Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

다중혈관 관상동맥 환자에서 y-문합을 이용하여 양쪽 내흉동맥만을 사용한 우회술의 조기 성적

Natural gas hydrates are solid, non-stoichiometric compounds of small gas molecules and water. They form when the constituents come into contact at low temperature and high pressure. The physical properties of these compounds, most notably that they are non-flowing crystalline solids that are denser than typical fluid hydrocarbons and that the gas molecules they contain are effectively compressed, give rise to numerous applications in the broad areas of energy and climate effects. In particular, they have an important bearing on flow assurance and safety issues in oil and gas pipelines, they offer a largely unexploited means of energy recovery and transportation, and they could play a significant role in past and future climate change.

Fundamental principles and applications of natural gas hydrates

This chapter assesses long-term projections of climate change for the end of the 21st century and beyond, where the forced signal depends on the scenario and is typically larger than the internal variability of the climate system. Changes are expressed with respect to a baseline period of 1986-2005, unless otherwise stated.

Chapter 12 - Long-term climate change: Projections, commitments and irreversibility

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A User’s Guide

We performed a sequence of tests on a partially water-saturated sand sample contained in an x-ray transparent aluminum pressure vessel that is conducive to x-ray computed tomography (CT) observation. These tests were performed to gather data for estimation of thermal properties of the sand/water/gas system and the sand/hydrate/water/gas systems, as well as data to evaluate the kinetic nature of hydrate dissociation. The tests included mild thermal perturbations for the estimation of the thermal properties of the sand/water/gas system, hydrate formation, thermal perturbations with hydrate in the stability zone, hydrate dissociation through thermal stimulation, additional hydrate formation, and hydrate dissociation through depressurization with thermal stimulation. Density changes throughout the sample were observed as a result of hydrate formation and dissociation, and these processes induced capillary pressure changes that altered local water saturation.

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Methane Hydrate Formation and Dissocation in a Partially Saturated Sand--Measurements and Observations - eScholarship


 Low-frequency distributed acoustic sensing (LF-DAS) has recently received much attention for its ability to monitor fracture propagation at offset wells. Hydraulic Fracture Test Site-2 (HFTS-2), a DOE- sponsored field-based research experiment, has acquired LF-DAS datasets during the stimulation of many horizontal wells in the Wolfcamp Formation of the Permian Basin. Over 100 stimulated stages with different completion designs in four horizontal wells were monitored by two horizontal offset wells and one vertical pilot hole with permanent fibers. The parent well depletion affected all four horizontal wells in almost half of their lateral section. Several studies have been done on the acquired comprehensive dataset. In this study, we apply our novel Green's function-based inversion algorithm to calculate the fracture width of each stage and investigate the impact of parent well depletion on fracture geometry. The Green's function-based inversion algorithm (Liu et al., 2021) has been successfully applied to a few stages of field case studies. The Green function matrix was built based on linear elasticity theory (Three- Dimensional Displacement Discontinuity Method) to relate the fracture openings with strain responses measured along the length of the fiber. This novel algorithm only relies on measured strain to solve fracture geometry at the monitoring well. Therefore, it is independent of the physics of the fracture propagation process and can be used to validate hydraulic fracture modeling results. Using our inversion algorithm, we can efficiently and quantitatively interpret LF-DAS data to provide information on fracture geometry and completion efficiency.
 We analyze more than 100 stages of the LF-DAS measurements obtained at the fiber wells B3H and B4H during B1H, B2H, and B4H stimulations. We apply our inversion algorithm for four datasets covering the stimulation of the above mentioned three wells. The fracture growth at stages in the parent well depleted zone is biased more towards upper formations and in the east direction. The fracture width at the stages in the parent well depletion zone is reduced compared to fracture widths at stages in the non- depleted zone irrespective of the monitoring well location relative to the treatment wells. This difference in fracture widths will affect the proppant distribution to a great extent, thereby, affecting the effectiveness of the stimulation. We also illustrate the application of the inversion algorithm for stages that have both conventional fracture hits with "heart-shaped" signals as well as fracture reopening signals. Our inversion algorithm gives a reasonable estimate of the fracture width that aligns with the qualitative analysis of microseismic datasets and statistic summary of fracture hit numbers of HFTS-2. We believe that this study provides us with insights into the fracture geometry due to parent well depletion effects quantitatively.

Investigate the Impact of Parent Well Depletion on Fracture Geometry Based on Low-Frequency Distributed Acoustic Sensing in Hydraulic Fracture Test Site – 2


 The primary objective of this work are (a) to analyze the impact of salinity on the attempt to sequester CO2 in saline aquifers by means of vertical wells over long times—covering both the early short injection period and the subsequent long ‘rest’ period of inactivity—by determining its effect on key well and aquifer properties, conditions and rates of mass and heat transport, and (b) to investigate possible methods to mitigate the problems caused by salt precipitation (halite). We use a widely used numerical simulator that accounts for all the multiphase flow and transport processes associated with CO2 sequestration, including halite precipitation. The non-isothermal simulations use a high-definition cylindrical grid with radial subdivisions as small as 0.01 m in the vicinity of the well to accurately capture the critically importance effect of halite precipitation (the evolution and impact of which may be eluded when using coarser grids). Using two different well configurations of vertical wells, we inject CO2 as realistic rates into aquifers with varying salinity levels until a geomechanically safe maximum pressure is attained, and allow the system to rest for 100 yrs. During the entire simulation period, we monitor the evolutions of (a) pressures and temperatures in the near-well region and in the entire aquifer and (b) of mass and heat losses through the overburden. Additionally, we investigate possible methods to alleviate injectability problems caused by halite precipitation. The study shows that (a) wells completed in the entire aquifer depth are more effective in low-salinity aquifers, but shorter wells completed at the base of the aquifer are superior in high-salinity cases, (b) the storage potential of CO2 dissolved in the aqueous phase is limited (both in terms of the magnitude of the dissolved concentration and its spatial extent), and its benefit decreases rapidly with an increasing salinity, (c) salinity can have pronounced adverse effects on the well injectivity, as halite precipitation can cause severe blockage in the vicinity of the well and a rapid rise of the near well pressure to geomechanically unsafe levels, thus potentially reducing the usefulness of an aquifer as a CO2 storage facility and (d) the mitigation of the effects of precipitated halite in the well vicinity is a challenging endeavor, with concurrent H2O-CO2 injection being ineffective.

Practical Aspects and Implications of Long-Term CO2 Sequestration in Saline Aquifers Using Vertical Wells

Simulation-based evaluation of the effectiveness of fiber-optic sensing in monitoring and optimizing water circulation in next-generation enhanced geothermal systems

Effect of geomechanics and of grid discretization on the predictions of production from natural hydrate deposits and of the associated geomechanical system response

George J. Moridis

Papers

Methane Hydrate Formation and Dissocation in a Partially Saturated Sand--Measurements and Observations - eScholarship

Investigate the Impact of Parent Well Depletion on Fracture Geometry Based on Low-Frequency Distributed Acoustic Sensing in Hydraulic Fracture Test Site – 2

Practical Aspects and Implications of Long-Term CO2 Sequestration in Saline Aquifers Using Vertical Wells

Simulation-based evaluation of the effectiveness of fiber-optic sensing in monitoring and optimizing water circulation in next-generation enhanced geothermal systems

Effect of geomechanics and of grid discretization on the predictions of production from natural hydrate deposits and of the associated geomechanical system response