Average shear stress yield criterion and its application to plastic collapse analysis of pipelines

It is known that available analytical and empirical solutions for the burst pressure of defect-free line pipes cannot broadly fit experimental data for different materials. Usually the Tresca prediction provides a lower bound to the burst pressure, and the von Mises prediction provides an upper bound to the burst pressure. A new multiaxial yield criterion, referred to as the average shear stress yield (ASSY) criterion for isotropic hardening materials, is developed in this paper based on the traditional Tresca and von Mises yield criteria so that the burst pressure of a pipeline at plastic collapse can be accurately predicted.     

Simulating the effect of subsurface stresses and transient pore pressure on wellbore stability in subsea horizontal wells

The most prominent challenge associated with offshore horizontal drilling is wellbore stability. In this paper, simulation of in-situ stresses around the wellbore is conducted to study the effects of transient pore pressure on the stability of horizontal wells. The rock mechanical analysis based on finite element technique lead to investigate a unique behavior found in subsea horizontal wells known as transient pore pressure behavior and near wellbore pressure gradients. The results demonstrate that near wellbore pore pressure gradient is only active in rock formations which possess transient pore pressure behavior; therefore, simulated solutions require adjustment to achieve accurate results.     

Stress Analysis of a Borehole in Saturated Rocks Under in situ Mechanical,Hydrological and Thermal Interactions

Abstract

A novel approach is developed to represent coupled thermal-hydraulic-mechanical (THM) behavior of porous systems that incorporates the non-isothermal free and forced convection of a single component fluid in a non-boiling thermoelastic medium. The three-way simultaneous coupling between the THM triplet is currently linear, but no restriction is placed on incorporating material nonlinearities. The coupled PDEs are solved in space by grid-adaptive finite elements. The model is validated against solutions for linear non-isothermal consolidation of a column. We demonstrate the utility of the model by analyzing the behavior of a deep wellbore in a themoelastic medium circulated by a pressurized, but chilled fluid. Model results illustrate the significant importance of the cross-couplings between individual THM processes for the evaluation of wellbore stability.     

页岩气水平井钻井液技术的难点及选用原则

我国页岩气资源勘探开发已全面铺开。针对页岩气的成藏特征,页岩气开发以浅层大位移井、丛式水平井布井为主。由于页岩地层裂缝发育、水敏性强,长水平段钻井中不仅易发生井漏、垮塌、缩径等问题,且由于水平段较长,还会带来摩阻、携岩及地层污染问题,从而增大了产生井下复杂情况的几率。解决井壁稳定、降阻减摩和岩屑床清除等问题是目前页岩气水平井钻井液选择和设计的关键。选择页岩气水平井钻井液的原则是确保井壁稳定、润滑、防卡和井眼清洗。油基钻井液可提高水润湿性页岩的毛细管压力,防止钻井液对页岩的侵入,从而有效保持井壁稳定,同时还具有良好的润滑、防卡和降阻作用,是国内外目前采用最多的钻井液体系。当采用水基钻井液的时,利用低活性高矿化度聚合物钻井液或CaCl2钻井液以降低页岩和钻井液相互作用的总压力;采用浓甲酸钾、Al3+盐,可以通过脱水、孔隙压力降低和影响近井壁区域化学变化的协同作用产生良好的井眼稳定作用;对于有裂隙、裂缝或层理发育的高渗透性页岩应使用有效的封堵剂进行封堵;但对于强水敏性页岩地层,水基钻井液在抑制性方面仍然存在局限性。从环境保护的角度出发,甲基葡糖苷钻井液在井壁稳定机理方面与油基钻井液相似,未来可望作为有效的钻井液体系之一。随着以页岩气为代表的非常规油气资源的不断开发,为了满足安全钻井和环境保护的需要,未来页岩气水平井钻井液技术的研究应围绕高效、低成本水基钻井液和低毒油基钻井液来展开。     

Hydrogen production via sulfur-based thermochemical cycles: Part 1: Synthesis and evaluation of metal oxide-based candidate catalyst powders for the sulfuric acid decomposition step

The present work concerns the synthesis of various single and mixed oxide materials and their study as catalysts for the sulfuric acid dissociation reaction via which the production of SO₂ and O₂ is achieved. This is the most energy intensive step of sulfur-based thermochemical cycles for the production of hydrogen. Commercial (i.e. FeO, Fe₃O₄, Fe₂O₃, CuO, Cr₂O₃, γ-Al₂O₃, Pt/γ-Al₂O₃) and in-house binary and ternary compositions of the Cu-Fe-Al system as well as Fe-Cr mixed oxide materials prepared by the Solution Combustion Synthesis (SCS) technique were comparatively tested. The materials were studied in powder form, in a fixed bed reactor at 850 °C and ambient pressure. The feedstock was concentrated liquid sulfuric acid (95-98% wt) and all tests were performed at a Liquid Hourly Space Velocity-LHSV of 50-52 h⁻¹. It was found that systems of both in-house synthesized mixed oxide families (Cu-Fe-Al and Fe-Cr) with proper composition could achieve SO₃ conversions comparable to that of the reference Pt/γ-Al₂O₃ system, being at the same time much less expensive. The results, in combination with characterization results of fresh and spent catalysts are employed to identify possible mechanisms of the reaction and streamline the synthesis of more efficient catalytic systems.     

Poro-thermoelastic borehole stress analysis for determination of the in situ stress and rock strength

The in situ stress state and rock strength are key parameters in a number of problems concerning petroleum and geothermal reservoir development, particularly in well stimulation and optimum wellbore trajectory analyses. Inversion techniques utilized to determine the in situ stress and rock strength based on the observation of borehole failure and its analysis often assume elastic rock behavior. However, when drilling through high-pressure and high-temperature rocks, coupled poro-thermo-mechanical processes result in a time-dependent stress and pore pressure distribution around the borehole. In this work, the poro-thermoelastic effects on borehole failure are studied and their impact on wellbore stability and the estimations of the in situ maximum horizontal stress and rock strength using wellbore failure data are investigated. It is shown that coupled poro-thermo-mechanical effects influence both failure mode and potential. Also, when considering shear failure, neglecting heating and cooling effects will underestimate and overestimate rock strength, respectively. Therefore, for accurate assessment of wellbore stability and inversion of wellbore failure data, poroelastic and thermal factors should be considered.     

An in depth investigation of deactivation through carbon formation during the biogas dry reforming reaction for Ni supported on modified with CeO2 and La2O3 zirconia catalysts

The dry reforming of biogas on a Ni catalyst supported on three commercially available materials (ZrO₂, La₂O₃ZrO₂ and CeO₂ZrO₂), has been investigated, paying particular attention to carbon deposition. The DRM efficiency of the catalysts was studied in the temperature range of 500–800 °C at three distinct space velocities, and their time-on-stream stability at four temperatures (550, 650, 750 and 800 °C) was determined for 10 or 50 h operation. The morphological, textural and other physicochemical characteristics of fresh and spent catalysts together with the amount and type of carbon deposited were examined by a number of techniques including BET-BJH method, CO₂ and NH₃-TPD, XPS, SEM, TEM, STEM-HAADF, Raman spectroscopy, and TGA/DTG. The impact of the La₂O₃ and CeO₂ modifiers on the DRM performance and time-on-stream stability of the Ni/ZrO₂ catalyst was found to be very beneficial: up to 20 and 30% enhancement in CH₄ and CO₂ conversions respectively, accompanied with a CO-enriched syngas product, while the 50 h time-on-stream catalytic performance deterioration of ～30–35% on Ni/ZrO₂ was limited to less than ～15–20% on the La₂O₃ and CeO₂ modified samples. Their influence on the amount and type of carbon formed was substantial: it was revealed that faster oxidation of the deposited carbon at elevated temperatures occurs on the modified catalysts. Correlations between the La₂O₃ and CeO₂-induced modifications on the surface characteristics and physicochemical properties of the catalyst with their concomitant support-mediated effects on the overall DRM performance and carbon deposition were revealed.     

Investigations of multiple injection strategies for the improvement of combustion and exhaust emissions characteristics in a low compression ratio (CR) engine

The experimental analysis was conducted for a better understanding of the combustion stability and reduction of exhaust emission in low compression ratio (CR) engine. The combustion stability was analyzed in terms of combustion pressure, the rate of heat release (ROHR), the indicated mean effective pressure (IMEP), and coefficient of variation of indicated mean effective pressure (COV_IMEP), and formation of exhaust emissions such as CO, HC, NO_X, and soot was measured and compared in the low compression ratio single cylinder CI engine.