Deformation Mechanisms and Safe Drilling Fluids Density in Extremely Thick Salt Formations

Hydrocarbons are very often associated with salt structures. The oil and gas industry is often required to drill along and through long salt sections to reach and recover hydrocarbons. The unique physical properties of salt require special techniques to ensure borehole stability and adequate casing design. This paper assumed that the mechanical behavior of salt is regulated by the magnitude of mean stress and octahedral shear stress and under the influence of different stress conditions the deformation of rock salt can be represented by three domains, i.e. compression domain, volume unchanged domain, and dilatancy domain, which are separated by a stress dependent boundary. In the compression domain, the volume of salt decreases until all microcracks are closed, with only elastic deformation and pure creep; in the volume unchanged domain the deformation is considered steady incompressible flow controlled by pure creep; and in the dilatancy domain the volume of salt increases during deformation due to micro-cracking, causing damage and accelerating ＂creep＂ until failure. This paper presents a hypothesis that the borehole is stable only when the magnitude of octahedral shear stress is below the dilatancy boundary. It gives the design method for determining drilling fluids density, and calculates the closure rate ofborehole with the recommended drilling fluids density. If the closure rate of the borehole is less than 0.1%, the drilling fluids density window can be used during drilling through extremely thick salt formations.

Compact intense electron-beam accelerators based on high energy density liquid pulse forming lines

This paper provides a review of the compact intense electron-beam accelerators (IEBAs) based on liquid pulse forming lines (PFLs) that havebeen developed at the National University of Defense Technology (NUDT) in China. The history and roadmap of the compact IEBAs used todrive high-power microwave (HPM) devices at NUDT are reviewed. The properties of both de-ionized water and glycerin as energy storagemedia are presented. Research into the breakdown properties of liquid dielectrics and the desire to maximize energy storage have resulted in theinvention of several coaxial PFLs with different electromagnetic structures, which are detailed in this paper. These high energy density liquidPFLs have been used to increase the performance of IEBA subsystems, based on which the SPARK (Single Pulse Accelerator with spark gaps)and HEART (High Energy-density Accelerator with Repetitive Transformer) series of IEBAs were constructed. This paper also discusses howthese compact IEBAs have been used to drive typical HPM devices and concludes by summarizing the associated achievements and theconclusions that can be drawn from the results.

Study on the Two-Dimensional Density Distribution for Gas Plasmas Driven by Laser Pulse

We perform an experimental study of two-dimensional（2D） electron density profiles of the laser-induced plasma plumes in air by ordinarily laboratorial interferometry. The electron density distributions measured show a feature of hollow core. To illustrate the feature, we present a theoretical investigation by using dynamics analysis. In the simulation, the propagation of laser pulse with the evolution of electron density is utilized to evaluate ionization of air target for the plasma-formation stage. In the plasma-expansion stage, a simple adiabatic fluid dynamics is used to calculate the evolution of plasma outward expansion. The simulations show good agreements with experimental results, and demonstrate an effective way of determining 2D density profiles of the laser-induced plasma plume in gas.

Time-dependent borehole stability in hard-brittle shale

Rock damage appears in brittle shale even prior to peak stress(i.e.,before failure)due to the occurrence of microcracks in these rocks.In this work,a coupled hydromechanical model was built by incorporating the mechanical and fluid seepage induced stresses around a wellbore during drilling.The borehole instability mechanism of hard-brittle shale was studied.The results show that even if a well is simply drilled into a hard-brittle shale formation,the formation around the borehole can be subjected to rock damage.The maximum failure ratio of the formation around the borehole increases with drilling time.A lower drilling fluid density corresponds to a faster increase in the failure ratio of the borehole with time and a shorter period of borehole collapse.When the initial drilling fluid density is too low,serious rock damage occurs in the formation around the borehole.Even though a high-density drilling fluid is used after drilling,long-term borehole stability is difficult to maintain.While drilling in hard-brittle shale,drilling fluid with a proper density should be used rather than increasing the density of the drilling fluid only after borehole collapse occurs,which is more favorable for maintaining long-term borehole stability.

Geoacoustic inversion based on reflection model of effective density fluid approximation

In order to obtain the physical and geoacoustic properties of marine sediments,an inverse method using reflection loss of different grazing angles is presented.The reflection loss is calculated according to the reflection model of effective density fluid approximation.A two-step hybrid optimization algorithm combining differential evolution and particle swarm optimization along with Bayesian inversion is employed in estimation of porosity,mean grain size,mass density and bulk modulus of grains.Based on the above physical parameters,geoacoustic parameters,including sound speed and attenuation,are further calculated.According to the numerical simulations,we can draw a conclusion that all the parameters can be well estimated with the exception of bulk modulus of grains.In particular,this indirect inverse method for bottom geoacoustic parameters performs high accuracy and strong robustness.The relative errors are 0.092%and 17%,respectively.Finally,measured reflection loss data of sandy sediments at the bottom of a water tank is analyzed,and the estimation value,uncertainty and correlation of each parameter are presented.The availability of this inverse method is verified through comparison between inverse results and part of measured parameters.

Cylindrical effects in weakly nonlinear Rayleigh Taylor instability

The classical Rayleigh–Taylor instability（RTI） at the interface between two variable density fluids in the cylindrical geometry is explicitly investigated by the formal perturbation method up to the second order. Two styles of RTI, convergent（i.e., gravity pointing inward） and divergent（i.e., gravity pointing outwards） configurations, compared with RTI in Cartesian geometry, are taken into account. Our explicit results show that the interface function in the cylindrical geometry consists of two parts： oscillatory part similar to the result of the Cartesian geometry, and non-oscillatory one contributing nothing to the result of the Cartesian geometry. The velocity resulting only from the non-oscillatory term is followed with interest in this paper. It is found that both the convergent and the divergent configurations have the same zeroth-order velocity, whose magnitude increases with the Atwood number, while decreases with the initial radius of the interface or mode number. The occurrence of non-oscillation terms is an essential character of the RTI in the cylindrical geometry different from Cartesian one.

Rotating Squeezed Vacua as Time Machines

Squeezed quantum vacua seems to violate the averaged null energy conditions (ANEC’s), because they have a negative energy density. When treated as a perfect fluid, rapidly rotating Casimir plates will create vorticity in the vacuum bounded by them. The geometry resulting from an arbitrarily extended Casimir plates along their axis of rotation is similar to van Stockum spacetime. We observe closed timelike curves (CTC’s) forming in the exterior of the system resulting from frame dragging. The exterior geometry of this system is similar to Kerr geometry, but because of violation of ANEC, the Cauchy horizon lies outside the system unlike Kerr blackholes, giving more emphasis on whether spacetime is multiply connected at the microscopic level.

Effects of oceanic ice cover and density stratification on the propagation of hydroacoustic waves

The propagation characteristic of hydroacoustic waves is studied for an ideal compressible two-layer fluid with different densities covered by an elastic ice sheet.Boundary conditions are simplified by adopting linear assumption and then the dispersion relation is derived.The analysis and visualization of the dispersion relation present the introduction of compressibility leads to the appearance of hydroacoustic wave modes while the density stratification leads to the appearance of interfacial wave mode.Larger ice thickness and the density ratio of the two fluid layers increase the wave number and group speed of hydroacoustic waves at the same frequency while the phase speed decreases.

WAVE DISSIPATING PERFORMANCE OF AIR BUBBLE BREAKWATERS WITH DIFFERENT LAYOUTS

The wave dissipating performance of air bubble breakwaters with different layouts is studied by experimental and numerical methods in this article. Based on the assumpation that the mixture of air and water is regarded as a variable density fluid, the mathematical model of the air bubble breakwater is built. The numerical simulation results are compared with the experimental data, which shows that the mathematical model is reasonable for the transmission coefficient Ct m. The influencing factors are studied experimentally and numerically, including the incident wave height H i, the incidentt wave period T , the air amount Qm , the submerged pipe depth D and the single or double air discharging pipe structure. Some valuable conclusions are obtained for further research of the mechanism and practical applications of air bubble breakwaters.