PERT methods in this project, an estimated schedule can be obtained to speed up the project. The cost difference using the CPM, PERT and CPM-PERT methods for the planned schedule is IDR 304,000,000 (Three Hundred and Four Million Rupiah) , Rp. 264,000,000 (Two Hundred and Sixty Four Million Rupiah) and Rp. 264,000,000 (Two Hundred and Sixty Four Million Rupiah). The difference in costs using the CPM, PERT, CPM-PERT method for the actual schedule is IDR 344,000,000 (Three Hundred Forty Four Million Rupiah), IDR 304,000,000 (Three Hundred Forty Four Million Rupiah) and IDR 304,000,000 (Three Hundred Forty Four Million Rupiah). These costs will be the contractor's profit

Improved Algorithms for Online Rent Minimization Problem Under Unit-Size Jobs

We consider the Online Rent Minimization problem, where online jobs with release times, deadlines, and processing times must be scheduled on machines that can be rented for a fixed length period of T. The objective is to minimize the number of machine rents. This problem generalizes the Online Machine Minimization problem where machines can be rented for an infinite period, and both problems have an asymptotically optimal competitive ratio of O(log(p_max/p_min)) for general processing times, where p_max and p_min are the maximum and minimum processing times respectively. However, for small values of p_max/p_min, a better competitive ratio can be achieved by assuming unit-size jobs. Under this assumption, Devanur et al. (2014) gave an optimal e-competitive algorithm for Online Machine Minimization, and Chen and Zhang (2022) gave a (3e+7) ? 15.16-competitive algorithm for Online Rent Minimization. In this paper, we significantly improve the competitive ratio of the Online Rent Minimization problem under unit size to 6, by using a clean oracle-based online algorithm framework